International Concrete Abstracts Portal

Autogenous deformation of concrete is the free deformation of sealed concrete at a constant temperature. A number of observed problems with early-age cracking of high-performance concrete can be attributed to this phenomenon. During the last 10 years, this has led to an increased focus on autogenous deformation within both concrete practice and concrete research. The papers in this publication were presented at the American Concrete Institute’s Fall Convention in Phoenix, Arizona, October 2002, and will help readers understand the complexity of the autogenous deformation of concrete. The 12 papers from eight different countries indicate the broad, global research efforts dealing with autogenous deformation, and illustrate that interest in autogenous deformation is shared throughout the worldwide concrete community. Note: The individual papers are also available as .pdf downloads.. Please click on the following link to view the papers available, or call 248.848.3800 to order. SP220

High Performance Concretes (HPC) with water-to-binder (w/b) ratios of 0.40 and from 0 to 15% silica fume have been tested under 20° C isothermal conditions and under realistic (semi-adiabatic) temperature developments with maximum temperature in the range 60 to 65° C. The coefficient of thermal expansion is not very sensitive to silica fume content and its time/temperature dependence may be expressed by the maturity concept. The autogenous shrinkage is extremely temperature dependent, and, importantly, isothermal data cannot be used to predict the behavior during realistic temperature histories. The effect of silica fume (1:1 replacement of cement) is generally to increase the autogenous shrinkage; however, the increase depends strongly on the temperature history, and occurs primarily the first 2 days. Thus, the consequence for crack sensitivity is by no means obvious, and must be calculated for each particular structure.

Recent experimental results (creep tests and indentation tests at a nanometer scale) on Ductal®, a non-brittle (fiber-reinforced) ultrahigh-performance concrete (UHPC), show that only one constituent of this composite, the C-S-H phase, exhibits creep. Former creep tests on hydrated cement paste have shown a very high creep rate of the cement gel which decelerates very slowly (much more slowly than concrete creep). Furthermore, these results provide a clear explanation for the observations of a strong correlation between shrinkage and creep values. The reason is, when hydration rate becomes negligible (typically after a few weeks), the dominant part of shrinkage is nothing but the viscoplastic response of the cement gel to the internal stress which is applied by the liquid phase on the pore surface. This statement makes wrong the last argument against the explanation of shrinkage by capillary tension, the so-called argument of reversibility. Creep aging, as well as the very low creep of high-strength concretes can be explained by the consumption of creep potential by the hygral stress. Several coupling effects between creep and shrinkage can be explained, as for example the so-called PICKETT effect.

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.