Concrete Mix Design
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The various grades of concrete shown in the Table of Designed Concrete Mixes shall be designed by the Contractor with particular attention to durability, strength, workability and surface finish, and to satisfy the Engineer on these qualities. The water content of all concrete shall be rigidly controlled and kept to the minimum required to obtain a workable concrete suitable for the nature of the work to be executed.
The water cement ratio (kg of free water/kg of cement) shall not exceed 0.55 with the exception of Grade C35A for which the water cement ratio shall not exceed 0.50.Each grade of concrete shall be designed in accordance with Clause 6.3 of BS 8110 : Part 1and shall comply with the requirements of BS 5328. In addition grade C35A concrete shall comply with the requirements of BS 8007.
The addition of proprietary admixtures intended to change the flow characteristics cohesion or rate of setting of the concrete shall not be made without the approval of the Engineer. No admixture shall contain chloride ions.
Each Grade of concrete mix shall be designed to have a mean strength greater than the specified characteristic strength by at least an amount termed the Current Margin. Unless otherwise approved by the Engineer the Current Margin for the initial mix design shall be taken as 4.6 N/mm2 for concrete Grade C10 and 15 N/mm2 for all other grades of concrete
specified in the Table of Designed Concrete Mixes.
The Engineer may at his discretion approve a change in the Current Margin for the initial mix design if the Contractor produces evidence to show that he can satisfy the requirements for the determination of the mean 28 day compressive strength given in BS 5328.
No concrete shall be placed in the Works until the relevant mix has been approved by the Engineer. Approval will not be given to any concrete mix until it has been successfully subjected to Preliminary Mix and Trial Mix Tests. The Contractor shall carry out Preliminary Mix Tests as specified hereinafter in order to determine for each grade of concrete shown in the Table of Designed Concrete Mixes the minimum practicable water/cement ratio and the required mix proportions of the fine and coarse aggregate, the necessary allowance being made for the moisture content of the aggregate.
After the value of the water/cement ratio and the mix proportions have been approved by the Engineer Trial Mixes shall be carried out by the Contractor as specified hereinafter. The water/cement ratio and mix proportions which have been approved as a result of the Preliminary Mix Tests shall be used throughout the course of the Works and no amendment shall be made without the approval of the Engineer.
Notwithstanding any such amendment the Contractor shall ensure that cube crushing strengths satisfy the compliance
requirements specified hereinafter. In concrete made with sulphate resisting cement the maximum total content of chlorides (as chloride ions) shall not exceed 0.2% by weight of cement and the total acid soluble sulphates (as sulphate ions) shall not exceed 4.0% by weight of cement.
Further tests shall be carried out if any feature of materials or mixes is changed during the course of the work.