Can you tell me ...

A. According to BSI standard BS 5328, groups of four test results are used, each result being the average of two cube tests on concrete from the same batch. For the highest classes of concrete (C20 and above), the average strength of a group of four consecutive test results must exceed the characteristic strength by 3N/mm², and the strength of any individual result must be no less than the characteristic strength minus 3N/mm².

For the lowest classes of concrete (C15 and under), the average strength of a group of four consecutive test results should exceed the specified characteristic strength by 2N/mm².

For all strength classes, the average of the first two and the first three test results in the very first sets of test results for a particular concrete on a new project may be lower than the requirements for the mean of four by 2N/mm² and 1N/mm² respectively.

If in your case BS EN 206-1 was specified, then during the initial stages of production (that is, until at least 35 test results have been obtained) the results are assessed in overlapping or non-overlapping groups of three results. The mean strength of each group of three test results must be at least 4N/mm² greater than the specified characteristic strength while the occasional individual test result can be 4N/mm² less.

After 35 test results have been generated within no more than 12 months, the initial production period is over and continuous production achieved. The standard deviation is calculated and the test results are assessed in groups of at least 15. The mean strength of each group of results must be not less than the specified characteristic strength plus 1.48 multiplied by the standard deviation. As in the initial production period, the occasional individual result may be 4N/mm² less than the specified characteristic strength.

For more information, turn to BS 5328, Concrete. Part 4: 1990, Specification for the procedures to be used in sampling, testing and assessing compliance of concrete and BS EN 206-1: 2000, Concrete: Specification, performance, production and conformity.

Q. I am planning to convert a building into a hotel. How many of the rooms need to be accessible for wheelchair users? Similarly, in the hotel's restaurant and bar, what proportion of the seating area needs to be accessible to wheelchair users?

A. Part M of the Building Regulations lays down that one in every 20 guest bedrooms in a new hotel should be made accessible to wheelchair users, as should at least half the seating area in a restaurant/bar. However, converting an existing building into a hotel represents a material change of use rather than a new build, so Part M does not apply and you can ignore its stipulations. Before proceeding, though, confirm this with your local authority: better safe than sorry.

Q. As a contractor working on a project on an IFC 98 contract, I would like some advice on clause 2.3 regarding extension of time. The problem is that impending delays have been notified but the architect is in no hurry to make a decision. Is there any mechanism in the contract to force the architect to make a decision?

A. Under IFC 98 clause 2.3 there is no time limit on when the decision has to be made. However, the clause says "so soon as he is able", and as failure to grant an extension properly due could result in time being at large, the architect should deal with the matter reasonably quickly. It would be unwise to set the matter to one side until the end of the project. Seek legal/contractual expert advice.

Q. I have been advised to use self-compacting concrete on my project. Can you tell me something about it?

A. Self-compacting concrete (SCC) was originally developed in Japan more than a decade ago to offset a growing shortage of skilled labour. It has proved beneficial economically because of a number of factors, including:

• Faster construction
  
• Reduction in manpower
  
• Better surface finishes
  
• Easier placing
  
• Improved durability
  
• Greater freedom in design
  
• Thinner concrete sections
  
• Reduced noise levels, absence of vibrations
  
• Safer working environment

Overall SCC is considered a high-performance material with far greater workability, higher strength and longer durability than concrete. It has excellent deformability and resistance to segregation and can be filled in heavily reinforced formwork without vibrators.

Large-scale construction companies have used SCC in practical structures. With the increasing use of congested reinforcements in mat foundations and moment-resisting reinforced concrete structures, there is a growing interest in specifying highly flowable concrete.

For further information, please contact British Cement Association. Phone 01344 762676, email library@bca.org.uk, or visit the BCA's website at www.bca.org.uk.