International Concrete Abstracts Portal

The measurement of the electrical resistivity of concrete is a nondestructive technique that is rapidly gaining acceptance as a means to evaluate the severity of reinforcement corrosion when used in conjunction with potential mapping methods. The concrete resistivity can be determined in situ from placing four equally spaced surface electrodes in contact with the structure and passing a current between the outer electrodes. A measurement of the voltage between the inner electrodes leads to an assessment of the resistivity of the concrete. One practical difficulty in interpreting resistivity measurements is allowing for the error caused by a surface layer with a resistivity lower or higher than that of the underlying concrete. This could, for example, be due to recent wetting of the concrete or due to carbonation of the surface zone. This effect is discussed, and practical correction curves permitting a true assessment of the resistivity of the underlying concrete are given. A very dramatic error in resistivity measurement can occur when there are two surface layers with resistivities--one lower and one higher--than the underlying concrete, such as might be caused by a recent wetting of concrete already having surface carbonation. This wetting can cause a paradoxical increase in the apparent resistivity of the concrete in excess of one order of magnitude. The results of experimental and theoretical studies are presented. The reasons for this effect are discussed, and practical guidance for in situ resistivity measurement is given.

Determines the relationship between the cracking in loaded reinforced concrete and the corrosion of embedded steel. The test specimens used in this work are 3 m long beams and their constructive dispositions and loadings are in conformity with French specifications (BAEL 83). A salt fog and gas mixture with the same percentage of carbon dioxide and atmospheric air are the two aggressive environments. The authors use the single-replica technique, which enables crack openings of 0.1 micrometer to be discerned by scanning electronic microscope. The loadless initial state of concrete and the load damage state after bending can also be described. At this resolution, the reinforced concrete beams exhibit an absence of microcracks in both initial and loaded states. The study of the diffusion aggressive ions in concrete allows the microstructural state of cement paste-aggregate interfaces to be defined. The authors conclude that damage of the aureole of transition in the tensile zone of bending beams occurs. The aggressive ions quickly reach the reinforcement through the cracking, whatever their widths, and then progress along the embedded tension steel. The influence of concrete cover is clearly proved, as well as the aggressive difference between conservation environments. The authors follow the corrosion development by using steel electrode potential measurements. Previous results are corroborated; in particular, crack existence appears to be an essential parameter but not crack width. The aggressive environment is an important factor that should be taken into consideration by building regulations. Concrete thickness cover is also important, as well as its permeability, but the latter cannot be studied due to the use of only one concrete composition.

Compared artificial portland cements (CPA type) without mineral addition and blended portland cements (CPJ) containing 15 to 25 percent filler, while attempting to keep the mechanical strength class of both types constant. The properties studied were: binder hydration, strength variations, types of hydrates, porosity, etc.; effect of seasonal temperature variations; and resistance to various aggressive environmental agents (carbonation, freezing and thawing, seawater, diffusion of chlorides). For constant mechanical strength class, the durability of CPJ cements with fillers is identical to that of CPA without mineral addition.

With special attention to durability of concrete, the author reviewed the proceedings of the cement chemistry congresses as well as other symposia held during the last 50 years by ACI, ASTM, and RILEM. What is presented here is not a comprehensive progress report on the subject of concrete durability but rather a state-of-the-art report from the author's perspective. It seems that, in spite of some important discoveries valuable from the standpoint of durability enhancement, today more concrete structures seem to suffer from lack of durability than was the case 50 years ago. In order of decreasing importance, the major causes concrete deterioration today are as follows: corrosion of reinforcing steel, frost action in cold climates, and physico-chemical effects in aggressive environments. There is a general agreement that the permeability of concrete, rather than normal variations in the composition of portland cement, is the key to all durability problems. There is also a general agreement that rapid growth of the concrete construction industry after the 1940s led to the production and use of wet concrete mixtures, which are able to meet the strength requirement via a change in the composition of portland cement, but were unsatisfactory from the standpoint of corrosion of reinforcing steel, resistance to freezing and thawing cycles, and chemical attacks. A rise in chemical aggressivity of the environment through the increasing use of deicer salts, and an increase in land, water, and air pollution, has also contributed to concrete durability problems. Although significant advancements have been made in regard to understanding and controlling various physical and chemical phenomena responsible for concrete deterioration, the trend towards less durable concrete structures has yet to be reversed. One of the reasons is that most of the information from tests on durability is in fragmentary form and cannot be easily synthesized into a complete understanding of actual, long-term, effects on field concrete. An over-reliance on test methods and specifications dealing with different aspects of durability has therefore become a part of the problem since accelerated laboratory tests do not correlate well with behavior of concrete structures in practice.

The durability of concrete is generally regarded as its ability to resist the effects and influences of the environment while performing its desired function. In an offshore or marine environment, the concrete can be subjected to the influences of wetting and drying, freezing and thawing, abrasion by ice and other debris, chemical attack or mineral depletion by water it is in, salt accumulations, and attack by marine organisms. The paper reviews these dteriorating mechanisms and also reviews the recent trends in strength development for concretes made with modern materials. Chloride ion penetration into concrete information from 33-year old Gulf of Mexico offshore concrete platforms is presented. The advantages of supplementary cementing materials in offshore and marine concretes are discussed along with recommendations for producing durable marine concretes.