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Concrete is a variable material. Judgment of concrete acceptability is consequently probabilistic. The acceptance criteria stipulated in the British Codes as well as the ACI Code are reviewed. The efficiency of the respective acceptance tests is examined by means of the operating Characteristic curves. It is shown that the criteria of acceptance stipulated in ACI 318-77 involve a high risk of accepting substandard concrete (consumer’s risk), while the CP 114 criteria for designed mixes involve a high risk of rejecting good quality concrete (producer ’S risk). The CP 110 criteria involve a reasonably low level of risks to the two parties. However, the test is suitable for continuous assessment of concrete quality in large projects, but less efficient in small projects where concreting is performed at intermittent periods. The BS 5328 criteria, though applicable to small projects, result in a varying distribution of risk between the producer and the consumer, depending on the variability of the concrete. A set of acceptance criteria more suitable for small projects is proposed. The proposed criteria involve a level of risks similar to that
of CP 110.
A concrete with epoxide resin as the sole binder having a high compressive strength (150 MN/m2) and stiffness (40 GN/m2 at 70 °C) has been produced by a technique involving vacuum impregnation of a prepacked mould. Tests demonstrate satisfactory creep and fatigue performance. while vacuum impregnated epoxide resin concrete has been developed for the stator structure of superconducting turbogenerators, it is likely to be used in other applications, as it also has resistance to aggressive environments, impervious nature, low shrinkage, and electrical resistance.
B.W. Staynes and J.S.H. Ross
Timber floor joists
It is always gratifying tofind in our correspondence a reply to a problem,associated with a Code, coming from such an authoritative source as a representative of the responsible committee. It encourages us to believe that the column may be read more widely than we are sometimes inclined to believe and that it is, apparently, regarded as a useful vehicle for the dissemination of information and the clarification of misunderstandings. Mr J. G. Sunley, Chairman of the BSI CSB 32 (CP 112) committee, has responded thus to a seeming conflict between CP 112 and Building Regulations. He writes:
In February you published a letter criticising the apparent conservatism of the Building Regulations Schedule 6 ‘Span tables for timber floor joists’. Your correspondent, Mr Redman, has kindly provided the CP 112 committee responsible for timber design stresses with copies of his calculations and we are now able to answer his letter.