International Concrete Abstracts Portal

Curing of concrete is impaired by exposure to drying at early ages. Removing water from the surface layers restricts the binder reactions and pore structure development. High porosity in the surface region will allow the ingress of deleterious agents, which can lead to durability problems. Present work reports results obtained by hydrating a flyash blended cement under drying conditions. Comparisons are made with similar results from a portland cement. Small samples of OPC/pfa (70/30) paste with a water binder ratio of 0.59, initially cured under saturated conditions for 7 days, were exposed at 20 C in a CO2-free environment, to various preselected relative humidities. After 28 and 91 days, the extent of reaction and the porosities of the samples were measured by thermogravimetry and methanol adsorption, respectively. Results show the extent of hydration falls when changing from saturated to 70 percent relative humidity (rh) conditions; below this rh, it is virtually constant. From the shape of the TGA curve, it seems that there is little change in the nature of the gel phase. The pozzolanic reaction appeared to cease below 80 percent rh. Total porosity only fell very slightly with increasing relative humidity even after 91 days exposure. Under drying conditions (70 percent rh) the large-diameter porosity was three times greater than large-diameter porosity obtained under saturated conditions. From these tests it is clear that to promote reaction and to effect a decrease in the volume of large pores, the relative humidity must be greater than 95 percent, at least during early-age curing.

Presents results regarding the effects of various temperature and humidity environments on the compressive strength of concretes containing blast furnace slag cement (BFSC) and ordinary portland cement (OPC). Three types of blended cements containing 4.5, 35, and 68 percent slag weight replacements of portland cement were used. The specimens were stored in locations with controlled environments, such as 35 C (95 percent relative humidity), standard ambient temperature 23 C (lime water, sealed with polypropylene bag, 100 percent relative humidity fog room, and 50 percent relative humidity drying room), and 10 C. Test results indicate that the temperature effect on the initial rate of strength development of BFSC concrete is more sensitive than that of OPC concrete; high temperature accelerates the strength gain and low temperature suppresses the initial strength increase of BFSC concrete. Higher ultimate strength was achieved for the 4.5 and 35 percent BFSC well-cured concretes as compared to OPC concrete. However, the inadequate supply of reactive materials resulted in lower compressive strength for the 68 percent BFSC concrete. Under dry conditions, concrete with high slag content stopped its strength development as excess loss of moisture hindered the hydration process of cement. Strength degradation was also found for high slag content BFSC concrete.

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.