Soo-Duck Hwang

Performance-Based Specifications of Self-Consolidating Concrete Used in Structural Applications

Proper selection of test methods and workability specifications are key concerns in the optimization and control testing of self-consolidating concrete (SCC). An experimental program was carried out to evaluate the suitability of various test methods for workability assessment and to propose performance specifications of such concrete used in structural applications. Various workability characteristics were determined for approximately 70 SCC mixtures made with water-cementitious material ratios (w/cm) of 0.35 and 0.42. Workability responses included the slump flow, J-Ring, V-funnel flow time, L-box, filling capacity, and surface settlement tests. Comparisons of various test methods indicate that the L-box blocking ratio (h 2 /h 1 ) and the J-Ring flow diameter can be related to filling capacity values determined using the caisson test. It is recommended that SCC used in structural applications should have slump flow values of 620 to 720 mm. To ensure proper filling capacity greater than 80%, such concrete should have high passing ability that corresponds to L-box blocking ratio (h 2 /h 1 ) ≥ 2 0.7, J-Ring flow of 600 to 700 mm, slump flow minus J-Ring flow diameter ≤ 50 mm, or V-funnel flow time ≤ 8 seconds. Such SCC should have a settlement rate of 0.16%/h at 30 minutes, corresponding to 0.5% maximum settlement.

Effect of Mixture Composition on Restrained Shrinkage Cracking of Self-Consolidating Concrete Used in Repair

When compared with conventional concrete with normal slump consistency, self-consolidating concrete (SCC) can inherently exhibit greater restrained shrinkage cracking and drying shrinkage risk, given a mixture design in which incorporation of relatively high content paste content and low coarse aggregate volume is often involved. For the influence of various mixture parameters on structural repair-designated high-performance SCC shrinkage cracking, an experimental program was undertaken. Hybrid fiber (HF) use, shrinkage-reducing admixture (SRA) dosage, synthetic fiber type and content, and high-range water-reducing admixture types are among the included parameters. Cracking potential evaluation was performed through an instrumented ring-type setup (ASTM C158). That a fiber volume increase can lead to a concrete cracking potential decrease is indicated by test results, regardless of fiber type. Synthetic fiber volume increased, on average, from 0 to 0.25% and 0.25% to 0.50%, which led to an approximately 40% elapsed time increase before initiation of restrained shrinkage cracking. After a 2.4-folds longer time before cracking and 56 days of drying, a 40% lower drying shrinkage was developed in SCC made with a high SRA concentration (and no fibers) when compared with similar concrete prepared without any SRA. Designing high-performance SCC with low cracking potential is effective through combined SRA use with either HF or synthetic fibers at a 0.25% or 0.50% dosage rate. Elapsed crack time initiation due to restrained shrinkage estimation can be performed to through drying shrinkage values at 50% relative humidity after 7 and 56 days of drying and modulus of elasticity at the beginning of restrained shrinkage setup (3 days of age) drying. For a given drying shrinkage level, a cracking potential increase can occur when, at the time of drying initiation, higher elastic modulus is developed by concrete. For concrete of a given elastic modulus, the extent of drying shrinkage deformation, similarly, correlates with a cracking potential increase.

Performance-Based Specifications of Workability Characteristics of Prestressed, Precast Self-Consolidating Concrete—A North American Prospective

Adequate selection of material constituents and test methods are necessary for workability specifications and performance of hardened concrete. An experimental program was performed to evaluate the suitability of various test methods for workability assessment and to propose performance specifications of prestressed concrete. In total, 33 self-consolidating concrete (SCC) mixtures made with various mixture proportioning parameters, including maximum size and type of aggregate, type and content of binder, and w/cm were evaluated. Correlations among various test results used in evaluating the workability responses are established. It is recommended that SCC should have slump flow values of 635–760 mm. To ensure proper filling capacity greater than 80%, such concrete should have a passing ability that corresponds to L-box blocking ratio (h2/h1) ≥ 0.5, J-Ring flow of 570–685 mm, slump flow minus J-Ring flow diameter ≤75 mm. Moreover, Stable SCC should develop a column segregation index lower than 5%, and rate of settlement at 30 min of 0.27%/h for SCC proportioned with 12.5 or 9.5 mm MSA. It is recommended that SCC should have a plastic viscosity of 100–225 Pa·s and 100–400 Pa·s for concrete made with crushed aggregate and gravel, respectively, to ensure proper workability.

Effect of Various Admixture-Binder Combinations on Workability of Ready-Mix Self-Consolidating Concrete

This paper describes an experimental investigation that was carried out in order to evaluate the effects of high range water reducing admixture (HRWRA), viscosity-enhancing admixture (VEA), and binder type on key workability characteristics of self-consolidating concrete (SCC), including retention of deformability, passing ability, and stability. Concrete-equivalent mortar (CEM) mixtures were prepared to evaluate the effect of admixture-binder combinations on flow characteristics, including minimum water content (MWC) to initiate flow and relative water demand (RWD) to increase a given fluidity. Four polycarboxylate-based HRWRAs, a polynaphthalene sulfonate-based HRWRA, four types of VEAs, and three blended cements were evaluated. In total, 16 SCC mixtures with initial slump flow consistency of 660 +/- 20 mm and air volume of 6.5 +/- 1.5%, and 17 CEM mixtures were investigated. Flow characteristics of SCC and CEM mixtures made with a number of admixture-binder combinations indicate that the efficiency of admixture-binder combination depends on water-to-cementitious material ratio (w/cm), type of binder, and type of admixtures. The CEM approach can be used to evaluate the effect of admixture-binder combination on flow characteristics because the increase in MWC to initiate flow of CEM corresponds to higher demand in HRWRA in SCC mixtures. Binder type was shown to have marked influence on the retention of slump flow, L-box and V-funnel passing ability, filling capacity, and surface settlement characteristics. The binder type also affects HRWRA and air-entraining admixture (AEA) demand. As established from CEMs, B3 quaternary cement with the smallest 50% passing diameter had the highest MWC (lowest packing density) needed to initiate flow and the highest RWD (highest robustness to changes in water). SCCs made with such quaternary cement and polycarboxylate-based HRWRA also exhibited the highest HRWRA demand compared those prepared with other blended cements. Both sets of SCCs made with 0.35 w/cm and 0.42 w/cm plus VEA had similar HRWRA demand and static stability when the polycarboxylate-based HRWRA was used.

Pull-Out Strength and Bond Behavior of Prestressing Strands in Prestressed Self-Consolidating Concrete

With the extensive use of self-consolidating concrete (SCC) worldwide, it is important to ensure that such concrete can secure uniform in-situ mechanical properties that are similar to those obtained with properly consolidated concrete of conventional fluidity. Ensuring proper stability of SCC is essential to enhance the uniformity of in-situ mechanical properties, including bond to embedded reinforcement, which is critical for structural engineers considering the specification of SCC for prestressed applications. In this investigation, Six wall elements measuring 1540 mm × 2150 mm × 200 mm were cast using five SCC mixtures and one reference high-performance concrete (HPC) of normal consistency to evaluate the uniformity of bond strength between prestressing strands and concrete as well as the distribution of compressive strength obtained from cores along wall elements. The evaluated SCC mixtures used for casting wall elements were proportioned to achieve a slump flow consistency of 680 ± 15 mm and minimum caisson filling capacity of 80%, and visual stability index of 0.5 to 1. Given the spreads in viscosity and static stability of the SCC mixtures, the five wall elements exhibited different levels of homogeneity in in-situ compressive strength and pull-out bond strength. Test results also indicate that despite the high fluidity of SCC, stable concrete can lead to more homogenous in-situ properties than HPC of normal consistency subjected to mechanical vibration.

Effect of Material Constituents and Mix Design on Performance of SCC for Precast, Prestressed Girders

An extensive investigation was undertaken to evaluate the influence of binder type, water-to-cementitious materials ratio (w/cm), type of coarse aggregate, and maximum size of aggregate (MSA) on workability and strength development of self-consolidating concrete (SCC) designated for precast, prestressed bridge girders. In total, 30 SCC mixtures were investigated. In total, 24 mixtures were non air-entrained and were prepared using crushed aggregate or gravel with three MSA of 10, 14, and 20 mm. The w/cm was set to a relatively low value of 0.33 and a higher value of 0.38. Three different binder compositions were investigated. The effect of slump flow consistency on performance was also investigated. Test results indicate that SCC made with the higher w/cm exhibited superior fluidity retention, passing ability, and filling capacity. Mixtures proportioned with the lower w/cm developed greater stability. Mixtures made with the crushed aggregate of 10 mm MSA exhibited greater passing ability, higher filling capacity, and better retention of air content than similar SCC prepared with 14 and 20 mm MSA. Mixtures proportioned with the gravel had superior passing ability, filling capacity and similar stability compared to those made with crushed aggregate of the same MSA.

Performance of Cast-in-Place Self-Consolidating Concrete Made with Various Types of Viscosity-Enhancing Admixtures

This article will discuss viscosity-enhancing admixtures (VEAs) which are water-soluble polymers that increase viscosity and cohesion of cement-based materials. Such enhancement is essential in highly flowable concrete—including self-consolidating concrete (SCC)—to control the risk of segregation. For a given mixture composition, the performance of SCC can widely vary with the type and dosage rate of the VEA in use. The main objective of this investigation is to compare the performance of five VEA systems in SCC. More specifically, the investigation seeks to determine the effect of VEA type on key workability and engineering properties of SCC designated for cast-in-place building applications. The concrete is designed with a characteristic compressive strength of 35 MPa (5076 psi) at 28 days. In total, five VEAs incorporated with two compatible high-range water-reducing admixtures (HRWRAs) are investigated. A finely ground limestone filler that can be used as a stabilizer in flowable concrete is also included in the study.