Pre-packed Concrete

Pre-packed Concrete

Pre-packed aggregate concrete (PAC) is concrete that is made by forcing grout into the voids of a mass of clean ,graded coarse aggregate densely prepacked in formwork .PAC is used where placing conventional concrete is extremely difficult ,such as where massive reinforcing steel and embedded items are present ,in water repair ,concrete and masonry repair ,or where shrinkage of concrete must be kept to a minimum.Graded aggregates similar to the substrate concrete will be placed firstly in the formwork. After the placement of graded aggregates, cement grout will be injected into the formwork from THE BOTTOM. The resulting concrete will be in excellent quality in proper implication.For the purpose of this repair method,grout typically consist of sand,cement ,pozzolana ,plasticizer/super plasticizer and an air entraining agent (for anticipated freeze and thaw problem,if required). The pozzolana and the plasticizer /super plasticizer are used to impact flowability to the grout. The coarse aggregate is washed to remove all fines and screened just prior to placement .grout is then injected through forms to provide the cementing matrix.grouting is begum at the bottom of the pre-placed aggregate. For the purpose of this repair method,grout typically consist of sand ,cement ,pozzolana ,plasticizer/super plasticizer and an air entraining agent (for anticipated freeze and thaw problem,if required). The pozzolana andthe plasticizer /super plasticizer are used to impact flowability to the grout. The coarse aggregate is washed to remove all fines and screened just prior to placement .grout is then injected through forms to provide the cementing matrix.grouting is begum at the bottom of the pre-placed aggregate.

Characteristics of the grout are affected by the water content,sand grading,cement, pozzolana and the types and amount of admixtures .For each designof grout mixture, there are optimum amounts of fillers and admixtures to producethe best pumpability or consistency .Proper proportioning for the structural groutmix components is necessary to get the required strength and durability of thefinished pre-placed aggregate concrete .trail mix design is necessary for each job.In underwater repair ,injection of grout at the bottom of the PACdisplaces water,leaving a homogeneous mass of concrete with minimum of paste wash out. In such application ,addition of anti wash admixtures minimizesthe paste wash out .for underwater PAC , the quality of underwater should alsobe tested to determine its influence onPAC over a period of time for takingappropriate corrective action.

Materials :

Grout for preplaced aggregateconcrete may be mixed with sand either of the gradation specified for conventional concrete orwith fine sand, pozzolanicor fly ash fillers,water reducing admixtures, and pumpingadmixtures as dictatedby the minimum size of the coarse aggregate. With 1-1/2-inch minimumsizecoarse aggregate, the sand gradation is thatspecified for conventional concrete.Theportland cement, water, and sand are mixed using high speed centrifugalgrout mixers thatproduce well mixed grouts of a creamy consistency. For usewith 1/2-inch minimumsize coarse aggregate, a grout mixture is preparedcontaining fine sand passing a No. 8screen 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.

admixture improves the flowability of the groutand the ultimate strength. Proprietary pumping admixtures are commonly used toincrease the penetration and pumpability of the final grout.The consistency of grout for preplaced aggregated should be uniform from batchto batch and should be such that it can be readily pumped into the voids atrelatively low pressure. Consistency is affected by water content, sandgrading, 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 optimumproportions. The maximum size coarse aggregate used with both types of groutis the largest available, provided that the aggregate can be easily handledand placed. Coarse aggregate should meet all the requirements of coarseaggregate for conventional concrete. It is essential that thecoarse aggregated be clean. The aggregate should be well graded fromminimum size (1/2- inch minimum or 1-1/2-inch minimum) up to the maximumsize, and when compacted into the forms, should have a void content of 35 to 40percent. If grout containing sand of concrete grading is used, the minimumcoarse aggregate size should be 1-1/2 inches.

Considerations for practical use

All loose and spalled concrete should be repaired prior to the application.

•Target compressive strength of the pre-packed concrete should be compatiblewith that of the substrate.

•Trials on compressive strength before operation are recommended.

•Careful design on aggregate size and grading is recommended.

•Aggregate size shouldn’t be too fine in order to avoid blockage of grout patch.

•Similar to micro-concrete, formwork with leakproof is required.

•Pre-soaking of the substrate with water is necessary.

•Aggregate should be clean and free from silt before application