International Concrete Abstracts Portal

Early-age cracking can occur in cementitious materials when volumetric changes caused by temperature or moisture fluctuations are prevented by the surrounding structure. This paper describes a preliminary study in which early-age damage development was monitored in restrained cementitious mortar specimens using acoustic emission. A steel testing frame was used to provide passive restraint to uni-axial specimens. Free shrinkage characteristics were measured using geometrically similar specimens. Acoustic sensors were mounted on the surface of the mortar and acoustic activity was recorded continuously. The experiment revealed that the acoustic activity in the free and restrained specimens was initially similar, however the restrained specimens generated an increase in acoustic activity at later ages. This presumably occurs as a result of the increase in the residual stress to strength ratio. The age of visible cracking was observed to correlate well with a discrete, sudden increase in acoustic activity. Acoustic energy was used to indicate the change in the properties of the acoustic events as the concrete grows closer to the age of cracking. The location of damage was determined in the specimes using a linear approach that corresponded well with visual observation.

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.

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.

Restrained tests are used to evaluate the risk of early age cracking and the craking sensitivity of concrete mixtures. One test that has become common in recent years is the active uniaxial restrained test in which the length change due to shrinkage is recovered by applying external load to maintain the concrete sample at constant length. The length change is measured by linear variable differential transformer (LVDT), which is used as the control signal in this test. In such tests, the dog-bone geometry is used to grip the ends. To ensure a fully restrained test, the LVDT response to the loads and to shrinkage specimen interaction should not interfere with the measurement of deformation, and this depends on the instrumentation and how the LVDT is attached to the concrete specimen. Some experiments in the literature have the LVDT attached to the steel grips, a practice vulnerable to possible error due to the interaction between the grip and the concrete. This study considered two methods of attaching the LVDT. First, the LVDT is attached to the steel grips, second, the LVDT is attached to the concrete within the zone of reduced cross-section. The results indicate that attaching the LVDT to the grips results in errant measurement of the shrinkage stress, creep, and elastic strains due to the grip-specimen interaction. The consequences will be false interpretation of fully restrained shrinkage and creep characteristics because the grip-specimen interaction leads to a partially restrained test. The study suggests mounting the LVDT to the concrete sample away from the grips to achieve a fully restrained test. Results for two concrete mixtures with w/c ratio of 0.51 and 0.56 are discussed for both methods of attaching LVDTs.

Concrete is the most widely used construction material in the world with over 8 billioin tons of it being produced yearly. Much of this concrete is steel reinforced since the concrete/steel composite has improved ductility over concrete alone, and the concrete provides a protective environment for the steel. However, reinforced concrete must be used in severe corrosive environments such as found in marine and deicing salt applications. The ingress of chloride leads to corrosion of the steel resulting in early repairs of the structure. The subsequent costs are over $50 billion/year in the United States, and represent a major drain on infrastructure resources throughout the world. In this paper the use of improved concrete designs to control corrosion of steel in concrete are addressed. These designs incorporate the use of low permeability concrete, corrosion inhibiting admixtures, reduced shrinkage and increased toughness with fiber reinforcement. It is demonstrated that this holistic approach to the concrete design provides a lower life-cycle cost.