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50 The Masterbuilder - May 2012 • www.masterbuilder.co.in
Literature Survey on Geopolymer Concretes and A Research Plan in Indian Context
The literature survey on Geopolymer Concretes (GPCs) presented in the first part of this paper indicated that qualitative information on available on the mechanical properties of GPC mixes is sufficient to develop GPCs for use in civil engineering structures. However, it is seen that with understanding of the similarities and difference between Portland cement and Geopolymer technologies, a rational research plan giving various steps involved can be formulated to achieve desired level of structural and durability related characteristics in structural grade GPC mixes. This aspect is discussed in this paper.
An Overview on Geopolymer Concretes
The general literature study on GPs, as presented earlier, indicate that often GPs are studied by many scientists at paste level, using processed materials such as Metakaolin and industrial waste materials such as fly ash and slag. GPCs are studied for structural applications by few scientists. It would be worthwhile to have an overview on GPC technology so that further studies on actual implementation of GPC technology for civil engineering applications as a rational alternate to P-C can be planned.
The geopolymers (GPs) initiated by Davidovits (1988) has great potential for adoption by concrete construction industry as an alternative binder to the Portland cement (Duxson et al, 2007). GPs could significantly reduce the CO2 emission to the atmosphere caused by the cement industries Gartner (2004). The alkaline liquid was proposed by Davidovits (1988; 1994) for reacting with the silicon (Si) and the Aluminium (Al) present in an alumino-silicate source material which may be either of geological origin or by-product materials such as fly ash and rice husk ash. Since, the chemical reaction involved is an inorganic polymerization process (but under alkaline condition), the word polymer in ‘Geopolymer’ to represent the new binders seems to be logical. The chemical composition of geopolymer material is similar to zeolitic materials, but the microstructure is amorphous. The fast chemical reaction
under alkaline condition of Si- Al minerals results in a three-dimensional polymeric chain and ring structure consisting of Si-O-Al- O bonds (Davidovits, 1994). The formation of geopolymer material can be described by following two Equations (1) and (2) (Davidovits, 1994; van Jaarsveld et al., 1997):
Rajamane N. P.1, Nataraja M. C.2, Lakshmanan N3, and Ambily P S 4
1Head, CACR, SRM University, 2Professor, Dept. of Civil Engg, SJCE,3Former Director, CSIR-SERC, 4Scientist, CSIR-SERC
The Equation 2 indicates that water is released during formation of geopolymers and forms discontinuous nano-pores in the matrix.
The atomic ratio Si: Al in the polysialate of geopolymer is selected based on the particular application and low Si: Al ratios are suitable for most of the civil engineering applications. Low-calcium (ASTM Class F) fly ash is more common and can be adopted to manufacture geopolymer concrete (GPC). Usually, 80% of the fly ash particles were smaller than 50 μm (Gourley, 2003; Gourley and Johnson, 2005; Hardjito and Rangan, 2005; Wallah and Rangan, 2006; Sumajouw and Rangan, 2006; Fernandez-Jimenez
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et al, 2006a; Sofi et al, 2006a; Siddiqui, 2007). The reactivity of low-calcium fly ash in geopolymer matrix is found to be adequate (Fernandez-Jimenez et al, 2006b). Coarse and fine aggregates of the conventional concretes are suitable to produce GPCs and grading curves are usually applicable to GPC mixes also (Hardjito and Rangan, 2005; Wallah and Rangan, 2006; Sumajouw and Rangan, 2006; Gourey, 2003; Gourley and Johnson, 2005; Siddiqui, 2007). It is recommended that the AAS is prepared at least 24 hours prior mixing of GPCs. Alkali silicate solutions are commercially available with different solid contents and molar ratios (MR). The ratio of [SiO2]/[M2O] is defined as MR where [SiO2] and [M2O] are contents of SiO¬2 (silica) and M2O (alkali oxide) in the alkali silicate. The sodium hydroxide is commercially available in the form of in flake or pellet.
The primary difference between geopolymer concrete and Portland cement concrete is the binder. The silicon and aluminum oxides in the low-calcium fly ash reacts with the alkaline liquid to form the geopolymer paste that binds the loose coarse aggregates, fine aggregates, and other un-reacted materials together to form the geopolymer concrete. The compressive strength and the workability of geopolymer concrete are influenced by the proportions and properties of the constituent materials that make the geopolymer paste. Experimental works of (Hardjito and Rangan, 2005) indicate:
- Higher concentration of sodium hydroxide results in higher strength of GPC.
- Higher the ratio of sodium silicate solution-to-sodium hydroxide solution ratio by mass, higher is the compressive strength of geopolymer concrete.
- The addition of naphthalene sulphonate-based super plasticizer can improves the workability of the fresh geopolymer concrete; however, there is degradation in the compressive strength of hardened concrete.
- The slump value of the fresh geopolymer concrete increases when the water content of the mixture increases.
Geopolymer concrete can be manufactured by adopting the conventional techniques used in the manufacture of Portland cement concrete. The GSM and the aggregates (SSD condition) are first mixed together dry a concrete mixer. The AAS, liquid component of GPC mixture, is then added and the mixing continued. The fresh GPC is cohesive and can be handled up to about 2hours (depending upon the formulation) without any sign of setting and without much effect on the compressive strength. Moulding of specimens, compaction, including workability measurement are similar
to Portland cement concrete (Hardjito and Rangan, 2005; Wallah and Rangan, 2006).
Rangan and his team at Curtin University had steam-cured the fly ash based GPC test specimens at 60ºC for 24 hours and then storing in ambient conditions till testing. Heat-curing substantially assists the chemical reaction that occurs in the geopolymer paste. Both curing time and curing temperature influence the compressive strength of geopolymer concrete and can be manipulated to fit the needs of practical applications. (Hardjito and Rangan, 2005).
A two-stage steam-curing regime was adopted by Siddiqui (2007) in the manufacture of prototype reinforced geopolymer concrete box culverts. It was found that steam curing at 80 °C for a period of 4 hours provided enough strength for de-moulding of the culverts; this was then followed by steam curing further for another 20 hours at 80 °C to attain the required design compressive strength. Also, the start of heat-curing of geopolymer concrete can be delayed for several days (say up to 5 days) without any degradation in the compressive strength. A delay in the start of heat-curing substantially increases the compressive strength of geopolymer concrete (Hardjito and Rangan, 2005). The role and the influence of aggregates in GPCs are considered to be the same as in the case of Portland cement concrete. Hardjito and Rangan (2005) suggests that the ratio of sodium silicate solution-to-sodium hydroxide solution by mass may be taken approximately as 2.5
Test data show that the strain at peak stress for fly ash based GPCs is in the range of 0.0024 to 0.0026 (Hardjito and Rangan, 2005). Collins et al (1993) have proposed that the stress-strain relation of Portland cement concrete in compression can be predicted using the parameters; peak stress and strain at peak stress.
The tensile splitting strength of geopolymer concrete is only a fraction of the compressive strength, as in the case of Portland cement concrete, but, larger than the values recommends by most of the Standards (such as AS3600, IS:456-2000), and Neville (2000) for Portland cement concrete (Sofi et al, 2007a; Hardjito and Rangan, 2005).
The unit-weight of concrete primarily depends on the unit mass of aggregates used in the mixture. Tests show that the unit-weight of the low-calcium fly ash-based geopolymer concrete is similar to that of Portland cement concrete (Hardjito and Rangan, 2005).
The drying shrinkage of heat-cured fly ash-based GPCs over a period of one year is significantly smaller than that experienced by Portland cement concrete. (Wallah and Rangan, 2006).
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The behaviour and failure modes of reinforced geopolymer concrete columns are similar to those observed in the case of reinforced Portland cement concrete columns and hence, reinforced low-calcium (ASTM Class F) fly ash-based geopolymer concrete structural members can be designed using the design provisions currently used in the case of reinforced Portland cement concrete members. (Sumajouw and Rangan, 2006). The studies carried out by Chang, et al (2007), Sarker, et al (2007a, 2007b), and Sofi, et al (2007b) demonstrate the application of fly ash-based geopolymer concrete.
Geopolymer has also been used to replace organic polymer as an adhesive in strengthening structural members. Geopolymers were found to be fire resistant and durable under UV light (Balaguru et al 1997)
Scientists, van Jaarsveld, van Deventer, and Schwartzman (1999) carried out experiments on geopolymers using two types of fly ash. They found that the compressive strength after 14 days was in the range of 5 – 51 MPa. The factors affecting the compressive strength were the mixing process and the chemical composition of the fly ash. A higher CaO content decreased the microstructure porosity and, in turn, increased the compressive strength. Besides, the water-to-fly ash ratio also influenced the strength. It was found that as the water-to-fly ash ratio decreased the compressive strength of the binder increased.
Palomo, Grutzeck, and Blanco (1999) studied the influence of curing temperature, curing time and alkaline solution-to-fly ash ratio on the compressive strength. It was reported that both the curing temperature and the curing time influenced the compressive strength. The utilization of sodium hydroxide (NaOH) combined with sodium silicate (Na2SiO3) solution produced the highest strength. Compressive strength up to 60 MPa was obtained when cured at 85ºC for 5 hours.
Xu and van Deventer (2000) investigated the geopolymerisation of 15 natural Al-Siminerals. It was found that the minerals with a higher extent of dissolution demonstrated better compressive strength after polymerisation. The percentage of calcium oxide (CaO), potassium oxide (K2O), the molar ratio of Si-Al in the source material, the type of alkali and the molar ratio of Si/Al in the solution during dissolution had significant effect on the compressive strength.
Swanepoel and Strydom (2002) conducted a study on geopolymers produced by mixing fly ash, kaolinite, sodium silica solution, NaOH and water. Both the curing time and the curing temperature affected the compressive strength,
and the optimum strength occurred when specimens were cured at 60°C for a period of 48 hours.
Van Jaarsveld, van Deventer and Lukey (2002) studied the interrelationship of certain parameters that affected the properties of fly ash-based geopolymer. They reported that the properties of geopolymer were influenced by the incomplete dissolution of the materials involved in geopolymerisation. The water content, curing time and curing temperature affected the properties of geopolymer; specifically the curing condition and calcining temperature influenced the compressive strength. When the samples were cured at 70ºC for 24 hours a substantial increase in the compressive13strength was observed. Curing for a longer period of time reduced the compressive strength.
Fly ash had been used in the past to partially replace Portland cement to produce concretes. An important achievement in this regard is the development of high volume fly ash (HVFA) concrete that utilizes up to 60 percent of fly ash, and yet possesses excellent mechanical properties with enhanced durability performance. The test results show that HVFA concrete is more durable than Portland cement concrete (Amphora 2002). Recently, a research group at Montana State University in the USA has demonstrated through field trials of using 100% high-calcium (ASTM Class C) fly ash to replace Portland cement to make concrete. Ready mix concrete equipment was used to produce the fly ash concrete on a large scale. The field trials showed that the fresh concrete can be easily mixed, transported, discharge, placed, and finished (Cross et al, 2005).
Pioneering research on fly ash-based geopolymer concrete was conducted at Curtin University and the results is described great details in widely referred Research Reports GC1 to GC4 (Hardjito and Rangan 2005, Wallah and Rangan 2006).
Directives from Literature for Research
The literature survey related to the use of aluminosilicate based binder, nomenclated now as geopolymer, is actually involves the activation of alumina and silica of any source material (of mineral origin such as fly ash, GGBS, MK, etc) or synthetically produced alumina, silica and their compounds in various forms. Since zeolites now have many commercial applications, the technology of production of these materials [which are also aluminosilicate in nature] is very advanced. Many attempts were made to use the test results of zeolites to geopolymers. But, still the field of geopolymers needs significant amount of research from engineers usage point of view as the standardized simpler models of chemical reactions are not yet available as it
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had happened in case of Portland cement where simple concepts as given below have helped engineers to develop the wide variety of uses of P-C based concrete, without need for deeper understanding of chemistry of hydration of P-C in the field:
- P-C is basically made of 4 Bogue’s compounds – C3A, C4AF, C3S and C2S.
- Bogue’s composition of any P-C can be computed arithmetically based on its oxide contents.
- Properties of P-C concretes can be accounted by relative amount of Bogue’s compounds in the cement. For e.g. for high early strength, C3S should be more and for high Sulphate Resistance, C3A should be less, etc.
- Water for complete chemical hydration reaction of any P-C is about 0.22 to 0.25 by weight of cement. This means, any hydrated cement portion of any matrix contains 22% to 25% by weight of unhydrated cement.
- Lower the W/C ratio, the strength and the denseness (hence degree of impermeability, thereby durability) increase.
- Setting and rheology (ie, behaviour of cement paste in freshly mixed state) can be controlled fairly reliably by well formulated and commercially available chemical admixtures.
- The microstructure of hydrated P-C consists of gel pores and capillary pores whose magnitude can be estimated by simple computations and the microstructure itself can be manipulated easily by processing conditions, addition of admixtures, etc.
- Degrees of hydration of cement can be measured approximately by many techniques, the simplest being the determination of non-evaporable water content of hydrated paste.
Apart from above, many practically useful tips are available to engineers, as a result of R&D on P-Cs spread over more than 2 centuries. Such a stage has not reached yet in case of geopolymers and hence, there is a need for more intense research. Towards this objective, the present study was taken up. Though the chemical nature of geopolymers is not fully understood, since geopolymers can be made from a variety of source materials and the activation of these source materials can be also carried out by many activating chemicals besides the numerous process conditions themselves. Keeping this in view, using the information available in the literature, following methodology can be utilised to develop various experimental programmes in the investigation taken up.
Stage 1: Selection of GSMs
The literature mentions Metakaolin as almost pure source for production of geopolymers, since it is mainly consists of alumina and silica which are basic building blocks of any geopolymer. Since MK is a processed material hence costlier both cost considerations and ‘Embodied Energy’ point of view, it was decided to adopt fly ash and blast furnace slag as the geopolymeric source materials (GSMs) since these are actually industrial waste products. Disposal problems of these wastes can be efficiently solved by using them in GPCs which can even become a good replacement material for conventional Portland cement based concrete.
To avoid the possibility of different types of unexpected/undesirable chemical reaction during GPs, due to variation in chemical and physical characteristics of FA and GGBS, one producer for each of them was identified and the same source materials were used throughout the experimental programme.
Stage 2: Selection of AAS
Alkali Activator Solution (AAS) required for initiating chemical reactions in GSMs consist of sodium hydroxide (NaOH) and sodium silicate solution (SSS). These are actually industrial chemicals used by many industries. For cost consideration and from practical point of view, it is necessary to use as much as possible the commercially available chemicals (instead of laboratory or reagent grades). Preliminary trials, with these chemicals, on production of GPCs indicated the adequacy of the industrial quality of chemicals for use in GPC mixes which themselves consist of fly ash, GGBS, sand and coarse aggregates, which are not themselves of any particular chemical purity, though for quality control purposes for their use in concretes, general practical guidelines for characterizing them are available in Standard Codes.
Sodium hydroxide, in flake from, (SHf) is readily available in market and hence adopted to prepare sodium hydroxide solution (SHS).
Sodium Silicate Solution (SSS) are actually available in different Molar Ratios whose chemical natures vary much from geopolymerisation consideration. Hence, a commercial producer was directly contacted and the SSS was obtained; the physical and chemical characteristics of this material were supplied by the producer himself.
Since, potable water may contain ions which can be taken as acceptable for drinking purpose, but may not be desirable from geopolymerisation, it was decided to use
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distilled water (DW) only in the experiments. However, it was noted that DW is very common commodity available commercially.
Since, a large number of combinations of alkali hydroxide and alkali silicate solution are reported in the literature to form AAS, a series of preliminary experiments was conducted on the typical GPC mixes to arrive at the acceptable proportion of SHf:SSS:DW to prepare AAS and the GPS/AAS ratio (i.e., solids/liquid ratio) for achieving desired workability of freshly mixed GPC mix.
Stage 3: Curing Type
By varying the proportion of AAS, it was possible to achieve desired level of setting / hardening rate in GPC mix such that the test specimens could be demandable after 24 hours of casting, without any need for applying heat or other external treatment for accelerating the geopolymerisation reaction. The main aim of the present work was to avoid steam or hot air exposure to the moulds containing GPC mix, before demoulding. However, information available in the published literature on the contribution of each component of AAS to setting was utilized in preparing AAS such that the moulding and demoulding operations remain essential, same as in the case of CCs.
Stage 4: Selection of typical GPC mixes for structural usage
The literature showed that slag can be activated at room temperature with AAS containing SHS and SSS. Hence, GGBS was basically utilized to formulate AAS and GPC mixes. It is widely reported in literature that generally fly ash alone when forms the GSM, it requires high temperature for activation and this was also the experience in the present work. However, the GPC mix made with GGBS (which was setting at room temperatures for the purpose of demoulding next day after casting) was modified by replacing GGBS partially with FA and still the modified GPC mixes (containing both GGBS and FA) enabled demoulding within 24 hours of casting. However, when the GGBS replacement level was high, there was need for changing the proportions of the AAS.
Stage 5: Mechanical And Durability Properties
Selected GPC mixes were evaluated for various mechanical strengths [fc, ft, fb, etc] including stress strain behaviour. This information is needed to decide whether the GPC mixes developed are different from CCs from structural behaviour considerations. Civil engineering structures are usually subjected to durability problems due to exposure to chloride ions, sulphates and acidic environment.
Hence, currently developed GPC mixes were also studied for durability against the above mentioned aggressive conditions.
Stage 6: Studies of Steel Reinforced GPCs
For any structural usage of concrete, steel reinforcement is a necessity. Hence, the GPCs developed were used to prepare typical beam and column specimens. For confirming the satisfactory structural behaviour of GPCs, the bond between the GPC mix and steel reinforcement should be also adequate. Therefore, this aspect is also to be studied in any planned investigation.
Stage 7: Ecological and economic benefits and practical application
Since the GPCs are required to be assessed for ecological benefits so that they can be accepted for structural application by engineers, the Embodied Energy (EE) and the CO2 Emission (ECO2e) per unit volume of GPCs were estimated using the information available in the literature on the EE of ECO2e of the individual ingredients of GPCs.
For practical application the utility of self curing nature of GPCs developed can be used to join precast slab panels and the joined whole slab could be tested for structural behaviour just after 24 hours of casting. Selected GPC mixes can be employed to produce building blocks of various types in actual block making factory.
A Research Plan in Indian Context
In order to achieve the above objectives and ensure desirable properties listed under Para 1.4, in concretes made with the new binder in the form of geopolymers, a comprehensive research work can aim towards:
- Selection of sources of FA and GGBS suitable to produce geopolymeric binders
- Formulation of ‘Alkaline Activator Solution’ (AAS), to activate FA and GGBS
- Development of ‘geopolymer concretes’ (GPCs) using different combinations of FA, GGBS and appropriate AAS, besides the aggregate system consisting of river sand and crushed granite stone aggregate
- Identification of proper curing regime
- Determination of demoulding time
- Evaluation of rates of strength development in concrete mixes
- Selection of GPC mixes suitable for use as structural grade concretes
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- Determination of stress-strain characteristics of GPCs- Investigation of the bond behaviour of GPC with steel
bars.- Evaluation of behaviour of reinforced GPC flexural
specimens (with different percentages of reinforcement) under flexural and shear
- Structural behaviour of steel reinforced GP concrete columns under uni-axial loading
- Durability aspects of concretes by testing for :- Permeability to water - Permeability to chloride ions- Corrosion of embedded rebar- Resistance to sulphate attack - Resistance to sulphuric acid attack - Statistical variability of characteristic strengths of GPCs - Effect of addition of steel fibres to GPCs- Effect of addition of replacement of ‘normal weight
coarse aggregates’ by fly ash based ‘lightweight coarse aggregates’ in GPCs
- Thermal properties of GPCs- Non-destructive evaluation by Ultrasonic Pulse Velocity,
electrical resistivity, etc.- Techno-economic feasibility of GPCs Vis a Vis Portland
Cement Concretes- Eco-friendliness of GPCs- Practical utilities of GPC such as self curing high early
strength jointing material for precast concrete, building/paver blocks
Concluding Remarks
Based on the literature study using the often mentioned qualitative guidelines and the much quantitative information available, it is possible to practically formulate the AAS for different combination of GGBS and class F fly ash to achieve strength levels useful in most common civil engineering application, in the present investigation. It is therefore possible to prepare typical structural members such as beams and columns using the GPCs [capable of being cured under ambient conditions only] for studying the possibility of applicability in existing structural design
guidelines to new composites. Durability related studies and a few practical applications should be carried out to understand comprehensively the utility of newly developed GPC mixes which are eco-friendly because of their lower carbon foot prints, compared to conventional Portland cement based concretes based on the computation for “Embodied Energy” and “Embodied CO2 emission” of the concretes.
The research plan proposed can generate enough data for engineers to start considering GPCs as desirable material of construction.
Abbreviations/Notations
AAS = Alkaline Activator Solution Alumina = Al2O3 CCs = Conventional concretes CGA = Crushed granite aggregates C-S-H = Calcium-silicate-hydrate DW = Distilled WaterECO2 = Embodied carbon dioxideEE = Embodied energyFA = Fly ashFAA = Fly Ash Aggregates GGBS = Ground Granulated Blast Furnace Slag GP = Geopolymer GPC = Geopolymer concreteHVFA = High volume fly ash IR = Infrared MK = MetakaolinMR = Molar ratios NMR = Nuclear Magnetic ResonanceOPC = Ordinary Portland CementP-C = Portland CementSHf = Sodium Hydroxide flakes SHS = Sodium hydroxide solution SiO2 = Silica SSD = Saturated surface drySSS = Sodium Silicate Solution W/C= Water-cement ratio
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Recycled Concrete as Aggregate for Structural Concrete Production
Demolition of old and deteriorated buildings and traffic infrastructure, and their substitution with new ones, is a frequent phenomenon today in a large part of
the world. The main reasons for this situation are changes of purpose, structural deterioration, rearrangement of a city, expansion of traffic directions and increasing traffic load, natural disasters (earthquake, fire and flood), etc. For example, about 850 millions tones of construction and demolition waste are generated in the EU per year, which represent 31% of the total waste generation [1]. In the USA, the construction waste produced from building demolition alone is estimated to be 123 million tons per year [2]. The most common method of managing this material has been through its disposal in landfills. In this way, huge deposits of construction waste are created, consequently becoming a special problem of human environment pollution. For this reason, in developed countries, laws have been brought into practice to restrict this waste: in the form of prohibitions or special taxes existing for creating waste areas.
On the other hand, production and utilization of concrete is rapidly increasing, which results in increased consumption of natural aggregate as the largest concrete component. For example, two billion tons of aggregate are produced each year in the United States. Production is expected
to increase to more than 2.5 billion tons per year by the year 2020 [2]. This situation leads to a question about the preservation of natural aggregates sources; many European countries have placed taxes on the use of virgin aggregates.
A possible solution to these problems is to recycle demolished concrete and produce an alternative aggregate for structural concrete in this way. Recycled concrete aggregate (RCA) is generally produced by two-stage crushing of demolished concrete, and screening and removal of contaminants such as reinforcement, paper, wood, plastics and gypsum. Concrete made with such recycled concrete aggregate is called recycled aggregate concrete (RAC). The main purpose of this work is to determine the basic properties of RAC depending on the coarse recycled aggregate content, and to compare them to the properties of concrete made with natural aggregate (NAC)—control concrete. Fine recycled aggregate was not considered for RAC production because its application in structural concrete is generally not recommended [3-6].
2. Basic Properties of Concrete with Recycled Concrete Aggregate
Based on available experimental evidence, the most
Prof. Dr. Vlastimir Radonjanin1, Mirjana Malešev1 and Snežana Marinković2
1Department for Civil Engineering, Faculty of Technical Sciences, Trg Dositeja Obradovica 2Faculty of Civil Engineering, University of Belgrade
A comparative analysis of the experimental results of the properties of fresh and hardened concrete with different replacement ratios of natural with recycled coarse aggregate is presented in the paper. Recycled aggregate was made by crushing the waste concrete of laboratory test cubes and precast concrete columns. Three types of concrete mixtures were tested: concrete made entirely with natural aggregate (NAC) as a control concrete and two types of concrete made with natural fine and recycled coarse aggregate (50% and 100% replacement of coarse recycled aggregate). Ninety-nine specimens were made for the testing of the basic properties of hardened concrete. Load testing of reinforced concrete beams made of the investigated concrete types is also presented in the paper. Regardless of the replacement ratio, recycled aggregate concrete (RAC) had a satisfactory performance, which did not differ significantly from the performance of control concrete in this experimental research. However, for this to be fulfilled, it is necessary to use quality recycled concrete coarse aggregate and to follow the specific rules for design and production of this new concrete type.
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important properties of recycled concrete aggregate (RCA) and concrete made with recycled aggregate (RAC) are briefly presented in this chapter. Recommendations for production of RAC are also presented.
When demolished concrete is crushed, a certain amount of mortar and cement paste from the original concrete remains attached to stone particles in recycled aggregate. This attached mortar is the main reason for the lower quality of RCA compared to natural aggregate (NA).
RCA compared to NA has following properties:
- Increased water absorption [7-9]- Decreased bulk density [3,10]- Decreased specific gravity [3] - Increased abrasion loss [3,11,12] - Increased crushability [3]- Increased quantity of dust particles [3] - Increased quantity of organic impurities if concrete is
mixed with earth during building demolition [3] and - Possible content of chemically harmful substances,
depending on service conditions in building from which the demolition and crushing recycled aggregate is obtained [3]
Available test results of recycled aggregate concrete vary in wide limits, sometimes are even opposite, but general conclusions about the properties of concrete with recycled coarse aggregate compared to concrete with natural aggregate are:
- Increased drying shrinkage up to 50% [13,14]- Increased creep up to 50% [13,15]- Water absorption increased up to 50% [3,16]- Decreased compressive strength up to 25%
[3,7,8,10,17]- Decreased splitting and flexural tensile strength up to
10% [3,8,17]- Decreased modulus of elasticity up to 45% [7,8,17] - Same or decreased frost resistance [3,18,19]
Technology of RAC production is different from the production procedure for concrete with natural aggregate. Because of the attached mortar, recycled aggregate has significantly higher water absorption than natural aggregate. Therefore, to obtain the desired workability of RAC it is necessary to add a certain amount of water to saturate recycled aggregate before or during mixing, if no water-reducing admixture is applied. One option is to first saturate recycled aggregate to the condition “water saturated surface dry”, and the other is to use dried recycled aggregate and to add the additional water quantity during mixing. The additional water quantity is calculated
on the basis of recycled aggregate water absorption in prescribed time.
Experimental Investigation
The aim of this investigation is to compare the basic properties of control concrete (concrete made with natural aggregate) and the properties of concrete made with different contents of recycled aggregate. Three concrete types were tested within the research program [20]. Mixture proportions of the tested concrete types were determined in accordance to the following conditions:
- Same cement content, - same workability after 30 min- Same maximum grain size (32 mm)- Same grain size distribution for aggregate mixture, -
same type and quantity of fine aggregate - Variable type and quantity of coarse aggregate
The type and quantity of coarse aggregate were varied in the following way:
- The first concrete mix had 100% of natural river coarse aggregate (R0), control mixture, - the second concrete mix had 50% of natural river coarse aggregate and 50% of recycled coarse aggregate (R50)
- The third concrete mix had 100% of recycled coarse aggregate (R100)
As all the other variables were kept constant, this research enabled us to determine the influence of the coarse recycled aggregate amount (0%, 50% and 100%) on tested concrete properties. The following properties of concrete were selected for testing:
- Workability (slump test) immediately after mixing and 30 minutes after mixing
- Bulk density of fresh concrete- Air content - Bulk density of hardened concrete - Water absorption (at age of 28 days) - Wear resistance (at age of 28 days) - Compressive strength fc (at age of 2, 7 and 28 days) - Splitting tensile strength (at age of 28 days)- Flexural strength (at age of 28 days) - Modulus of elasticity (at age of 28 days) - Drying shrinkage (at age of 3, 4, 7, 14, 21 and 28 days) - Bond between ribbed and mild reinforcement and concrete
Ninety nine specimens were made for testing of the listed properties of hardened concrete.
Component Materials
Component materials for concrete mixtures were:
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- Portland-composite cement CEM II/A-M(S-L) 42.5R, (Lafarge-BFC)
- Fine aggregate (river aggregate, separation Luka Leget, grain size 0/4 mm)
- Two types of coarse aggregate: river aggregate, separation Luka Leget, and recycled concrete aggregate, grain sizes 4/8, 8/16 and 16/31.5 mm
- Water
Fine and coarse natural aggregates were derived from River Sava and dominantly consist of quartz grains.
Recycled concrete aggregate was produced by crushing of “old” concrete cubes used for compressive strength testing and one precast reinforced concrete column, which had inappropriate dimensions (Figure 1). The strength class of old concrete cubes was C30/37 and the corresponding value of compressive strength for precast column was C40/50, nomenclature according to Eurocode 2 [21]. The primary crushing was done with a pneumatic hammer (Figure 1) and the secondary crushing was performed in a rotating crusher. The obtained material after the primary and secondary crushing is shown in Figure 2.
Crushed concrete particles were separated into standard fractions of coarse aggregate (4–8 mm, 8–16 mm and 16–31.5 mm), as seen in Figure 3.
Figure 1. Waste concrete for recycling: concrete cubes and precast column.
Figure 2. Recycled material after (a) primary and (b) secondary crushing.
Figure 3. Recycled concrete aggregate fractions. From left to right; 4–8 mm, 8–16 mm and 16–31.5 mm coarse aggregates.
All component materials were tested prior to mix proportion design. The results of natural aggregate testing are shown in Table 1 and grading curves are shown in Figure 4.
Tested property Measured value
Grain size Quality requirement
0/4 4/8 8/16 16/32
Crushing resistance (in cylinder) mass loss (%) - 14.0 18.6 23.8 <30
Freezing resistance test mass loss (%) 1.8 1.6 1.4 1.5 <12
Content of weak grains (%) - 0 0 0 <3(4)
Crushing resistance (Los Angeles test) mass loss (%) - 26.3 29.0 29.2 <30
Water absorption after 30 minutes (%) 0.7 0.4 0.4 0.3 -
Fines content (%) 1.6 0.23 0.15 0.12 <5(<1)
Specific gravity kg/m3 2,655 2,666 2,669 2,671 2,000–3,000
Bulk density, uncompacted kg/m3 1,611 1,490 1,470 1,460 -
Bulk density, compacted kg/m3 1,729 1,590 1,570 1,560 -
Table 1. Results of natural aggregate testing.
Figure 4. Grading curves of natural aggregate.
The results of recycled concrete aggregate testing are shown in Table 2 and grading curves in Figure 5.
Properties of natural and recycled concrete aggregate were tested according to Serbian standards for natural
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aggregates and quality requirements given in Tables 1 and 2 are also according to Serbian standard for natural aggregates: SRPS B.B2.010:1986 [22]. As it can be seen from Table 2, recycled aggregate with grain sizes of 8/16 and 16/32 don’t satisfy the weak grains content and crushing resistance quality requirements for natural aggregates. This was expected because of the mortar and cement paste attached to the stone particles in the recycled aggregate.
According to test results, natural river aggregate satisfies quality requirements given in [22] and cement satisfies prescribed quality requirements given in EN 197-1:2,000 [23].
3.2. Mix Proportion Design
Concrete mix proportions were calculated according to above listed conditions and are shown in Table 3. Dried recycled aggregate, basic water content and additional water quantity were used to achieve the required workability of RAC.
Water absorption of recycled aggregates was studied in time intervals for a total of 24 hours. By analyzing the results, it was found that the major changes in the quantity of absorbed water occur in the first 30 minutes. On the other hand, it is known that the major change in the consistency of “ordinary concrete” (without chemical admixtures) occurs in the first 20–30 minutes. Also, after production, concrete must be transported to the site. Taking into account the underlying attitudes, 30 minutes from the moment of adding water to the concrete mixer was adopted as the reference time for the required workability.
Additional water quantity was calculated on the basis of water absorption of recycled aggregate after 30 minutes, Table 2.
The substitution of natural coarse aggregate with recycled aggregate is made by weight, provided that all mixtures have the same granulometric composition, corresponding to the Fuller’s curve (Dmax = 31.5 mm). Percentage participation of each aggregate fraction in aggregate mixture is given in Table 4 and corresponding quantity of each aggregate fraction is given in Table 5.
3.3. Results of Fresh Concrete Testing
Calculated real amounts of component materials and test results of workability (Figure 6), air content and bulk density for all three concrete types are presented in Table 6.
By analyzing the results of fresh concrete, shown in Table 6, it was concluded that:
- Approximately the same workability after 30 minutes was achieved for all three concrete types using the additional water for concrete R50 and R100 (Figure 6b). Figure 5. Grading curves of recycled concrete aggregate.
Tested property Measured value Grain size Quality requirement
4/8 8/16 16/32
Crushing resistance (in cylinder) mass loss (%) 18.3 26.7 30.7 <30
Freezing resistance test mass loss (%) 2.0 1.4 1.0 <12
Chemical testing (mortar part of recycled aggregate)
chloride content 0 0 0 <0.1
sulfate content in traces in traces in traces <1.0
pH 9.85 9.85 9.85 -
Content of weak grains (%) 0 3.7 7.1 <3 (4)
Crushing resistance (Los Angeles test) mass loss (%) 29.6 33.7 34.0 <30
Water absorption after 30 minutes (%) 4.59 2.87 2.44 -
Fines content (%) 0.45 0.23 0.36 <1.0
Specific gravity kg/m3 2,346 2,458 2,489 2,000–3,000
Bulk density, uncompacted kg/m3 1,275 1,239 1,236 -
Bulk density, compacted kg/m3 1,388 1,323 1,325 -
Table 2. Results of recycled concrete aggregate testing.
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Concrete mixture
Cement (kg/m³
Effective water (kg/m³
Aggregate (kg/m³)
Additional water (kg/m³)
Effective water-cement
ratio
Total water-ce-ment ratio
Bulk density (kg/m³)
R0 350 180 1857 0 0.514 0.514 2,387
R50 350 180 1816 19 0.514 0.569 2,365
R100 350 180 1776 37 0.514 0.620 2,343
Table 3. Design quantities of component materials.
Concrete type Natural river aggregate Recycled concrete aggregate
0/4 4/8 8/16 16/32 4/8 8/16 16/32
R0 33 16 21 30 0 0 0
R50 33 8 10.5 15 6.5 7.5 19.5
R100 33 0 0 0 13 15 39Table 4. Percentage participation of each aggregate fraction in aggregate mixture.
Concrete mixture Content of natural river aggregate (kg/m³) Content of recycled aggregate (kg/m³)
0/4 4/8 8/16 16/32 4/8 8/16 16/32
R0 612 298 390 556 0 0 0
R50 600 145 191 272 118 136 354
R100 586 0 0 0 231 266 693Table 5. Design amounts of different aggregate fractions.
Figure 6. Slump test (a) after mixing and (b) after 30 minutes
- Concrete mixture R50 requires about 10% more total water quantity in comparison to mixture R0, and the corresponding value for concrete mixture R100 is about 20%.
- Differences in air content ( p) are insignificant. Air content in fresh concrete was determined by standard test method that is based on Boyle-Mariotte’s Law. In [26] was concluded that the air content of the RAC is higher than concrete made with NA at 100% replacement. However, the author used a gravimetric method for calculation of total air content, including aggregate porosity.
- Bulk density of concrete depends on aggregate type and quantity. The highest bulk density has concrete with natural aggregate (R0) and the lowest concrete with maximum content of recycled aggregate (R100). The bulk density decrease is about 3%.
3.4. Results of Hardened Concrete Testing
Measured compressive strengths of concrete R0, R50 and R100 at age of 2, 7 and 28 days [24], are shown in Table 7 and they represent average values. For each concrete type the following number of specimens (15 cm cubes) were used: three specimens/age 2 days, three specimens/age 7 days and six specimens/age 28 days. Standard deviation for the compressive strength results at age of 28 days is also shown in Table 7.
Measured values of drying shrinkage of concrete R0, R50 and R100 are shown in Table 8. The specimens were three prisms (10 × 10 × 40 cm) for each concrete type. An extensometer with 25 cm base was used for measuring.
Results of the testing of other properties of the hardened concrete are presented in Table 9. Each property of hardened concrete was tested on a group of three appropriate specimens at the age of 28 days. Water absorption of concretes R0, R50 and R100 was tested on 15 cm cubes. Splitting tensile strength of concrete was tested on 15 cm cubes, and flexural strength on 10 × 10 × 40 cm prisms. All tests were performed according
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Concrete mixture
Cement (kg/m³)
Total water (kg/m³)
Aggregate (kg/m³)
Water/ cement ratio1
Aggregate/ cement
ratio
Slump2 (cm)
Slump3 (cm)
Air content (%)
Bulk den-sity (kg/m³)
R0 352 181 1866 0.514 5.306 16 10 1.5 2,399
R50 352 200 1826 0.568 5.188 14.5 8.5 1.4 2,378
R100 348 216 1765 0.620 5.074 11 9 1.3 2,329
Table 6. Results of fresh concrete testing.
Concrete type Concrete age (days) Standard deviation
(MPa)2 7 28
R0 (MPa) 27.55 35.23 43.44 1.5769
R50 (MPa) 25.74 37.14 45.22 1.2089
R100 (MPa) 25.48 37.05 45.66 3.5016R50/R0 (%) 93 105 104
R100/R0 (%) 92 105 105
Table 7. Concrete compressive strength and relative compressive strength at different ages.
Con-crete type
4 days (mm/
m)
7 days (mm/
m)
14days (mm/
m)
21 days (mm/
m)
28 days (mm/
m)
Rela-tive
drying shrink-age*,
%
R0 0.017 0.124 0.203 0.277 0.339 100
R50 0.036 0.086 0.176 0.254 0.306 90
R100 0.091 0.204 0.251 0.335 0.407 120
Table 8. Drying shrinkage at different concrete ages.
to Serbian standards for testing the hardened natural aggregate concrete properties.
Cylindrical specimens with a diameter of 10 cm and height of 15 cm and with embedded ribbed and mild reinforcement (12 mm diameter) were used for testing the bond between reinforcement and concrete R0, R50 and R100. The length of the embedded part of reinforcement was 15 cm. For this testing, an axial tension procedure and tearing device were used (Figure 7).
Relative values R50/R0 and R100/R0 for properties presented in Table 9 are shown graphically in Figure 8.
Concrete type R0 R50 R100
Water absorption, (%) 5.61 6.87 8.05
Splitting tensile strength, (MPa) 2.66 3.20 2.78
Flexural strength, (MPa) 5.4 5.7 5.2
Wear resistance, (cm³/50 cm) 13.40 15.58 17.18
Modulus of elasticity (GPa) 35.55 32.25 29.10
Bond between mild reinforcement and concrete, MPa
6.48 5.87 6.76
Bond between ribbed reinforcement and concrete, MPa
8.22 7.50 7.75
Table 9. Other properties of hardened concrete at age of 28 days.
Figure 7. Testing of bond between concrete and reinforcement.
3.5. Discussion of Hardened Concrete Properties
To describe the development of concrete compressive strength fc with time (t), a fraction Function (1) was adopted:
Figure 8. Relative values R50/R0 and R100/R0 for properties of hardened concrete.
(1)
Calculated parameters of this functional relation (“a” and “b”) for concrete R0, R50 and R100, together with correlation coefficient (“r”), are presented in Table 10. Values of correlation coefficients point to the fact that the chosen fraction function realistically represents the development of compressive strength with time for all three tested concrete types.
The test results of concrete compressive strength at age
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of 2, 7 and 28 days (Table 7) and established functional relations fc(t) for concrete R0, R50 and R100 are illustrated in Figure 9.
Concrete type a b r
R0 44.242 1.320 0.976
R50 47.556 1.761 0.997
R100 48.116 1.856 0.996
Table 10. Parameters of functional relationship between the compressive strength and age of the concrete.
Analysis of the concrete compressive strength values points to the following:
- All three concrete types have approximately the same compressive strength development with time
- All three concrete types have 28-day compressive strength that is larger than 40 MPa
- Differences between compressive strengths of concrete R0, R50 and R100 are negligible for the same concrete age
To find out if differences between obtained compressive strengths of concrete R0, R50 and R100 at age of 28 days are significant or not, differences between their mean values were statistically tested according to method in [25]. For that purpose, pairs of corresponding 28-day strength were formed (R0–R50, R0–R100 and R50–R100). Tested value is defined with expression:
(2)
(3)
Criterion: (4)
where:
t0 = quintile of Student distribution for number of degree of
freedom = n1 + n2 − 2 xav,1 = average value (set I) xav,2 = average value (set II) n1 = number of test results (set I) n2 = number of test results (set II) t = critical value of Student distribution for number of degree of freedom = n1 + n2 − 2 1 = standard deviation (set I) 2 = standard deviation (set II)
Results of this statistical test are shown in Table 11.
On the basis of the results presented in Table 11 and Criterion (4), it was concluded that differences between measured compressive strengths of concrete R0, R50 and R100 are insignificant (all results belong to the same set of results). This conclusion led to the fact that coarse aggregate type didn’t influence the concrete compressive strength value in this experimental research. This conclusion is opposite to results of other authors [8,31,33], who found that compressive strength decreases with increasing quantity of recycled aggregates in concrete with the same effective water-cement ratio. However, in these experiments, recycled aggregate was obtained from demolished concrete structures of unknown compressive strength. Hansen [3] find out that substitution of natural aggregate with recycled concrete aggregate up to 30% has no significant influence on concrete compressive strength.
Figure 9. The compressive strength of concrete at various ages.
Test pairs n1 n2 s to t , for = 0.05
(R0 and R50) 6 6 1.406523 2.1899242.2281(R0 and R100) 6 6 2.7163 1.417718
(R50 and R100) 6 6 2.61943 0.29425
Table 11. Testing of difference significance for concrete compressive strength.
Our results confirm the statement that compressive strength of RAC depends more on the quality of recycled aggregate than on the quantity.
According to the analysis of the 28-day drying shrinkage values (Table 8), it is concluded that:
- The lowest shrinkage rate was for concrete R50 (0.3 mm/m), and the highest for R100 (0.4 mm/m),
- Drying shrinkage of concrete R100 is 20% higher than shrinkage of concrete R0,
- Difference between 28-day shrinkage of concrete R0 and R50 is less than 10%.
The obtained results for drying shrinkage of RAC correspond to results of other authors, who found larger or smaller values for drying shrinkage compared to NAC [10,14,29]. The test results of wear resistance are shown in Figure 10. It is concluded that the highest material loss
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occurs for concrete R100 and the lowest for concrete R0.
The analysis of water absorption values (shown in Figure 11), points to the following:
- The lowest water absorption was registered in concrete R0 and the highest in R100,
- Concrete R50 has 22% higher absorption, while concrete R100 has 44% higher absorption than control concrete R0.
Figure 10. Test results of concrete wear resistance.
By using the same statistical method as for the analysis of measured values of splitting tensile strengths (Table 9), it was concluded that differences between measured splitting tensile strengths are insignificant (all results belong to the same set of results). The same conclusion is drawn for flexural strength results (Table 9). Hansen [3] states that both tensile strengths of RAC are maximum 10% less than the tensile strength of NAC. Other papers [27,28] and [29] also confirmed that RAC tensile strength is not significantly affected by the amount of recycled coarse aggregate.
Our analysis of the obtained values of bond between mild and ribbed reinforcement and concrete R0, R50 and R100 (Table 9) shows that:
- Difference between lowest and highest bond for both reinforcement types is about 10%,
- Bond between tested concretes and ribbed reinforcement is higher at least 15% than bond between tested concretes and mild reinforcement.
Although all three concrete types have similar compressive strength, the modulus of elasticity is lower for the concrete with recycled aggregate, Table 9. This decrease depends on the content of the recycled aggregate and maximum decrease is for the concrete with maximum recycled aggregate content. The modulus of elasticity of concrete R100 is lower than the modulus of elasticity of control concrete R0 by about 18%. The same decreasing of modulus of elasticity was found in research [14].
3.6. Load Testing of Reinforced Concrete (RC) Beams
Tested concrete types (R0, R50 and R100) were used for producing RC beams (beams “R0”, “R50” and “R100”). Three beams with a length of 3.0 m and rectangular cross section of 15/25 cm were prepared for flexural testing. Beams were reinforced with ribbed reinforcement 3R 12 in the lower zone, 2R 10 in the upper zone and with stirrups 6/20 (Figure 12).
Details of the production of the beams for experimental testing is shown in Figures 13 and 14.
The maximum (failure) load was calculated for the “RO” beam. Stresses in concrete and reinforcement, deflections and characteristic cracks width were calculated using the program CREEP (authors M. Tatomirovic, P. Pavlovic). Calculated values for the beam with referent concrete R0—beam “R0” are shown in Table 12.
Figure 11. Test results of concrete water absorption.
Figure 12. Characteristic dimensions of RC beams and arrangement of reinforcement.
At the age of 28 days, the beams were subjected to load testing (bending with concentrated force in the middle of the span). The arrangement of measuring spots for registering deflections and strains in the concrete and reinforcement is shown in Figure 15. The load was increased in six phases
Figure 13. Moulds with placed reinforcement.
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Figure 14. Finishing of the beams concrete surface.
until failure of the beams. During load testing, the following data were registered: deflections, reinforcement strains, concrete strains, arrangement and width and length of cracks.
An example of the appearance and development of cracks during the load testing of beam R50 is presented graphically in Figure 16. The photo of a crack pattern in the middle part of the span, after failure, is shown in Figure 17.
By analysis of registered cracks on all tested RC beams, it was concluded:
- First crack appears in the middle of the span in the third load phase (P = 20 kN).
- The maximum width of cracks after collapse is between 2.0 and 2.7 mm.
Phase Load (kN) Beam edge Stress in concrete (MPa) Stress in reinforcement (MPa) Deflection (mm) Crack width (mm)
I 5 UpperLower
4.075 65.569
0.46 0.017
II 10 UpperLower
7.278 117.092
1.54 0.062
III 20 UpperLower
13.683 220.139
3.87 0.137
IV 30 UpperLower
20.087 323.186
6.10 0.207
V 40 UpperLower
26.492 426.233
8.33 0.276
VI 50 UpperLower
32.897 529.279
failure
Table 12. Calculated values for cross-section in the middle of the span (beam “R0”).
Figure 15. Arrangement of measuring spots throughout the beam. (U—deflection; T—strain in reinforcement; D—strain in concrete).
- Similar disposition and width of cracks was registered on all tested RC beams.
The measured deflections and stresses in concrete in the middle of the span are presented in Table 13. Measured stresses in the concrete are based on measured concrete strains.
For the purpose of comparing beam behavior during loading, the calculated deflections of beam “R0” and measured deflections of all three beam types are presented in Figure 18.
In the elastic area all tested beams have similar deflection, which means that for appropriate load level the quantity of coarse recycled concrete aggregate has no significant influence on the beam behavior. Fanthifazl [34] had the similar conclusion in regard to the behavior of beams exposed to bending.
Figure 17. Crack pattern after collapse of beam R50.
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Figure 16. Development of cracks during load testing of beam R50.
Phase Load kN Deflection, (mm) Concrete compressive stress, (MPa)
R0 R50 R100 R0 R50 R100
I 5 0.55 0.67 0.73 - - -
II 10 0.89 1.21 1.37 1.32 2.64 3.04
III 20 2.68 2.78 2.94 7.00 8.05 8.71
IV 30 4.66 5.97 6.89 12.80 10.96 14.78
V 40 7.43 10.52 11.78 20.20 21.12 24.02
VI 50 failure
Table 13. Measured deflections and stresses in concrete in the middle of the span.
At the higher values of the test load, deflection depends on the type and quantity of used aggregate (with increasing quantity of recycled aggregate, the deflection value is increasing also). The different values of modulus of elasticity of used concrete types are the main cause for recorded behavior of tested beams in the post elastic area.
According to these test results, concrete compressive stresses depend on the type and quantity of used aggregate. With increasing of recycled aggregate content up to 100%, concrete compressive strength is increasing up to 25%.
Conclusions
On the basis of our comparative analysis of test results
Figure 18. Calculated and measured values of deflection of all tested beams.
of the basic properties of concrete with three different percentages of coarse recycled aggregate content (0%, 50% and 100%), the following conclusions are made.
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The way of preparing recycled aggregate for concrete mixtures influences the concrete workability: workability of concrete with natural and recycled aggregate is almost the same if “water saturated—surface dry” recycled aggregate is used. Also, if dried recycled aggregate is used and additional water quantity is added during mixing, the same workability can be achieved after a prescribed time. Additional water quantity depends on the time for which the same workability has to be achieved. It is determined as water quantity for which the recycled aggregate absorbs for the same period of time.
Bulk density of fresh concrete is slightly decreased with increasing quantity of recycled aggregate. The type of coarse aggregate has no influence on the air content in concrete.
Concrete compressive strength mainly depends on the quality of recycled aggregate. If good quality aggregate (obtained by crushing higher strength class concrete as in this case) is used for the production of new concrete, the recycled aggregate has no influence on the compressive strength, regardless of the replacement ratio of natural coarse aggregate with recycled aggregate. The same conclusion is valid for concrete tensile strength (splitting and flexural).
The water absorption of concrete depends on the quantity of recycled aggregate. The amount of absorbed water is proportionally increased with increasing recycled aggregate content. Water absorption depends on the porosity of cement matrix in the new concrete and porosity of cement matrix of the recycled concrete: if recycled aggregate is produced from low porosity waste concrete, water absorption of the new concrete depends on the achieved structure of the new cement matrix.
Wear resistance of the concrete depends on the amount of recycled aggregate. Concrete wear resistance decreases with increasing recycled aggregate content, due to the increased quantity of hardened cement paste, which wears easier than grains of natural aggregate.
The modulus of elasticity of concrete also decreases with increasing recycled aggregate content as a consequence of lower modulus of elasticity of recycled aggregate compared to natural aggregate.
Shrinkage of concrete depends on the amount of recycled concrete aggregate. Concrete with more than 50% of recycled coarse aggregate has significantly more shrinkage compared to concrete with natural aggregate. Increased shrinkage is a result of the attached mortar and cement paste in the recycled aggregate grains.
The bond between recycled aggregate concrete and
reinforcement is not significantly influenced by recycled concrete aggregate, because it is realized through new cement paste.
According to these test results, the performance of recycled aggregate concrete, even with the total replacement of coarse natural with coarse recycled aggregate, is mainly satisfactory, not only in terms of the mechanical properties, but also the other requirements related to mixture proportion design and production of this concrete type. The only two properties those are lower than for the natural aggregate concrete properties are the modulus of elasticity and shrinkage deformation. Because of that, it is not recommended to apply this type of concrete for structural elements for which large deformations can be expected. Also, this type of concrete shouldn’t be used for structures exposed to aggressive environment conditions without appropriate previous testing, as there are opposing conclusions about durability-related properties of RAC in existing literature [3,16,30,32].
Based on the results of the load tests on the reinforced concrete beams, it is concluded that used coarse aggregate type and quantity has no significant influence on the pattern and width of cracks. First, crack appears in the middle of the span at a load level equal to about one third of the ultimate load regardless of the concrete type. The measured crack widths were approximately the same for all three tested beams.
On the other hand, concrete compressive stresses depend on the quantity of recycled concrete aggregate for all load phases. Increasing the quantity of coarse RCA up to 100% increased the concrete compressive stress up to 25% in these tests.
Deflections of tested beams do not depend on the type and quantity of used aggregate in the elastic area—similar deflections were registered regardless of the concrete type. However, in the post elastic area, with increasing quantity of coarse recycled aggregate the deflection value increased. The deflection increase compared to deflection of the “R0” beam is 4% for the “R50” beam and 10% for the “R100” beam for the service load level. The main reason for such behavior of the tested beams is a lower modulus of elasticity of concrete types R100 and R50 in comparison to referent concrete R0.
All the conclusions made in this work about the tested properties of fresh and hardened concrete and consequently, about the behavior of beams subjected to bending, are valid for recycled aggregate concrete produced with quality recycled aggregate, obtained from demolished concrete with good mechanical properties, as it was the case in this experimental research.
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Acknowledgements
The work reported in this paper is a part of the investigation within the research project TR 16004—Utilization of recycled aggregate concrete in reinforced concrete structures, supported by the Ministry for Science and Technology, Republic of Serbia.
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Steel structures have been extensively used in seismic areas worldwide because of their favorable mass-to-stiffness ratio, ductility and hence, enhanced
energy absorption capacity. Indeed, the typical steel frame configurations, i.e., moment resisting frame (MRF), con-centrically braced frame (CBF) and eccentrically braced frame (EBF), exhibit different behavior with regard to stiffness, strength and ductility. Indeed, MRFs provide a satisfactory strength and possess an excellent ductility but they suffer large story drifts due to low lateral stiffness. By contrast, CBFs are capable of ensuring both required strength and stiffness, but buckling failures limit the global ductility. EBFs combine the strength and the stiffness of the CBF with the ductility of MRF; therefore their intermediate behavior results in agreement with the stiffness, strength and ductility required in seismic design, thus limiting the
structural damage during earthquake loading. On the other hand, MRFs exhibit damage generally limited to nonstructural components, while structural and nonstructural may be found in CBFs. Similarly, composite MRFs show damage concentrated in infills, claddings and other non-structura components; while buckled and/or yielded braces characterize the seismic response of CBFs.Steel bridges constitute a large number of the existing bridges worldwide. Corrosion, lack of proper maintenance, and fatigue sensitive details are major problems in steel bridges. It has been reported that 88,000 bridges in the US and Canada are structurally deficient, while the number of functionally obsolete bridges is over 82,000.
During the earthquakes in Northridge (1994) and Kobe (1995), extensive and unexpected damage was observed
Sonjoy DebB.Tech,’Civil’. N.I.T.Silchar, Research Scholar, Indian Institute of Technology
Retrofitting Techniques for Steel Structures
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in many framed steel and composite buildings. Indeed, several brittle fractures were detected in welded MRFs particularly at beam to column connections. The damage was found in a great population of buildings, with different heights (one story to about 25 stories) and ages (up to 30 years old), thus showing that steel structures are vulnerable to seismic loads (Refer Figure 1,2,3). Some other ateel structure failure have been shown in Figure 4 and Figure 5.
Many steel bridges are in need of upgrading to carry larger loads and increasing traffic volumes. The cost for retrofitting in most cases is far less than the cost of replacement. In addition, retrofitting usually takes less construction time, and therefore, reduces service interruption time.
Current methods of retrofitting steel bridges and structures
It typically utilize steel plates that are bolted or welded
Figure 1. Damage to old steel buildings in the Kobe earthquake: collapse (left), construction with light gaugedsections (middle) and corroded sections (right)
Figure 2. Damage to nonductile braces in Kobe earthquake: net fracture at bolt holes (left) and severe distortion ofunstiffened beam in chevron braces (right)
Figure 3. Brittle fracture of beam bottom flanges in welded MRF connection during Northridge earthquake: fracture propagating through column web and flange (left) and fracture causing a column divot fracture (right)
to the structure. However,constructability and durability drawbacks are associated with this method. Steel plates require heavy lifting equipment and can add considerably more dead load to the structure, which reduces their strengthening effectiveness. The added steel plates are also susceptible to corrosion, which leads to an increase in future maintenance costs. In many cases, welding is not a desired solution due to fatigue problems associated with weld defects. On the other hand, mechanical details such as bolted connections, which have better fatigue life, are time consuming and costly.Figure 6 and Figure 7 demonstrates typical repair procedures in steel structures.
Figure 4. Yielding and buckling of diagonal braces in San Fernando earthquake
Figure5 . Local buckling in box column of Pino Suarez high rise buildings in Mexico City
The need for adopting durable materials and cost-effective retrofit techniques is evident. One of the possible solutions is to use high performance, nonmetallic materials such as fiber reinforced polymers(FRP). The superior mechanical and physical properties of FRP materials make them quite promising for repair and strengthening of steel structures.
The use of FRP systems for retrofit of concrete structures has been successful. Its effectiveness has been demonstrated for a variety of retrofitting mechanisms. Today, the use of glass and carbon FRP materials for retrofit of concrete bridges is becoming more widely accepted in practice. FRP is used in the form of sheets or plates attached to the concrete surface for flexural and shear retrofitting or as sheets for wrapping columns to increase their ductility and axial strength. The
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US Department of Transportation(USDOT) and Federal Highway Administration (FHWA) have sponsored several projects that have led todesign guidelines for bridge repair and both have implemented these retrofitting schemes into severalprojects.
Bonding of FRP materials to metallic structures was first used in mechanical engineering applications. Carbon fiber reinforced polymer (CFRP) laminates have been successfully used to repair damaged aluminum and steel aircraft structures. Bonding of composite laminates was also shown tohave many advantages for marine structures. For civil engineering structures, previous work conducted on the strengthening of metallic structures using CFRP has been focused in three main areas:strengthening of iron or unweldable steel girders, rehabilitation of corroded steel girders, and repair offatigue damaged riveted connections.
The low-tensile modulus of glass fiber composites (GFRP), ranging between 72 GPa, and 87GPa, makes them less desirable for retrofitting steel structures. On the other hand, CFRP display soutstanding mechanical properties, with a typical tensile strength and modulus of elasticity of more than1,200 MPa and 140 GPa, respectively. In addition, CFRP laminates weigh less than one fifth of the weight of a similar size steel plate and are also corrosion resistant. CFRP plates or sheets can be bonded to the tension face of the member to enhance its strength and stiffness. By adding CFRP layers, the stress level in the original member will decrease, resulting in a longer fatigue life.
Figure 6. Typical repairs for buckled (left) and fractured (right) flanges of steel beams
When considering the retrofit of steel structures using FRP materials versus retrofit using steelplates, there are two considerations that favor FRP materials. First, the costs associated with retrofitting are often more associated with time limitations for completing the project, as well as labor costs and the costs to divert traffic, and to a lesser extent, with material costs. Due to the light weight of FRP composite materials, it is expected that they could be installed in less time than by strengthening with the equivalent number of steel plates. The second factor that favors composites, especially CFRP, is its higher tensile strength in comparison to the yield strength of steel, provided that adequate means of bonding are introduced.
A case study was performed to examine the economic advantages of rehabilitation of damaged steel girders with CFRP pultruded laminates as compared to replacement of the girders in Delaware,bridge girders, with a total length of 180 meters. The Girders were replaced due to severe and extensive damage. The replacement costs were compared with the cost of rehabilitation at an assumed 25 percent section loss. It was concluded that total replacement cost was 3.65 times higher than the cost of rehabilitation.
Figure 7. Retrofitting measures for beam-to-columns (post-Northridge): (a) cover plates, (b) triangular ribplates, (c) haunches and (d) RBS
Retrofit of Steel Girders
Research efforts to examine the feasibility and efficiency of retrofit of steel girders have been mainly conducted using one or combination of the following approaches:
- Repair of naturally deteriorated steel girders.- Repair of an artificially notched girder to simulate fatigue
cracks or section loss due to corrosion.- Strengthening of an intact section to increase the
flexural strength and stiffness.- Retrofit of steel girders in composite action with a
concrete deck.
Fatigue Behavior of Steel Sections Retrofitted with FRP
The use of steel plates to repair and strengthen existing steel structures has been traditionally used forrehabilitation of steel girders. However, the welded detail of steel plates is sensitive to the fatigue loads.Various researchers have examined the effectiveness of using bonded CFRP sheets or plates to improvethe fatigue strength.
Gillespie et al conducted fatigue testing on two strengthened girders, which were removed from an old bridge as indicated
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earlier. The two specimens were tested in fatigue under 10 millioncycles, within the stress range expected in the field. Throughout the 10 million cycles, the CFRP plateswere periodically monitored and inspected for debonding. No evidence of CFRP plate debonding wasfound.
Bassetti et al tested the effect of bonding prestressed CFRP plates on reducing the rate ofcrack propagation and increasing the fatigue life of riveted steel structures. Two research programs wereconducted on both small- and full-scale specimens. CFRP plates of 1.2 mm thickness were attached tocentral-notched specimens. The specimens were loaded with a stress range of 80 MPa and a stress ratio of 0.4. The results showed that the crack growth rate was drastically decreased and the fatigue life wasincreased by a factor up to twenty, depending on the prestressing level. The authors, however, did not report the details of the prestressing technique or procedure.
Surface Preparation and Bonding of FRP
Surface preparation is the key to a strong and durable adhesive bond. Since rehabilitation takes place onsite, surface treatment must also be environmentally friendly, and easily accomplished in field conditions.Brockmann has shown that application of the CFRP material can occur up to 150 hours after completion of the surface preparation. If strengthening occurs after this time, a lower bond strength coul dresult.Surface grinding or sand blasting is recommended to remove all rust, paint, and primer from the steel surface. Additionally, the bare steel may be pretreated using either an adhesion promoter or aprimer/conditioner, which leaves a thin layer attached to the metal oxide surface. This type of bond significantly improves the long-term durability because water displacement through this coating isunlikely since the hydrolysis of the primary bonds is a slow process. The bonded side of the FRP plates may be sanded to increase the surface roughness using medium grit sandpaper or a sandblaster, and wiped clean with acetone. However, excessive surface preparation of FRP plates may expose the surface of the carbon fibers leading to possible galvanic corrosion if placed in direct contact with the steel surface. Theadhesive is then applied to the pretreated steel surface, bonding either FRP laminates or sheets to the steel. The adhesive typically used is a two-component viscous epoxy. A less viscous epoxy is used for bonding the laminates to each other. It is recommended to leave the bonded plates to cure for a sufficient time, not less than 48 hours. Milleretal. suggested application of an accelerated curing method, suchas heating blankets or induction heater to increase the curing rate of the adhesive.An adhesive for a particular rehabilitation scheme must perform three functions. First, the adhesive must have adequate bond strength so that
the composite material can be optimally utilized.
Force Transfer
Force transfer between FRP and steel takes place through bond at the interface between the two materials,which is influenced by several factors including bonded length, types of fiber and resin, surface preparation, thickness of adhesive and thickness of FRP laminate. Experimental and analytical studies were performed by Miller to quantify the force transfer of a 457 mm CFRP plate bonded to the tension flange of a steel girder. It was found that approximately 98 percent of the total force transfers within the first 100 mm of the end of the bonded plate. Therefore, it was determined that the development length was on the order of 100 mm for this type of plate and adhesive.
Analysis has also shown that the epoxy failure at the ends of the FRP laminates or plates is due to high peeling stresses normal to the surface. Abushaggur and El Damatty developed a finite elementmodel to predict the distribution of peeling stresses for a beam subjected to four-point bending. The distribution shows a symmetric behavior about the center of the beam. It was found that the critical locations for peel off failure are towards the edges of the FRP sheets.
Durability of Steel Members Retrofitted with FRP
One of the most important factors affecting durability is the environmental surroundings. The FRP retrofitting system itself is non-corrosive, however, when carbon fibers become in contact with steel, agalvanic corrosion process may be generated. Three requirements are necessary for galvanic corrosion to occur between carbon and steel: an electrolyte (such as salt water) must bridge the two materials, there must be electrical connection between the materials, and there must be a sustained cathodic reaction onthe carbon. By eliminating any one of these requirements, the galvanic cell is disrupted. A good selection of adhesives with inherent durability and high degree of resistance to chlorides, moisture, and freeze-thaw cycles is also very important.In order to test the durability of the bond between the composite and steel, tests were conducted using the wedge test. This test has great sensitivity to environmental attack on the bond between materials. The test was proved to have a very high degree of correlation with service performance and isconsidered more reliable than conventional lap shear or peel tests.
Field Applications
Field installations demonstrate that the rehabilitation technique can be applied under actual fieldconditions. The rehabilitation and associated pre- and post-field diagnostic testing allow for furtherevaluation of the effectiveness of the
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retrofit system in providing stiffness and strength increases forstructures.
The 1-704 bridge, which carries southbound I-95 traffic over Christina Creek, just outside of Newark, was chosen by Delaware Department of Transportation to assess the CFR Prehabilitation process conducted by the University of Delaware. One layer of CFRP plates was bonded to the outer face of the tension flange of the steel girder, which has a span of 7500 mm and a W24x84 cross section. Six CFRP plates were placed side-by-side to cover the entire flange width. The CFRP plates were installed over thefull length by using four over lapped sections. Each section was 1500 mm long and had staggered joints.At the joints between the plate sections, a 300 mm stagger of every other CFRP plate was used.Consecutive CFRP plates were beveled at a 45o angle to form a scarf joint instead of a typical butt joint.Load tests were performed on the chosen girder, prior to and after the rehabilitation. A comparison between the load test data indicated that adding a single layer of CFRP plates resulted in 11.6 percent increase in girder stiffness, and 10 percent decrease in strain.Two historic metallic bridges in the UK were also strengthened with CFRP plates . The firstbridge was the Hythe Bridge, which had eight inverted T sections (cast iron beams) of 7800 mm span.Four prestressed CFRP plates were bonded to each beam by epoxy adhesive in addition to the end anchorages. The prestressing level was designed to remove all tensile stresses. The second bridge was Slattocks. The bridge beams were 510 mm deep and 191 mm wide, and supported a reinforced concrete deck. CFRP plates of 8 mm thickness were bonded to the bottom flanges of 12 innermost beams. A feasibility study was done and indicated that it would have cost much more to install a set of special traffic lights to control vehicle flow for traditional bridge repairs as it has for the total strengthening workusing CFRP plates, where the traffic was allowed to keep moving over the bridges during the strengthening process.
Conclusions
Research interests in the field of retrofit of steel structures using FRP materials are gradually increasing.Although using FRP for retrofit of steel structures has not yet gained the same popularity and wide spreaduse as in concrete structures, the literature to date shows positive and promising evidence of success. Thefollowing conclusions could be drawn:
1. The use of FRP sheets and strips is not only effective for restoring the lost capacity of a steel section,as a repair technique, but is also quite effective in strengthening of steel structures to resist higher loads.
2. Epoxy bonded FRP sheets and laminates are quite
promising in extending the fatigue life of steel structures. The FRP has a significant effect on reducing the crack propagation.
3. Strengthening using FRP results in increasing the yielding load of the steel section. Consequently, the service load can be increased.
4. The galvanic corrosion may be initiated when there is a direct contact between steel and CFRP, the steel and the CFRP are bridged by an electrolyte and there is a sustained cathodic reaction on theCFRP. Precautions can be taken to eliminate this problem by using a non conductive layer betweenthe carbon and steel or by protecting the area from moisture ingress.
5. Delamination of FRP in the compression side of the girder could occur before delamination in the tension side due to buckling of the laminate. Therefore, bonding the FRP reinforcement to the compression side may not be as effective as bonding it to the tension side.
6. The lower value of modulus of elasticity of all currently available FRPs, including CFRP, incomparison to steel, may result in increasing the number of layers required to increase the stiffness ofthe section and consequently could affect the cost effectiveness of such technique.
7. As the number of FRP layers increases, the efficiency for utilizing the full strength of the FRP material decreases, since the stress in the FRP laminate for one layer was much higher than that formultiple layer system. It has been shown that the thicker the reinforcing material, the higher the chance of bond failure. Consequently, balanced design should be considered to effectively utilize thestrength of CFRP laminates.
8. Strengthening the tension flange of I-sections with FRP will result in increasing their moment capacity. Consequently, lateral torsional buckling of the compression flange may control the failure.Therefore, such strengthening technique is more effective when sufficient lateral supports of compression flange are provided as in composite sections.
9. Applying prestressing force to CFRP plates is very efficient in retrofitting of steel structures subjected to fatigue loads. It prevents further cracking by promoting crack closure effect, which increases the stiffness of the cracked sections.
10. Four-point bending tests show small influence of the FRP bond length on the delamination failure mode, especially when the entire bonded length of FRP is located within the constant moment zone.Three-point bending tests have shown the importance of the bond length on this particular mode offailure, due to the
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presence of shear stresses along the entire span of the beam.
Reference
- Armstrong, K. B. “Carbon Fibre Fabric Repairs to Metal Aircraft Structures.” The Third TechnologyConference on Engineering with Composites, London, England, SAMPE European Chapter, 8.1-8.12
- Karbhari, V. M., and Shulley, S. B. “Use of Composites for Rehabilitation of Steel Structures Determination of Bond Durability.”Journal of Materials in Civil Engineering. ASCE, Vol. 7, No. 4,November 1995, pp. 239-245.
- Allan, R. C., J. Bird, and J. D. Clarke.“Use of Adhesives in Repair of Cracks in ShipStructures.”Materials Science and Technology. Vol. 4, No. 10, October, 1988, pp. 853-859.Hashim, S. A. “Adhesive Bonding of Thick Steel Adherents for Marine Structures” MarineStructures. Vol. 12, 1999, pp. 405-423.
- Gillespie, J. W., Mertz, D. R., Kasai, K., Edberg, W. M., Demitz, J. R., and Hodgson, I.Rehabilitation of Steel Bridge Girders: Large Scale Testing. Proceeding of the American Society forComposites 11th Technical Conference on Composite Materials, 1996, pp. 231-240.
- Miller, T. C., Chajes, M. J., Mertz, D. R., and Hastings, J. N. “Strengthening of a steel bridge girder using CFRP plates.” Journal of Bridge Engineering,ASCE, Vol.6, No. 6, November/December 2001,514-522.
- Bassetti, A., Liechti, P., and Nussbaumer, A. Fatigue Resistance
and Repairs of Riveted Bridge Members. Fatigue Design ’98, Espoo, Finland, pp. 535-546.
- Brockmann, W. Steel Adherents. In Kinloch, A. J., Ed. Durability of Structural Adhesives. AppliedScience Publishers, London, 1993, pp. 281-316.
- Mays, G. C. and A. R. Hutchinson.Adhesives in Civil Engineering. Cambridge University Press, New York, NY, 1992.
- Miller, T. C. The Rehabilitation of Steel Bridge Girders Using Advanced Composite Materials. Master’s thesis.University of Delware, Newark, Del., 2000.
- Abushaggur, M., and El Damatty, A. A. Testing of Steel Sections Retrofitted Using FRP Sheets. Annual Conference of the Canadian Society for Civil Engineering.Moncton, Nouveau-Brunswick,Canada.June, 4-7, 2003 (CD-ROM).
- Francis, R. Bimetallic Corrosion. Guides to Good Practice in Corrosion Control, National PhysicalLaboratory, 2000.
- Scardino, W. M., and Marceaue, J. A. Comparative Stressed Durability of Adhesively Bonded Aluminum Alloy Joints.Proc., Symp. On Durability of Adhesive Bonded Struct., U.S. Army Armament Res.And Devel.Command, Dover, N. J., 1976.
- Luke, S., and Mouchel Consulting. The Use of Carbon Fibre Plates for the Strengthening of Two Metallic Bridges of a Historic Nature in the UK.FRP Composites in Civil Engineering, Vol. II J.-G.Teng (Ed.), pp. 975-983.
- Retrofit of Steel Structures Using Fiber Reinforced Polymers (FRP): State-of-the-Art, AmrShaat, David Schnerch, Amir Fam and Sami Rizkalla
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Steel curving/bending is a manufacturing process by which metal can be deformed by plastically deforming the material and changing its shape. The material is
stressed beyond its yield strength but below its ultimate tensile strength. There is little change to the materials surface area. Bending generally refers to deformation about one axis only. Bending is a flexible process by which a variety of different shapes can be produced though the use of standard die sets or bend brakes. The material is placed on the die, and positioned in place with stops and/or gages. It is held in place with hold- downs. The upper part of the press, the ram with the appropriately shaped punch descends and forms the v-shaped bend. Bending is done using Press Brakes. Press Brakes can normally have a capacity of 20 to 200 tons to accommodate stock from 1m to 4.5m (3 feet to 15 feet). Larger and smaller presses are used for diverse specialized applications. Programmable back gages, and multiple die sets currently available can make bending a very economical process.
Steel Curving/ Bending allows to create various architectural shapes, which is not feasible with traditional way of construction. It also allows considerable savings in the construction cost and the durability aspect of the structure is an added bonus. This story aims to provide the readers with the existing methods of bending, types of bending processes. At the end a few case studies are taken to give readers an over view of what wonders can be created with steel curving/ bending.
Methods of Bending
There are five typical methods of bending in the industry: rolling, incremental bending, hot bending, rotary-draw bending, and induction bending. Each method has its advantages. Some methods are more commonly used in the steel construction industry, while others are more common in the automobile or manufacturing industries:
- Rolling (cold bending) is the typical method of curving steel for construction and is usually the most economical for rolling members with tighter radii. A steel member is placed in a machine and curved between three rolls. Cold bending may also be called “pyramid rolling”
because of the three rolls’ pyramid arrangement. Bending occurs when the distance between these rolls is manipulated before each successive pass.
- Incremental bending or gag pressing is usually used for cambering and curving to very large radii. Bending is achieved by applying point loads with a hydraulic ram or press at the member’s third point.
- Hot bending is where a structural member is heated directly and then bent. The heat source could be a direct flame or furnace. This application is used extensively in repair.
Sonjoy DebB.Tech,’Civil’. N.I.T.Silchar, Research Scholar, Indian Institute of Technology
The Fine Art of Steel Bending
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- Rotary-draw bending is where the structural member is bent by rotating it around a die. The member is clamped into a form and then is drawn through the machine until the bend is formed. This method produces tight radii and is mainly used for complicated bends in the machine and parts industry.
- Induction bending uses an electric coil to heat a short section of a structural member, and then that member is drawn through a process similar to rotary-draw and cooled with water directly after. In some cases, this process can produce a smaller, tighter radius.
Types of Bending
(A) Air Bending
It is a bending process in which the punch touches the work piece and the work piece does not bottom in the lower cavity. As the punch is released, the work piece springs back a little and ends up with less bend than that on the punch (greater included angle). This is called spring-back. The amount of spring back depends on the material, thickness, grain and temper. The spring back will usually range from 5 to 10 degrees. The same angle
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is usually used in both the punch and the die to minimize set-up time. The inner radius of the bend is the same as the radius on the punch. In air bending, there is no need to change any equipment or dies to obtain different bending angles because the bend angles are determined by the punch stroke. The forces required to form the parts are relatively small, but accurate control of the punch stroke is necessary to obtain the desired bend angle.
(B) Bottoming
Bottoming is a bending process where the punch and the work piece bottom on the die. This makes for a controlled angle with very little spring back. The tonnage required on this type of press is more than in air bending. The inner radius of the work piece should be a minimum of 1 material thickness. In bottom bending, spring-back is reduced by setting the final position of the punch such that the clearance between the punch and die surface is less than the blank thickness. As a result, the material yields slightly and reduces the spring-back. Bottom bending requires considerably more force (about 50%~60% more) than air bending.
(C) Coining
Coining is a bending process in which the punch and the work piece bottom on the die and compressive stress is applied to the bending region to increase the amount of plastic deformation. This reduces the amount of spring-back. The inner radius of the work piece should be up to 0.75 of the material thickness.
(D) V Bending
In V-bending, the clearance between punch and die is constant (equal to the thickness of sheet blank). It is used widely. The thickness of the sheet ranges from approximately 0.5 mm to 25 mm.
(E) U Die Bending
U-die bending is performed when two parallel bending axes are produced in the same operation. A backing pad is used to force the sheet contacting with the punch bottom. It requires about 30% of the bending force for the pad to press the sheet contacting the punch.
Figure 1. Tube bending to form 60ft-high parabolic arches for McDonald’s
Figure 2. Supersized tube bending for supersized golden arches.
Figure 3. The tube bending created multiple radii to minimize costly and time-consuming weld splices.
(F) Wiping Die Bending
Wiping die bending is also known as flanging. One edge of the sheet is bent to 90 while the other end is restrained by the material itself and by the force of blank-holder and pad. The flange length can be easily changed and the bend angle can be controlled by the stroke position of the punch.
(G) Double Die Bending
Double die bending can be seen as two wiping operations acting on the work piece one after another. Double bending can enhance strain hardening to reduce spring- back.
(H) Rotary Bending
Rotary bending is a bending process using a rocker instead of the punch. The advantages of rotary bending are:
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Figure 4. Tube bending of 402 tons of 12x12 for the roof trusses of the University of Phoenix Stadium.
- Needs no blank-holder- Compensates for spring-back by over-bending- Requires less force- More than 90 degree bending angle is available
General bending guidelines are as follows:
- The bend radius should, if possible, be kept the same for all radiuses in the part to minimize set up changes.
- For most materials, the ideal minimum inner radius should be at least 1 material thickness.
- The minimum flange width should be at least 4 times the stock thickness plus the bending radius. Violating this rule could cause distortions in the part or damage to tooling or operator due to slippage.
- Slots or holes too close to the bend can cause distortion of these holes. Holes or slots should be located a minimum of 3 stock thickness plus the bend radius. If it is necessary to have holes closer, then the hole or slot should de extended beyond the bend line
- Dimensioning of the part should take into account the stack up of dimensions that can happen and mounting holes that can be made oblong should be.
- Parts should be inspected in a restrained position, so that the natural flexure of the parts does not affect measurements. Similarly inside dimensions in an inside bend should be measured close to the bend.
Some Structures with Steel Curving (Bending)/ Examples of Steel Curving (Bending)
Figure 5. Contractor’s curving technology for tube bending saved more than 80,000 lbs. of steel.
Figure 6. Beam bending for a series of half ellipses.
Figure 7. Reserve Curving of Steel member
McDonald’s Arches
Chicago Metal Rolled Products have curved large rectangular tubing to form the parabolic arches for the new flagship McDonald’s which opened in downtown Chicago. The tube bending company matched the customer-supplied templates putting multiple radiuses into 50-foot-long tube to minimize costly weld splices and to reduce the time required for fabrication and erection on the fast-paced project. To meet the project’s tight schedule, Chicago
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Metal completed all the tube bending within three days after the customer supplied the material. The new design has two 60-foot-tall arches that span much of the entire site and help support the roof of the two-story restaurant. Each large arc is comprised of two 20 x 12 tubes covered by plate on all four sides. The arches are 20 inches wide and vary in thickness from 36 inches at the base to 24 inches at the top (Refer to Figure 1, 2 and 3).
University of Phoenix Stadium
For the roof trusses of the University of Phoenix Stadium in Glendale, Arizona, Chicago Metal Rolled Products’ tube bending machines curved 402 tons of 12 x 12 x 5/8 and 12 x 12 x ½ square tubing to a variety of radiuses from 1000 to 1200 feet. Across the width of the field span 256-foot-long lenticular trusses so-called because both the top and bottom chords are curved, creating a profile that resembles a convex lens. Tube bending from Chicago Metal Rolled Products of sixteen such trusses are incorporated in the two retractable roof panels. (See Figure 4 and 5).
Illinois Institute of Technology Train Tube
A new McCormick Student Center at the Illinois Institute of
Figure 8. A 20 Tonne beam bent to the shape of a semi circle.
Technology in Chicago, designed by Dutch architect Rem Koolhaus, was to be linked to Chicago’s elevated “El” train system. Koolhaas’ solution to train noise was to create a steel-and-concrete tube to encase trains as they pass over the single-story, building. Beam bending provided by Chicago Metal Rolled Products produced 104,000 pounds of W12 x 58# beams the “hard way” to form a series of half ellipses with radiuses of approximately 12’, 24’ and 34’. (See Figure 6).
Ratner Athletic Center at the University of Chicago
Beam bending to form a reverse curve saved over $24,000 worth of weld splices (See Figure 7).
Fabricators in U.S. claim to put curves into wide-flange beams up to 44 inches tall that weigh 285 pounds per foot and do it “the hard way”—along the longest axis of the cross section. Its latest equipment acquisition is the largest beam roller ever built for anyone (See Figure 8).
Conclusion
It has often been observed that steel bending is more often an art than a mere skill. And this is true. But it is also true that in order to produce quality bends on time and at a competitive price for an ever more demanding OEM (original equipment manufacturer) market, a machine operator must be trained not only in machine operation but also in the principals of lean manufacturing. Whether the operator is doing steel bending in the factory of an OEM or in the factory of a contract manufacturer that supplies the OEM, he or she should understand the principals of lean manufacturing which is also called Just-in-time manufacturing, world-class manufacturing, Japanese manufacturing techniques, or the Toyota way. There are differences in what principals are promulgated in each of these approaches, but they all share some fundamental values. And it is important for the machine operator to understand and apply them.
Reference
- http://www.cmrp.com/documents/Fabricator%20Article%20May%202001.pdf
- www.maxweiss.com
- www.kubesteel.com
- www.bendtec.com
- ht tp:/ /www.modernsteel .com/Uploads/ Issues/May_2007/30765_bender-rollers.pdf
- http://www.225steel.com/steel-bending.htm
- http://www.cmrp.com
Photo courtesy
www.steelmasterusa.com
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In high-rise construction and bridge construction the need for predetermined erection procedures and temporary support systems has long been established
in the industry. Low-rise construction does not command a comparable respect or attention because of the low heights and relatively simple framing involved. Also the structures are relatively lightly loaded and the framing members are relatively light. This has lead to a number of common fallacies which are supported by anecdotal evidence. This article will guide you how to carry out the erection of steel structures for low-rise buildings step by step.
Three of the most important things that are to be noted before starting the job is as follows
(a) After receiving the building package and before storing, all the items are required to be checked for any defects
and quantities as per the list, if a single part is missing, the entire work will suffer. Hence the owner should make a check list of all the items and verify it while receiving.
(b) Materials needs to be properly stored and handled at site during construction to avoid any undue damage.
(c) The owner should ask the contractor an errection plan as well as safety action plan for executing the job.
Following are the broad steps for erection of bracing and other parts of low-rise structured steel buildings
A. Site and Foundation PreparationB. Building Delivery and StorageC. Erection of Primary, Secondary Structural and Doors
and WindowsD. Sheeting (Wall and Roof)
Sonjoy DebB.Tech,’Civil’. N.I.T.Silchar, Research Scholar, Indian Institute of Technology
Erection Bracing of Low-Rise Structured Steel Buildings
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A. Site and Foundation Preparation
Before doing the concrete foundation it is extremely important that foundation should be properly checked for its width and length and most importantly for equality of both the diagonals. After this all the column locations to be marked on the foundations very accurately and anchor bolts to be fixed then. This is a very important step, as any mistake in this step will effect the entire erection programme, since all the structures are fabricated for predetermined sizes and mistake in any span would give birth to complexity in the erection process. Hence great care to be taken while carrying out the anchor bolt setting plan. All anchor bolts should be held in place with a template or similar means, so they will remain plumb and in the correct location during placing of the concrete. Check the concrete forms and anchor bolt locations prior to the pouring of the concrete. A final check should be made after the completion of the concrete work and prior to the steel erection. This will allow any necessary corrections to be made before the costly erection labor and equipment arrives.
B. Building Delivery and Storage
If observed closely, massive threaded steel rods sticking up from the cement in groups of four can be seen. This is where the steel structure will be bolted down to the cement.
While receiving the materials at site, place the parts around the foundation so they will be in the most convenient locations for installation. For example: place the end columns and rafters at the ends of the building and the mainframe columns and rafters at the sides. Place the bolts and nuts in a place where they will be accessible to the parts. You may want to screw the bolts and nuts together and place them with the corresponding parts. This will save time as you begin assembling the parts and also will reduce your time and cost for re shifting the materials again to the location of erection. Purlins and girts, depending on the number of bundles, are usually stored near the sidewalls clear of other packages or parts. Sheet packages are usually located along one or both sidewalls off the ground and sloping to one end to encourage drainage in case of rain. Accessories are usually unloaded on a corner of the slab or off the slab near one end of the building to keep them as much out of the way as possible from the active area during steel erection. For storage of sheets it is recommended to be stored under roof if at all possible. If sheets are to be stored outside, the following precautions should be observed:
1. The storage area should be reasonably level, and located so as to minimize handling.
2. When stored on bare ground, place a plastic ground cover under the bundles to minimize condensation on the sheets from ground moisture.
3. Store bundles at least 12 inches above ground level to allow air circulation beneath the bundle, and to prevent damage from rising water.
4. Elevate one end of each bundle slightly to permit runoff of moisture from the top of the bundle or from between sheets. A waterproof cover should be placed over the bundles to allow for air circulation under the cover.
5. Inspect stored bundles daily and repair any tears or punctures in the waterproof cover.
6. Re-cover opened bundles at the end of each workday to prevent subsequent moisture damage.
C. Erection of Primary, Secondary Structural and Doors and Windows
General
Many methods and procedures are in use for erecting the structural portion of metal buildings. The techniques of raising frames vary from erecting small clear spans and endwall frames in units to erecting the larger clear spans and modular frames in sections. The erection methods used depend strictly on the type of building, the available equipment, the experience level of the crews, and the individual job conditions. The variations in these factors preclude the establishment of a firm or specific set of
It is essential to keep the materials required at the site before hand to facilitate and fast and speedy process
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erection rules and procedures. Consequently, the erection operation must be tailored by the erector to fit individual conditions and requirements. However, there are certain erection practices, pertaining to structural members, which are in general use and have proven sound over the years and which can be followed for erection in all the places. In every condition Erectors are cautioned not to cut primary members (rigid frame columns, rafters, end bearing frame rafters, interior columns). These are the primary support members for the frame and are designed as such. Any cutting of these members may affect the structural stability.
The intermediate or interior frames nearest the bearing endwall are usually erected first. This bay usually contains the diagonal bracing. The proper completion and plumbing of this first bay is extremely important to the successful completion of the building. Although several methods are used to erect rigid frames, it has been found most satisfactory to erect the columns first, tie them together with the girts and tighten the anchor bolts. On small spans and short eave heights, columns can often be set in place by hand without the use of hoisting equipment. Temporary bracing should always be installed as soon as sections are lifted in place (See Figure 1).Once this is over the structure is ready for girt erection. At first it is needed to put one screw in the end of each girt. At the corners put 2 screws in the end of the girts. The girt erection is shown in Figure 2a and 2b.
Once the sidewalls have been stood and all girts are on them, wind rod braces needs to be installed (See Figure 3a
Figure 1. Truss column anchored to foundation concrete.
and 3b). These go in the same bay on both sides, preferably near the center of the length of the building. Care should be taken that these are not installed where there is a door or window opening. Once installed, these can be used to adjust the columns to be plumb. Once the columns are plumb make sure everything is snug.
Figure 2. Girt erection and bolting procedure.
Figure 2. (a) Installation of Wall wind brace rods, (b) Detail of wind brace rod mounting bracket
(a) (b)
After this attach the girts to the clips on the column and end wall legs using a single tek screw in each end. After ensuring that columns are straight then only attach the knee braces to the girts. The knee braces will bolt to the column or truss and attach to the girt with a tek screw. If columns are twisted it is needed to straighten them before attaching the knee brace (See Figure 4).
Figure 4. Attaching knee braces to truss leg
After erection of columns and installation of girts on the sidewalls the structure is ready for the roof trusses erection (See Figure 5,6). Roof trusses should be bolted together on the ground and lifted into place. Two important point to be noted before truss erection is as below
(a) Knee brace angles should be installed on the truss before lifting into place.
(b) If building is wider than forty feet it is recommended that a spreader bar should be used to pick up the trusses. If it is not used, the truss may fold in the middle and cause damage.
Knee brace fixing to the roof truss and attaching roof truss to the legs is shown in Figure 7 and 8 below.
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Figure 5. Lifting roof truss arms into place
Figure 6. Using lift to raise roof trusses into place
Figure 7. Knee braces attached to truss
Figure 8. Attaching roof trusses to legs
Figure 9. Attaching purlins to completed trusses
Figure 10. Detail of attached purlins showing tek screw spacing
Attaching purlins to the truss top with joint details is shown in Figure 9 and 10
Installation of wind braces in the roof truss in the same panel where it is attached in the wall is shown in Figure 11. Figure 12a and 12b shows installation of eave clip to the end of pulin.
Figure 11. Installation of wind brace rods in roof in the same panel where it is installed in wall
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Figure 12. (a) 6” Track installed to eave clip before nested (b) 6” Track on ends of purlins. purlin is put in.
How to Make a Framed Opening (See Figure 13a and 13b)
1. Measure the size of the door or window to determine the size opening required.
2. Mark the girts where you want to position the opening. 3. Cut the girts with your abrasive saw. 4. Slide the track over the end of the cut girts to form the
opening. 5. Insert the door or window into the opening and square
it so that it functions properly. 6. Fasten the track to the girts with tek screws and fasten
the door or window to framed opening. 7. Attach J trim with colored screws if it isn’t part of the
door or window frame.
(a) (b)Figure 13. (a) Fixing of Window (b) Fixing of Doors
Once this step is complete the sheeting job can be started. However before proceeding for sheeting job, it is necessary to recheck and make sure everything is properly installed at this point. If anything is missing it is the time to go back and fix it before sheeting job is started.
Checklist for the erection job (To be carried out prior to start if sheeting job)
1. Columns and Endwallcolumns
- All columns are properly located- All columns are plumb- All bolts are in place and tightened
2. Roof Trusses
- All roof trusses are properly in place- All bolts are in place and tightened- Building is plumb
3. Purlins and Girts
- All purlins and girts are in place and well attached - Fascia purlins are installed - 6” gable track is installed
4. Bracing
- All knee braces are bolted and tightened on columns and roof trusses
- All knee braces are screwed to girts and purlins- All wind brace rods are in place - All wind brace rod nuts should be snugged down
against the clip after building is plumb
D. Sheeting (Wall and Roof)
After completing the check the structure is ready for sheeting erection. Gang drill is recommended for drilling the sheets. The process is shown in Figures 14, 15 and 16. Not more than 20 sheet panels to be stacked together.
Figure 15. Not more than 20 sheets to be stacked
Figure 16. Drilling of sheets in progress
Figure 14. Gang drilling is recommended for even screw lines.
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Figure 17. Wall panel erection and check for verticality
Figure 18. Roof Sheet installation
roofing sheets (See Figure 18). After the roofing installation it is required to check whether all the bolts caps and washers are in place or not otherwise there is possibility of leakage during rains.
Final step is to put the corner corner trim and gable trim (See Figure 19a) and end cap ( See Figure 19b).
Conclusion
It is recommended to go through across the building after completion of the work to ensure that there are no missing screws or loose parts. Everything should be checked twice to make sure it is tight and secure. During the entire construction period the owner should look after the safety aspect of the work very carefully viz. personal protective equipment’s are allotted to every worker or not, safety nets are there or not, lifting capacity of the crane/chain pulley etc. It is also recommended to make a proper work schedule day wise before starting the work in along with the contractor which will help the owner to keep a track on the erection process and avoid getting undue delay in the work.
Reference
- Erection Bracing of Low-Rise Structural Steel Buildings, James M. Fisher, PhD, P. E. and Michael A. West, P. E.Computerized Structural Design Milwaukee, Wisconsin.
- Prefabricated Steel Building Installation Manual, MUELLER INC, METAL BUILDING, ROOFING AND COMPONENTS.
- Building Erection Manual, Worldwide Steel Buildings.
Photo Courtesy
www.people.fsv.cvut.czwww.trinityrising.blogspot.inwww.newburymarket.com
Figure 19. (b) Installation of end plate at top corner
Figure 19. (a) Installation of corner trim
Wall panels are installed before roof. Base trim must be attached first with 4 screws per piece. At every location the verticality of the wall panel is to be checked.
Once the wall panel is installed, next step is installation of
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Sonjoy DebB.Tech,’Civil’. N.I.T.Silchar, Research Scholar, Indian Institute of Technology
Flat Plate Flooring Systems:The “Win-Win” Solution
A flat plate floor system is a two-way concrete slab supported directly on columns with reinforcement in two orthogonal directions (Figure 1). Primarily
used in hotels, multi-family residential buildings, and hospitals, this system has the advantages of simple construction and formwork and a flat ceiling, the latter of which reduces ceiling finishing costs, since the architectural finish can be applied directly to the underside of the slab. Even more significant are the cost savings associated with the low-story heights made possible by the shallow floor system. Smaller vertical runs of cladding, partition walls, mechanical systems, plumbing, and a large number of other items of construction translate to large cost savings, especially for medium and high-rise buildings. Moreover, where the
total height of a building is restricted, using a flat plate will result in more stories accommodated within the set height. The thickness of a flat plate is controlled by the deflection requirements given in Sect. 9.5.3 of ACI 318-05. Minimum slab thicknesses for flat plates with Grade 60 reinforcing bars, are laid out in ACI 9.5.3 and it is a function of the longest clear span between supports. Flat plate systems are economically viable for short to medium spans and for moderate live loads. Up to live loads of about 50 psf, the deflection criteria usually govern, and the economical span length range is 15 ft to 25 ft. For live loads of 100 psf or more, punching shear stresses at the columns and bending moments in the slab control the design. For these cases, the flat plate is economical for spans between 15 ft and 20 ft.
Floor Systems Flat Plate Construction
Flat Plate Flooring Systems:The “Win-Win” SolutionSonjoy DebB.Tech,’Civil’. N.I.T.Silchar, Research Scholar, Indian Institute of Technology
www.masterbuilder.co.in • The Masterbuilder - May 2012 129
A flat plate floor with a live load of 100 psf is only about 8% more expensive than one with a live load of 50 psf, primarily due to the minimum thickness requirements for deflection. Floor panels with an aspect ratio of 2 would be about 30% more expensive than panels with an aspect ratio of 1; the thickness of the rectangular panel is governed by the greater span length, resulting in a loss of economy. On average, the formwork costs for flat plates represent approximately 46% of the total floor cost. Concrete material, placing, and finishing account for about 36% of the cost. The remaining 18% is the material and placing cost of the mild reinforcement. Application of flat plate floor system in realtime structure is shown in Figure 2a, 2b and 2c.
The advantages of the flat plate system are thin structure, simple formwork, and flat soffits. The integral interaction of 2-way slab allows for wider distribution of moment capacity and therefore a large effective width for carrying moment. This results in the ability to use a thin structure to support the required loads. The simplicity of a flat concrete slab with repetitive bays lends itself well to construction efficiency. Flat soffits are of particular advantage to construction of an apartment building or hotel where ceiling finishes will
Figure 1. Plate Plate System
Figure 2. (a) Flat Plate Floor System (Practical Applications)
Figure 2. (b) & 2c Flat Plate Floor System (Practical Applications)
be applied directly to the underside of the slab. This allows for a reduction in story height and ease of construction. Due to the nature of the building being a research facility there is an extensive amount of MEP systems. Thus, a large amount of plenum space is necessary making ceiling finishing not of particular advantage. However, the flat soffit also means there is are no complexities when hanging or installing MEP fixtures due to uniformity of the supporting structure.
The flat slab has overcome all the drawback of the traditional system of beams framing into columns and supporting slabs spanning between the beams. Though the relatively deep beams of traditional floor system provide a stiff floor which is capable of long spans, and which is able to resist lateral loads, yet the complications of beam formwork, co-ordination of services, and overall depth of floor have led to a decrease in the popularity of this type of floor.
Benefits of using Flat Plate Floor System
(A) Larger Span Length Achieved
The span ‘L’ of a reinforced concrete flat-plate is approximately D x 28 for simply supported, D x 30 for an end span of a continuous system, to D x 32 for internal continuous spans. The economical span of a flat plate can be extended by prestressing to approximately D x 30, D x 37 and D x 40 respectively, where D is the depth of slab. Whereas for the traditional reinforced concrete beam-and-slab floor has an economical span ‘L’ of D x 15 for a single span and D x 20 for a multi-span, where D is the depth of the slab plus beam. The depth of slab between the beams can be initially sized using the span-to-depth ratios for a flat plate.
(B) Flat Soffit i.e. Flat Ceiling
The main and unique feature of this system is that it provides a way for the architect to achieve the concept of high and completely flat ceiling with no beam protrusion. The services can be installed within or below the slab and there are flexibilities in relocating vertical small penetrations. The soffit is often flat and high ceiling height can be achieved. Whereas traditional beam column slab system, the ceiling
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is not flat and hence many locations it is required to use false ceiling to get a flat ceiling, which is again going to increase the cost of construction. Moreover the lifespan of false ceiling few years and hence it needs to be changed several times in the lifespan of the structure. This problem can be avoided with flat plate system. As already the soffit of the slab is flat, there is no need of providing false ceiling. Because of this flat plate slab system has found immense use in hotels, malls, public buildings. The difference can be very easily made out from Figure 3 and Figure 4.
(C) Savings in Shuttering Cost
Shuttering/ Formwork constitutes a major cost of construction of reinforced concrete structure. In a traditional beam column slab system, the need of shuttering area is more and so the cost of formwork is also more. Whereas flat plate system requires only soffit shuttering of slabs, which makes flat plate slab system very popular amongst the builders as it has many fold benefits.
Figure 3. Beam and slab system
Figure 4. Ceiling of Flat plate slab system against ceiling of beam column slab system
(D) Savings in Construction Time
As formwork and stagging time is reduced, the overall construction time also gets reduced considerably in flat plate slab system. Keeping in mind of the tight schedule of the projects these days, if construction time can be saved in some means, it will give the builder/ owner early commissioning time of the project, which in turn will reward them with early revenue generation.
(E) Prestressing
Prestressing is not possible in traditional beam column system, whereas post-tensioned flat plat/slabs are a common variation of the conventional plate structure where most of the reinforcement is replaced by post-tensioned strands of very high strength steel. The structural advantage of post tensioning over conventional RCC is that the slab is nearly crack free at full service load. This leads to a smaller deflection compared to conventional RCC because of the higher rigidity of the un-cracked section. Hence reduction in thickness of the slab compared to conventional RCC is the rationale for using post-tensioning system for spans over 10m and above. Further the lack of cracking leads to a watertight structure. Flat plat/slab design and build contractors in India claim a 20% cost reduction compared to conventional RCC.
(F) Building Height
Traditional beam column slab system produces building/ structure heigher than flat plat slab system. The reason behind is absence of beams in the flat plate slab system. Which is very much beneficial for malls, theatres, hotels etc. . In malls, theatres, hotels, because of higher span requirement, the depth of beam is very high, which adds to the floor height making the overall height of each floor more. This again has cost impact as well as aesthetic impact on the structure. This problem can be avoided by adopting flat
Figure 5. Prestressing Flow diagram of Flat Plate Slab System
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plate floor system. Also by adopting to suitable prestressing system, it is possible to do construction of higher span slabs, without any increase in floor height which is a major concern with beam column slab system.
(G) Service
In traditional beam column slab system the penetrations through beams for large ducts difficult to handle. This is a common need in hotels, malls, public buildings, as the service lines are more in these time of buildings. Since making holes in large size beams is not feasible the service lines needs to be taken through longer routes, which again increases the cost of installation and effects the aesthetics by a great deal. With the adoption of flat plate slab system, the large and bulky sized beams are avoided and service lines can be very easily taken through the slab by keeping suitable and required sized openings in the slab. Figure 6 shows one such work, where service lines were routed through the openings in the slab.
Drawbacks of Flat Plate Floor System
Though Flat plate slab system promises a world of benefits over the traditional beam column slab system, still all is not well with this kind of system too. The main disadvantages of the flat plate system are deflection control, punching shear at columns, and future core drilling. The relatively thin slab of the structure makes it susceptible to excessive deflections and floor vibrations, in a laboratory facility such as the MSC this could be an issue. The uniformity of the flat plate system may lend itself to an ease of construction, however, it is not very efficient at resisting shear forces at critical locations, namely columns. If the slab is found to be inadequate to resist punching shear, certain measures can be introduced to strengthen these locations. These include increasing the depth of the slab over the entire panel, increasing the column size, adding a shear capital, or adding shear reinforcement. Furthermore, in a research facility experiments and equipment is often changing to meet the needs of the current industry. This often results in retrofits to the structure involving core drilling of the slab. In a 2-way system this can be problematic because it significantly lowers strength capacity of the floor system. The most dominant failure type in flat plate slab system is brittle failure caused by shear failure. But it does not mean that these drawbacks will limit the use of flat plate floor system. These limitations and drawbacks can be overcome by adopting suitable design practice.
General Consideration for use of Flat Plate Floor System
The following are the key factors to be considered before adopting the use of the concrete flat plate with steel/concrete column system:
Figure 6. Services through slab with provision for opening
- Architectural layout should be well planned to fully enhance the main area where high flat ceiling with neatly arranged steel/concrete columns are required in the design
- Spacing of columns- Punching shear checks at column areas- Long term deflection of the flat plate- Early planning of routing for M&E services, opening for
voids and location of staircase
The design of flat slab structures involves three steps- Framing system- Engineering analysis- Reinforcement design and detailing
Framing System
Initial framing system formulation provides a detailed geometric description of the column spacing and overhang. Even though the architect provides this part of the design, the engineer should emphasize on the following
- Three continuous spans in each direction or have an overhang at least one-forth times adjacent span length in case of only two continuous spans.
- Typical panel must be rectangular- The spans must be similar in length i.e. adjacent span
in each direction must not differ in length by one-third
Engineering Analysis
Flat plate/slab may be analyzed and designed by any
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method as long as they satisfy the strength, stiffness and stability requirements of the IS 456:2000 or ACI-318 codes. A typical flat plate/slab can be analyzed by direct design method or equivalent frame method as prescribed by the code. However, if the flat plate/slab is atypical with unusual geometry, with irregular column spacing, or with big opening then the designer may have to use finite element method model analysis using computers. The design of flat plate/slabs irrespective of the methodology used must first assume a minimum slab and drop thickness and a minimum column dimension to ensure adequate stiffness of the system to control deflection. The IS 456:2000 code is not clear on these minimums. However ACI specifies empirical formulas to arrive at these minimums. Refer to Table 1 for minimum slab thickness.
Once the slab thickness and column dimensions with boundary conditions are selected, the structure is loaded for different load cases and combinations prescribed by the code. The computed forces and moments in the members should be used for reinforcement design.
Critical reactions for the load combinations are used for the design of the supporting columns and foundations.
Seismic Design of Flat Plate/Slab
Seismic design lateral force is based on the provisions of Indian Standard IS 1893 (Criteria for Earthquake Resistant Design of Structure), however due to non-clarity of IS1893 designer, in addition may have to use, other codes like UBC-2000 (Uniform Building Code) to design an effective lateral system. Based on these codes a common practice is to determine lateral force by either using static or a dynamic procedure.
Reinforcement Design and Detailing
Reinforcement design is one of the critical parts of flat plate/slab design; maximum forces from the analysis shall be used in the design of the reinforcement. Reinforcement required for flexure by using minimum slab thickness per table 1 typically will not require compression reinforcement. The tension steel area required and detailing for appropriate strips can be per IS 456:2000 or ACI-318, both being similar. However design for punching shear force (including additional shear due to unbalanced moment) per IS 456:2000 is 32% conservative compared to ACI-318, because Indian code underestimates the concrete two-way shear strength by 32% compared to ACI.
Conclusion
Flat Plate slab system often provide the most economical solution for high-rise residential/ commecial construction. The system’s low floor height, compared to traditional beam
column slab system results in overall reduction of buiding height which further results lesser dead load, leading to lower foundation costs. Flat plate/slab construction is a developing technology in India. Flat plate/slab can be designed and built either by conventional RCC or Post-tensioning. Design of conventional RCC flat plate/slab in India, utilizing Indian codes, has many shortcomings, which have to be addressed and revised soon. Until then Indian engineers will continue to use Indian codes in combination with other standards like the ACI, BS or Euro Code to design and analyze Flat slabs/plates.
Reference
- Holbert, David H, P.E. Concrete: Floor Framing Systems, lecture 10/4/2010.
- Fanella, David A. “Concrete Floor Systems: Guide to Estimating and Economizing”. 2nd Ed. Portland Cement Association, 2000.
- Guide to Long-Span Concrete Floors, Cement and Concrete Association of Australia.
- Review and Design of Flat Plate/Slabs Construction in India, Gowda N Bharath; Gowda S. B. Ravishankar; A.V Chandrashekar
- Indian Standard IS 456:2000, Plain and Reinforced Concrete Code of Practice.
- Purushothaman P., Reinforced Concrete Structural Elements, Tata McGraw-Hill Publication Company Ltd. New Delhi. 1984
- Verghese P.C., Advanced Reinforced Concrete Design, Prentice-Hall of (India Private Ltd. New Delhi. 2003
- Notes on ACI 318-2000, Building Code Requirement For Reinforced Concrete, Portland cement association. USA 2000
- Structural Design Guide to the ACI Building code, Third edition, Van Nostrand Reinhold Company. New York. 1985
- Kenneth Leet and Dionisio Bernal, Reinforced Concrete Design, Third edition, McGraw-Hill, USA. 1997
- Structural Engineering Handbook, Forth Edition, McGraw-Hill, USA1997
- Alaa G. S. and Walter H.D., Analysis and Deflection of Reinforced Concrete Flat Slabs, Canadian Journal of Civil Engineering, Vol. 25. 1998
- Branson, D.E, Deformation of Concrete Structures, McGraw-Hill Company, New York.1977
- Nilson A.H. and Walter D.B., Deflection of Two-way Floor Systems by the Equivalent Frame Method, ACI Journal, Vol. 72, No.5 1975
- Indian Standard IS 1893 (Part 1): 2002, Criteria for Earthquake Resistant Design of Structures.
- Uniform Building Code, International Conference of Buildings Officials, California. 2002
- John W. W., Thomas H.K. and Changsoon R.H.A, Dynamic Response of Flat Plate System With Shear Reinforcement, ACI Journal, Vol. 102, No.5
Photo Courtesy www.diomisengineering.com
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Formwork - Future Approach in India
“It is not the strongest of the species that survives, or the most intelligent. It is the one that is the most adaptable to change that does it.” - Charles Darwin.
Winds of change are blowing across every sphere of construction in India. Same is the case with the formwork and scaffolding systems in India. An approx. data on formwork derived from the cement consumption in India reveals that in India, formwork executed is around 750 million Sqm. out of which formwork executed using system or engineered formwork hardly constitutes around 10%. It is a known fact that formwork constitutes around 6%-8% of the cost of concrete and 60% of the time of the structure. So it is the right time an emphasis is laid on the right approach on formwork for the future of the Indian construction.
With the increased growth in high-rise construction, demanding infrastructure projects shaping up the metros and tier 2 cities in India, the questions that arise now are - “Are the formwork systems available in India today sufficient enough for executing such demanding projects? Are the major formwork suppliers across the world that have entered the Indian market able to give end to end solutions to the Indian construction industry? “ Though the utilisation of formwork has gone up by leaps and bounds over the years our approach is still old fashioned. Have we modernized our approach is still a question to be answered by all the stake-holders.
This paper deals with focal points which will shape up the Future Approach of Formwork for the Indian construction industry. They can be broadly defined as Value Chain Linkage, Safety Integration in Formwork, Comprehensiveness in Quality, Standardisation, Green concept and sustainability and finally the Costing of Formwork.
Value Chain Linkage
Formwork is one of the vital links in the total Value Chain,
the other two links being reinforcement and concrete. At present, the Indian construction industry’s major concern is the stringent timelines (duration) in the projects. With the clients’ demands increasing day by day, the construction companies’ focus is mainly on the ‘floor to floor cycle’ time to meet the timelines of the projects. But to achieve this, a good engineered formwork system is alone not a solution. A good formwork system by itself might not give all the desired results; it only enables to reduce the timelines in one of the vital links of the value chain. There should be a wholistic approach considering the other two links of rebar and concrete. Also there should be emphasis on the development of the skill levels of the supervisors, labour and the approach of the engineers rather than just on the
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selection of the right system. To put it in a simpler way, our future approach while selecting a formwork system should be such that it should integrate the necessary features to support the other two links of the value chain i.e. rebar and concrete which will enable us to carry out these two activities also in a fast track manner. Only such a comprehensive approach would yield the desired results and help us to meet the demands of the customers.
Safety
Safety in formwork is another major concern today especially in high-rise construction and large infrastructure projects like metros, flyovers, airports etc. It is a known fact that in India, Safety levels are yet to catch up to the International Standards. There is a lot of pressure on the Indian construction companies today to improve the same by the Govt. of India, Foreign Investors and also the increased number of PMC’s (which are basically reputed MNC’s). Safety cannot be treated as a separate entity, rather it should be an integral part of the formwork system. Formwork & scaffolding being the major contributors to the safety in construction sites as they are also used for the rebar and concreting works, it is time we pay proper heed to how these have to be integrated with safety so as to ensure the overall safety at sites. The various areas of safety that we need to focus and integrate with formwork are:
- Access (both Vertical & Horizontal)- Working platforms- Lifelines and Safety Catch Nets- Erection & Dismantling of Formwork- Storage & Maintenance of Formwork- Simple Tools & Tackles- Design and Engineering
So our future approach when choosing a formwork system should address the above aspects and how they are integrated into the formwork system. Only then, in our way forward, we will be able to live upto the expectations of the customers and also reach to the level of International Standards.
Quality
Quality of the finished product is another aspect resulting from a good and efficient formwork system. For achieving a good concrete surface, the right kind of sheathing member should be used in any formwork system, depending on the type of finish demanded by the client. Invariably, plywood has been the most commonly used sheathing member world-wide and has yielded the best results till date with regards to form finish. Nevertheless considering other factors in choosing the right kind of formwork system for the right job, today aluminium formwork has started penetrating and off late captured the market rapidly with a share of about 15% of the overall formwork value in India. Due to its easier handling, good quality surface finish, repeatability and durability, and best suited for high-rise residential buildings which are the trend today, aluminium in future might be a strong contender as far as sheathing is concerned in formwork. Apart from this, to achieve a good quality product, the formwork system should deal with critical issues such as Grout tightness, Deformation, Facilitating Concrete Compaction, Provision of Clean-out doors and Box-outs etc. Only when all these issues are addressed along with the selection of the right sheathing member, a good quality product can be delivered. Looking into the future, our approach in selection of the system should keep all these aspects in view to deliver quality products.
Standardisation
Standardisation of the various formwork systems is also an aspect to introspect because we cannot afford to have too many systems at sites which leads to lot of complications in terms of usage as well as accountability. The formwork systems should be standardized such that a single system is adaptable to various structural elements and also across various projects. Though it has its own limitations, still standardization can be done to an extent which reduces the number of components involved in a system, increase efficiency of the components involved and the flexibility in usage of these (in terms of sizing and detailing). This automatically reduces the pain for the engineers / supervisors and also the labour who are the end-users of the system and gives better results as they can easily account for the materials and use them efficiently. In this particular aspect, our future approach should be “Using less for more output through Innovative Solutions”.
Green Formwork
Rapid industrialization, growth in population and urbanization in the two previous millennia and in the current century have not only taken a heavy toll on non-renewable natural resources of the planet but also caused unprecedented
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202 The Masterbuilder - May 2012 • www.masterbuilder.co.in
rise in global warming. Most leading business houses and industries across the world have adopted Corporate Social Responsibility (CSR) as the roadmap of their current and future business ethics and principles. Whether this principle is adhered to while manufacturing of formwork systems? A confident ‘YES’ may not be forthcoming. Currently no importance is being given to this aspect of Green Concept and Sustainability. Stepping into the future, our approach should be “Greener Formwork Systems” to do our part for the betterment of environment. The focus here can be on some of the important parameters like Energy Consumption, Wastage, Recycling and Depletion of Natural Resources. If these aspects are dealt with in the sourcing of raw materials, manufacturing of the products involved in the formwork systems as well as utilisation of the system as a whole, it helps in delivering “Greener Formwork Systems”.
(Mobilisation delay, work front delay, delay due to shortage of other resources & demobilization)
Also the associated costs like the upkeep and maintenance can be dealt with a central approach by building it up in the investment cost or with a localized approach to create a sense of ownership for the sites using the formwork systems.
The above example clearly indicates that the ‘life of formwork’ plays a major role in the Costing of Formwork. Formwork cannot be a scapegoat for inefficiency within and across sites which revolve around these time-bound methods of costing. However if the realistic costing is done as per the cost incurred per use, it can help construction companies in India to take a positive call on purchase or hire of modern formwork systems and change their approach in future.
Conclusion
Finally to conclude, Formwork Systems cannot be decided just by suppliers alone as they might not think of all the related elements in the value chain, instead it has to be decided by the end-users and engineers who are the future change-managers. And the guiding principle should be - Formwork must be approached not in isolation, but in a comprehensive manner to include the entire Value Chain, Safety, Quality and Sustainability. Also the thrust should be on realistic costing of formwork to enable viable usage of Modern Formwork Systems.
Description Hire charges or WDV method (5% per month)
Cost per use method
Investment Cost Apportioned
9960 4800
Fixing and Removing Cost
3000 3000
Upkeep and Maintenance Cost
1000 1000
Total Cost 13960 8000
Costing of formwork
With the rapid growth in the construction industry, introduction of modern formwork systems is essential to meet the delivery requirements of the customers and at the same time be competitive. However the modern formwork systems come with a high-price tag. Hence costing of formwork for a particular project is very critical for the engineers. However different costing methods are used by different contractors. Considering the factors like the efficiency of formwork being linked to the succeeding & preceding activities, idling at sites and poor planning; the time-bound costing method (Written Down Value or Hire-charges) ends up with higher formwork costs especially on materials for no fault of formwork. A small example below gives a clear picture of how the time-bound costing methods can be compared:
Sample Calculations of Formwork Costing for Aluminium Formwork
- Cost of formwork - Rs. 16000 / Sqm. (say)- Duration of the project - 10 Months (Only for Structure)- No. of possible re-uses - 100 (say)- No. of re-uses expected / month - 3- Actual duration considering all delays - 20 Months (say)
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