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SP-259CD The American Concrete Institute (ACI) Committee 231, Properties of Concrete at Early Ages, has sponsored a full-day technical session on the Transition from Fluid to Solid: Re-examining the Behavior of Concrete at Early Ages at the ACI Spring Convention, San Antonio, Texas, March 15-19, 2009. This special publication contains the twelve papers presented at this session. The subject matter of these papers includes: (1) the development of concrete properties and microstructure at early ages, (2) test methods for assessing early-age volume change and cracking potential, (3) construction operations timing, (4) computer simulations of early-age behavior, and (5) mechanisms that end the concrete dormant period.

Flowable fill is a self-compacted, cementitious material used primarily as a backfill in lieu of compacted fill. It generally consists of sand, Portland cement, fly ash/slag and water. Flowable fill, does not settle, does not require vibration or other means of compaction, can be excavated, is fast to place, and safer than other forms of fill. Because of its versatile applications, the construction industry utilize flowable fill as a means of reducing cost and completion time for their projects. Among its many uses, flowable fill mixtures used for pavement base design for placement under flexible pavements received reviews due to its curing or setting time. Review of literature shows that flowable fill is highly considered and used by numerous state department of transportation (DOTs). In that study, state DOTs that were surveyed discussed problems of getting flowable fill to set and harden within a reasonable amount of time. Hardening time is the approximate period of time required for flowable fill to go from plastic state to a hardened state with sufficient strength to support the weight of a person. This paper presents the results of a laboratory study which evaluate the effects of accelerating admixtures on setting time and long term strength of flowable fill. Samples were cast in Limerock Bearing Ratio (LBR) cylinders and rectangular wooden molds. Samples were categorized as undrained and drained. Undrained samples contained plastic sheets in their interior and the drained samples contained geofabric or filter fabric in its interior. The results show flowable fill containing accelerating admixture hardened and set at somewhat earlier time than control mixtures containing no accelerating admixture. Thus, accelerating admixtures help reduce minimally both the setting and harden times in flowable fill. The findings from this study, show promising sign for field application. Such information, although small, can be of beneficial usage for engineers deciding on whether to add accelerator to a flowable fill mixtures for reducing the setting and hardening time.

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.