International Concrete Abstracts Portal

Roller-compacted concrete (RCC) pavements are now an economical alternative to those constructed from asphalt and conventionally placed portland cement concrete, particularly for those pavements experiencing heavy-duty, low-speed traffic. However, a major concern related to the use of RCC pavement is its frost resistance. RCC pavements can be constructed with aggregate that are not susceptible to frost, and can be cured to an appropriate degree of maturity so as to reduce the fractional volume of freezable water on saturation to limits that can be accommodated by elastic volume change and by the air-void system. However, the ability to effectively entrain proper air-void systems in RCC pavements has remained a question due to the low water contents required to place the mixtures. An investigation was conducted by the U.S. Army Engineer Waterways Experiment Station Structures Laboratory to determine if proper air-void systems can be entrained in RCC pavement mixtures proportioned with several types and dosage rates of air-entraining admixtures, and with various aggregate types and gradings. Results of the investigation indicated that air-void systems sufficient to protect critically saturated RCC pavement mixtures from deterioration due to cycles of freezing and thawing could be created in a wide range of the mixtures produced without following special mixing procedures.

Discusses the utilization of silica fume concrete admixture to prevent reinforcing steel corrosion. The mechanism of steel corrosion in salt-impregnated concrete is described, along with laboratory test date showing how ordinary concrete's corrosion-prone characteristics are altered by the use of silica fume. The mineral admixture significantly lowers the concrete permeability to prevent chloride ingress to the reinforcing steel level, while simultaneously increasing the concrete's electrical resistance to corrosion currents. Test data from the FHWA 90 day Chloride Ponding Test indicates a 98 percent reduction in chloride penetration. The AASHTO T277 rapid chloride permeability test shows a 10-times impermeability and 25-times resistivity improvement with the use of 12 percent silica fume. T he Time-to-Corrosion FHWA/NCHRP 244 slab test is scaled-down steel-reinforced deck, from which macrocell corrosion current, AC resistance, half-cell potential, and chloride absorption are measured. Zero corrosion current was measured after NaCl was ponded in alternate soak/dry cycles for 48 weeks. The second phase test program evaluated the corrosion performance of full-sized concrete bridge sections, including beams, columns, piles, and bridge deck panels. The test members were subjected to environments simulating salt water and deicing agents for 370 days. Test results show that silica fume admixture prevents salt-induced corrosion of steel a reinforcing bar and tensioning strands.

The durability in seawater of high-strength concretes produced with the addition of silica fume replacing a part of the cement was investigated. The influence of the wet-curing time on the behavior in seawater of high-strength mortars (strength in excess of 60 MPa) in which a part of the cement was replaced by densified silica fume, was determined. The various curing times applied to the specimens, after mold removal, were 48 hr, 7 days, and 28 days at 100 percent relative humidity, followed by storage for 28 days at 20 C and 50 percent relative humidity before the start of tests for resistance to seawater for 1 year. Investigation of the porosity of these mortars shows that, just after curing, the silica fume, as expected, reduces the total porosity of the reference mortar (25 to 45 percent) and substantially alters the pore-size distribution--the shorter the curing time, the more marked this effect. However, as hydration continues at 50 percent RH, the porosity of the reference mortar decreases and the differences in total porosity with respect to the mortars containing silica fume become smaller--the longer the initial curing time and the higher the C3 A content of the cement, the greater this effect. This explains the results of resistance to seawater, where it is found that silica fume contents of less than 10 percent do not lead to any significant improvement in behavior in seawater. This shows that the type of curing and the ambient conditions under which strength increases may limit the beneficial effects of silica fume on durability, when the addition of the silica fume is accompanied by a corresponding reduction of the cement content. It is also found that the best curing method is the specimens in fresh water for the first 7 days, while a curing time of only 48 hr is highly detrimental in terms of the subsequent behavior of the mortars in seawater.

An investigation has been carried out on the effects of cement replacement by fly ash on the carbonation rate of concrete. The research was mainly devoted to portland blast furnace slag cement because this cement has a major market share in the Netherlands. It has been concluded that in the case of portland blast furnace slag cement concrete, replacement up to 25 percent by mass results in a substantially higher carbonation rate, while for a similar portland-cement concrete, the difference between concrete with and without replacement is relatively small. This observation corresponds with the finding that the pH development of the pore water in concrete with portland blast furnace slag cement is too low to initiate a substantial fly ash dissolution. As a consequence, pozzolanic activity will be slight. However, in portland-cement concrete, pozzolanic action can develop more effectively and contribute to strength and densification of the matrix. A useful relation exists between the carbonation depth after 1 or 2 years and compressive strength after 28 days and, even better, after 7 days for each type of cement. This relation might cover all types of cements when the lime content of the binder is involved.

Ten different polymers, six latices, and four epoxy systems were added to two base mortars of water-cement ratio (w/c) = 0.55 and 0.40. The latices were PMMA, PMMA/PBA (two compositions), PVAc/PE/PVC, PVAc/Veo Va and SBR. Two of the epoxies were based on Bisphenol A resin (epoxy F1 and L), while the other two also contained a reactive thinner based on hexyleneglycol diglycidylether (epoxy F0 and V). The hardeners were both polyamine (epoxy L) and a water-soluble hardener based on polyamide. The cement paste in the base mortars was partly substituted with 5, 10, 15, and 20 volume percent polymer (10 and 20 volume percent for the epoxies). The binder volume was kept constant in all mortars, and the w/c was constant within each series. The carbonation rate of the PCCs with w/c = 0.40 is equal to or higher than the unmodified mortar. The only exceptions are the 15 and 20 percent PVAc/PE/Pvc-modified mortars, which withstand carbonation significantly better than the control. The epoxies L and V, together with the SBR PCCs, performed particularly poorly. Among the PCC with w/c = 0.55, PVAc/PE/PVC/PCCs, together with the 10 percent PMMA/PBA I PCC, performed better than the control. All the other PCCs resisted carbonation equal to or less than the control. In the latter case, however, it is difficult to state the contribution from the air content which may imply that the performance of the polymer actually is even better. The results are assessed with respect to the degree of hydration of the PCCs, their air content, the replacement of cement binder with polymer, and the neutralization effect following a possible hydrolysis of the polymer.