International Concrete Abstracts Portal

The exhaust from the auxiliary power unit (APU) of the modern F/A-18 aircraft has caused spalls and erosion on portland cement concrete (PCC) pavements. The exhaust gas has a maximum temperature of 385 F (196 C) and a maximum velocity of 140. At this temperature, PCC seems to lose its integrity when subjected to repeated and prolonged exposure. Spills of hydraulic fluid and jet fuel on the pavement aggravate the spalling process. The main objective of this investigation was to determine effects of cyclic heating on the strength of portland cement concrete subjected to high temperature, and compare the effects of cyclic heating on concrete contaminated with hydraulic fluid and jet fuel with noncontaminated concrete. Five different concrete mixtures were investigated. Twenty-one prisms and 21 cylinders were made from each mixture and tested for compressive strength, flexural strength, pulse velocity, and dry unit weight. Within each group, specimens were tested after each of the following heating/cooling cycles: 0, 15, 30, 60, 120, 240, and 400. A heating and cooling cycle is defined as heating in an oven at 400 F (204.4 C) for 60 min and cooling at room temperature for 30 min. After every 15 heating/cooling cycles, the contaminated specimens were soaked in jet fuel or hydraulic fluid overnight before the next heating/cooling cycles. Test results indicate that jet fuel contamination is more detrimental than hydraulic fluid contamination. Compressive strength, flexural strength, and pulse velocity are adversely affected by the cyclic heating.

A survey was conducted of failures of prestressing steel in bridge members exposed to potentially aggressive environments. It appears that the main cause of corrosion of prestressing tendons is the ingress of moisture laden with corrosion-inducing agents. Moisture can make its way to the prestressing steel by penetrating through leaking joints from a deck slab, or by diffusion from the underside of a bridge. Moisture may penetrate through concrete cover, sheathing and grout (or grease in unbonded tendons), as well as through anchorage systems. The rate of penetration of moisture depends primarily on permeability of concrete, type of sheath employed for protection of a tendon, and condition of grout or grease inside the sheath. Brittle fracture of reinforcing steel can occur due to pitting corrosion and/or stress corrosion cracking (SCC). Two types of SCC have been identified in prestressing steel in bridges: hydrogen embrittlement and fatigue corrosion. The rate of corrosion in prestressed concrete components can be minimized by using proper preventive and remedial measures.

Discusses the findings from the study of bond that forms a part of a major laboratory evaluation of the characteristics of repair materials carried out in the Building Research Centre. The important properties of repair patching materials that can affect the bond of a repair, such as shrinkage, thermal movement, compressive, shear, and tensile strength, are evaluated. The importance of surface preparation is also discussed.

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.