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Synopsis Estimates of concrete strength are often made from compression tests on cores which have a diameter considerably less than the recommended 100 mm. The paper examines the results of a laboratory investigation in which 44 mm diameter cores were cut and tested. The influence of both specimen and aggregate size upon height/diameter ratio and orientation effects is examined, and observed relationships between core strengths and measured control cube strengths are compared with those normally used for larger cores. It is shown that core strength is affected by both specimen size and aggregate size, and it is proposed that conversion to corresponding cube strengths should take this into account. The variability of results is also assessed in relation to the above factors, and an estimate made of the accuracy of predicted actual cube strength that is likely to be achieved from testing cores of this size.

Carbonation of the concrete leading to reduced alkalinity around the steel is one of the main reasons for the corrosion of reinforced concrete. Studies of carbonation-induced corrosion and of rehabilitation methods, such as electrochemical re-alkalization, require the convenient preparation of realistically large specimens of carbonated concrete in a sufficiently short time. This paper describes a rapid method of preparing carbonated concrete by exposing concrete, which has been dried to an internal relative humidity of 60%, to a pure atmosphere of carbon dioxide gas at 15 bar pressure (1500 kPa). The pressure chamber used can accommodate specimens up to 150 mm diameter or 100 × 100 mm section and such specimens can be fully carbonated in 2 weeks, much more quickly than by other methods. Carbonation increases the electrical resistivity and strength of the concrete and reduces the water absorption. Optical and electron microscopical investigations on the carbonated concrete confirm that the microstructure is no different from that produced in concrete by carbonation under natural exposure. The accelerated carbonation method can be used for development work on materials and repair methods and has been used by the authors in preparing carbonated concrete specimens for re-alkalization tests.

The pull-off technique has grown in popularity for testing the surface zone strength of concrete, and when used in conjunction with partial coring is particularly suitable for the bond testing of repairs. Theoretical and laboratory experimental studies have been undertaken to examine a number of important practical factors which influence the results obtained during surface and partially cored pull-off tests. Finite-element methods were used to analyse failure zone stress distributions. Disc material and proportions were shown to be particularly significant in both types of test, and in some circumstances the results are also affected by the elastic modulus of the concrete. These factors may influence correlations with compressive strength, demonstrating the importance of standardization of disc characteristics to permit direct comparisons of results. This will also be an important consideration when comparing measured test results with specified values. A very wide range of loading rates is provided by currently available test equipment, and this feature merits further study, together with other factors relating to partially cored tests as applied to repairs.

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