International Concrete Abstracts Portal

The paper deals with internal curing of High-Strength Concrete using pre-soaked lightweight aggregate (LWA). The effect of internal curing depends directly on the distance over which the internal curing water can travel. The effectiveness of internal curing is a function of the ratio between the water penetration depth and the paste-lightweight aggregate proximity, which is related to the spacing between the aggregates. Estimates of these parameters were developed in this study, based on a combination of modeling and experimental work. The results indicate that water can penetrate from the LWA into the surrounding matrix to a distance of up to several millimeters during the first seven days of hydration. The water penetration was sensitive to the pore structure of the aggregate, ranging from about 1 to 6 mm, and it was reduced in systems having lower w/b ratio and silica fume by almost a factor of 2.

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.

An experimental investigation of HPSCC, is outlined. Optimizations were performed on a laboratory scale according to an ideal grading of the particles in the fresh concrete for SCC, with high strength, high durability in marine environment or with fire spoiling safety. SCC was introduced in the full-scale production of beams and piles. The results showed high slump flow and robustness that allowed for a reasonable variation of the water-cement ratio, w/c, keeping the fresh concrete properties within the limits of the full-scale production even at elevated temperature. Creep, shrinkage, salt frost scaling and sulphate resistance did not differ much from the corresponding properties of vibrated concrete, NC. Internal frost resistance was improved for SCC compared with NC but the chloride migration was larger in SCC with limestone powder than in NC. Spoiling of the concrete during fire, especially in low-w/c concrete, was avoided by use of polypropylene fibers.

Investigation into the mechanism of shrinkage is essential for the engineering purposes to mitigate cracking of concrete structures. Because comprehensive analysis of unit processes of self deformations is inevitable for systematic understanding of interaction between various self deformations, such as drying shrinkage, autogenous shrinkage and thermal deformation etc.,or their influences on stress generation and crack extension or crack propagation. In this paper one example of this trial is presented based on analysis of relationship between weight loss and drying shrinkage measured by ASTM method. By this analysis, it is shown that unit processes involved in these physical phenomena are not so simple as can be stated by Fick's diffusion theory. Depending upon duration of underwater curing or degree of hydration, unit processes are drastically varied suggesting that variation in microstructure gives rise to quite different situations for desiccation and deformation. Based on these models, relation and interaction between drying shrinkage and autogenous shrinkage is discussed.