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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.
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.
Fig 1 shows a bar chart indicating the normal, classical design programme as given in the RIBA plan of works. This process is essentially linear, even though in practice it moves in progressive spirals within which decisions at one stage make it necessary to reconsider and refine decisions that were made earlier. The plan of work presumes a briefing stage (A and B), followed by the sketch plan stage which includes the town planning application at stage D. These are followed by working drawing stage which incorporates refinement of the design, sufficient detailing for quantities to be taken off, application for building regulations approval and, subsequently, the normal process of tendering, assessment of tenders, the placing of the contract and its execution. By and large subcontracts await appointment of the main contractor. I want to highlight three steps in particular that determine the time taken to get the design to tender stage.