Farshad Rajabipour

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.

Influence of Shrinkage-Reducing Admixtures on Development of Plastic Shrinkage Cracks

Cracking that forms between concrete placement and concrete setting is generally described as plastic shrinkage cracking. The authors discuss how concave menisci may form on fresh concrete surfaces due to water evaporation. Potential development of plastic shrinkage cracks may occur when concrete surface tensile stress and concrete settlement develop because of the menisci. The authors focus on how mortars containing a shrinkage-reducing admixture (SRA), which is commercially available, develop plastic shrinkage cracks. When exposed to the same environmental conditions, fewer and narrower plastic shrinkage cracks are shown in SRA-containing mortar than plain mortar. The authors propose that pore fluid of SRA-containing mortar has lower surface tension, which can result in lower crack-inducing stresses at the mortars topmost layer, reduced capillary tension, reduced settlement, and less evaporation.

Investigating the Alkali-Silica Reaction of Recycled Glass Aggregates in Concrete Materials

Application of crushed recycled glass in concrete materials can offer significant economical and environmental benefits provided that the alkali-silica reaction (ASR) of glass in concrete is properly controlled. Previous work on the use of glass sand in mortars shows that the reactivity of glass is influenced by its particle size as mortars containing finer glass sand show reduced ASR expansions. This may be counterintuitive since ASR is considered to be a surface reaction and should accelerate by increasing the surface area (i.e., reducing the size) of reactive aggregates. This paper presents a more in-depth investigation of the size-effect phenomena using scanning electron microscopy (SEM)/energy dispersive spectroscopy imaging of mortars containing different size glass particles. The SEM micrographs reveal that ASR does not occur at the glass-paste interface; rather, it occurs inside microcracks that exist inside glass particles which were generated during the glass bottle crushing operations. Larger size glass particles show larger and more active microcracks which render their high alkali-silica reactivity. At its interface with cement paste, glass shows evidence of pozzolanic reaction which leads to the formation of nonexpansive CSH. For particles smaller than #30 sieve (0.6 mm), the intraparticle ASR is minimal and only the interfacial pozzolanic reaction proceeds. This agrees well with the results of ASTM C1260 tests showing that mixed color glass aggregate smaller than #30 sieve does not produce deleterious ASR expansions in mortars even when no ASR suppressant (e.g., fly ash) is used.

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.

Quantifying the Influence of Specimen Geometry on the Results of the Restrained Ring Test

Over the last decade, the restrained ring test has trequently been used to assess the cracking susceptibility of a concrete mixture when it is restrained from shrinking treely. Despite the trequent use of the ring test, limited analysis has been performed to understand how the specimen geometry influences the results of the test. This paper discusses the influence of specimen geometry on the results of the ring test considering three conditions: (1) uniform shrinkage of the concrete ring, (2) shrinkage caused by drying from the top and bottom surfaces of the concrete ring, and (3) shrinkage caused by drying from the outer circumference of the concrete ring. The role of moisture gradients, thickness of the concrete and the restraining (i.e., steel) rings, and the stiffness of concrete are considered in a series of numerical simulations. Results from these simulations can enable better selection of test specimen geometries and interpretation of the results from the ring test. Analytical expressions are provided to use for determining the geometry of the ring specimen that better simulates specific field conditions while providing the most useful information from the test.

Shrinkage Characteristics of Alkali-Activated Slag Cements

AbstractRecent interest in creating green construction materials has sparked the development of portland cement–free binders. Alkali-activated slag (AAS) concrete has a low embodied energy and comparable or superior strengths to ordinary portland cement (OPC) concrete. However, one factor limiting AAS usage is its durability performance, specifically its susceptibility to shrinkage. Before declaring AAS a marketable product, the mechanisms behind its volumetric instability need to be understood. This paper presents a preliminary study of the shrinkage deformations of various AAS mixtures, wherein four unique AAS mortars were designed and tested for autogenous, chemical, and drying shrinkage; time of setting; and compressive strength. All results were compared to those obtained for a control OPC mortar. Alkali-activated slag mixtures with comparable strength to OPC show a higher autogenous and drying shrinkage. A lower elastic stiffness, higher degree of saturation, and potentially higher chemical shrinkag...

Comparative Life Cycle Assessment of Conventional, Glass Powder, and Alkali-Activated Slag Concrete and Mortar

AbstractThis study compares the cradle-to-gate greenhouse gas emissions (GHGs), energy use, water use, and potential environmental toxicity of conventional (Conv), glass powder (GP), and alkali-activated slag (AAS) concrete and mortar. The comparison is based on 1  m3 of concrete/mortar with similar 28-day compressive strength, so the same concrete/mortar member with same dimensions may be manufactured from Conv, GP, or AAS materials and used for same applications. The result shows that compared to a 35-MPa Conv concrete, a 35-MPa GP concrete has, on average, 19% lower GHGs, 17% less energy, 14% less water, and 14–21% lower environmental toxicity. A 35-MPa AAS concrete has 73% lower GHGs, 43% less energy, 25% less water, and 22–94% lower effects for all environmental toxicity categories except an 72% higher ecotoxicity effect. Environmental impact reductions are also found for using GP as a cement replacement in concrete with lower strengths and replacing cement with GP or AAS in mortars with different st...

Effect of Drying Rate on Shrinkage of Alkali-Activated Slag Cements

The volumetric instability of alkali-activated slag (AAS) binders has raised concerns and impeded the acceptance of this Portland cement-free material. The objective of this article is to characterize the influence of drying rate on drying shrinkage behavior of AAS mortars to better understand the mechanisms responsible for its large shrinkage deformation. A series of four AAS mortar mixtures with varying activator composition, as well as a reference Portland cement mortar, was cast and dried at different relative humidities, that is, 30, 50, 70, and 85% RH. Drying took place inside nitrogen-purged environmental chambers for the purpose of eliminating the contribution of carbonation to the total volumetric change of AAS. The shrinkage and corresponding mass loss of 1.27 cm × 1.27 cm × 12.7 cm prisms were measured as a function of time. The results show that shrinkage of AAS varies largely depending on the drying rate, that is, ambient RH. Interestingly, even though the drying mass loss increases with reducing the RH, the magnitude of shrinkage is the largest for samples stored at 50 and 70% RH, depending on the mixture type. Possible causes of these irregular behaviors are discussed. It is concluded that the drying rate has a much more significant influence on AAS than on ordinary Portland cement (OPC), which implies a more complicated shrinkage mechanism for AAS samples stored at various relative humidities.

A prediction method of Tensile young's modulus of concrete at early age

Knowledge of the tensile Youngs modulus of concrete at early ages is important for estimating the risk of cracking due to restrained shrinkage and thermal contraction. However, most often, the tensile modulus is considered equal to the compressive modulus and is estimated empirically based on the measurements of compressive strength. To evaluate the validity of this approach, the tensile Youngs moduli of 6 concrete and mortar mixtures are measured using a direct tension test. The results show that the tensile moduli are approximately 1.0–1.3-times larger than the compressive moduli within the materials first week of age. To enable a direct estimation of the tensile modulus of concrete, a simple three-phase composite model is developed based on random distributions of coarse aggregate, mortar, and air void phases. The model predictions show good agreement with experimental measurements of tensile modulus at early age.

Fresh and Hardened Properties of Concrete Incorporating Recycled Glass as 100% Sand Replacement

AbstractThis paper investigates the use of glass cullet as a 100% sand replacement in Portland cement concrete (glasscrete) systems. Specifically, this paper evaluates the fresh and hardened properties of these systems in comparison with conventional natural sand concretes on the basis of similar 28-day design compressive strength, or the same w/cm. The results show that glasscrete mixtures need a lower w/cm to match the 28-day compressive strength of conventional concrete. In addition, glasscrete mixtures have greater elastic modulus, less drying shrinkage, less water sorptivity, and greater resistance against chloride ion penetration. Empirical curves are developed to provide material engineers and suppliers with necessary design specifications on the proper w/cm to implement when proportioning glasscrete mixtures. This study concludes that glasscrete mixtures are producible with adequate consistency and mechanical and durability performance, as long as the alkali-silicate reaction is properly controlle...