SELF-CONSOLIDATING CONCRETE FOR SLIP-FORM CONSTRUCTION: PROPERTIES AND TEST METHODS Kejin Wang (1), Surendra P. Shah (2) and Thomas Voigt (3) (1) Department of Civil, Construction and Environmental Engineering, Iowa State University, USA (2) Department of Civil Engineering, Northwestern University, USA (3) Research and Technology Innovation Center, USG Corporation, Europe Abstract In this paper, the development of a new concrete that can not only self-consolidate but also hold its shape right after casting is described. This concrete may have a great potential for slip form construction, such as slip form paving - providing a more workable mixture, eliminating internal vibration, increasing concrete production rate, and enhancing pavement smoothness and durability. The present study focuses on the test methods developed for evaluation of the concrete flow ability, compactibility, and shape-holding ability. The effects of materials and mix proportions on the concrete properties are also investigated. The results indicate that concrete materials and mixture proportions can be tailored to obtain sufficient self-consolidating ability and timely shape stability, thus suitable for slip form construction. 1. INTRODUCTION Slip form construction has been extensively used by the worldwide paving industry. Different from the fixed form paving, slip-form paving brings together concrete placing, casting, consolidation, and finishing into one unique process without requirement for formwork. In this process, concrete mixture with a slump less than 2 inches (in.) is placed in front of a paver. As the paver moves forward, the mixture is spread, leveled, consolidated (by equally spaced internal vibrators), and then extruded. After extrusion, the fresh concrete slab can hold in shape without any lateral support for further surface finishing, texturing, and curing until the concrete sets. Because of the low consistency of the mixture, a great deal of vibration is needed to move entrapped air and consolidate the concrete. 161
Over consolidation of pavements is noted visually in finished pavements. Longitudinal trails are observed on the surface of the concrete pavements (Figure 1). These trails are also called vibrator trail, and they run parallel to each other with spacing similar to that of vibrators on pavers. Cores taken from vibrator trails of some pavements have revealed in many instances the hardened concrete contains less than 3% air, rather than 6-7% as designed, thus significantly reducing concrete freeze-thaw durability [1]. Although some measures are taken to monitor frequency of the vibrators, improper vibrations are sometime still inevitable. (a) Vibrator trials (b) Segregation of concrete on a vibration trial Figure 1: Vibrator trails on concrete pavements (Photo courtesy: Robert F Steffes, CP Tech Center, Iowa State University) It is believed that it will be a revolutionary advance in paving technology if vibration of the slip-form paved concrete can be eliminated. Such a new self-consolidating concrete for slip-form application (SF SCC) would permit the concrete paving industry to have not only more uniform, durable, and smoother pavements but also much faster, safer, and quieter construction. Elimination of internal vibration will also reduce energy consumption in construction. Such concrete technology could also be applied to slip form construction of many other structures, such as water towers and tanks. A challenge in developing SF SCC is that the new SCC needs to possess not only excellent self-consolidating ability without segregation before extrusion but also extraordinary shape stability to sustain its self-weight, or to hold the slab in shape, without support from any framework right after casting. It is understood that to obtain self-consolidating ability, a concrete mixture must overcome the stress generated by the friction and cohesion between the aggregate particles; while to hold the freshly cast products in shape, the fresh concrete must have certain strength or stability. A key issue is to achieve these two conflicting needs for the concrete at appropriate time. 162
2. RESEARCH APPROACH Concrete is a shear-thin material. At a high shear rate (such as during mixing), concrete microstructure is disturbed, and its yield stress and viscousity are reduced. Thus, the concrete becomes more flowable and self-compactible. While at a low or zero shear rate (after casting), concrete microstructure can be re-built, green strength can develop, and the concrete becomes less deformable. Extrusion in a slip-form paving process can further help in rearranging solid particles in the concrete for packing, enhancing the concrete consolidation, and facilitating green strength development. Based on the current knowledge of concrete materials, a rational balance between self-consolidating ability and shape stability of a concrete mixture can be achieved timely by tailoring concrete materials and mix design, and therefore, there is a great potential for development of a desirable SF SCC. An approach to the concrete mix design development is to start from a conventional SCC mix design and then modify it gradually with fine materials and/or chemical admixtures so as to achieve improved its shape stability without loss of its self-consolidating ability. Thus, SF SCC may not be as fluid as the conventional SCC but it will be (1) workable enough for machine placement, (2) self-consolidating without segregation, (3) able to hold shape right after casting, and (4) of compatible/superior performance properties (such as strength and durability) to/over conventional concrete. To meet these criteria, concrete mixtures have to be evaluated for flow ability, compactibility, and shape-holding ability. 3. TEST METHODS FOR SF SCC PERFORMANCE Various test methods have been developed for evaluation of flow ability, compactibility, and shape-holding ability of SF SCC. Some non-standard test methods are described as below. 3.1 Modified slump cone test Flow ability of concrete is commonly measured by a standard slump cone test (ASTM 143). For conventional concrete, the standard test requires the concrete sample to be placed with three layers and rodded 25 times for each layer. The test measures the slump of the concrete right after the slump cone mold is removed. For conventional SCC, a modified slump cone test is often used, where no rodding, tamping, or any vibration is allowed for the sample preparation. The test provides two measurements: slump and spread (or slump flow). Conventional SCC generally has a slump spread ranging from 20 to 32 inches (50 to 80 cm). With such a large spread, conventional SCC can flow well and self-consolidate, but it shapes like a big pancake after the slump cone mold is removed and has no timely shape-holding ability. It, therefore, requires formwork for construction. For SF SCC, the modified slump cone test that is used for conventional SCC can also be applied. The test is able to provide three parameters: slump, spread, and shape of the mixture right after the slump cone mold is removed. The measurements of the concrete slump and spread are related to the concrete flow ability; while the shape of the mixture after the slump cone removal provides an insight into the concrete compactibility. 163
When a fresh concrete mixture is placed into the slump cone from a constant height without any rodding, tamping, or vibration, the following observations can be made and explained [2,3]: If a concrete mixture has good compactibility or it is well compacted, the shape or deformation of the mixture after the slump cone is removed should be plastically isotropic, as shown in Figure 2(a). The mixture has a uniform aggregate particle distribution and good cohesion. If a concrete mixture has no good compactibility or it is not well compacted, the shape or deformation of the mixture after the slump cone is removed may be irregular, due to the weak zones in the fresh concrete, as shown in Figure 2(b). remove slump cone or or little/no slump 4"-5" slump 5"-8" slump (a) well compacted Weak Zone Weak Zone (b) poor compacted Figure 2: Slump cone shape versus concrete compactibility The flow behavior of SF SCC is generally between those of conventional pavement concrete and SCC mixtures. That is, a SF SCC mixture often has certain slump and spread values so as to be able to flow. It should also have a good cone shape, as shown in Figure 2 (a), after the slump cone is removed, thus ensuring a good self-consolidating ability. The criteria of these three slump cone test parameters, slump, spread, and shape, have to be met together for SF SCC mix design. 3.2 Compaction factor tests A modified compaction factor test method was used to evaluate the self-consolidating ability or compactibility of a concrete mixture. In this test, an inverse slump cone is placed on a 4 x8 or 10 cm x20 cm cylinder. Freshly mixed concrete is filled in the slump cone and falls into the container under its own weight. The unit weight of the concrete cylinder is then measured and compared with that of concrete cylinder prepared with three layers and rodded 25 times for each layer. The 164
compaction factor of the concrete is expressed by the ratio of the unit weights of the non-rodded and rodded concrete. 3.3 Green strength test A simple test was initially developed to assess the green strength of fresh concrete. In this test, a plastic cylinder mold (4 in. x 4 in. or 10 cm x 10 cm, without bottom) was used for concrete casting. During the casting, a concrete mixture was placed into the cylinder mold at a given height (6 in. or 15 cm) with no additional consolidation applied. Immediately after the cylinder was filled up, the plastic mold was removed, and the shape of the concrete sample was examined. If a mixture demonstrated little or no deformation after the mold was removed, the mixture was considered to have good shape-holding ability, and the green strength test of the sample was then pursued. A large plastic cylinder was placed on the top of the fresh concrete sample. A small amount of sand was then slowly but continuously poured into the large plastic cylinder until the sample collapsed. The maximum amount of the sand applied during the test divided by the loading area of the sample defined the green strength of the concrete. Figure 3 illustrates the test procedure for the concrete green strength measurement. (a) Test setup: slump cone placed on top of a plastic mold without the bottom (b) Casting: mold is filled with fresh concrete without rodding or vibration (c) Demolding: after plastic mold is removed, some concrete holds its shape (d) Loading: a big cylinder is placed on top of fresh concrete sample; sand is gradually loaded into cylinder until sample fails Figure 3: Test procedure for concrete green strength measurement 165
(a) Drop table (b) After 25 drops (c) After loading Figure 4: Device and samples used for modified green strength measurement In the later course of the SF SCC development, the above green strength test method was further modified. A standard drop table was used and the shape stability of the tested materials was evaluated after compaction. As shown in Figure 4, in the modified test, a 4x8 inch cylinder was loosely filled up with fresh concrete. This cylinder was then placed on the drop table and subjected to 25 drops. After the compaction, the cylinder was turned over and demolded. A vertical force was applied to the cylinder until the specimen collapsed. The maximum load was used to calculate the green strength of the tested cylinder. 3.4 Mini-paver test Mini-paver test is developed to simulate field paving using SF SCC in laboratory. As shown in Figure 5, the system consists of three parts: (1) an L-box with a platform on top, (2) a towing system (a towing cable and a crank), and (3) a working table. The L-box was 18 in. (46 cm) wide, 24 in. (60 cm) long, 18 in. (46 cm) high, and 3-6 in. (7.5-15 cm) thick. It could pave an 18 in. (46 cm) wide, 3-6 in. (7.5-15 cm) thick, and 48 in. (122 cm) long slab in the lab using two cubic feet of concrete mixture. Before the paving test, approximately 200 pounds of weights were placed in the back chamber of the paver (Figure 5 (b)). A stop plate was positioned at the end of the horizontal leg of the L-box. Freshly mixed concrete was 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 (3 5 ft/min). As the mini-paver moves forward, it extrudes the concrete slab, out of the horizontal leg of the L- box. 166
(a) front view (b) back view (c) sketch Figure 5: Mini-paver system 4. TEST RESULTS 4.1 Effect of fine materials on the flowability of cement pastes As mentioned before, the SF SCC mix design development started with a common selfconsolidating concrete (SCC). This SCC was modified by gradually adding different fine materials, such as fly ash, clay and cement, until the concrete reached a shape stable condition. Figure 6 shows the effects of different fine materials (FM) and water-to-fine material ratio (W/FM) on flowability and shape stability of concrete pastes, where the paste flow was measured by the flow drop table as described in ASTM C 230. The high effectiveness of the fine materials in shape stability improvement appeared closely related to the finer particle size. With the fine material addition, a high flowable, low shape stability paste was changed into a nonflowable, highly shape stable paste. 167
Figure 6: Effectiveness of different fine materials on shape stability of cement pastes [4] 4.2 Interrelation between concrete flow ability and compactibility Figure 7 presents the slump and spread values of various concrete mixtures, where the first term of the mix symbols represents the percentage and type of fly ash in the mixture, and the second term is the water-to-binder ratio (w/b) of the mixture. For example, 40C-0.4 indicates that the mixture has 40% class C fly ash and w/b of 0.4. C3 represents a conventional pavement concrete mixture and SCC is a conventional SCC mixture. Table 1 illustrates the shape of the concrete mixture after the slump cone was removed. It was observed that mixtures not having regular cone shape after the slump test generally had a low compaction factor, indicating that they were not compacted well. Mixtures having w/b of 0.40 and containing 25% or 30% class C fly ash (such as mixes 25C-0.40*, 30C-0.40, 30C-0.40*) not only had a desirable slump, but also a regular shape after the slump cone was removed. These mixtures were expected having not only good flowability and selfcompactibility but also good shape-holding ability, and they can be considered as SF SCC. As circled in Figure 7, the slump and spread values of these concrete mixtures were approximately 8 in. (200 mm) and 14 in. (350 mm), respectively. The flow behavior of these SF SCC mixtures were between those of the conventional slip-form paving concrete (C3) and conventional SCC, much closer to that of conventional SCC. 168
12 SCC Slump (in) 10 8 6 4 30F-0.35 REF-0.35 25C-0.4* 40C-0.4* 50F-0.35 30C-0.4 30F-0.4 40C-0.35 30C-0.4* 2 C3 0 5 7 9 11 13 15 17 19 Spread diameter (in) Figure 7: Relationship between concrete slump and spread Table 1: Concrete flow ability and compaction ability Compaction Slump/Spread Shape of the Concrete Mixture after the Loosely-filled Slump Cone Test Factor 0.8 2-6 (50-150 mm)/ 8-11 (200-275 mm) (irregular shape) 1.0 3-7 (175-225 mm)/ 12-15 (200-275 mm) (regular cone shape) 4.3 Relationship between green strength and flowability The relationship between green strength and flowability of some tested mixtures is given in 169
Flow Diameter after 25 Drops Figure 8: Relationship between green strength and flowability [4] Figure 8. It can be seen that most of the mixtures follow a main trend: green strength decreased with an increased flow diameter of the concrete mixtures subjected to 25 drops on the drop table. It can also be seen that certain mixtures did not follow this main trend. These mixtures show a higher green strength (corresponding to a better shape stability) for a given flowability than other mixtures. These mixtures are potential candidates for being used as SF SCC or low compaction energy concrete. Flow Diameter after 25 Drops 170
Figure 8: Relationship between green strength and flowability [4] 4.4 Mini-paver test results Mini-paver simulates field slip-form paving process. Figure 9 shows a SF SCC slab extruded from the mini-paver and the cross section of the concrete slab. The top surface of the final pavement section was smooth, and little or no edge slump was observed (Figure 9 (a)). The cross-section of the SF-SCC showed no visible honeycomb and segregation (Figure 10 (b)). It demonstrates that a well-designed SF SCC mixture could not only self-consolidate but also hold their shape very well after coming out of the paver. After the concrete was hardened, three 2 in. (5 cm) diameter cores and three 4 in. (10 cm) diameter cores were taken at the age of 9 days for compressive and split tensile strength tests, respectively. The average concrete compression strength was 4,900 psi, and the average split tensile strength was 420 psi. Further research has indicated that well designed SF SCC mixes also had comparable properties, such as set time, heat of hydration, and freezing-thawing resistance, to conventional pavement concrete [5]. Selected SF SCC mixtures have been successfully used for some field applications. The short-term performance of the field SF SCC appeared satisfactory, and the long-term performance of the SF SCC is under monitoring. (a) Concrete slab from a mini-paver test TOP BOTTOM (b) Cross section of the above concrete slab Figure 9: Mini-paver test results of SF SCC 171
5. CONCLUSIONS The new type of self-consolidating concrete for slip-form paving (SF SCC) shall have sufficient flowability for self-consolidation but be stiff and strong enough to hold its shape right after paving, thus not requiring consolidation and formwork in construction. Modified (non-rodding) slump test, compaction factor test, green strength test, and min-paver test can be used to characterize flow ability, self-consolidating ability, and shape holding ability of SF SCC concrete mixture. Well designed SF SCC can have comparable strength and performance to conventional concrete. SF SCC has a great potential for slip-form construction. ACKNOWLEDGEMENTS The present study is a part of a pool-funded project sponsored by five state departments of transportation (DOTs), the concrete admixture industry, the Federal Highway Administration (FHWA), and the National Center for Concrete Pavement Technology (CP Tech Center). The authors greatly appreciate the technical inputs and financial supports from the sponsors. Special thanks are given to Dr. Gang Lu and Mr. Bob Steffes at Iowa State University for their participation and help in this research project. REFERENCES [1] S. Tymkowicz and R. F. Steffes, Vibration Study for Consolidation of Portland Cement Concrete, Semisesquicentennial Transportation Conference, Iowa State University (1996) [2] G. Christensen, "Modeling the flow of fresh concrete," Ph.D. dissertation, Princeton University (1991) [3] W. R. Schowalter and G. Christensen, Toward a rationalization of the slump test for fresh concrete: comparisons of calculations and experiments, Journal of Rheology, 42(4)(1998): 865-870. [4] B. Y. Pekmezci, T. Voigt, K. Wang, and S. P. Shah, Low Compaction Energy Concrete for Improved Slip Form Casting of Concrete Pavements ACI Materials Journal, 104(3)(2007): 251-258. [5] Kejin Wang, Surendra P. Shah, Thomas Voigt, Bekir Yilmaz Pekmezci, David J. White, Joseph Gray, Lu Gang, Jiong Hu, Clinton Halverson, Self-Consolidating Concrete Applications for Slip-Form Paving (Phase I) Final Report, Center for Portland Cement Concrete Pavement Technology, Iowa State University (2005) 172