SELF-CONSOLIDATING CONCRETE FOR SLIP FORM CONSTRUCTION ...demo.webdefy.com/rilem-new/wp-content/uploads/2016/10/pro065-077… · Second International Symposium on Design, Performance

Second International Symposium on Design, Performance and Use of Self-Consolidating Concrete SCC2009-China, June 5-7 2009, Beijing,China

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SELF-CONSOLIDATING CONCRETE FOR SLIP FORM CONSTRUCTION: MIX DESIGN METHOD AND APPLICATION

K. Wang (1), S. Shah (2), G. Lu (3) and J. Grove (4)

(1) National CP Tech Center, Iowa State University, USA

(2) ACBM Center, Northwestern University, USA

(3) SHAW GROUP, USA

(4) FHWA, USA

Abstract

Recent research has demonstrated that the concept of self-consolidating concrete (SCC) can be used to design a special concrete that not only can self-consolidate but also hold its shape right after casting. Such a special concrete is of a great potential for slip form (SF) construction, and it is therefore called SF SCC. In this paper, the relationships among the flow ability, self-consolidating ability, and shape-holding ability of various concrete mixtures are examined. The mix design procedure for SF SCC is developed, and the performance criteria for designing SF SCC are established. The proposed method has been applied for designing SF SCC mix proportions using materials from different field sites. The results from a lab simulation of a field slip form paving process and a field paving construction trial have approved that the proposed SF SCC mix design method is applicable and reliable.

Kay words: self-consolidating concrete, slip form construction, mix design

1. INTRODUCTION Slip form construction, particularly slip form paving, has been widely applied in the

United States due to its labor saving and rapid construction speed [1]. Because of the requirement for holding its shape right after casting, conventional slip form concrete mixtures are generally dry, having very low flowability, and a great deal of vibration is necessary for proper consolidation. Over vibration is often reported due to high frequency of vibrators and/or slow paving speed. As a result, vibrator trails, sometimes together with longitudinal cracks, are observed on the pavements resulting from significant aggregate segregation under the vibration and loss of air in the top paste layer under freezing-thawing conditions [2].


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To overcome the above-mentioned problem, use of self-consolidating concrete (SCC) is desirable. In SCC, consolidation is provided by the self-weight of the concrete mixture, rather than external energy (such as use of a mechanical vibrator) [3]. SCC is increasingly used by the concrete industry due to its high rates of production and high quality. Unfortunately, conventional SCC generally has high flowability, and it requires firm formwork for construction. This conflicts with the concept of a slip form construction process, where freshly cast concrete shall hold its shape and no fix form work is necessary.

Recent studies have shown that the material properties and mix proportions have great effects on concrete flowability and shape-holding ability [4-5]. A rational balance between concrete flowability, self-consolidating ability, and shape-stability can be achieved through a careful selection of concrete materials and tailoring concrete mix proportion [6]. As a result, the concept of SCC can be used to design a novel concrete that not only can self-consolidate but also hold its shape right after casting. Such a concrete is of a great potential for slip form construction, such as slip form paving, and it is therefore called SF SCC. This paper presents a summary of SF SCC mix design methodology, procedure, performance criteria, and its field application.

2. SF SCC MIXING DESIGN CONCEPTS

SF SCC shall have (1) sufficient flowability for self-consolidation, (2) adequate viscosity for resisting aggregate segregation, and (3) a proper yield stress for holding the shape of the concrete right after being extruded from the slip form equipment. The flowability, self-consolidating ability, and shape-holding ability shall be considered simultaneously in the concrete mix design and achieved timely in the slip form concrete construction.

Conventional SCC is generally characterized by its special rheological properties: low yield stress, which ensures high flowability, and adequate viscosity, which prevents aggregate segregation. For SF SCC, high flowability is not necessary because it will have adverse effect on concrete shape holding ability. To balance self-consolidating ability and shape holding ability, flowability of SF SCC shall be designed just enough to ensure self-consolidating ability. Since slip form construction is actually an extrusion process, a certain external pressure is often applied to the concrete by a slip form, which helps the concrete in consolidation. Thus, the self-consolidating ability of SF SCC can even be slightly less than conventional SCC.

To obtain good shape-holding stability, a SF SCC mixture should have sufficient yield stress right after casting. A recent study has indicated that the yield stress of a concrete mixture is mostly dependent upon the yield stress of its matrix material, or mortar, and the interlock and friction of its aggregate particles [5]. In SF SCC, mortar shall be designed to be able to overcome its yield stress so as to be able to flow into the voids among coarse aggregate particles. The amount of mortar shall be sufficient to fill up the void and coat the aggregate particles slightly, thus ensure a good self-consolidating ability. On the other hand, the mortar shall also have adequate viscosity and cohesion so as to be able to drag the coarse aggregate particles when the concrete flows, thus preventing the concrete mixture from excessive flow. The coarse aggregate particles form a skeleton in concrete. The interlock and friction of the aggregate particles also provides the concrete with a certain shear resistance. An optimal aggregate gradation and volume fraction shall be selected to maximize the shear resistance for desirable shape-holding ability.


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Hence, there are two key components in the SF SCC mix design: (1) to design a proper mortar and (2) to find adequate mix proportion between mortar and coarse aggregate. In each of the design components, flowability or self-consolidating ability and shape stability of the designed material need to be evaluated.

3. SF SCC MIX DESIGN PROCEDURE AND PERFORMANCE CRITERIA Based on the discussion above, a performance-based method is proposed for the SF SCC

mix design, and the design procedure contains three steps as described as the following: Step 1: Design SF SCC mortar mix proportion A modified flow table test, adapted from ASTM C230, Standard Specification for Flow

Table for Use in Tests of Hydraulic Cement, is proposed to be used for balancing the flowability and shape-holding ability of a SF SCC mortar. ASTM C230 was originally designed to determine the water content needed for a cement paste sample to obtain a given flow spread (1105 mm) after a standard flow table drops 25 times. In the modified test, a potential mortar sample is placed on the ASTM C230 flow table. Right after the placement, the initial flow spread of the mortar is measured. Then, the table is dropped so that the sample reaches a flow spread of 2005mm, and the number of drops is recorded.

Based on results from a series of trial-and-error tests, a SF SCC mortar that has good flowability and shape stability shall have an initial flow value equal or slightly less than 10% and the number of drops, at which the mortar mixture reaches a flow spread of 2005mm, is less than 18. If the tested mortar does not meet the design requirements, adjustments can be made by selecting different cementitious materials, water-to-cementitious material ratio (w/cm), and sand content, and/or using various admixtures.

Step 2: Find coarse aggregate content in SF SCC After the mortar mix proportion is achieved, coarse aggregate particles are added into the

mortar gradually to form SF SCC. The amount of coarse aggregate to be used for a SF SCC can be determined by a modified slump test, where ASTM C143, Standard Test Method for Slump of Hydraulic Cement Concrete, is followed but no rodding is applied. Both slump and slump spread are measured at the end of the test. In addition, the shape of the concrete mixture is evaluated right after the slump cone is removed.

Figure 1: Slump, spread, and compaction factor (CF) of various concrete mixes

I

IIIII

0

2

4

6

8

10

12

8 12 16 20 24 28 32 36 40Spread (in)

Slum

p (in

)

CF=1.00.95


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To verify the self-consolidating ability of the designed SF SCC, a modified compaction factor test has been employed in the development of SF SCC mixtures [1]. In this test, fresh concrete was dropped from a constant height into a container through an inverted, standard slump cone. The container was specified by ASTM C138, Standard Test Method for Density (Unit Weight), Yield, and Air Content (Gravimetric) of Concrete. After striking off the excess concrete and smoothing the surface, the weight of the concrete sample was measured and the uncompacted density of the fresh concrete (without rodding) was calculated. In the meantime, the compacted density of the same fresh concrete (with standard rodding) was also obtained according to ASTM C138. The ratio of uncompacted-to-compacted density values is defined as the compaction factor. The larger the compaction factor, the better self-consolidating ability the concrete mixture has.

Figure 1 shows the results from the modified slump and compaction factor tests of various concrete mixtures. As observed from the figure, the tested concrete mixtures can be divided into three different groups. In Group I, the concrete mixtures generally have low flowability, with a slump lower than 150 mm (6 in.) and a spread less than 280 mm (11 in.). The self-consolidating ability of this group of concrete mixture is also low, with a compaction factor (CF) less than 95%. The bent cone shape of the mixtures at the end of the modified slump test indicates that honeycombs exist inside the tested concrete, and the aggregate particles in the mixture are not uniformly distributed [6]. Therefore, this group of mixtures can not be used as SF SCC. In Group III, the concrete mixtures have very high flowability, with a slump higher than 250 mm (10 in.) and a spread over 610 mm (24 in.). This group of mixtures has a CF value of 100%, indicating high self-consolidating ability. However, due to their large slump spread, the mixtures in this group are actually the conventional SCC, and they are unable to hold its shape right after casting. Therefore, it can not be used as SF SCC either. In Group II, the concrete mixtures have a slump approximate 175-230 mm (7-9 in.) and a spread approximate 300-380 mm (12-15 in.). The regular cone shape of the mixtures at the end of the modified slump test implies that the aggregate particles in the mixtures are well uniformly distributed, and the mixtures are able to hold its shape at a certain degree after casting. The CF values of the mixtures are larger than 95%, slightly lower than that of conventional SCC. This group of mixtures appears suitable for SF SCC application. As discussed previously, the consolidating ability of the designed SF SCC can be further improved during field construction because an external pressure is often applied to the concrete by the slip form construction equipment.

Principally, the features of the concrete mixtures in Group 2 of Figure 1 indicate that SF SCC should be designed to have a maximum self-consolidating ability with a minimum flowability. Based on these features and results from more trial-and-error tests, it is proposed that a successful SF SCC mixture shall have 125-200 mm (5-8 in.) of slump, approximate 300 mm (12 in.) of the slump spread, a regular cone shape at the end of the modified slump test, and a compaction factor approximate 98%.

Depending on the aggregate properties (such as gradation, particle shape and surface texture), an optimal volume fraction of coarse aggregate in SF SCC is found to be approximate 40-45%. If the tested concrete does not meet the proposed mix design performance criteria, adjustments can be made by modifying aggregate gradation, aggregate volume fraction, and using various admixtures.

Step 3: Verify SF SCC mix proportion with a lab simulation After the initial SF SCC mix proportion is achieved from Steps 1 and 2, this SF SCC


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candidate must be verified with a lab simulation of a slip form construction process so as to final approve its self-consolidating ability and shape holding ability. A mini-paver that simulates a field slip form paving process has been designed for the purpose of this verification [1, 6].

Figure 2: Sketch of the mini-paver system

Figure 3: A flow chart of SF SCC mix design procedure

Step 1: Design of SF SCC mortar

Flow table test

F010% 2005mm @ 18 drops

Step 2: Design of coarse aggregate content

Non-rodding slump and compaction test

Step 3: Verification of SF SCC with a lab simulation

S=125~200mm, D300mm, Cone shape, CF0.98

No

Yes

Yes

No

Little or no edge slump, no visible honeycomb and segregation

Yes

Yes

SF SCC

No


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As shown in Figure 3, the designed mini-paver system consists of an L-box with a platform on top, a towing system (a towing cable and a crank), and a working table. The L-box is 46 cm (18 in.) wide, 60 cm (24 in.) long, 46 cm (18 in.) high, and 7.5-15 cm (3-6 in.) thick. Before the paving test, approximately 100 kg (220 pounds) of weights are placed in the back chamber of the paver. A stop plate is positioned at the end of the horizontal leg of the L-box. Freshly mixed concrete is stored on the platform. To begin paving, the concrete is pushed from the platform into the vertical leg of the L-box up to a certain height, which generates a pressure to consolidate the concrete. Then, the crank system is turned and it pulls the mini-paver forward at a designed speed (90-150 cm/min or 35 ft/min). As the mini-paver moves forward, it extrudes the concrete slab, out of the horizontal leg of the L-box.

The SF SCC mix design shall be acceptable only when the concrete made by the mini-paver shows satisfactory shape of the freshly cast slab (little or no edge slump) and no visible honeycomb and aggregate segregation observed on the cross section of the hardened slab.

Figure 3 presents a flow chart of the procedure and criteria for the SF SCC mix design.

4. DESIGN SF SCC MIX PROPORTIONS FOR FIELD APPLICATIONS

4.1 Field Materials and Properties In order to verify the applicability of the mix design method proposed above, three SF

SCC mix proportions were developed using field concrete materials. The materials were obtained from three different locations, one in Wisconsin (Alma Center) and two in Iowa (Ottumwa and Guthrie) of the United States. The aggregate properties, such as sieve analyses, absorption, and void ratios, are shown in Figures 4 and 5 and Table 1. Type I cement and class C fly ashes are used in the three SF SCC mixes.

0

20

40

60

80

100

1.5"1"3/4"1/2"3/8"No. 4No. 8No. 16

Sieve size

Per

cent

age

pass

ing,

by

mas

s

Alma center, WI (F.M.=7.49)

Ottumwa, IA (F.M.=6.48)

Guthrie, IA (F.M.=6.89)

Figure 4: Sieve analysis results of field coarse aggregates


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0

20

40

60

80

100

3/8"No. 4No. 8No. 16No. 30No. 50No. 100No. 200

Sieve size

Per

cent

age

pass

ing,

by

mas

s Alma center, WI (F.M.=2.95)

Ottumwa, IA (F.M.=2.96)

Guthrie, IA (F.M.=3.32)

Figure 5: Sieve analysis results of field fine aggregates

Table 1: Coarse aggregate properties

material properties source Material Absorption (%) Voids in aggregate. (%) Alma Center, WI Rock 0.33 45.0

Ottumwa, IA Limestone 3.69 39.4 Guthrie, IA Gravel 1.95 38.9

4.2 SF SCC Mix Design According to the mix design procedure as described in Figure 3, the mortars were first

designed (Step 1) to have an initial flow of 10% and a drop number of 16-17 when the flow spread reached 200mm. Figure 6 shows the flow table results of the mortars.

Figure 6: Mortar flow table results

After obtaining desirable mortar mixes, coarse aggregates were added to the mortars to

obtain the concrete mixes (Step 2). The modified slum cone and compaction factor tests were used for evaluating the flowability and self-consolidating ability of the mixtures. The shapes of the mixtures were inspected at the end of the modified slump cone tests to assess the shape-holding ability of the mixtures. As shown in Figure 7, both slump and slump spread (in diameter) were in the required range, and all concrete samples had a cone shape at the end of

Ottumwa, IA F0=10% 200mm@17Drops

Alma center, WI F0=10% 200mm@16Drops

Guthrie, IA F0=10% 200mm@16Drops


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the slump cone tests. The CF values of these mixtures were equal or close to 98%. Thus, these mixes (Table 2) were acceptable as SF SCC mixes for mini-paver tests in lab.

Guthrie, IA

S=190mm, D=320mm Cone shape

Alma center, WI


Ottumwa, IA


Figure 7: Slump results of modified concrete

Table 2: Final mix designs for mini-paver tests

Source CEMENT FA-C WATER SAND C. Agg Alma Center 380 164 210 850 760

Ottumwa 360 155 195 800 850

Guthrie 334 143 181 750 955 Note: all values are kg/m3. Using the initial mix proportions presented in Table 2, three SF SCC mixtures were

prepared for mini-paver tests. Figure 8 shows the concrete slab after paving. All three mini-paver tests were successful. The concrete was extruded from the mini-paver by its self-weight with no form work and additional consolidation. The edges of the concrete slabs all looked vertical and sharp. The top surface was smooth, and no further finishing was necessary. Therefore, the mixture proportions in Table 2 were finally approved suitable for the field applications.

Figure 8: Mini-paver results

Alma center, WI Ottumwa, IA Guthrie, IA


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5. FIELD APPLICATION OF SF SCC A field trial of SF SCC was for the first time performed in Iowa in August 2006. An

asphalt paver was used in the field trail, and it paved a 10 cm (4 in.) thick, 240 cm (8 ft.) wide, and 1040 cm (34 ft.) concrete pavement segment.

Following the procedure as discussed previously, a mix design of SF SCC was developed using the locally available materials. The mix proportion is shown in Table 3, in a comparison with a conventional pavement concrete mix (C3). The differences between these two concrete mixes include the decreased amount of coarse aggregate (25 mm or 1 in. nominal maximum size limestone) and the additional using of fly ash, viscosity modifying admixture (VMA) and super-plasticizer (SP). The SF SCC was mixed in a mixing truck and delivered from a local ready mixed concrete plant 5 minutes away from the construction site.

Table 3: Mix design comparison of SF SCC used for the field trial and conventional pavement concrete (C3)

Cement Fly Ash Water Sand Coarse Aggregate VMA SP

SF SCC 333 148 160 750 765 1.4 56

C3 333 - 150 750 945 - -

Note: all values are kg/m3. The slip form paving started after the non-rodding slump test reached the specified value,

180 mm (7 in.) slump and 305 mm (12 in.) slump spread, and with a regular cone shape of the mixture after the slump test (Figure 9).

Figure 9: Cone shape of the field SF SCC mixture after the non-rodding slump test

To start the paving, the concrete was dumped into the paver. As the paver moved slowly

forward, the concrete was extruded out of the paver under its self weight. Figure 10 shows the SF SCC pavement after paving. The pavement surface was smooth, and the edge was sharp and vertical. This field trail indicated that the well designed SF SCC can be applied to the field slip form construction.


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(a) SF SCC pavement (b) Edge of the SF SCC slab

Figure 10: The first field SF SCC pavement

6. CONCLUSIONS Following conclusions can be drawn from the present study: (1) Concept of SCC can be applied into a slip form construction. A desirable balance

between the self-consolidating ability and shape holding ability that are typically required by SF SCC can be achieved by tailoring concrete materials and mix proportion.

(2) A SF SCC mixture shall be workable enough for machine placement without external consolidation, but it shall not be as fluid as conventional SCC so as to have sufficient shape holding ability. Principally, SF SCC should be designed to have a maximum self-consolidating ability (compaction factor 98%) with a minimum flowability (Slump =125~200mm, Spread 300mm).

(3) There are two key components in the SF SCC mix design: (1) to design a proper mortar and (2) to find adequate mix proportion between mortar and coarse aggregate. In each of the design components, flowability, self-consolidating ability, and shape-holding ability of the designed material need to be checked against the proposed design criteria.

(4) The performance-based mix design method proposed in this paper is applicable and reliable for SF SCC mix design development.

ACKNOWLEDGEMENT

Five state departments of transportation (Iowa, Kansas, Nebraska, New York, and Washington States) in the United State, some concrete admixture companies, the Federal Highway Administration (FHWA), and the National Center of Concrete Pavement Technology (CP Tech Center) sponsored the present study. The technical inputs and financial supports from the sponsors on the research project are greatly appreciated.


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REFERENCES

[1]. Wang, K., Shah, S. P., White, D. J., Gray, J., Voigt, T., Lu, G., Hu, J., Halverson, C., and Pekmezci. B. Y., Self-consolidating concrete-Applications for slip-form paving: Phase I (Feasibility Study), National Concrete Pavement Technology Center, Iowa State University, Ames, Iowa, US, 2005

[2]. Tymkowicz, S., and Steffes, R. F., Vibration Study for Consolidation of Portland Cement Concrete, Proceedings of the Semisesquicentennial Transportation Conference. Ames, Iowa: Iowa State University, 1996

[3]. Okamura, H., and Masahiro, O., Self-Compacting Concrete: Development, Present Use and Future, Proceedings of the 1st International RILEM Symposium on Self-Compacting Concrete, Stockholm, Sweden, September 1314, 1999

[4]. Pekmezci, B., Voigt, T., Wang, K., Shah, S. P., Low Compaction Energy Concrete for Improved Slipform Casting of Concrete Pavements, ACI materials journal 2007, vol. 104, no3, pp. 251-258

[5]. Lu, G., Wang, K., Theoretical and Experimental Investigation into Shear Failure Behavior of Fresh Mortar, to be submitted

[6]. Wang, K., Shah, S. P., and Voigt, T., Self-consolidating Concrete for Slip-form Construction: Properties and Test Methods Proceedings of the International Conference on Microstructure Related Durability of Cementitous Composites, Nanjing, China, October. 13-15, 2008 (in press)

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SELF-CONSOLIDATING CONCRETE FOR SLIP FORM CONSTRUCTION ...demo.webdefy.com/rilem-new/wp-content/uploads/2016/10/pro065-077… · Second International Symposium on Design, Performance