International Concrete Abstracts Portal

The fresh state of concrete is becoming increasingly important in furthering the types of applications in today’s construction world. Processing techniques that have resulted in new technologies such as self-consolidating concrete depend on the microstructural changes within the cement paste during the first hours after mixing and placing. These changes to the microstructure reflect flocculation between particles in suspension. The ability to modify this behavior allows control over the balance between flowability and shape-stability of concrete. This study uses a centrifuge method to determine the relationship between local volume fraction (volume fraction of the sediment region) and compressive yield stress within cement pastes. Based on this relationship, the effectiveness that different admixtures such as clays and fly ash have on the balance between flowability and shape-stability can be measured. Results are consistent with green strength tests performed on example concrete mixes derived from the cement paste mixes.

The objective of this study was to evaluate a quick heat generation test to flag changes in cementitious materials in the field. The effects of initial water temperature and initial cement temperature on the quick heat generation curve were evaluated. The effects of different cement chemistries were also studied. Parameters measured include maximum paste temperature at 15 minutes, cement fineness, and cement chemistry. A relationship exists between the both the initial water temperature and the temperature of the paste at 15 minutes and the initial cement temperature and the temperature of the paste at 15 minutes. A linear relationship also exists between the initial paste temperature and the final paste temperature for a single cement source. Laboratory results showed that the quick heat generation test is capable of identifying changes in cement chemistry between different cement sources and the results are reproducible.

Structural build-up that occurs during the induction period is of particular interest to users of self-consolidating concrete (SCC) since it can affect the workability of concrete. A novel experimental device based on scanning laser microscopy was used to directly monitor particle flocculation in SCC cement pastes. This is one of the few studies in which this experimental method has been used to study flocculation in concentrated suspensions. This paper discusses the results from a study that was carried out to investigate the flocculation and floc properties in SCC cement pastes. Results show that the floc network is immediately broken down by superplasticizers and that the rate of reflocculation decreases when the water-to-cement (w/c) ratio is decreased. An increase in w/c ratio resulted in a reduction in floc strength. Results show that viscosity modifying agents can induce flocculation due to different flocculation mechanisms.

Assessing the fl uid-to-solid transition in cementitious systems at early-ages is crucial for scheduling construction operations, for determining when laboratory testing can begin, and for assessing when computer simulations of restrained stress development should be initiated. This transition has been traditionally assessed using mechanical penetration techniques (e.g., Vicattest), which, though easy to perform, do not directly relate to the evolution of fundamental material properties or the microstructure. This paper assesses the fl uid-to-solid transition of a cementitious material at early ages using measures that relate to the formation of a solid-skeleton in the material. The increase in the ultrasonic wave velocity is correlated to the percolation of a solid structure that occurs during the fl uid-to-solid transition. Results of computer modeling (using CEMHYD3D) indicate that solidifi cation as determined from the percolation of the solids is similar to experimental observations (Vicat test). It is noted that the rate of change in the pulse velocity is not a rigorousmethod for assessment of the time of solidifi cation, especially in systems containing air. Rather, an increase in the pulse velocity beyond a threshold value appears to be a more appropriate method to assess structure formation. Further, the isothermal calorimetry (heat release) response is observed to not correspond to a fundamental aspect related to solid percolation or structure formation in the material.

The propensity for early-age shrinkage cracking in low w/c concretes has spawned the development of new technologies that can reduce the risk of cracking. One such technology is internal curing. Internal curing uses saturated lightweight aggregate to supply ‘curing water’ to low w/c paste as it hydrates. Significant research has been performed to determine the effects of internal curing on shrinkage and stress development in sealed samples. However, relatively little detailed information exist about how water is released from the lightweight aggregate to the surrounding cement paste. This study examines the timing of moisture release from saturated lightweight aggregate (LWA). Specifically this paper focuses on fluid transport around the time of set. X-ray absorption is used to trace the time at which water moves from the lightweight aggregate to the paste. X-ray observations are compared with results from the Vicat needle, autogenous shrinkage, and acoustic emission tests. These results are contextualized in terms of structure formation and vapor space cavitation in the cement paste.