International Concrete Abstracts Portal

To meet the development in structural design and construction methods, a continuous effort has been made to advance the concrete mix design process. Paper gives a survey of the research work in this process. The mix design has to take care of several contradictory requirements, such as high strength and low permeability on one side and moderate heat development and extraordinary workability on the other. From 1972 to 1986, the concrete grade has been increased from C45 to C70 while the workability (slump) has been increased from 120 to 240 mm mainly due to extreme dense reinforcement (above 1000 kg/m3 in local areas). The chemical composition of the cement is aimed at an optimized 28 day compressive strength with moderate heat development. The fine aggregates are produced through a hydraulic process to obtain the desired particle distribution. Minor changes in the finer part of the grading have resulted in remarkable improvements in the workability and pumpability by stabilizing the paste-aggregate matrix. The practical use of very high strength normal density concrete (C80-C100) and high-strength concrete with lightweight aggregates are new challenges for the concrete mix design so as to satisfy new advancements in construction methods.

Effect of chemical and mineral admixtures on the durability of concrete in an undersea environment has been studied. Addition of slag and slag-like materials and water-reducing agent improves the durability of concrete. However, alkali-silica aggregate reaction aggravated by the marine environment and the corrosion of reinforcing steel bars are enhanced. By the addition of rust inhibitors, these effects can be controlled.

A project was devoted to the experimental examination of durability of reinforced concrete beams cast with either normal gravel or artificial lightweight ARGI 16 aggregates. Two series of three identical beams, different only by the nature of concrete, were exposed in the following conditions. One couple comprising a beam of each type is left in conditions simulating the normal tidal zone; the remaining beams are stored in a similar exposure combined with a thermal treatment during the immersion position (ventilation with air heated at 32 C). All tested elements were subjected to a bending moment by means of a negative prestressing, letting a permanent cracking of tensioned concrete. The examination period lasts five years. The paper deals with observations done during this period and results gained in the final loading tests achieved. The conclusion drawn from this particular study is very promising for durability of normal or lightweight concrete structures in marine environment, at least with the normally required quality.

Coastal defenses in the United Kingdom have often been constructed using natural stone armor or plain concrete armor. This paper reports on the novel use of slender section reinforced concrete units in the marine splash zone. A program of monitoring the durability performance of the coastal defenses was commenced by the University of Liverpool in 1985. A visual survey together with electrical potential and resistivity monitoring of a sample of 51 units is conducted annually. Overall, the slender reinforced concrete units are performing very well both hydraulically and structurally. Some minor impact damage soon after construction has been easily repaired using the steel reinforcement as a bonding key. The results of three years of potential monitoring have shown that the steel reinforcement has been passivated in all but one of the sample units. The resistivity monitoring has shown that the fly ash concrete has a significantly higher resistivity than the ordinary concrete and hence; its use should lead to lower rates of corrosion. Further studies on the durability of reinforced concrete in the splash zone are in progress.

Concrete berth faces (wharves) in Saint John, New Brunswick, are subject to a tidal range of 8.5 meters and over two hundred freezing and thawing cycles a year. This location and application is perhaps one of the most severe tests for concrete, and when failure of the in situ concrete occurs, repairs must be made in adverse circumstances. In order that repairs be effective, the underlying cause for the initial deterioration must be understood so that it will not adversely affect the repair procedure. The wharves in Saint John are showing distress as evidenced by concrete spalling predominantly in the saturated area of the tidal zone. This deterioration is attributed to concrete breakdown caused by freezing and thawing actions, as well as expansion from alkali-aggregate reaction and reaction with marine salts. The method of surface preparation used and the repair method selected are described. The tidal cycles had a profound effect on the site schedule and construction details. Repair costs are also discussed. To evaluate the effectiveness of the repairs and determine the properties of the wet mix shotcrete, a comprehensive pre-construction testing program was adopted.