Gaurav Sant

Measurement of Volume Change in Cementitious Materials at Early Ages: Review of Testing Protocols and Interpretation of Results

Early-age cracking in concrete bridge decks, pavements, and superstructure elements has served as the impetus for substantial research on early-age shrinkage in cementitious materials. Much of this research has indicated how mixture proportions, constituent materials, and construction operations can be altered to reduce the risk of cracking. Unfortunately, many unrestrained shrinkage-testing protocols do not provide a comprehensive picture of the early-age shrinkage exhibited by cementitious materials, especially those used in higher-strength concrete. In this paper, the authors review several early-age shrinkage testing procedures. A testing protocol is presented to show how chemical shrinkage can be measured by using buoyancy measurements. A comparison of the measured autogenous shrinkage is made by using four measurement methods: a sealed membrane, a corrugated tube, a noncontact measurement in a rigid mold, and the ASTM C 157 standard. The results of the autogenous and chemical shrinkage tests are compared with one another to describe fully early-age length change. It is shown that through careful experimentation and interpretation, the results of these tests can be completely correlated with one another. This can provide the end user with reliable test procedures to compare different paste compositions and different admixtures and can provide inputs for models that quantify cracking potential.

Performance of Shrinkage-Reducing Admixtures at Different Humidities and at Early Ages

Shrinkage-reducing admixtures (SRAs) have been developed to reduce the risk of early-age shrinkage cracking in concrete. While substantial results are available in the literature to illustrate how SRAs influence the free shrinkage of mixtures tested in accordance with ASTM C157, fewer results have shown the influence of SRA on volume changes that occur from the time of casting until the standard shrinkage tests begin. In addition, few tests have been conducted in environments other than 50% relative humidity (RH). This paper discusses the initial shrinkage (that is, measured from the time of casting), long-term shrinkage, and residual stress development of a plain cement paste and a paste containing 5% SRA over a wide range of RH. The results indicate that pastes containing SRA demonstrate an expansion at early ages. In addition, the results provide evidence for the idea that smaller pores are emptied upon drying in specimens containing SRA. This phenomenon can substantially change the RH levels where capillary stresses are the main cause of shrinkage in concrete and can explain the reduction in drying shrinkage that occurs when SRAs are used in concrete.

Shrinkage Mitigation Strategies in Cementitious Systems: A Closer Look at Differences in Sealed and Unsealed Behavior

Shrinkage reducing admixtures (SRAs) and saturated lightweight aggregates (LWAs) are increasingly used to reduce shrinkage cracking of concrete mixtures. While both methods show great potential, to obtain the full anticipated benefits of either SRA or LWA the boundary conditions of the concrete element must be carefully considered and understood. Addressed are shrinkage and shrinkage cracking behavior of concrete with sealed and unsealed boundaries. The sealed concrete undergoes self-desiccation, while the unsealed concrete simultaneously experiences both self-desiccation and external drying. The research work presented provides a theoretical and experimental demonstration of the differences in the shrinkage behavior of mixtures containing SRA and LWA. Data are provided from experiments that demonstrate the benefits of SRA and LWA under sealed and unsealed conditions. Theoretical considerations explain the influence of boundary conditions on shrinkage and cracking of concrete. This has important implications for selecting an adequate shrinkage mitigation strategy. In addition, it is demonstrated that the experimental results are consistent with the theoretical predictions.

Using X-Ray Absorption to Assess Moisture Movement in Cement-Based Materials

This paper discusses the use of x-ray absorption as a technique for measuring fluid transport in cementitious systems. First, the paper describes how a material’s x-ray attenuation spectrum can be combined with composite modeling concepts to determine the extent of moisture movement (wetting or drying) that can be measured in cementitious systems. Second, the paper discusses the influence of beam power (voltage and current) and camera integration time on the variation in the measured x-ray intensity. Third, the paper discusses two devices for measuring the transmitted x-ray intensity (camera and detector) to describe differences in their measurements and resolution. Finally, a demonstration is provided to show how x-ray absorption can be used to detect moisture ingress and determine the transport properties of the material under evaluation.

Comparison of Ca(NO3)2 and CaCl2 Admixtures on Reaction, Setting, and Strength Evolutions in Plain and Blended Cementing Formulations

AbstractDue to the role of Cl− ions in inducing the corrosion of steel rebar in reinforced concrete, it is of interest to use chloride-free accelerators in concrete construction. Ca(NO3)2 (CN) is a Cl− free admixture, which accelerates set and is thought to inhibit steel corrosion. This study carefully compares the influences of both Ca(NO3)2 and CaCl2 (CC) additions on reaction and property evolutions across a range of cementing materials including ordinary portland cements (OPCs) and blended OPCs, i.e.,xa0those containing Class F fly ash, slag, or fine limestone. The results indicate that CN acts as a set accelerator, though in relation to its dosage a slight drop in early age (i.e.,xa01xa0day) strength is noted. However, this effect is transient, with similar strengths being recorded across all dosage levels by 28xa0days. While CC is noted to be a superior (i.e.,xa0setting and hardening) accelerator, both additives show similar performances by 28xa0days. The results from a phase boundary nucleation and growth (BNG...