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Sunday, February 22, 2009

Conditions of underwater concrete

Concrete poured underwater must have good workability and, thus, should meet the following conditions:

(1) The mixture must incorporate the proper proportions of sand and gravel (preferably not crushed material) in a rich paste of Portland cement and freshwater.

(2) The mixing water must not exceed 5.5 gallons per bag of cement. (Mixing water includes the water entering the batch in the form of free, surface moisture on the sand and/or gravel; this free water must, therefore, be deducted from the total water to be
added.) If the aggregate particles are surface-dry and not saturated, they will absorb some of the gross mixing water; allowance must, therefore, be made for extra mixing water, taking care that the W/C ratio of 5.5 gallons per bag is not exceeded.

(3) The mixture should not contain less than 8 bags and not more than 10 bags of cement per cubic yard of ASTM Type V concrete.
- Type V is the recommended product for such structures because of its high resistance to sulfate attack (a form of disintegration occurring in seawater and other high salt environments)

(4) For improved workability, the concrete should incorporate an admixture to provide not less than 3°% and not more than 6% entrained air as determined
by standard ASTM methods.

(5) The sand and gravel should be physically sound, and the maximum gravel size should be 3/4 inch The aggregate should be graded as indicated in Table 1 .

(6) The formwork in which the concrete is poured must be rigid, carefully fitted, and designed so that no underwater currents can pass through it. Provision must be made for the seawater displaced by the concrete to escape from within the form. Timber is generally the most suitable material for construction of the formwork. Joints between the formwork and the intact portion of a structure should be caulked.

(7) Low temperatures during mixing and curing of concrete (i.e., below 50°F) can delay strength development for periods as long as one year and so should be avoided.

(8) An enclosed chute or "trunk" should be specified so that there is no mixing with water during placement

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