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.

At the University of Salford, UK., a research team led by the author has developed a form of composite concrete construction in which fiber reinforced cement (FRC) units are employed as surface reinforcement. As part of an extensive program of investigation, full-sized rectangular and T-section beams, with and without FRC units as surface reinforcement, have been tested under fatigue in up to three million repetitions of loading. Companion beams having surface reinforcement have also been tested under short-term static loading. After a brief review of the concept of FRC composite concrete construction, the paper describes and details the test program. The behavior of the beams is examined regarding ultimate load, deflection, and cracking--the criteria of safety and serviceability. The performance under fatigue loading of beams with surface reinforcement is compared with that of companion beams without surface reinforcement but subjected to similar fatigue loading, and with surface reinforcement but tested under short-term static loading. It is concluded that the use of FRC as surface reinforcement is effective in controlling deflection and cracking well within the permissible limits without affecting ultimate load-carrying capacity for the beams subjected to fatigue loading.

Durability of concrete is commonly associated with the effects of aggressive environments, such as freeze-thaw cycles, or the penetration of liquids with high chloride ion concentrations into the matrix of the concrete that can degrade the physical and mechanical properties of concrete. Cyclic loading that causes a progressive disruption of the matrix structure by crack initiation and propagation allows aggressive environments to accelerate the rate of deterioration of the concrete and/or the reinforcement. The growth of cracks and their propagation during cyclic loading can be retarded or arrested by fibers incorporated in the concrete and thereby delay the formation of pathways for the penetration of aggressive environments or the formation of corrosion products that disrupt the matrix structure of the concrete. This paper presents test data on the flexural fatigue strength and toughness index of concrete reinforced with three types of metallic and one type of synthetic fiber in volume percentages ranging from 0.1 to 2.0. The beams reinforced with metallic fibers exhibited greater fatigue strength than beams reinforced with synthetic fibers. The fatigue strength increased with fiber volume percentage for each type of fiber. The fatigue strength of the beams varied with the deformed shape of the metallic fibers. The toughness index of the fiber reinforced beams was computed from the area under the static load-deflection curve. The toughness indexes for two of the three types of metallic fiber reinforced beams were greater than for the synthetic fiber reinforced beams. The toughness index increased with fiber volume percentage.

A part of the Statpipe Development Project is a landfill for two gas pipelines on the exposed western coast of Norway. The pipelines are placed inside a submerged concrete tunnel that acts as an underwater protecting bridge over the rocky sea bed. The 590 m long tunnel was cast in five separate elements produced in two dry docks. The tunnel starts at a water depth of 30 m and ends up at water level. The tunnel elements were produced and placed during the summer of 1982. The splash zone element encompassed the following characteristics; 400 kg ordinary portland cement and 32.5 kg silica fume per m3 concrete. The water-cement-sand ratio was 0.36, the slump value was approximately 200 mm, and the 28-day cube strength was approximately 78 Mpa. After 7 years in service, cores were drilled from the splash zone element. The testing of the cores included compressive strength, capillary absorption, chloride profile, thin-section analyses, x-ray diffraction, scanning electron microscopy, and element analysis. The results indicate that in such a low-porous concrete, the reaction products between seawater and cement paste will fill up the original low porosity and tighten the concrete so that the ingress of chlorides will cease. For concrete exposed to seawater, ingress of clorides and risk of reinforcing bar corrosion represents the most severe problem. The tightening effect of seawater in such a high-performance concrete seems to reduce this problem to a minimum.

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.