i additional plasticizers added to increase the slump one time. Concrete in the pipeline should be removed before the slump decreases. When concreting is completed, all parts of the pump and pipeline should be thoroughly cleaned.
The concrete should be pumped as near to its final underwater position as possible. The diver who has control of the discharge end should not permit lateral flow within the open-top form of more than 2 or 3 feet. The discharge end of the line must always be kept buried in the mass of fresh concrete; otherwise, washout will occur at the point where the concrete comes out. Aluminum pipe should not be used because an adverse chemical reaction with the concrete may occur. The pipeline should be protected from any excessive heat (solar included).
CAUTION Use extreme caution when a hose blockage requires opening a coupling due to the pressure from the concrete column.
One method of placing concrete underwater, especially at easily accessible locations, involves a tremie - a steel tube with a hopper for filling at its upper end. A plug, consisting of either a rubber ball or a wad of burlap that fits snugly inside the tremie, is inserted below the loading hopper. The freshly mixed concrete, introduced at the hopper, forces the plug down and displaces the seawater. The tremie is continually replenished with concrete while the lower end is kept embedded in the newly deposited concrete. Compaction by vibration is not permitted, as it generally results in excessive laitance. Tremie concrete must be quite workable so that it flows readily into place.
Fine aggregate contents of 45 to 55 percent by volume of total aggregate, and air contents of 4 to 5 percent, are generally desirable. A typical aggregate gradation is indicated in Table 2-26. A typical mix design for tremie concrete is given in Table 2-27. The size of the coarse aggregate should be restricted as follows:
• Very large pours: 1.5 inch maximum
• Normal pours: 0.75 inch maximum
• Restricted access pours: 0.375 inch maximum
It is general practice to use a steel tremie, but a rigid rubber hose could be substituted. Joints in the tremie must be well-gasketed and sealed, otherwise the water could leach the cement from the concrete. An aluminum alloy tremie should not be used because an adverse chemical reaction could occur, producing inferior concrete. For deep placements, the tremie should be fabricated in sections, with joints that allow the upper sections to be removed as the placement progresses.
The size of the tremie depends on the maximum size of gravel and on the quantity of concrete to be placed; the usual range in diameter is from 4 to 10 inches, and should be at least eight times the maximum coarse aggregate size. The slump of tremie concrete should be maintained between 6 and 9 inches.
The quality of tremie concrete is greatly dependent on proper mix design and placement. Reinforcing should be designed to minimize segregation due to a screening effect. The largest size bars possible should be used, and they should be spaced a minimum of three to four times the aggregate size. Admixtures are used to improve the flowability of the concrete. Otherwise, much laitance, due to washing out of cement, and gravel pockets, due to lifting of the tremie pipe to facilitate placement, can result. Water reducing retarders in combination with an air entraining agent, and/or superplasticizers, are used to provide increased flowability at low water cement ratios. This increased flowability permits wider spacing of the tremie pipes, longer pipe lengths, wider pipes, and longer placing times.
The tremie method of pumping concrete is best suited for footings. The tremie is difficult to handle and will often hang up in spacers typically used in the encasement of piles.
Prepacked concrete is used on large underwater repair jobs where placement of regular concrete would be either difficult or impossible, and/or where minimization of shrinkage is a requirement. It is most economical for isolated locations, where pumping distances are excessive.
Prepacked concrete involves placing coarse aggregate in a form and then filling the voids in the aggregate mass with grout, as shown in Figure 2-55.
The preplaced aggregate concrete components should meet die requirements specified in Chapter 7 of ACI 304.2R-91. Form materials are essentially the same as for regular cast-in-place concrete, except that they must be sufficiently tight to prevent leakage.
The cement, aggregates, and water for preplaced aggregate concrete are also similar to those for regular cast-in-place concrete, however, the concrete mix contains a higher percentage of coarse aggregate. Aggregate gradation is in accordance with Table 7.2 of ACI 304.2R-91. A typical gradation is indicated in Table 2-28. For repair depths exceeding 12 inches, gradations with larger size aggregates are used. The
Gradation of Aggregates for Tremie Concrete
Gradation of Aggregates for Tremie Concrete
Was this article helpful?