K.C.G. Ong

Effects of the Parent Concrete Properties and Crushing Procedure on the Properties of Coarse Recycled Concrete Aggregates

AbstractFine tuning of concrete recycling processes requires a comprehensive understanding of the overlapping interactions and effects of the various influencing parameters. The most important parameters associated with recycled concrete aggregate (RCA) production that may affect quality and yield include such properties of the parent concrete as the composition, strength and aggregate grading, type of crushers used, number of crushing stages, the size of the RCA particles, and the size reduction sequence. The effects of such parameters overlap and interact with one another and render it impossible for firm conclusions to be drawn on the effects of one parameter in isolation as evidenced by results reported in available literature. The present paper reports on the findings of an experimental study on the concomitant effects of the strength of the parent concrete, size of the natural aggregates used in the parent concrete, and the number of crushing stages on the properties of coarse recycled concrete aggr...

Acid Treatment Technique for Determining the Mortar Content of Recycled Concrete Aggregates

The presence of old cementitious mortar in recycled concrete aggregates (RCA) has been reported to be one of the main factors contributing to a lowering of the performance of concrete made with RCA relative to those made with natural aggregates (NA). This difference in performance has been attributed to the proportion of mortar present in the RCA vis-a-vis the NA used to cast the original parent concrete. Therefore, determining the mortar content might provide an effective means of assessing the overall quality of RCA. Also, as reported in the available literature, it is essential to take into account the amount of mortar present in RCA as part of the total mortar present when designing concrete mixes cast with RCA. However, an easy-to-use standard method for accurately determining the cementitious mortar content of RCA is not available at present. In this study, an acid treatment testing method is proposed to determine the mortar content of RCA. The method is able to remove most, if not all, of the cementitious mortar and takes approximately 24 h to carry out. In addition, the effects of mortar content on properties of RCA including 24-hour water absorption, bulk density, and Los Angeles abrasion resistance were investigated. The effects of the original parent concrete strength on the relationships between mortar content and RCA properties were also investigated. The study was confined to only granitic coarse aggregates.

Time Zero Value Based on S-Wave Reflection Loss Measurement for Early Age Shrinkage Monitoring of Cementitious Specimens Subjected to Sealed and Unsealed Curing Conditions

Estimating the time at which to start monitoring early age shrinkage, especially of high performance cementitious materials, can be crucial. Despite numerous studies on early age shrinkage monitoring, there is no general agreement on the time at which early age shrinkage measurements should start (i.e., the “time zero” value [TZV]). The present paper proposes the use of the stiffening time measured via S-wave reflection loss as the TZV for early age shrinkage monitoring of high performance cementitious materials subjected to both sealed and unsealed curing conditions. The results imply that in the case of autogenous or sealed specimens, similar stiffening times occur throughout the specimen depths. In the case of unsealed specimens, however, the results show that the stiffening time might vary with depth depending on the water-to-cementitious ratio used. It is postulated that water redistribution within the specimen, as well as the formation of a “skin layer,” might significantly affect the stiffening time measured in the case of unsealed specimens used for early age drying shrinkage monitoring.

Image Analysis Technique for Monitoring Early Age Shrinkage Strains with Depth from the Top Surface of Monolithic Cementitious Prism Specimens

This paper describes the use of the image analysis technique for monitoring the variation of early age shrinkage strains with respect to the depth from the top surface of ordinary Portland cement mortar and concrete prism specimens starting as early as 30 min after adding water to the mixture. The applicability of the image analysis technique was demonstrated on both sealed and unsealed cementitious prism specimens. As expected, in situations where the top trowelled surface is protected from moisture loss, the shrinkage strains are similar with depth from the top trowelled surface. On the other hand, magnitudes of shrinkage strains monitored vary with depth when the top trowelled surface is exposed to a dry environment from an early age. The results also showed that the effect of settlements on early age shrinkage strains monitored can be minimized by adopting the current image analysis testing arrangement. Furthermore, when the early age shrinkage strains monitored using the image analysis technique were benchmarked against the results obtained via laser sensors, comparable shrinkage strains values were observed on the sealed prism specimen. On the other hand, the laser sensors were found to provide an “average” value of shrinkage strains that develop across the whole cross section of the unsealed prism specimen.