International Concrete Abstracts Portal

This paper describes recent experimental efforts to quantify autogenous volume change, stress development, and cracking in low water-to-cement ratio (w/c) mixtures. A non-contact laser was used to measure length changes from the time that the mortar was placed in the forms. An analytical solution was used to illustrate how the restrained ring test could be used to quantify the level of residual stress that develops from the time of casting. Piezoelectric sensors were used to measure the acoustic activity that develops in the mortar. It is hypothesized that this acoustic activity is generated by microcracking at the aggregate interface caused by differential shrinkage. Length change measurements show an initial period of shrinkage before setting, a slight expansion during setting, and continued shrinkage after final set. The residual stresses were observed to develop several hours after the time of set, suggesting that some movement may be permitted while the structure of the paste is developing. Acoustic activity was observed to occur a few hours after residual stresses developed, suggesting that some critical strain level is needed for microcracking to occur.

This paper evaluates a test method for measuring bulk, autogenous volume changes in cement paste and mortar. In this test method, paste and mortar were sealed in latex membranes and submerged in water. The weight of the specimens was recorded periodically, both in air and submerged in water, and their volume change was calculated using Archimedes' principle. Several sources of error in the test method were identified, and measures were taken to account for some of this error. It was concluded that the experimental error for this test may be quite substantial as the test duration increases, and therefore this test method is most suited for measuring the early age volume changes of cement paste and mortar.

In this paper, measurements of non-evaporable water content, chemical shrinkage, autogenous deformation, internal relative humidity (RH), pore solution composition, and early-age elastic modulus are presented and discussed. All experiments were performed on Portland cement and blast-furnace slag (BFS) cement pastes. Self-desiccation shrinkage of the BFS cement paste was modeled based on the RH measurements, following the capillary-tension approach. The main findings of this study are: 1) self-desiccation shrinkage can be related to self-desiccation both for Portland and for BFS cement pastes, taking into account the influence of the dissolved salts in the pore solution, 2) the BFS cement paste studied shows pronounced self-desiccation and self-desiccation shrinkage, mainly caused by its very fine pore structure.

High-performance concrete is very vulnerable to early cracking because it does not bleed and it develops within the first 24 hours a significant autogenous shrinkage when it still has a very weak tensile strength. Plastic shrinkage cannot always be fought with curing membrane: fog spraying is much more appropriate. Among the different means already available to fight early autogenous shrinkage external water curing is a very efficient one. Water curing must be extended for 7 days. At the present state of the technology concretes having a water/cement ratio of 0.36 are the more robust against early cracking when an external water curing is used. The cost of water curing can represent from 0.1 to 1.5% of the total construction cost of concrete structures, a good investment in a sustainable development perspective.

A micro-macro experimental study has been performed, from the end of mixing up to several months, on a set of plain cement pastes prepared with the same type I ordinary Portland cement (OPC) and various water-to-cement ratios (W/C), and cured at various constant temperatures. Chemical shrinkage, volumetric and one-dimensional autogenous deformations have been measured and analyzed in relation to the hydration process (degree of hydration of the cement a, Ca(OH)7 content, ...) and to the microstructural characteristics of the material. The effects of the curing temperature at early age (< 24 hours) in the range 10-50°C, and of W/C in the range 0.25-0.60, have been investigated. The temperature-induced changes recorded on both the magnitude and the kinetics of volumetric autogenous shrinkage clearly show the irrelevance of using the usual maturity concept to describe such phenomena within the whole early-age period. In addition, a threshold is pointed out at about a = 7%, both defining the range where autogenous shrinkage is linearly related to a and corresponding to the precipitation of Ca(OH)2. Moreover, a W/C threshold is pointed out both at the macro-level (autogenous deformations, ...) and at the micro-level (characteristics of the hydration products, MIP porosity and pore size distribution, ...). The chemical and (micro)structural basic effects of calcium hydroxide are in particular distinguished.