Field engineers have observed that Jointed Plain Concrete Pavements (JPCP) exhibit irregular joint cracking patterns. Upon inspection, it was seen that most of the joints remain uncracked at early ages, while many of those joints that do crack experience excessively large crack widths. This results in a quicker localized deterioration of these joints, nd ultimately a shorter life span of the highway. This phenomenon, as well as many of the early-age mechanical properties of concrete, was investigated in this study. This paper describes the study of the early-age JPCP joint cracking. On-site highways. The pavements were monitored for slab temperature profile histories, ambient temperature histories, transverse joint crack developemnt and overall behaviors. The concrete temperature histories were obtained at selected locations for each investigation using embedded thermocouples and a contact infrared thermometer. Crack growth histories were obtained for each site by measuring the crack widths at each joint. A time and location dependent analysis was developed which gives an acceptable representation of the observed cracking pattern. The analysis is based on such factors as frictional forces between the JPCP and its underlying layers. The results of this study can be used to help control the locations of joint cracking and crack widths of early-age concrete pavements.

Results from flexural fatigue tests on plain concrete and two fiber reinforced concrete (FRC) mixes (hooked-end steel fiber and polyolefin fiber) are presented and discussed. The specimens were made using the same concrete materials used for the MoDOT field test program. MoDOT's RDT Division was responsible for field implementation, which included design and construction of unbonded fiber-concrete overlays in the southbound lanes of Interstate 29 in Atchison County, Missouri, beween Route A and US 136. The fatigue performance of both the FRC mixes investigated in this study were superior to that5 of the Control mix. Crack widths in the Steel Fiber Reinforced Concrete (SFRC) specimens were typically smaller than those in the Polyolefin Fiber Reinforced Concrete (PFRC) specimens under comparable levels of fatigue loading (stress level as well as number of fatigue cycles). This property influences the long-term durability of the material for pavement applications. The difference between the FRC mixes and the Control mix becomes readily apparent at the higher levels of upper limit of fatigue stress. Fatigue failure in FRC can be characterized by a three-stage process. Iin the fist stage fatigue damage is accumulated in the concrete matrix. Rapid growth in net-deflection occurs with increasing fatigue cycles. The second stage is characterized gy little or no growth in net-deflections, attributable to stable and steady growth of damage along fiber-matrix interfaces. Only when this damage reaches a threshold level does the third stage begin. The third stage is characterized by a rapid growth in net-deflections resulting in fiber-pull out and /or fractures at the critical cross-section and associated catastrophic growth of the main matrix crack.

An experimental study was conducted to evaluate the restrained shrinkage cracking in plain and fiber reinforced concrete. The experiment utilizes a constant humidity chamber holding the restrained shrinkage specimens. The chamber is subjected to constant flow of air around the specimens. The strain in the restraining steel and the crack width in the concrete samples were monitored continuously. The experimentally obtrained results are affected by geometry of the specimen, the humididty and shrinkage conditions, and the restraing offered by stiffness of the steel ring. In addition, concrete properties such as the stiffness, shrinkage and creep affect the response. In order to better understand the restrained shrinkage of concrete under the proposed test method and eliminate the influence of test conditions, an analytical approach was developed. The model incorporates key influential parameters of shrinkage, creep, aging, and microcracking, in the stress analysis of a restrained concrete section. The theoretical model was used to calibrate and interpret the experimental test results.

A current research project on hydration kinetics, mechanical properties and early age stress behavior of blended cement conducted at the University of Michigan is reviewd in this paper. A number of experiments including calorimetry and differential thermal analysis were performed to investigate hydration kinetics. The mechanical properties investigated included the compressive strength, splitting tensile strength, Young's modulus, creep compliance, relaxation modulus, and coefficient of thermal dilation. The early age stress behavior was studied by measuring the stress developed in a uniaxially restrained concrete member. In addition, the deformation due to autogeneous shrinkage was also measured experimentally. The experimental data could be used to quantify degree of hydration,, and temperature effects on hydration, and could be used as imputs for predicting the early age stress development in concrete.

The influence of superplasticizer on the chemical (total) and autogeneous (external) shrinkage of hydrating cement pastes was investigated. Three different commercial CSA Type 10 cemeents were tested. Test variables also included type of superplasticizer (melamine-based and naphtalene-based) and dosage in admixture (three different dosages). All neat paste mixtures were prepared at a water/cement ratio of 0.35. Chemical shrinkage measurements were carried out using the classical dilatometric method initially developed by Le Chatelier. Autogeneous shrinkage measurements were performed according to the immersion method. All tests were performed in a temperature-controlled bath kept at 20 degrees C. Test results indicate that the dosage in admixture influences the kinetics and magnitude of both chemical shrinkage and autogeneous shrinkage, especially during the first 24 hours. Beyond that period, the overall effects of dosage were observed to be less pronounced. Data also emphasize the potentioal importance of the type of superplasticizer upon early volume changes. Though the investigated cements are known to sometimes exhibit quite different early-age behaviors in the field, no significant differences were observed as far as chemical shrinkage and autogeneous shrinkage are concerned.