Microgravity Investigation of Cement Solidification

Concrete is the most widely used man-made material in the world, second only to water. The large-scale production of cements contributes to approximately 5% anthropogenic CO2 emission. Microgravity research can lead to more durable and hence more cost-effective material

Reliability-based analysis of early-age cracking in concrete

With recent concern regarding the environmental impact of the construction and urban development, there has been an increased emphasis on understanding how the concrete industry can become more sustainable. Sustainability relates to the application of energy efficient materials with low impact on environment and ensured durability. By improving the long-term durability of concrete elements, the life of the infrastructure can be extended, saving resources and environment. One problem that leads to premature deterioration in concrete structures is the development of cracks. As a result, there is an interest in developing procedures to produce crack free concrete elements. This research describes how experiments and computer simulations can be used to relate fundamental material properties and variability to the cracking performance of cement and concrete materials. When thermal, hygral or chemical volume changes in concrete are restrained, residual stresses arise. If the residual stresses exceed the tensile strength of concrete, cracking occurs. Previous research has focused on the development of test methods and computer models to predict cracking in concrete. While these models are a great step forward, they are generally deterministic and do not consider inherent variability in material properties, construction processes, and environmental conditions. As a result, these models do not accurately capture the true risk of cracking in concrete elements. In this research, Monte Carlo method and Load and Resistance Factor Design (LRFD) approach have been applied to incorporate different sources of variability in investigating the probability of cracking in restrained concrete members. Simulations are performed to determine the extent of free shrinkage reduction that is required to minimize the probability of cracking to an acceptable level. An approach is presented that allows engineers to select and incorporate the probability of cracking during the material design process. With this information, concrete can be designed using new materials, like shrinkage reducing admixtures (SRA) or by internal curing using for example lightweight aggregates (LWA), to meet the specified shrinkage performance

Relating Material Properties to Exposure Conditions for Predicting Service Life in Concrete Bridge Decks in Indiana

Bridges in the US are deteriorating at an alarming rate. It has been estimated that transportation agencies across the US invest more than 5 billion dollars on concrete bridge repair and renovation annually. To meet the needs of transportation industry, high performance concrete (HPC) has been developed for the construction of bridges. However, to date, the link between material properties and field performance is not completely established. Goodspeed et al. [1996] defined the performance of concrete using four material parameters that describe durability and four material parameters that describe mechanical properties. It should be noted however that material properties alone cannot entirely define field performance. Rather some consideration is needed to quantify the conditions to which the concrete will be exposed. The exposure conditions vary based on the geographical location. This work relates material properties with the exposure conditions typical of those in the state of Indiana to estimate the performance of concrete bridge decks. The exposure conditions in the state of Indiana have been assessed. Specifically, temperature, rainfall, wetting events, freeze thaw cycles, and relative humidity have been classified. To assess the variation in these parameters across the state, contour maps were developed using information from cities in the state of Indiana as well as cities in surrounding states. The eight parameters suggested by Goodspeed et al. [1996] have been reviewed. Three key distresses behavior (chloride ingress, freezing and thawing, and shrinkage cracking) have been investigated in depth. Relationships have been developed to relate measured material properties (from the results of AASHTO/ASTM tests) with the predicted performance of the concrete structure under different exposure condition. First, a model is presented that relates the results of Rapid Chloride Permeability Test (RCPT) with the anticipated service life of bridge deck against corrosion due to chloride ingress. Second, a model is presented that relates results of sorptivity, porosity, and critical saturation with the anticipated service life of concrete exposed to freezing and thawing. Third, a model is presented that relates the shrinkage of concrete with the potential for premature cracking. The results of each of the models have been presented for conditions that are typical of the state of Indiana

Experimental Investigation of Cement Hydration in Gravity-Free Environment

In this work, cement hydration in terrestrial and microgravity environment was compared. This was for the first time, when the International Space Station was utilized to fully investigate the complex process of cement solidification. Microstructural development of hydrating cement occurs in stages during the hydration reaction and hardening process, which results in elaborate combinations of amorphous and crystalline phases. The morphology, volume fraction, and distribution of these phases ultimately determine the hardened cements material properties. Minimizing gravity-driven phenomena, such as thermosolutal convective flow and sedimentation ensures crystal growth strictly by diffusion and a microstructure forms differently from that observed in typical laboratory conditions on Earth. A test matrix was developed that includes samples with various w/c ratio and various compositions; incorporating alite, pure water and cement system, cement with chemical admixtures, as well as commercial products. This paper reports on main changes initially observed in the microstructural development of hydrated pure compound of tricalcium silicate (C3S)