International Concrete Abstracts Portal

Heat of hydration of cement and resulting thermal gradient has a great influence on the quality of concrete and concrete structures particularly in the mass concrete for dams. In roller compacted concrete (RCC) method for dam construction, surface of concrete layers are very large in comparison with thickness of the layers. Therefore the thermal condition in the center of layers is almost adiabatic in horizontal direction. t means that, the generated heat of hydration mostly flows in the vertical direction and a great proportion of heat dissipates through the upper face of the layer before the next layer is placed. Low thermal conductivity of concrete layers has a great influence on the dissipation of generated heat. Thermal gradient induced by generated and remaining heat in the layers can cause thermal cracking in RCC dams which have no post-cooling system. In this investigation a laboratory model is set up to optimize the layer thickness and required time for dissipating of generated heat and controlling thermal cracking. The laboratory model consists of a 90x90 cm cylinder filled with 3 layers of concrete with 30cm thickness each layer. In this simulation the variables were the thickness of layers and the exposure time for each before placing the subsequent layer. Temperature variations were recorded at the center of each layer and at different distances from the center of the laboratory model. From the results of this research, the thickness of the layers and their related exposure time were determined for various concrete mixtures in order to minimize the heat problem and thermal crack prevention. The proposed guide for placing concrete in RCC dams seems to be beneficial for the construction of such dams under various conditions.

A beneficial use of carbonation technology to sequester exhaust CO2 in concrete through accelerated curing was studied. The carbonation took place in a chamber under 500 kPa pressure, at ambient temperature, for two hours and with a 100% concentration CO2 to simulate the recovered CO2 from a flue gas. Both Type 10 and Type 30 Portland cements were used in concrete containing 0%, 25%, 50% and 75% of either quartz aggregates or lightweight aggregates. The performance of carbonated concrete was evaluated by strength development and mass gain. For a 15-mm thick sample, a 9-16% CO2 uptake in two hours was achieved. Analysis by X-ray diffraction indicated calcite formation. Samples collected from the surface or the core, immediately after 2-hour carbonation or 7 days later, contained consistent carbon content. The strength after 2-hours carbonation was close to that of reference samples cured 7 days in a moist environment. Carbonated concrete demonstrated a much finer and denser microstructure and a much higher resistance to atmospheric carbonation shrinkage.

The progress of macro- and micro-cell corrosion of steel bars in cracked concrete from the very beginning to early age exposure was investigated under an artificially created marine splash environment. For this, concrete prism specimens (100 × 100 × 600 mm) were made and cracked in the laboratory. The investigated parameters were crack widths (different stress levels); cement types (ordinary portland and slag cements); and surface condition of the bar (polished and cement paste coated). Electro-chemical and physical evaluations of corrosion, chloride contents in concrete, and the deposits in the cracks and the de-bonded area beside the crack were investigated. Natural seawater was sprayed over the specimens to simulate the marine splash environment. A remarkable amount of macro-cell corrosion is generated over the steel bar at the cracked region immediately after seawater spray. However, it drops gradually and no significant influence of seawater spray is found later. The magnitude of micro-cell corrosion is very low compared with the macro-cell corrosion at the early age, however,, it increased gradually. The deposits over the steel bars at the cracked region (i.e. de-bonded area) and onto the cracked planes are confirmed as CaCO3 and ettringite. Pitting is observed for wider cracks. At the wider cracks, relatively larger corrosion pits are found for slag cements. Application of a cement paste coat over the steel bars is found to be an effective way to control corrosion at the cracked region.

Gypsum is a cheap binder but its utilization remains limited due to its bad water resistance. The solutions usually proposed to improve this water resistance consist in adding water repelling agents like organo-polysiloxanes to the mixing water. This paper shows that another solution is to mix calcium sulfoaluminate clinker and gypsum. When hydrating, such mixture becomes water-resistant and gypsum is converted into hydraulic binder. Therefore, it is possible to develop low cost housing materials, mainly based on gypsum. The experimental results show that the quantity of calcium sulfoaluminate clinker necessary to stabilize gypsum is in the range of 20 to 30% of the total amount of binder (phosphogypsum + calcium sulfoaluminate cement). In such case, resistance to wetting-drying cycles is very good. Building materials like concrete blocks, renders, and screeds were developed and behaved very well under natural weathering for more than 18 months. This can be a solution to valorize large amounts of gypsum by-products and develop low cost housing materials.

In recent years, the demand for super high strength concrete has been increasing for high-rise RC building, so that large amounts of high-range water-reducing agents have been used in Japan. However, there are some problems with conventional admixtures. On one hand, the viscosity of prepared fresh concrete is very high and it is not easy to cast by pumping on site. On the other hand, early age shrinkage cracking easily occurs due to autogenous shrinkage, because high strength concrete has low water-cement ratio (w/c) and high content of cement. Autogenous shrinkage is of importance for concrete with w/c lower than 0.4. Therefore, the authors have synthesized a new hybrid admixture, not only reducing the viscosity of fresh concrete, but also having the shrinkage reducing action on hardened concrete. The new admixture is one-liquid type of admixture prepared by blending a new high-range water-reducing agent with a shrinkage reducing agent. Several high strength concretes with 50-150 MPa were produced by using the new admixture at low w/c. In this paper we present data showing that the new admixture reduced the viscosity of fresh concrete and simultaneously reduced autogenous shrinkage compared with concrete containing conventional high-range water-reducing agent.