International Concrete Abstracts Portal

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.

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.

A review of available literature indicates that the "finger placing" technique of embedding pullout inserts in concrete after placement has not been used for determining an the in-situ concrete strength development durin construction. Early strength information may be a useful guide line for form removal, stressing or reshoring. This technique o embedding pullout inserts to determine the strength of hardene concrete was used during the construction of a box Culver requiring approximately 11,500 m3 of concrete. Thirty location on the structure were tested to determine the force required t extract embedded inserts at ages of two, four, and seven days higher pullout forces were required to extract insert as the concrete matured. Currently no criteria exist for eval uating the pullout test results; however, the data obtaine throughout this study appear to be normally distributed at earl The standard deviation and within test variation values ar higher than expected. Improvements in these parameters may b achieved by improvements in finger placing and testing techniques

This paper reports results of an investigation of the use of pullout testing and the maturity method to predict the early age strength of concrete. Concrete specimens, composed of 12 variations of 2 cement types, 2 aggregate types, and 3 water-cement cured at different temperatures: 37OF (2.8°C), and the outdoor environ-Cylinder compression and pullout tests were performed on specimens at ages ranging from 12 to 168 hours (7 days). Regression equations for cylinder strength and maturity, and pullout force and maturity are developed where the maturity is modified by changing the datum temperature from -10° C to 0° C to improve the predictive capabilities. A model for the prediction of the cylinder strength in terms of maturity is developed, as well as a oncrete strength by the pullout ombining cylinder strength and The reliability of the pullout model was affected by the comparative rates of strength gain of the cylindrical specimens and the slab specimens.

Pulse Velocity/Strength relationships can be estab-lished for concrete but they are influenced by many factors. Of particular significance are the proportions and composition of the components, age s curing conditions and moisture content of the concrete. Cement type, air-entrainment and curing temperatures -influence to a lesser degree. Pulse velocity correlates well with strength at early ages but is insensitive to even major increases in strength at later ages. A relationship established at early ages therefore is not applicable as the concrete matures. A rela-tionship determined on sound concrete during its development stage cannot be used to predict the strength of concrete that is deteriorating. Such a relationship should be established on cores from the concrete in question. A relationship established using laboratory-cured specimens, cannot be used with assurance to follow strength development in a structure. A Pulse Velocity Strength relationship can be confused by cracks, voids or other discontinuities in the concrete.