International Concrete Abstracts Portal

Controlled low-strength material (CLSM) mixture design remains a trial-and-error process. A new approach using relative proportioning of the constituent materials instead of prescribed mass contents is proposed. Relative proportions allow for independent adjustments that enable unbiased estimation of their effects on CLSM properties. For the CLSM mixtures studied, a central composite experimental design was defined using three relative proportions: volumetric paste percentage (VPP), water-cementitious material ratio (W/CM), and portland cement-total cementitious materials ratio (OPC/CM). Second-order response models for slump flow, subsidence, and 28-day compressive strength were obtained for different sets of constituents, including virgin and recycled concrete fine aggregates and two fly ash sources. Slump flow and subsidence were most affected by the VPP and W/CM, respectively, whereas strength was explained by the combined effect of the W/CM and OPC/CM. The W/OPC ratio was not a reliable predictor of CLSM strength.

This research examines the effects of sand characteristics and fly ash contents on fresh properties and on the unconfined compressive strength of flowable fill mixtures. Additionally, this research investigates the flowability of wet and dry sands and the influence of fly ash addition on this property using a number of sand sources and various levels of fly ash addition. It was found that adding fly ash into dry or wet sand substantially influenced its flowability. In addition, it was found that the flowability of sand-fly ash mixtures was influenced by such sand properties as fineness modulus and uncompacted air void content. Similarly, other flowable fill properties, such as bleeding, subsidence, penetration resistance, and unconfined compressive strength, were observed to vary with the sand property and the amount of fly ash added into the mixture.

Viscosity-enhancing admixtures (VEAs) 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.

The Segregation of coarse aggregate in cement-based materials can lead to heterogeneous properties of the hardened material with direct implication on mechanical properties and durability. The control of segregation is especially critical in flowing and selfconsolidating concrete (SCC), as well as in concrete made with supplementary cementitious materials or fillers used as partial replacement of cement. In the present study, an attempt was made to develop a rapid methodology for estimating segregation during the dormant period of cement hydration, using electrical conductivity to monitor changes in the local composition over a specimen height, as a function of time. The results of the multi-pair electrode conductivity method were compared with those obtained by studying the actual segregation determined on hardened concrete samples. This was done by an image processing technique of tracing and counting aggregate particles along hardened cylindrical sections. In total, nine concrete mixtures having slump values of 70 to 240 mm (2.76 to 9.45 in.) and five SCC mixtures with slump flow values ranging between 650 mm and an excessive value of 880 mm (25.6 to 34.7 in.) were tested. A high degree of correlation between the results of the electrical conductivity and image analysis methods was established. Data obtained from the electrical conductivity approach after 20 minutes of testing are shown to correlate well with bleeding, segregation, and homogeneity indexes determined from electrical conductivity approach at peak conductivity typically greater than 3 hours as well as with the segregation index determined from image analysis. The conductivity approach can then be used as a nondestructive method for evaluating the static stability following concrete placement.

This paper evaluates the fluid and hardened state properties of controlled low-strength materials (CLSM) with cement kiln dust (CKD) as the primary binding agent. The low-strength property of CKD can be advantageous when used in CLSM because most applications require future excavatability. Because CLSM is often produced with fly ash (FA), it inherently mitigates the higher water demand of CKD. Laboratory-prepared mixtures with CKD:FA ratios of 1:12, 1:6, and 1:1 were investigated by varying the water-binder ratio (w/b) from 0.95 to 1.10. Flowability, volume stability (bleeding), setting time, mass density, and 28-day unconfined compressive strength (UCS) were measured for each mixture. The results revealed that high flowability and setting times within 24 h could be achieved with most mixtures, although the bleeding levels are higher than normal. Fifteen different CKD-CLSM mixtures were proportioned to produce average strengths ranging from 34 to 460 kPa. The wide range of strengths is significant for two reasons. First, all mixtures are considered to be excavatable based on these 28-day strengths. Second, the stress-strain behavior of these mixtures is representative of soil types ranging from soft clays to very stiff clays. Thus, cement kiln dust can be beneficially added to produce a very low-strength material that offers comparable strengths to soils used for conventional fills and many other low-strength applications.