International Concrete Abstracts Portal

The underwater placement of small concrete volumes for repair operations necessitates that the fresh concrete be highly resistant to water erosion and segregation, as well as self-compacting and self-leveling. The hardened concrete must develop high wear resistance and excellent adhesion to underlying surfaces and reinforcing steel. Four potential repair concretes and one conventional tremie mixture were cast underwater in small and relatively shallow depressions using tremie pipes. Research findings indicate that an anti-washout admixture should be used to minimize the risks of water dilution and segregation and to enhance the spreadability and leveling of underwater-cast concrete. Proven concrete mixtures recommended in this research can produce relatively flat repair surfaces with in-place compressive strength in excess of 8000 psi (55.2 MPa) and relative density close to 100 percent of similar values for concrete cast and consolidated above water. Bond strength close to 400 psi (2.8 MPa) can develop between underwater-cast concrete and neighboring concrete repair surfaces.

An investigation was conducted to assess the structural integrity of a 17-year-old precast prestressed concrete conveyor bridge used to transport sodium chloride rock salt from a storage building to an outside stockpile area. The stockpile, depending on storage requirements, quite often buried most of the structure and/or subjected it to sodium chloride dust. The investigation revealed that the structure had performed remarkably well, considering the small concrete cover used to protect the reinforcing elements and the inadequate consideration of structural cracking induced by unanticipated loading from stockpiled salt. The concrete strength of the single tee members was estimated to be 7000 psi (48 MPa), with cover to the stirrups varying from virtually 0 to 1 1/2 in. (0 to 38 mm) and cover to the prestressing strands varying from 3/4 to 2 in. (19 to 51 mm). It was observed that aggressive prestressing strand corrosion causing pitting and some brittle wire failures occurred locally at flexural crack locations in single tee column members with little corrosion activity immediately adjacent to the cracks, even after 17 years of aggressive chloride exposure. This observation seems to conflict with the prevailing theory of the role of cracking on corrosion--that cracks perpendicular to steel reinforcement should result in limited early localized corrosion but, with time, chloride ions penetrate even uncracked concrete and initiate widespread corrosion.

The service conditions for concrete construction in the coastal areas of the Arabian Gulf are considered to be those of one of the most aggressive environments in the world. Deterioration of hardened cement paste due to salt attack is one of the leading reasons for poor performance of concrete structures in this region. Calcium, magnesium, sodium salts of sulfates, chlorides, and carbonates extensively contaminate the ground, groundwater, and the aggregates. In such an environment, structures built with concrete which can be rated as good in temperate climatic conditions can hardly last for a decade or two. Field and laboratory studies are in progress at King Fahd University of Petroleum and Minerals at Dhahran, Saudi Arabia, to formulate preventive measures. As a part of this endeavor, the performance of in-service concrete structures is monitored. This paper details the investigations carried out to evaluate the performance of these concrete structures. Data developed in this investigation show that the aggressive service environment is the major cause for concrete deterioration, as such appropriate mix design techniques and construction practices are to be adopted for the production of a very dense and impermeable concrete.

The commercial utilization of high-strength concrete with 60 to 120 Mpa compressive strength is a recent phenomenon; therefore, long-term field experience with regard to durability in corrosive environments is not available. In this paper, a critical review of the factors necessary to obtain high strength and high durability is presented. Typically, the concrete mixtures contain high cement content, low water content, and several admixtures, such as a superplasticizer, a pozzolan, and at times an air-entraining agent. When properly placed, consolidated, and cured, such mixtures should have low permeability and high durability to corrosive environments. However, there is some concern that microcracking in the aggregate-cement paste transition zone, possibly due to a variety of causes, may impair the impermeability and durability. The results of a recent investigation are discussed, which show that the aggregate type can play an important role in controlling the strength of the transition zone and, therefore, the degree of potential microcracking of concrete in service.

Long service life of concrete depends on correct choice and use of materials. Problems such as ASR (alkali silica reaction) and the prospect of sulfate attack and corrosion need early and proper identification and attention. Resistant materials must be selected and properly used to insure control of these adverse conditions. Low alkali cement or sulfate-resisting cement must be used as appropriate in these situations. Other requirements often overlooked are those essential to prevent or minimize thermal cracking of massive structural concrete, as in power plants, bridge piers, foundation elements, and thick linings of large tunnels. The ordinary concrete in municipal use, especially in new subdivisions, is often short of durability and exhibits much cracking, due to failure to follow the most fundamental rules of good practice, especially freezing weather protection, enough cement, control of slump, ample provision of joints, and curing. Sidewalks and driveways are too often disfigured and disappointing. Curing is often neglected. Specifications for the work must cite the requirements in complete detail and be followed explicitly when the work is done.