International Concrete Abstracts Portal

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.

Use of silica fume is an accepted technology in the Norwegian concrete environment and high-strength concrete incorporating silica fume is being used increasingly. However, in recent years, conflicting reports regarding the fire resistance of high-strength concrete have been published. This review indicates that eventual fire-safety problems seem to be linked less to the materials used than to physical properties of hardened concrete or conditions under which high strength concrete is used.

Reports performance of gravel concrete incorporating silica fume after 72 hr exposure at 150, 300, and 450 C. A total of eight concrete mixtures, each 0.09 m3 in volume, were made at water-cement ratios of 0.23, 0.35, 0.50, and 0.71. The mixtures at each w/c consisted of one control mixture and one incorporating 8 percent silica fume by weight of cement. The cylinders and prism specimens were cast for testing in compression and flexure before and after exposure to the elevated temperatures. Before exposure to the elevated temperatures, the test specimens were moist cured for 7 days and air dried for 21 days at ambient temperature and 50 percent relative humidity. After exposure, the test specimens were cooled to room temperature and tested in compression and flexure. The weight loss and pulse velocity determinations were made before and after heat exposure. The test results show that after 72 hr exposure at 150, 300, and 450 C, the performance of the control concrete in the compression testing mode is marginally superior to that of the silica fume concrete. The reverse is true when the two types of concretes are tested in the flexural mode.

Mixtures employing silica fume (SF) and superplasticizers with a worst-case combination of the available local gravel and cement sources were evaluated using 250 to 500 kg/m3 of Type I cement instead of Type III, and up to 15 percent silica fume. Many of the mixtures evaluated met or substantially exceeded the specified strength criteria at a cost below that of the concretes presently used. Hardened concrete air void parameters are also satisfactory for the SF concretes examined. A mixture using 300 kg/m3 of Type I cement with 10 percent SF comfortably met the present strength specifications at considerably reduced material cost when using accelerated curing. However, 20-hr strengths of 45 to 50 MPa and 28-day strengths over 50 MPa are possible with higher cement contents. A mixture using 450 kg/m3 of Type I cement with 10 percent SF can just meet the 20-hr strength requirement without accelerated curing, and its 28-day strength exceeds the current specification requirement by more than 50 percent. Slightly lower material costs and the saving associated with eliminating accelerated curing coupled with long-term strength in excess of 60 MPa make this concrete attractive in terms of both cost and strength if designers can fully utilize the higher strength.