Influence Of Shrinkage-reducing Admixtures Research Articles

Influence of shrinkage-reducing admixture on drying shrinkage and mechanical properties of high-performance concrete

High-performance concrete (HPC) has specific performance advantages over conventional concrete in strength and durability. HPC mixtures are usually produced with water/binder mass ratios (mW/mB) in the range of 0.2–0.4, so volume changes of concrete as a result of drying, chemical reactions, and temperature change cannot be avoided. For these reasons, shrinkage and cracking are frequent phenomena. It is necessary to add some types of admixture for reduction of shrinkage and cracking of HPC. This study used

Shrinkage-Reducing Admixture: Effects on Durability of High-Strength Concrete

Shrinkage-reducing admixture (SRA) is a mitigation strategy against autogenous shrinkage in high-strength concrete (HSC). It acts by reducing the surface tension of the fluid in the pore system of the cement paste. An experimental program was carried out to investigate the influence of SRA on concrete durability, as few studies about this subject have been developed. Its effectiveness in reducing autogenous shrinkage and its influence on compressive strength were also evaluated. The results showed a substantial reduction of autogenous shrinkage due to the presence of the SRA, with a slight reduction on compressive strength (up to 5%). Overall, concrete durability under the action of aggressive agents (such as water, CO2, and chloride ingress) was not influenced by SRA, as verified in the results of chloride penetration, natural carbonation, water permeability, capillary absorption of water, and absorption of water tests.

Influence of shrinkage-reducing admixtures on plastic and long-term shrinkage

Abstract The use of shrinkage-reducing admixtures (SRA) has been suggested to improve concrete performance in terms of lower risk of cracking related to drying shrinkage. Various forms of SRA are commercially available and they may act through different mechanisms. Some SRA mainly acts on drying and weight loss leading to shrinkage. In this paper, the influence of a liquid SRA on plastic shrinkage, long-term shrinkage, mechanical characteristics as well as concrete pore structure were investigated. Samples of concrete were prepared with two water-to-cement ratios (w/c) to design ordinary and high strength concrete. The effect of the shrinkage-reducing admixture was studied by adding 1% to the total mass of binder, while keeping the other parameters constant. At early age, the results indicate that the SRA lead to the same plastic shrinkage for w/c = 0.65 while it reduced the plastic shrinkage by 25% for w/c = 0.43. Drying shrinkage was assessed from 1 day on hardened concrete. The SRA reduced the 7 day drying shrinkage for w/c = 0.65 and w/c = 0.43 concrete mixtures by up to 56%–31%, respectively, and the 70 day drying shrinkage by up to 33%–25% when the specimens were cured for 24 h then stored at relative humidity of 50%. At equal water-to-cement ratios, the SRA is seen to be more efficient in reducing drying shrinkage at early ages. These findings suggest that the SRA is most effective when internal relative humidity is relatively high or when higher porosity exists in the material. In fact, the SRA modified the pore structure increasing the total porosity and eliminating the percentage of larger pores with diameters ranging from 300 to 1000 nm. When concerned with drying of concrete, the larger pores are the first ones to lose their internal water and consequently change the RH levels where capillary stresses are the main cause of shrinkage. Thus, this phenomenon may contribute in the reduction of drying shrinkage that occurs when this liquid SRA is used in concrete.

Influence of Shrinkage-Reducing Admixtures on Moisture Absorption in Cementitious Materials at Early Ages

The water-absorption behavior of cement pastes (w/c=0.30) containing varying concentrations (i.e., 0, 0.2, and 5%) of a shrinkage-reducing admixture (SRA) was measured. Moisture ingress was monitored using X-ray absorption. A decrease in both the depth of water penetration and the rate of water absorption was observed with increasing specimen maturity and admixture concentration. This agrees with theoretical considerations that suggest water sorption is a function of the surface tension and the viscosity of the fluid ingressing into the pores. The Boltzmann-Matano method was successfully employed to determine the moisture content dependent moisture diffusivity of the material, which exhibited a dependence on both the pore structure (specimen maturity) and the SRA concentration.

Influence of shrinkage-reducing admixtures on the reduction of plastic shrinkage cracking in concrete

Fresh concrete exposed to high evaporation rates is prone to plastic shrinkage cracking, especially in structures with large surface area/volume ratios. The present work shows that the reduction of the surface tension of the mixing water is an effective way for decreasing such cracking. In this study, conventional and high strength concretes with superplasticizers and shrinkage reducing admixtures (SRAs) were exposed to drying in the plastic state. Continuous monitoring of the surface displacement facilitated the identification of the different stages of plastic shrinkage cracking. Measurements of capillary pressure, settlement, internal temperature and evaporation rate were also made. The results show the effectiveness of SRAs in reducing plastic shrinkage cracking, even in high strength concrete. This is attributed to the reduction in the evaporation rate, delay of the peak capillary pressure due to the development of menisci in the pores and lower settlement.

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 mortar's topmost layer, reduced capillary tension, reduced settlement, and less evaporation.

Influence of Shrinkage-Reducing Admixtures on Early-Age Properties of Cement Pastes

Because most shrinkage-reducing admixtures (SRAs) significantly reduce the surface tension of a cement paste pore solution, they will naturally influence all physical properties and processes that are dependent in some way on surface tension. Such properties include internal relative humidity, capillary stresses, and freezing point depression, all via the Kelvin equation and its variants (Kelvin-Laplace, Gibbs-Thomson). Processes that will thus be strongly influenced by the presence of SRAs include drying, autogenous stress and strain development, and freezing. In this paper, experimental measurements of these processes in cement pastes and mortars with and without SRA additions will be presented in light of the Kelvin equation. The experimental measurements that are applied to early-age specimens include X-ray absorption measurements to quantify drying profiles, bulk mass loss measurements, measurements of internal relative humidity, assessments of autogenous deformation under sealed curing conditions, and low temperature calorimetry scans to quantify freezable water content. The results indicate that SRAs can provide benefits in several new applications beyond their conventional usage to reduce drying shrinkage.