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Preplaced-Aggregate Concrete

a. General

Preplaced-aggregate (PA) concrete is produced by placing coarse aggregate in a form and later injecting a portland-cement-sand fly ash grout, usually with chemical admixtures, to fill the voids. The smaller-size coarse aggregate is not used in the mixture to facilitate grout injection. It is primarily applicable to the repair of existing concrete structures. PA concrete may be particularly suitable for underwater construction, placement in areas with closely spaced reinforcing steel and cavities where overhead contact is necessary, and in areas where low volume change is required. It differs from conventional concrete in that it contains a higher percentage of coarse aggregate since the coarse aggregate is placed directly into the forms with point-to- point contact rather than being contained in a flowable plastic mixture. Therefore, hardened PA concrete properties are more dependent on the coarse aggregate properties. Drying shrinkage of PA concrete may be less than one-half that of conventional concrete, which partially accounts for the excellent bond between PA concrete and existing roughened concrete. The compressive strength of PA concrete is dependent on the quality, proportioning, and handling of materials but is generally comparable to that achieved with conventional concrete. The frost resistance of PA concrete is also comparable to conventional air-entrained concrete assuming the grout mixture has an air content, as determined by ASTM C 231 (CRD-C 41) of approximately 9 percent. PA concrete may be particularly applicable to underwater repair of old structures and underwater new construction where dewatering may be difficult, expensive, or impractical. Bridge piers and abutments are typical of applications for underwater PA concrete construction or repair. A detailed discussion of PA concrete is provided in ACI 304.R.
b. Applications

PA concrete has been used on different types of civil works construction including:

(1) Resurfacing of lock chamber walls.

(2) Underwater repair to lock guide walls.

(3) Resurfacing of spillways.

(4) Construction of plugs to close temporary sluices through a dam.

(5) Filling of temporary fish ladders through a dam.

(6) Scroll case embedment.

c. Materials and proportioning

Intrusion grout mixtures should be proportioned in accordance with ASTM C 938 (CRD-C 615) to obtain the specified consistency, air content, and compressive strength. The grout mixture should also be proportioned such that the maximum w/c complies with appropriate standards. Compressive strength specimens should be made in accordance with ASTM C 943 (CRD-C 84). Compressive strength testing of the grout alone should not be done to estimate the PA concrete strength because it does not reveal the weakening effect of bleeding. However, such testing may provide useful information on the potential suitability of grout mixtures. The ratio of cementitious material to fine aggregate will usually range from about 1 for structural PA concrete to 0.67 for mass PA concrete. A grout fluidifier meeting the requirements of ASTM C 937 (CRD-C 619) is commonly used in the intrusion grout mixtures to offset bleeding, to reduce the w/c and still provide a given consistency, and to retard stiffening so that handling times can be extended. Grout fluidifiers typically contain a water reducing admixture, a suspending agent, aluminum powder, and a chemical buffer to assure timed reaction of the aluminum powder with the alkalies in the portland cement. Products proposed for use as fluidifiers which have no record of successful prior use in PA concrete may be accepted contingent on successful field use. ASTM C 937 requires that intrusion grout made as prescribed for acceptance testing of fluidifiers have an expansion within certain specified limits which may be dependent on the alkali content of the cement used in the test. Experience has shown, however, that because of the difference in mixing time and other factors, expansion of the field-mixed grout ordinarily will range from 3 to 5 percent. If, under field conditions, expansion of less than 2 percent or more than 6 percent occurs, adjustments to the fluidifier should be made to bring the expansion within these limits. The fluidifier should be tested under field conditions with job materials and equipment as soon as practicable so that sufficient time is available to make adjustments in the fluidifier if necessary. If the aggregates are potentially alkali reactive, the total alkali content of the portland cement plus fluidifier added to increase expansion should not exceed 0.60 percent, calculated as equivalent sodium oxide by mass of cement. The grout submitted for use may exhibit excess bleeding if its cementitious material to fine aggregate ratio is different than that of the grout mixture used to evaluate the fluidifier. Expansion of the grout mixture should exceed bleeding at the expected in-place temperatures. Grout should be placed in an environment where the temperature will rise above 40 °F, since expansion caused by the fluidifier ceases at temperatures below 40°F. This condition is normally readily obtainable when PA concrete is placed in massive sections or placements are enclosed by timber forms. If an airentraining admixture is used in the PA concrete, adjustments in the grout mixture proportions may be necessary to compensate for a significant strength reduction caused by the combined effects of entrained air and the hydrogen generated by the aluminum powder in the fluidifier. However, these adjustments must not reduce the air content of the mixture to a level that compromises its frost resistance. The largest practical NMSA should be used to increase the economy of the PA concrete. A 37.5-mm (1- 1/2-in.) NMSA will typically be used in much of the PA concrete; however, provisions are made for the use of 75-mm (3-in.) NMSA when it is considered appropriate. It is not expected that many situations will arise where the use of aggregate larger than 50 mm (2 in.) will be practical. Pozzolan is usually specified to increase flowability of the grout.

d. Preplacing aggregate

Care is necessary in preplacing the coarse aggregate if excessive breakage and objectionable segregation are to be avoided. The difficulties are magnified as the nominal maximum size of the aggregate increases, particularly when two or more sizes are blended. Therefore, the Contractor’s proposed methods of placing aggregate should be carefully reviewed to ensure that satisfactory results will be obtained. Coarse aggregate must be washed, screened, and saturated immediately prior to placement to remove dust and dirt, and to eliminate coatings and undersize particles. Washing in forms should never be permitted because fines may accumulate at the bottom.

e. Contaminated water

Contaminated water is a matter of concern when PA concrete is placed underwater. Contaminants present in the water may coat the aggregate and adversely affect the setting of the cement or the bonding of the mortar to the coarse aggregate. If contaminants in the water are suspected, the water should be tested before construction is permitted. If contaminants are present in such quantity or of such character that the harmful effects cannot be eliminated or controlled, or if the construction schedule imposes a long delay between aggregate placement and grout injection, PA concrete should not be used.

f. Preparation of underwater foundations

Difficulty has been experienced in the past with cleanup of foundations in underwater construction when the foundation material was glacial till or similar material. The difficulty develops when as a result of prior operations, an appreciable quantity of loose, fine material is left on the foundation or in heavy suspension just above the foundation. The fine material is displaced upward into the aggregate as it is being placed. The dispersed fine material coats the aggregate or settles and becomes concentrated in the void spaces in the aggregate just above the foundation thus precluding proper intrusion and bond. Care must, therefore, be exercised to ensure that all loose, fine material is removed insofar as possible before placement of aggregate is allowed to commence.

g. Pumping

Pumping of grout should be continuous insofar as practical; however, minor stoppages are permissible and ordinarily will not present any difficulties when proper precautions are taken to avoid plugging of grout lines. The rate of pumping should be regulated by use of sounding wells so that the preplaced aggregate is slowly intruded to allow complete and uniform filling of all voids. The rate of grout rise within the aggregate should be controlled to eliminate cascading of grout and to avoid form pressures greater than those for which the forms were designed. For a particular application, the grout injection rate will depend on form configuration, aggregate grading, and grout fluidity.

h. Joint construction

A cold joint is formed in PA concrete when pumping is stopped for longer than the time it takes for the grout to harden. When delays in grouting occur, the insert pipes should be pulled just above the grout surface before the grout stiffens, and then rodded clear. When pumping is ready to resume, the pipes should be worked back to near contact with the hardened grout surface and then pumping resumed slowly for a few minutes. Construction joints are formed in a similar manner by stopping grout rise approximately 12 in. below the aggregate surface. Care must be taken to prevent dirt and debris from collecting on the aggregate surface or filtering down to the grout surface. If construction joints are made by bringing the grout to the surface of the coarse aggregate, the joint surfaces should be cleaned and prepared properly.

i. Grouting procedure

The two patterns for grout injection are the horizontal layer and the advancing slope. Regardless of the system used, grouting should start from the lowest point in the form.

(1) Horizontal layer 

In this method grout is injected through an insert pipe to raise the grout until it flows from the next insert hole 3 to 4 ft above the point of injection. Grout is then injected into the next horizontally adjacent hole, 4 or 5 ft away, and the procedure is repeated sequentially around the member until a layer of coarse aggregate is grouted. Successive layers of aggregate are grouted until all aggregate in the form has been grouted.

(2) Advancing slope

The horizontal layer method is not practical for construction of such slabs when the horizontal dimensions are large. In situations such as this, it becomes necessary to use an advancing slope method of injecting grout. In this method, intrusion is started at one end of the form and pumping continued until the grout emerges on the top of the aggregate for the full width of the form and assumes a slope which is advanced and maintained by pumping through successive rows of intrusion pipes until the entire mass is grouted. In advancing the slope, the pumping pattern is started first in the row of holes nearest the toe of the slope and continued row by row up the slope (opposite to the direction of advance of slope) to the last row of pipes where grouting has not been completed. This process is repeated, moving ahead one row of pipes at a time as intrusion is completed.

(3) Grout insert pipes and sounding devices

The number required and the location and arrangement of grout insert pipes will depend on the size and shape of the work being constructed. For most work, grout insert pipes will consist of pipes arranged vertically and at various inclinations to suit the configurations of the work. The guide specification provides for the option of the diameter of the grout insert pipes being either 3/4, 1, or 1-1/2 in. Generally, either a diameter of 3/4 or 1 in. would be allowed for structural concrete having a maximum size aggregate of 37.5 mm (1-1/2 in.) or less. If the preplaced aggregate has a maximum size larger than 37.5 mm (1-1/2 in.), the grout insert pipes should be 1-1/2 in. in diameter. Intrusion points should be spaced about 6 ft apart; however, spacing wider than 6 ft may be permissible under some circumstances, and spacings closer than 6 ft will be necessary in some situations. Normally, one sounding device should be provided for each four intrusion points; however, fewer sounding devices may be permissible under some circumstances. In any event, there should be enough sounding devices, and they should be arranged so that the level of the grout at all locations can be accurately determined at all times during construction. Accurate knowledge of the grout level is essential to:

(a) Check the rate of intrusion
(b) Avoid getting the grout too close to the level of the top of the aggregate when placement of the aggregate and intrusion are progressing simultaneously.
(c) Avoid damage to the work which would occur if a plugged intrusion line were washed out while the end of the line was within the grout zone.

Sounding devices usually consist of wells (slotted pipes) through which the level of the grout may be readily and accurately determined. If sounding devices other than wells are proposed, approval should be based on conclusive demonstration that such devices will readily and accurately indicate the level of the grout at all times. In repairing vertical surfaces, such as lock chamber walls or sloping surfaces which are substantial distances and are relatively thin (up to about 2 ft thick), the grout is brought up uniformly from the bottom. Intrusion points for such work should be arranged in horizontal rows with the rows spaced not more than 4 ft apart horizontally. Holes in adjacent horizontal rows should be staggered so that a hole in any row is at the midpoint of the space between holes in the adjacent rows above and below. Intrusion is controlled by pumping through all holes in each horizontal row until grout flows from all holes in the row above. Grouting then proceeds through the next row above after the holes below, which have just been grouted are plugged. The process is repeated until a section is completed. The bottom row of holes should be placed at the bottom of the form.

j. Finishing unformed surfaces

If a screeded or troweled finish is required, the grout should be brought up to flood the aggregate surface and any diluted grout should be removed. A thin layer of pea gravel or 3/8- to 1/2-inch crushed stone should then be worked into the surface by raking and tamping. After the surface has stiffened sufficiently, it may be finished as required. A finished surface may also be obtained on PA concrete by adding a bonded layer of conventional concrete of the prescribed thickness to the PA concrete surface. The PA concrete surface should be cleaned and grouted prior to receiving the topping.

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