International Concrete Abstracts Portal

Temperature effects are the predominant cause for volume change in concrete pavements. This paper describes an experimental investigation of thermal volume change conducted to improve the understanding of joint movement in concrete pavement. Four slab strips containing embedded strain gauges and thermocouples were monitored in a controlled environment under four heating rates. Each strip was monitored for translation, rotation, and warping height. Key findings of the experiment include the internal strain distribution and non-linear thermal gradients produced by asymmetrical heating. The laboratory data are compared with long-term data from an instrumented parking lot pavement. Analysis of the data provides insight into the prediction of thermal movements and determination of thermal stress development in pavements.

Creep and shrinkage data for two high strength lightweight aggregate concretes were collected over a two-year period. The concretes, with unit weight of 1922 kg/m3 (120 pcf), were developed using expanded slate as coarse aggregate. Strengths of 55.2 MPa (8,000-psi) and 69.0 MPa (10,000-psi) were obtained at 56 days. Creep specimens were loaded to 40 or 60 percent of the initial compressive strength at 16 or 24 hours after casting. Based on this preliminary study, AASHTO-LRFD creep estimates of high strength, lightweight aggregate concrete were within 20% accuracy for ages later than one month. ACI-209 estimated creep of the 55.2 MPa lightweight concrete and shrinkage of the 69.0 MPa concrete within 20% accuracy, but greatly underestimated shrinkage of the 55.2 MPa mix. When compared with normal weight, high strength concrete of similar strength and similar cement paste content from previous research, the 69.0 MPa lightweight mix experienced lower total strain after two years.

This paper illustrates how stress relaxation can be used to obtain valuable information regarding the behavior of concrete at early ages. Five concrete mixtures were investigated using a so-called discretized restrained shrinkage (DRS) testing device, allowing the determination (from the time of casting) of the increase in load induced by autogenous shrinkage and the evaluation of the different strain components (free shrinkage, elastic strain, creep). Test results indicate that the stress due to early-age restrained autogenous shrinkage is quite variable, in good part due to the variation in the relaxation capacity of the mixtures. Both the relaxation ratio, defined as the stress generated divided by the theoretical stress, and the relative relaxation, defined as the absolute value of stress relaxation divided by the average applied stress, can be used to illustrate and analyze the variation of the relaxation phenomena as a function of the type of mixture tested.

Creep and shrinkage of concrete were studied under constant load and restrained conditions during the first week after casting. Concrete behavior was characterized by a uniaxial test that measures shrinkage deformation and restrained shrinkage stress. The extent of stress relaxation by tensile creep was determined using superposition analysis. The experimental measurements were compared with current creep and shrinkage models to assess their validity for early age prediction. The ACI 209 equation for creep is currently not applicable to early age, but modifications are proposed that fit a database of early age behavior. The B3 model has been previously modified to accommodate early age creep, and this modification was employed in the current study. Test results for normal concrete with different w/c ratios are discussed.

Concrete shrinks as a result of drying, self-desiccation, chemical reaction, or temperature reduction. If this shrinkage is prevented by restraint, tensile stresses develop which may result in cracking. Various alternatives have been proposed to reduce the cracking propensity of a mixture including the use of commercial chemical admixtures called shrinkage reducing admixtures (SRA). To date however, little information has been presented to describe how the performance of mixtures containing SRA’s could be predicted. In addition, little guidance exists to determine the dosage rate of SRA that should be used to achieve a specific level of performance. This paper describes initial research results from a study that has been aimed at quantifying the role of SRA at early-ages. The surface tension of various solutions of water and SRA was measured. In addition, free shrinkage measurements were conducted for specimens exposed to sealed and drying conditions from the time of set. Results indicate that shrinkage can be directly related to the concentration of SRA. Restrained ring experiments were performed to investigate the influence of a SRA on residual stress development and cracking in mortar. It has been observed that the residual stress reduction is not directly proportional to the reduction in free shrinkage since the specimens with SRA demonstrate less stress relaxation.