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Preplaced aggregate concrete is an excellent repair material that has not been used much in recent years. Preplaced aggregate concrete is made by injecting portland cement grout, with or without sand, into the voids of a formed, compacted mass of clean, graded, coarse aggregate. The preplaced aggregated is washed and screened to remove fines before placing into the forms. As the grout is injected or pumped into the forms, it displaces any included air or water and fills the voids around the aggregate, thus creating a dense concrete having a high aggregate content.

Because the coarse aggregate has point contact prior to grout injection, preplaced aggregate concrete undergoes very little settlement, curing, or drying shrinkage during hydration.

Drying shrinkage of preplaced aggregate concrete containing 1-1/2 inch maximum size

aggregate is about 200 to 400 millionths, while conventional concrete drying shrinkage

containing the same size maximum aggregate is about 400 to 600 millionths.

Another advantage of preplaced aggregate concrete is the ease with which it can be placed in certain situations where placement of conventional concrete would be extremely difficult or impossible. Preplaced aggregate concrete is especially useful in underwater repair construction. It has been used in a variety of large concrete and masonry repairs, including bridge piers and the resurfacing of dams. It has been used to construct atomic reactor shielding and plugs for outlet works and tunnels in mine workings, and it has been used to embed penstocks and turbine scrollcases (American Concrete Institute, 1992). Figure 42 shows the upstream face of Barker Dam near Boulder, Colorado, which was resurfaced with prepacked aggregate concrete.

Although preplaced aggregated is adaptable to many special repair applications, it is essential that the work be undertaken by well qualified personnel who are willing to follow exactly the construction procedures required for this repair material. Form work for preplaced aggregate concrete requires special attention to prevent grout loss. The construction of forms should be with workmanship better than that normally encountered with conventional concrete.

Leaking forms can cause significant problems and should, by careful construction, be avoided whenever possible. The injected grout is more flowable than plastic concrete and takes slightly longer to set. Forms, therefore, must be

constructed to take more lateral pressure than would be necessary with conventional concrete.

Form bolts should fit tightly through the sheathing, and all possible points of grout leakage should be caulked.

(a) Preparation The preparation of concrete to be repaired by preplaced aggregate concrete is identical to the preparation required for replacement concrete (section 29) if the development of bond is required.

Figure 42.—The downstream face of Barker Dam, near Boulder, Colorado, was resurfaced with prepacked aggregate.

(b) Materials .—Grout for preplaced aggregate concrete may be mixed with sand either of the gradation specified for conventional concrete or with fine sand, pozzolanic or fly ash fillers, water reducing admixtures, and pumping admixtures as dictated by the minimum size of the coarse aggregate. With 1-1/2-inch minimum size coarse aggregate, the sand gradation is that specified for conventional concrete. The portland cement, water, and sand are mixed using high speed centrifugal grout mixers that produce well mixed grouts of a creamy consistency. For use with 1/2-inch minimum size coarse aggregate, a grout mixture is prepared containing fine sand passing a No. 8 screen and with at least 95 percent passing a No.

16 screen. Best pumping characteristics will be obtained with fineness modulus between 1.2 and 2 and with the rounded shape of natural sands as opposed to crushed sands.

Addition of fly ash and water reducing

admixture improves the flowability of the grout

and the ultimate strength. Proprietary pumping admixtures are commonly used to increase the penetration and pumpability of the final grout.

The consistency of grout for preplaced aggregated should be uniform from batch to batch and should be such that it can be readily pumped into the voids at relatively low pressure.

Consistency is affected by water content, sand grading, filler type and content, cement type, and admixture type. For each mix, there are optimum proportions that produce best grout pumpability or consistency, and tests are necessary for each job to determine these optimum proportions.

The maximum size coarse aggregate used with both types of grout is the largest available, provided that the aggregate can be easily handled and placed. Coarse aggregate should meet all the requirements of coarse aggregate for

conventional concrete. It is essential that the coarse aggregated be clean. The aggregate should be well graded from minimum size (1/2-inch minimum or 1-1/2-(1/2-inch minimum) up to the

Guide to Concrete Repair

maximum size, and when compacted into the forms, should have a void content of 35 to 40 percent. If grout containing sand of concrete grading is used, the minimum coarse aggregate size should be 1-1/2 inches.

(c) Application.—The grout piping system used with preplaced aggregate concrete must be designed to serve at least 3 purposes—to deliver and inject grout, to provide means for

determining grout level in the forms, and to serve as vents in enclosed forms for escape or air and water. Proper design and location of the grout piping system is essential for successful placement.

The grout delivery pipeline should be a

recirculating system. That is, the grout delivery pipeline should extend from the grout agitator or holding tank to the grout pump, then to the injection manifold, and return to the grout agitator tank. With this type of pipeline, the grout can be kept moving and circulating in the delivery pipeline even when no grout is being injected into the aggregate. Such a system prevents stoppages and clogging of the delivery line. Noncirculating or deadheaded grout delivery lines are not allowed on Reclamation projects. The delivery line should be kept as short as practicable, and the pipe size should be such that normal grout flow velocities range between 2 and 4 feet per second. For most applications, a 1-inch ID grout line will suffice.

All valves used in the grout piping system should be quick opening ball valves which can be readily cleaned.

The simplest piping system is a single recirculating delivery line attached via a manifold and valves to a single injection line.

The injection line should extend to the lowest point in the form. Multiple injection lines are used for larger projects. Spacing of the injection lines is variable, depending on the form configuration, aggregate gradation, and other factors, but spacings of 4 to 6 feet are common. In preparing the layout of the grout delivery system, it is normally assumed that the slope of the grout face will be 4:1 for work in the dry and 6:1 for underwater work. Much

flatter slopes are common with actual grout surfaces.

Sounding wells constructed from 2-inch-diameter slotted pipe are installed to allow determination of the level of grout during injection. Similarly, clear plastic windows can be installed in the forms to allow visual determination of grout levels. The number and location of sounding wells are determined by the size and configuration of the aggregate mass.

The ratio of sounding wells to injection pipes should be from 1:4 to about 1:8.

Grout injection should begin at the lowest point of the form and continue uniformly until the entire form is filled. After sufficient grout has been pumped to raise the level of grout in the form about 18 inches above the bottom outlet of the injection line, the injection line can be progressively raised, maintaining about 12 inches of embedment below the level of the grout at all times. A great deal of thought and planning is required if multiple injection lines are used. The objective is to entirely fill the form without trapping air or water. Vents must be located where needed and the injection sequence designed to promote complete filling. It is not possible to use internal vibrators to consolidate preplaced aggregate concrete. External vibrators, however, can be attached to the forms and used advantageously. External vibration will

eliminate the splotchy appearance that can occur where coarse aggregate particles contact the forms. Underwater applications of preplaced aggregate concrete require additional

considerations. During injection, grout pumping must continue until an undiluted flow of grout emerges from the top of the form. Formwork is usually closed at the top to prevent washout or dilution of the grout after placement if flowing water is encountered. Anti-washout admixtures might prove useful for underwater applications of preplaced aggregate concrete. Care must be taken, however, when using several different types of admixtures (e.g., anti-washout, pumping aids, or high range water reducers) that

undesirable combinations are avoided. It is known for example, that some anti-washout admixtures can significantly reduce the

pumpability benefits of some high range water reducers. Such problems should be detected during the mixture proportioning tests previously recommended in paragraph (b).

The minimum volume of the grout mixer tank and the grout agitator tank should be 17 cubic feet. The grout should be mixed using a high speed centrifugal mixer operating at a mini-mum of 1,500 rotations per minute. The grout pump should be of the helical screw, rotor-type (commonly known as a "Moyno" grout pump), capable of pumping at least 20 gallons of grout per minute at the specified injection pressure.

Quality control of preplaced aggregate concrete lies with proper compaction of the aggregate into the forms and maintenance of proper grout consistency throughout the job. Compaction requirements must be satisfied by visual inspection during placement and before grout is introduced into the forms. Grout consistency can be determined by using a Baroid Model 140 Mud Balance to measure grout density. Some practitioners promote using a flow cone to time the rate of flow of a known volume of grout through the cone as a measure of consistency.

Recent laboratory tests (Smoak, 1993), however, have proven that the flow cone is useless for measuring the consistency of grout containing high range water reducing admixture.

(d) Curing.—The curing requirements for preplaced aggregate concrete are the same as for replacement concrete (section 29). Preplaced aggregate concrete placed during underwater applications will normally receive excellent curing without further effort.

28. Shotcrete.—Shotcrete is defined as "mortar