International Concrete Abstracts Portal

Ultrasonic wave reflection (UWR) has been used to monitor hydration and strength development of concrete. UWR measures the changes in reflected ultrasonic waves at the interface between a buffer material and hydrating cement paste. To monitor the subtle changes during early hydration it is necessary to use a buffer with low acoustic impedance, close to that of cement paste. In this research, UWR measurements on hydrating Type I portland cement are performed using a high impact polystyrene (HIPS) buffer. Both S-waves and P-waves are analyzed simultaneously to develop and extend the use of UWR to monitor early stiffening of cement paste. The penetration resistance test (ASTM C 403) and temperature rise of cement paste are used to correlate stiffening characteristics. The UWR responses show good correlation with results from temperature rise and penetration resistance. The onset of stiffening is the same for penetration resistance and both P- and S-wave UWR, and nearly the same for temperature rise. It is found that the HIPS buffer can provide sensitive measurement on the early age stiffening of cement paste.

Drying shrinkage cracking can occur in concrete due to volumetric changes caused by temperature and moisture gradients. The purpose of this study is to acquire fundamental data on the relationship between internal relative humidity and drying shrinkage cracking. The detection of drying shrinkage cracking was performed using the Acoustic Emission method. It was found that the AE measurement technique was successful at detecting drying shrinkage cracking.

A simple but challenging experiment was carried out to measure concrete temperature, air content, unit weight, slump, setting (penetration resistance), heat release, maturity, and compression strength over a 28-day period beginning with discharge from the chute of a concrete truck. It was thus demonstrated that concrete’s transition from liquid to solid is represented continuously by maturity and by heat release, but it is more commonly recorded in terms of three phases in concrete development: slump loss, setting, and strength gain. The paper describes how these phases overlap each other and are related to concrete temperature, heat release, and maturity.

The evolution of early age mechanical properties and volume change of cement paste is performed through Finite Element analysis on a 3D computer-generated cement paste. The time evolution of the hydrating microstructure is generated by µic(mike), a vectorial hydration model which takes into account the Particle Size Distribution (PSD) of anhydrous cement particles, the w/c ratio, the filler content and different hydration kinetics mechanisms such as nucleation, growth and diffusion. The microstructure geometry is then discretized into a finite element mesh. At each hydration step, the capillary depression is computed according to Laplace-Kelvin equation and applied on the pore space generated by the hydration model. Then, the autogenous shrinkage corresponds to the overall load-free deformation of the computational volume. Two constitutive models are used. The first one is a purely elastic model where macroscopic stress depends on the total porosity only. The second one is a poroelastic model which takes into account the fluid-solid interaction and the de-saturation effect. In parallel to the modeling work, a systematic experimental study has been performed on series of white cement pastes prepared different finenesses and various water-cement ratios. Many characterization techniques were used in the experimental study: chemical shrinkage, evolution of relative humidity, mercury intrusion porosimetry (MIP), x-ray diffraction (XRD), linear and volumetric autogenous shrinkage and ultrasonic wave propagation measurements. The numerical results are compared with experiment data and it is shown that the poroelastic model provides the best agreement to the experimental results. The remaining gap between the modeling and the experiment is discussed and future developments are outlined.

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.