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PHILLIPS MINING, GEOTECHNICAL & GROUTING, INC. <br />only to suit the particular mode(s) of placement being used, but also to make sure the <br />optimum properties for the concrete are achieved. <br />The basis and rationale for the required concrete mix design is given below. <br />For this plug construction it is anticipated that ready-mixed concrete will be transported to <br />the adit portal where it is discharged into the hopper of a suitable concrete pump. The <br />addition of water and any other additives, not already pre-blended into the mix, will be <br />performed during the final mi~dng in the truck at, or close to, the concrete pump. Any <br />additives that are used to achieve the required properties of this concrete must be mutually <br />compatible and also compatible with the other concrete components. Equipment must be <br />provided, and accurately calibrated to ensure that the correct amounts of additives, <br />including water, are metered into the concrete mix. <br />For this underground work, especially with its restricted placing environment, where <br />concrete vibration is not possible during all phases of the pour, it is essential that high <br />workability concrete mixes are used. The concrete must be capable of flowing into the <br />irregular spaces created on the rough surface of the rock and also must bond to it. To <br />achieve the necessary flow characteristics, the slump of the concrete, at the point of <br />placement in the forms, must be of the order of 9 to ]0 inches. However, to achieve the <br />strength required and to obtain the necessary dense, low permeability and low shrinkage <br />concrete, a low water/cement ratio must be used. Under these circumstances the <br />workability of the mix must be achieved by the use of superplasticizers. <br />The concrete placement must be completed in one continuous operation to form a <br />monolithic concrete mass. in this way, horizontal construction joints, which are potential <br />leakage paths through the plug, can be avoided. However, when placing a relatively large <br />mass of concrete in one pour, care must be taken to avoid detrimental thermal effects that <br />Page 6 <br />