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CHARACTERISTICS OF PERVIOUS CONCRETE MADE WITH STEEL SLAG FOR ROAD CONSTRUCTION APPLICATION

Ivana Barišić (1), Krunoslav Ćosić (2) and Ivanka Netinger (2)

(1) Department of Geotechnics, Transportation and Geodesy, University of Osijek, Faculty of Civil Engineering Osijek, Croatia

(2) Department of Materials and Constructions, University of Osijek, Faculty of Civil Engineering Osijek, Croatia

Abstract Special concrete types made by special mix design and different additives are introduced

with the aim to achieve optimal performance for each specific application. One such concrete is pervious concrete. Pervious concrete has an advantage over ordinary concrete in form of: reduction of the runoff water, improvement of water quality near pavements and parking lots and, in case of urban areas, reduction of heat island effect and traffic noise. However, high porosity causes significant reduction of its strength so the pervious concrete has limited application in highway pavement structures. Since there is an amount of steel slag landfilled in Republic of Croatia, it would be interesting to use this waste material as an aggregate in pervious concrete pavements. The main goal of this paper is to find out the optimal mixture of pervious concrete prepared with available steel slags that is able to meet the requirements according to Croatian legislation. Compressive strength, flexural strength and dynamic modulus of elasticity of pervious concrete containing slag will be tested and compared with the same properties of commonly used pavement concrete. Based on test results, the guidelines for preparation of optimal pervious concrete mixture and its incorporation in pavements will be given.

Keywords: pervious concrete, steel slag, pavement, composition, strength characteristics

1. INTRODUCTION Concrete is dominant construction material whose application covers nearly 70% of all

material needs in civil engineering. While its application in building construction is well known and common, its application in road construction is slightly neglected, particularly in Croatian road construction where asphalt is dominant pavement material. Construction of concrete pavements, also known as rigid pavements is customary in developed countries such in North America and some European countries. For example, in Czech Republic nearly 50% of highways are built with concrete pavements [1].

The first use of dense concrete in street pavement date from 1865 when it was experimentally installed in Scotland [2]. First contemporary, classic concrete pavement was built in 1895 in Bellefontaine, Ohio using only humble technology while first concrete

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pavement built by modern machinery date from 1924, Germany [1]. Around 10 years later, first concrete pavement was built in Croatia.

First usage of pervious concrete in pavement construction follows the end of World War II [2]. Destructions during the war prompt utilization of new materials and building methods. In Germany, lack of raw materials and high amounts of waste, particularly demolition waste motivate investigation of porous concrete. Since then, pervious concrete has been used over 30 years in many countries, especially in USA and Japan [3].

Pervious concrete, also referred to as porous or permeable concrete is material with the same basic components as standard concrete but designed to have high porosity (void content is between 11% and 35% [4,5]) and permeability (typically about 2-6 mm/s). It is a mixture of Portland cement, uniform coarse aggregate and with either a small amount of or without fine aggregate and water [3]. Due to its high porosity, pervious concrete have good drainage properties and high noise absorption characteristics which are important elements for quality pavement. Taking advantages of its drainage properties, it has been used in construction of shoulders, bases and subbases of roads but more recent application, taking its good acoustic properties is in construction of top layer or overlay of concrete pavements, similar to that of porous asphalt [4]. High porosity of pervious concrete is also its main disadvantage since it is associated with strength decrease. Pervious concrete mixtures can develop compressive strengths in the range of 3,5 MPa to28 MPa (typical values are about 17 MPa) and flexural strengths generally ranging between 1 MP to 3,8 MPa [6]. Low strength of pervious concrete influence the stability and durability of structure because of susceptibility to frost damage and low resistance to chemicals. That is the reason of its limited application in construction of high traffic highways.

In order to address this issue, researches with different, new component in pervious concrete are conducted worldwide [3,7,8]. Since there is an amount of steel slag landfilled in Republic of Croatia, pervious concrete with aggregate completely made of this waste material is investigated. The main goal of this paper is to find out the optimal mixture of pervious concrete prepared with available steel slags that is able to meet the requirements placed on concrete for pavement construction according to Croatian legislation.

2. EXPERIMENTAL PROGRAM Steel slag from Croatian landfills is proven to be good substitute for natural aggregate in

classic, dense concrete [9-11]. So, with that assumption laboratory program included determination of fresh and hardened concrete properties, not including determination of substitute aggregate properties. Detailed experimental study was conducted in order to determine basic engineering properties of pervious concrete made with domestic steel slag as alternative aggregate to natural dolomite aggregate.

2.1 Material characteristics As a natural material commonly used in Croatian road construction, crushed dolomite

stone was used. Steel slag from the only two Croatian landfills near the towns of Sisak and Split was used as substitute aggregate material.

Slag disposed in landfill near Sisak plant is a combination of blast furnace and electric furnace slags while slag from Split depot originates from electric furnace. The total amount of disposed material is estimated of around 1,5 million tones (1,5 million tones at Sisak depot and 30 000 tones at Split depot). High utilisation costs of slags originated from Croatian steel

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plants for cement production can be found in high hardness of slags, where high milling costs are required. Utilisation of this material as an aggregate in concrete would be certainly more economically acceptable compared to utilisation as a binder. In addition, as far as aggregates occupy more than three quarters of concrete volume, much more of this waste material would be used than by theirs application as a part of binder in concrete.

For all mixes, ordinary Portland cement of grade 42,5 (CEM II/A-M(S-V)42,5N)was used as a binder.

2.2 Methodology For determination of the optimal mixture of pervious concrete, 3 different pervious

concrete mixtures were tested and 1 reference mix, standard dense concrete mixture. Mix compositions and nomenclatures are shown in Table 1.

Table 1: Mix compositions and nomenclatures for 1m3 of concrete Mix M1 – dolomite dense concrete M2 – dolomite pervious concrete

Property Mass (kg)

Density (kg/dm3) Volume (dm3) Mass

(kg) Density (kg/dm3) Volume (dm3)

Cement 350,0 3,0 116,7 300,0 3,0 100,0 Water 115,5 1,0 115,5 99,0 1,0 99,0

v/c 0,33 0,33 Superplasticizer 3,5 1,1 3,2 - - -

Air 2,5% - 25,0 15% - 150,0 Aggregate 2034,0 2,75 739,7 1783,7 2,7 651,0

Σ 2503,0 2,50 1000 2182,7 2,18 1000,0 Mix M3 – steel slag (Sisak) pervious concrete M4 – steel slag (Split) pervious concrete

Property Mass (kg)

Density (kg/dm3) Volume (dm3) Mass

(kg) Density (kg/dm3) Volume (dm3)

Cement 300 3,0 100 280,0 3,0 93,3 Water 99,0 1,0 99,0 92,4 1,0 92,4

v/c 0,33 0,33 Superplasticizer - - - 2,80 1,1 2,5

Air 15% - 150,0 15% - 150,0 Aggregate 2053,3 2,93 651 2003,7 2,86 661,7

Σ 2452,3 2,45 1000,0 2378,9 2,38 1000,0

Mechanical properties of hardened concrete were tested on 3 of the kind specimens (10/10/40 cm prisms and 15/15/15 cm cubes). All specimens were demoulded 24 h after the casting and placed in a water tank for 27 days. Specimens before testing are shown in figure 1.

a) b) c) d)

Figure 1: Hardened concrete specimens: a) mix M1; b) mix M2; c) mix M3; d) mix M4

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All testing on fresh and hardened concrete were performed according to relevant European Standards. Consistency was measured according to HRN EN 12350-2, density according to HRN EN 12390-7, compressive strength (HRN EN 12390-3) was measured on cube specimens while flexural strength (HRN EN 12390-5) and dynamic modulus of elasticity using ultrasonic pulse velocity (EN 12504-4) were measured on prism specimens. Poisson’s ration for pervious concrete was selected to 0,22 [12].

4. RESULTS AND DISCUSSION 4.1 Fresh concrete tests results

Test results on fresh concrete are shown in Table 2. In fresh concrete, water extraction or aggregate segregation was not observed so the material was embedded in moulds and compacted by vibrating table.

Table 2: Results of fresh concrete tests

Mix Consistency by Slump test (cm)

M1 0,0

M2 1,0

M3 0,0

M4 0,0

4.2 Hardened concrete test results Results of hardened concrete tests are shown in Tables 3 and 4 and Figures 2 to 5.The void

content of tested mixes ranged from 14 to 21 %. Porosity of pervious concrete with standard, dolomite aggregate is slightly higher than porosity of pervious concrete with steel slag aggregate. So, the results of hardened concrete tests can be compared in order to define influence of steel slag aggregate on pervious concrete characteristics. Since usual void content for pervious concrete is between 11% and 35% [4, 5] it can be concluded that here defined mix composition produces pervious concrete with satisfying void content.

However, porosity is one of the most important characteristics of pervious concrete, mainly influencing its strength and durability characteristics. In this research, porosity was determined by hydrostatic weighing (EN 12390-7: Testing hardened concrete - Part 7: Density of hardened concrete) which give us only isolated/closed pores. To characterize materials microstructure, several methods were developed such as: gas absorption by the Brunauer–Emmett–Teller (BET) technique, image analysis of thin sections or mercury intrusion porosimetry (MIP), NMR microscopy, small angle neutron scattering and X-ray computed micro-tomography (micro-CT) [13]. Micro-CT is particularly suitable since allow non-destructive 3D visualization of the internal microstructure of materials, pore-size distribution and the orientation of pores, but also offers a visualization of the internal structure of the specimen.

Total porosity (common ratio of isolated/closed and connected pores) was calculated using mathematical algorithm. Since hydrostatic weighing including only isolated pores (water contained in the connected pores is impossible to weigh because water flow out from specimen) total porosity is determined calculating bulk density which refers to mass of water

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saturated specimen plus water from connected pores. Water from connected pores was calculated using mass of oven-dried specimen, volume of specimen and apparent density by using Archimedes' principle.

The results of porosity determined by hydrostatic weighting (isolated pores) and by calculation (total pores) are presented in Table 3.

Table 3: Comparison of porosity results

Mix Void content (%) of isolated pores Hydrostatic weighting

Void content (%) of total pores Calculation

M1 3,8 6,3 M2 11,8 21,3 M3 11,3 14,2 M4 11,8 19,9

From the results shown in Table 4 and Figure 2 it can be seen that pervious concrete made with steel slag from Sisak landfill have higher compressive strength then pervious concrete made with standard aggregate. Increase in compressive strength is 9,1%. Results of flexural strength are showing different trend. Namely, substitution of natural aggregate with steel slag from Sisak landfill results in flexural strength decrease for 26,5%. Dynamic modulus is showing similar trend to that of flexural strength. Decrease in dynamic modulus of elasticity for pervious concrete made with steel slag aggregate is 26,6%.Pervious concrete made with steel slag from Split landfill has shown insufficient strengths ranging from 11,6 MPa to only 0,9 MPa for compressive and flexural strength respectively. That means reduction for 56,2% and 75,0% in compressive and flexural strength respectively comparing with pervious concrete with Sisak steel slag.

Table 4: Results of hardened concrete tests

Mix Density of hardened concrete (kg/m3)

Compressive strength (MPa)

Flexural strength (MPa)

Dynamic modulus of elasticity (GPa)

M1 2442 69,5 9,7 70,85 M2 2095 24,1 4,9 51,27 M3 2435 26,5 3,6 37,61 M4 2252 11,6 0,9 29,89

Comparing pervious concrete with standard dense concrete, there is a reduction in compressive and flexural strength for dolomite aggregate pervious concrete of 65,3% and 49,5% respectively. This is a significant reduction in strengths, which was expected since the main disadvantage of pervious concrete is reduction in strengths caused by its high porosity. Also, in this research pervious concrete was prepared with 300 kg of cement and dense concrete with 350 kg and no superplasticizer was used in order to achieve less expensive material.

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4.3 Evaluation of pervious concrete as pavement material According to General technical conditions for roadwork [14], concrete for pavement is

classified according to 28 day compressive and flexural strengths. Requirements according to traffic load are given in Table 4.

Table 5: Requirements for pavement concrete 28 day strengths [13]

Predicted traffic load Compressive strength (MPa) Flexural strength (MPa) Very heavy 35/45 5,0

Heavy 30/37 4,5 Other 25/30 4,0

Comparing results of hardened concrete tests shown in Table 3 and requirements given in Table 4 it can be concluded that natural aggregate pervious concrete do not satisfy required compressive strength, but satisfy required flexural strength values. On the other hand, pervious concrete made with steel slag aggregate from Sisak landfill satisfy compressive strength requirement for medium, light and very light traffic loads. This material does not satisfy flexural strength requirements. Solution for this problem can be addition of fibers to fresh concrete mixture. Incorporation of 54 mm long polypropylene fibers of 0,5 or 1 % by volume of the pervious concrete mixture generally were not found to influence the compressive strength to any significant degree [15]. However, the contribution of fibers towards improving the residual flexural capacity was higher at a higher porosity. This effect is explained with the fact that higher porosity means there are more number of pores, thereby mean free spacing between the pores is reduced. This increase the possibility that the pores are bridged by the fibers.

Pervious concrete made with steel slag from Split landfill can not be used as pavement material since it has significantly lower strength then those required by standard.

5. CONCLUSIONS The main goal of this research was to identify possible usage of steel slag disposed on

Croatian landfills as pervious concrete aggregate for its usage in pavements. Comparing the results of conducted laboratory testing leads to the following conclusions: Quality of pervious concrete is highly depending on aggregate type. Increase in porosity of 15% result in strengths decrease of approximately 60%. Pervious concrete made with steel slag from Sisak landfill have high potential for usage

as pervious concrete pavement material. Substitution of dolomite aggregate with steel slag in pervious concrete result in nearly

27% decrease in compressive strength and dynamic modulus of elasticity. Steel slag from Split landfill presents poor aggregate material for utilization in pervious

concrete intended for usage in pavements.

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[4] Putman, B.J, and A.I. Neptune: 'Comparison of test specimen preparation techniques for pervious concrete pavements', Constr BuildMater, 25 (2011) 3480-3485.

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