International Concrete Abstracts Portal

Studies on the carbonation of concrete with fly ash and corrosion of reinforcements have been going on since 1962 at 16 research organizations in Japan. As already reported for intermediate ages, it has been shown that the depth of carbonation can be evaluated by compressive strength at the age of 28 days, which is very common value for specifying the concrete quality regardless of whether or not fly ash has been added. A number of findings have been made concerning the influence of exposure conditions on the depth of carbonation. This paper compiles the final test results at 20-year age, to augment the results previously reported for the intermediate ages. These studies also provide much verification data with respect to the durability of fly ash concrete. These test results are reflected on the Japanese standard specification for concrete cover.

Penetration of chloride ions from the environment into reinforced concrete is important in relation to corrosion behavior of embedded steel. Blended cements containing ground granulated blast furnace slag (GGBFS) are expected to offer a greater degree of protection compared to that of OPC. The kinetics of chloride ion diffusion through hardened cement pastes made from SRPC, OPC, and OPC/GGBFS blends have been determined by a steady-state (thin disc) method. Concrete slabs containing similar cements and quartzite aggregate have been made and ponded regularly with 5 percent NaCl solution. Material taken from various depths within these slabs has been subjected to pore solution expression and analysis, and the concentration profiles of free and total chloride have been determined. Values of chloride diffusivity obtained by the steady-state method have been used to calculate the chloride concentration profiles expected when penetration is into a semi-infinite medium. Comparison between the two techniques shows the same general trends in relative performance of the various cements, but actual chloride concentrations, at a given depth, are greater in the concrete slabs. Results from total and free chloride measurements indicate that the chloride-binding capacity of slag cements exceeds that of OPC and SRPC.

During hydration of portland cement clinker and granulated slag in portland blast furnace slag cement, finely dispersed calcium silicate hydrates are formed as the major constituent of hydrated cement paste. With increasing slag content of cement, more C-S-H phases are formed, contributing to the well-known dense pore structure of pastes made of PBFS cements. Upon carbonation of the hydrated cement paste, all alkaline compounds are decomposed to form carbonates. Furthermore, the decomposition of CSH results in the formation of a porous silica gel. In an experimental investigation, different types of hydrated cement paste, mortars, and concretes manufactured with portland cement and portland blast furnace slag cements with different slag contents were subjected to carbonation and the resulting changes in the pore structure monitored. These tests demonstrated that the silica gel formed during carbonation shows pores in the range of approximately 300 nm pore radius. Where large quantities of silica gel are formed, carbonation leads to a coarser pore structure compared to the original structure. Permeability of these systems then increases significantly. The porous silica gel, however, proved to be reactive. Upon access of alkalies, new C-S-H phases may be rebuilt with a very fine port size distribution with pore radii ó 10 nm.

The use of silica fume (microsilica) to improve the compressive strength at a given cement level or as a cement replacement is on the rise. Additional benefits of adding silica fume to improve the corrosion resistance of embedded steel and improve concrete durability in erosive or severe chemical exposure were investigated. Concretes with embedded steel were produced with silica fume levels varying from 0 to 15 percent by mass of cement. Additional variables were water-cement ratio and calcium nitrite content. All concretes were air-entrained and had high-range water-reducers. Plastic properties of the concretes are reported as well as compressive strength, freeze-thaw, and resistivity and rapid chloride data. Corrosion rates and chloride contents are reported and show substantial improvements with silica fume and/or calcium nitrite. An accelerated hydraulic erosion test was conducted, in which ball bearings impact the concrete surface, simulating abrasive action of waterborn particles. Mass loss was measured for concretes with 0 to 15 percent silica fume by mass of cement. Silica fume significantly improved erosion resistance. Chemical testing was performed in 5 percent acetic acid, 1 percent sulfuric acid, 5 percent formic acid, and mixed sulfates. A cyclic method involving drying, weighing, and wire brushing was used. Results show that silica fume concretes had superior chemical resistance that improved as silica fume levels increase.

Purpose was to examine, under uniaxial compression, the monotonic and hysteretic stress-strain properties of normal and lightweight concrete with the addition of silica fume, and to propose a mathematical model of stress-strain curves based on experimental data. The mathematical model has been developed to predict the complete stress-strain curve of concrete in compression, in terms of the addition of silica fume to cement. The parameters of the model have been adjusted using system identification techniques to obtain the best match possible between the experimental data and the model predictions. The experimental data were obtained by testing normal and lightweight concrete cylinders; the amount of the addition of silica fume was varied, respectively, from 0 to 28 and from 0 to 24 percent by weight of cement.