Pumped Concrete

Pumping freshly mixed concrete is the most expeditious means of placing concrete underwater in spaces of limited accessibility. It is generally preferred over the tremie method, however, the following considerations should be accounted for:

• Pumped concrete should be used to fill the forms from the bottom upward, displacing the seawater as additional concrete is forced in at the bottom. (Recently, new admixtures have been developed that enable concrete to be dropped without a tremie.)

• High quality concrete is required, because the mixture must be both workable and cohesive, so as to pass through the pump without blockage.

• Workable mixtures containing relatively small coarse aggregate particles tend to provide an easily placed concrete.

• The slump must be carefully controlled to prevent segregation, as excessively wet mixtures will sometimes segregate and cause blockage in the hose or pipeline. (High-range water reducers used as an admixture in the concrete can provide the high workability and eliminate segregation.)

• Coarse aggregate should consist of rounded particles, as crushed stone mixtures are comparatively difficult to pump because the angular particles tend to increase friction in the pipeline.

• If it is necessary to use crushed rock, the maximum coarse aggregate size should not be greater than one-third the smallest inside diameter of the hose or pipe.

• The properties of the fine aggregate (sand) are more important than those of the coarse aggregate. In particular, the sand should have a relatively high proportion of the finer sizes (i.e., 15 to 30 percent should pass the no. 50 sieve, and 5 to 10 percent should pass the no. 100 sieve).

• Porous aggregates (e.g., expanded clay, foamed slag, pumice, and many coralline materials) should be avoided unless denser aggregates are not available. If porous aggregates are used for pumped concrete, they should be presoaked as described in ACI 304.2R-91, Placing Concrete by Pumping Methods.

• Inserting a pig between mixes is usually a good idea. Care must be taken with long vertical drops with the pump hose. With no backpressure, i.e., break seal or disconnect, the concrete wants to siphon and collapse hose, creating an instant plug.

The pumping method offers several advantages:

• Concrete can be transferred from a barge direcdy into the forms.

• The process is less subject to operator error than the tremie concrete process.

• Pumped concrete is more easily placed in awkward situations and locations with difficult access, such as under piers.

• The pressurization process consolidates the repair material, providing for full encapsulation of exposed reinforcing steel.

The concrete pump most often used by the UCTs is a direct-flow, reciprocating piston pump with a pumping rate of about 25 yd3/hr, a hopper capacity of 6 fit3, and a maximum pumping distance of about 400 feet. The pump, illustrated in Figure 2-54, is normally fitted with a 2-inch-di-ameter hose but can be fitted with a 2-1/2-inch-diameter hose for mixes that are hard to pump. It requires concrete with 3/8-inch maximum size aggregate, graded as indicated in Table 2-24. A recommended concrete mix design for pumped concrete that has proved successful in the past is given in Table 2-25. While this is suitable for placement in areas of limited accessibility, larger aggregate sizes and corresponding pump sizes should be used for large pours.

Carefully planning the location of the pump and hose routing before starting an operation can save subsequent moves throughout the project. The pipeline should be either horizontal or vertical rather than inclined, wherever possible. With an inclined pipeline, any water bleeding from the freshly mixed concrete within the pipeline will collect above the concrete and run down the inside of the pipe.

Figure 2-54. Typical concrete pump.

Table 2-24

Gradation of Aggregates for Pumped Concrete

Table 2-24

Gradation of Aggregates for Pumped Concrete

Was this article helpful?

0 0

Post a comment