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Roller Compacted Concrete

P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials


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Concept and significance

The development of Roller Compacted Concrete

(RCC) caused a major shift in the constructionpractice of mass concrete dams and locks. Thetraditional method of placing, compacting, andconsolidating mass concrete is at best a slow

process. Improvements in earth-movingequipment made the construction of earth androck-filled dams speedier and, therefore, morecost-effective.

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Early Developments

The first successful application of RCC technology

was demonstrated in 1974. The repair of thecollapsed intake tunnel of Tarbela Dam provedthat the material had more than adequatestrength and durability. The maximum placementof 18,000 m3 of RCC in one day, which is still theworlds record, was a clear evidence of thepotential of this new construction method.

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Definition

ACI 207.5R-89 defines roller compacted

concrete (RCC) as concrete compacted by rollercompaction. The concrete mixture in itsunhardened state must support a roller whilebeing compacted.

Thus RCC differs from conventional concreteprincipally in its consistency requirement. Foreffective consolidation, the concrete mixture mustbe dry enough to prevent sinking of the vibratory

roller equipment but wet enough to permitadequate distribution of the binder mortar inconcrete during the mixing and vibratorycompaction operations

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Advantages

By 1997, 150 projects using RCC, including 46

new dams, were completed in the UnitedStates.

The U.S. Army Corps of Engineers list thefollowing advantages of using RCC:

Costs: Depending on the complexity of thestructure, RCC costs 25 to 50% less thanconventional concrete.

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Advantages

Rapid Construction: For large projects, RCC

dams can be finished 1 to 2 years earliercompared to regular mass concrete dams.

Spillways: Compared to embankment damswhich normally require that spillways beconstructed in an abutment, RCC dams offer theattractive and cost-effective alternative ofconstructing the spillway in the main structure

of the dam.

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Japanese Experience

Cement consumption is lower because muchleaner concrete mixtures can be used.

Formwork costs are lower because of the layerplacement method.

Pipe cooling is unnecessary because of the low

temperature rise. Cost of transporting, placement, and

compaction of concrete is lower, becauseconcrete can be hauled by end dump trucks;spread by bulldozers and compacted byvibratory rollers.

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Sequence of placement

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Compaction

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Materials -- Cement

The consolidation by a roller does not require

special cements; however, when RCC is to beused in mass concrete, the recommendation ofselecting cements with lower heat generationshould be followed.

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Mineral Admixtures

Mineral admixtures are used extensively in RCC

mixtures. The use of large amounts of mineraladmixtures reduces both the adiabatictemperature rise of concrete and costs, andimproves durability. In the United States, Class F

fly ash is the most common mineral admixtureused in dams, however, in other parts of theworld Class C fly ash , slag , and natural pozzolanhave also been used.

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Chemical Admixtures

Air-entraining and water-reducing admixtures are

used in RCC compositions that contains highervolume of paste.

Set-retarding admixtures can extend the time upto which the concrete lift should remainunhardened, reducing the risk of cold joints withthe subsequence lift. In RCC mixtures of dryconsistency, however, chemical admixtures show

rather a limited effectiveness.

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Aggregates

Aggregates greater than 76 mm in diameter (3in.) are seldom used in RCC because they can

cause problems in spreading and compacting thelayer.

The size of coarse aggregate has a significantinfluence on the degree of compaction in small

layers. This influence is less marked in relativelythicker layers specially when large vibratoryrollers are employed.

The use of material finer than 75 mm (No. 200

mesh sieve) produces a more cohesive mixture byreducing the volume of voids.

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Concrete Mixture Proportioning

Method I

Uses the principles of soil compaction to producea lean RCC, where the optimum water content of

the concrete is the one that produces themaximum dry density of the mixture.

This method does not utilizes the conventionalconcept of minimizing the water-to-cement ratioto maximize the concrete strength; the bestcompaction gives the best strength, and the bestcompaction occurs at the most wet mix that willsupport the operating vibrating roller.

The overriding criteria for these mixtures are thecompressive and shear strength since the damusing this type of concrete typically will have animpermeable upstream face made either bytraditional mass concrete or precast panels.

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Concrete Mixture Proportioning

Method II

Uses traditional concrete technology methods to

produce high-paste RCC mixtures. UpperStillwater and Elk Creek Dams are examples ofdams that were built using this approach. Theoverriding criteria for these mixtures are the shear

strength between the lifts and low permeability ofconcrete since no protective, impermeable face isused upstream .

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Laboratory Testing

RCC is a zero-slump concrete whose properties

are strongly dependent on the mixtureproportions and on the quality of compaction.Concrete is consolidated in the field usingvibrating rollers.

Despite extensive research on this subject, thereis as yet no unanimously accepted methodologyto simulate the field condition in preparing

laboratory samples.

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Strength

For RCC mixtures made according to the concretetechnology approach, where the volume of the

paste exceeds the volume of the voids betweenthe aggregate, the compressive strength followsthe dependence on the water-to-cement ratio aspredicted by Abrams rule.

For RCC mixtures made according to the soilmechanics approach, where the cement pastemay not fill the voids between the aggregate,Abrams rule does not apply, and strength is oftenplotted as a function of the moisture content.

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Elastic Modulus and Poissons ratio

The thermal stresses generated by heat ofhydration are proportional to the elastic modulusof concrete. Therefore, lean RCC mixtures, whichproduce concrete with low elastic modulus, areattractive to designers.

As with regular concrete, the elastic modulus ofRCC depends on the degree of hydration, volumeand type of aggregate, and water-to-cementratio.

Poissons ratio for CCR typically ranges from 0.15to 0.20.

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Creep

The long-term deformation of RCC depends onthe amount and the type of aggregate, the water-to-cement ratio, the age of loading, and theduration of loading.

RCC with lower compressive strength and lower

elastic modulus will normally show high creepwhich is a critical factor in determining the stressrelaxation when thermal strain is restrained.

Lean concrete with large amounts of fines also

shows high creep.

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Durability

The coefficient of permeability of RCC is a criticalparameter for long-term performance of dams,particularly if no impermeable membrane has

been used at the upstream face of the dam. The construction process of RCC generates

porous zones between the lifts where water canpercolate. Depending on the mixture proportions

and construction process, the coefficient ofpermeability can very over 8 orders of magnitude.

For instance, the lean concrete at Willow Creekdam had a coefficient of permeability of 2 x 10-4

m/s, while the coefficient of permeability at UpperStillwater Dam was 4 x 10-12 m/s. Willow CreekDam, however, has an impermeable membrane atits upstream face.

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Durability

If the moisture content in concrete goes beyond

the critical saturation point, the performance ofnon-air entrained RCC to cycles of freezing andthawing will be poor; however, if the structuredoes not become saturated, the frost resistance

of RCC is satisfactory. Air-entrainment of very lean RCC mixtures has not

been very successful.

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Construction Practice

The overall planning of a RCC dam is conceptuallydifferent from a gravity dam. To minimizethermal stresses, traditional mass concrete is builtin separate, monolith blocks.

This process is slow but allows great flexibility; if

a problem develops in one of the blocks, theconstruction front moves to another block. RCCdams do not have such luxury.

The operation is continuous, building onehorizontal lift at a time.

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There are no special requirements for batchingand mixing of RCC which can be produced using

the same equipment as for conventional massconcrete.

Ready-mixed concrete trucks cannot be used totransport RCC because the zero-slump concrete is

too dry and cannot be discharged. To obtain significant economical benefits, special

care must be taken in the selection of equipmentand construction methods for fast placement and

consolidation of RCC. Conveyor systems can be an efficient method of

transporting RCC.

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The success of a RCC dam is often contingent onthe correct selection of lift thickness, whichdepends on the mixture proportions and on theequipment available.

If the lift is too thin, the placement rates will besmall, thereby reducing the advantages of usingRCC.

If the lift is too thick, the compaction will not beadequate, creating horizontal layers of higherporosity, thereby compromising the strength anddurability of the structure.

Normally, the thickness of the lifts ranges from0.15 to 0.90 m; in the U.S. a lift thickness of 0.3m is often used.

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Compaction of the lift is achieved by using avibrating steel-wheel roller.

Compaction of the lift should be performed assoon as possible, typically within 10 minutes afterspreading and no more than 40 minutes after

mixing. Once adequate compaction is achieved, good

curing conditions for the finished surface areessential; the surface should be kept in a

moistened condition until the next lift is placed.

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The dry consistency of RCC results in difficulty in

bonding fresh concrete to hardened concrete. This bond can be improved between the lifts by

reducing the time of casting the lifts or by increasingthe paste content in the mixture.

Typically, bedding mixtures contain 360 to 460kg/m3 of cement, 170 to 220 kg/m3 of fly ash, and4.75-mm maximum size aggregate.

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