chlorides; corrosion; maintenance; measurement; nondestructive tests; offshore structures; polarization; reinforcing steels; repair.

The nature of the stress waves found in the Schmidt Hammer after impact during the testing of concrete were examined experimentally. Using a specially designed plunger, the authors have been shown that the impact of the hammer mass produces a large compressive wav e and a large reflected stress waved at r the centre of the plunger. The ratio,or/or of the amplitudes of these waves and the time T between their appearance was found to depend upon the surface hardness of cured concrete. The rebound number was found to be approximately proportional to the ratio of the two stresses and was not significantly affected by the moisture conditions of the concrete. The magnitude of the first stress wave at the centre of the plunger is almost constant and is approximately 80 percent of the value calculated by Smith’s numerical solution, which does not consider the efficiency of the impact of the hammer. The authors have concluded that the principal of operation of the N-type Schmidt test hammer may be more complex than is assumed when consideration is given only to the simple problem of applying Newton’s laws to impacting bodies. It may involve considerable components of longitudinal wave transmission. It is further concluded that, to correctly measure the rebound number of hardened concrete, the Schmidt hammer can be calibrated by testing a material with a constant hardness and measuring the resulting impact stress wave. By observing the behaviour of the impact stress in the plunger the surface hardness of concrete canbe measured with higher accuracy.

Whilst compressive testing is by far the most common method of testing cores, there exists a need for a method of tensile strength estimation. The point-load test is investigated for its potential to satisfy this need, and the results of over 800 point-load core tests carried out on a plain, fibrous and sprayed fibrous concretes are discussed. A good straight-line relationship between flexural strength and 68mm core point-load strength is obtained for both plain and fibrous concretes. The average values of the coefficient of variation of the point-load strengths for plain and fibrous concretes were both between 8 and 9%; this compares with a value of 6% for the flexural strengths. The point-load method is shown to be of particular potential for assessing the strength properties of sprayed fibrous concretes.

During the past 40 years in-situ/non-destruc-tive testing of concrete has achieved increasing acceptance for the evaluation of existing concrete structures with regard to their uniformity, durability and other properties. This paper reviews critically the available in-situ/non-destructive tests for estimating concrete strength and for determining properties other than strength, and discusses their implications. The methods discussed for estimating concrete strength in-clude surface hardness and penetration resistance tests, pullout, ultrasonic pulse velocity, break-off, combined methods, and maturity techniques. The tests reviewed for determining properties other than strength include magnetic, electrical, radioactive, pulse echo, radar, microwave absorption, acoustic emission, nuclear, infrared thermography, and permeability methods. Some of the tests described are relatively easy to perform whereas others require sophisticated equipment and trained personnel, and there are others which are still in the development stage. Regardless of the type of test used, it is emphasized that interpretation of test data must be performed by specialists rather than by technicians performing the tests. Unless comprehensive laboratory correlations have been established between the strength parameters to be predicted and the results of in-situ/non-destructive tests, the use of the latter to predict compressive or flexural strength of concrete is discouraged.

Radiometric determination of density and moisture of soils and building materials has been carrie jout successfully. Many types of instruments are available. However, instruments are not available for measurement on fresh concrete so that it is not possible to repair inhomogeneities, if any even before hardening. The paper describes an analog and a digital radiation density meter and their application in the inspection of concrete walls against radiation. By repairing defective concrete in the course of concrete placement it is possible to attain a laboratory quality of concrete placed on site. Long-term and temperature stability, the measuring procedure, calibration of the instruments, biological shielding tests and the evaluation of the results are described.