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122 ConcreteProducts March 1999

technical talkby Henry G. Russell, P.E.

Why use high-performance concrete?

For many years, high-strength, high-per-formance concrete has been used in thecolumns of high-rise buildings. However, in

recent years, there has been increased use ofhigh-performance concrete (HPC) in bridges

where both strength and durability are im-portant considerations. The primary reasonsfor selecting HPC are to produce a more eco-nomical product, provide a feasible technicalsolution, or a combination of both.

At the present time, a cubic yard of HPCgenerally costs more than a cubic yard of con-ventional concrete. HPC requires addition-al quantities of materials such as cement, flyash, silica fume, high-range water-reducersand retarders to ensure that the concretemeets the specified performance. However,concrete is only one component in con-struction, and the total cost of the finishedproduct is more important than the cost ofan individual material. On the other hand,HPC should not be specified if there are noeconomical or technical advantages to begained from its use. Here, TECHNICAL TALKex-plains why HPC is used in buildings andbridges.

Buildings

The economic advantages of using high-strength, high-performance concrete in thecolumns of high-rise buildings have beenknown for many years. In simple terms, high-strength concrete provides the most eco-nomical way to carry a vertical load to thebuilding foundation. The three major com-ponents contributing to the cost of a columnare concrete, steel reinforcement and form-

work. By utilizing high-strength concrete, thecolumn size is reduced. Consequently, lessconcrete and less formwork are needed. Atthe same time, the amount of vertical rein-forcement can be reduced to the minimumamount allowed by the code. The net resultis that the least expensive column is achieved

with the small est size column, the lea stamount of reinforcement and the highestreadily available concrete strength.

According to a study by Moreno (1), theuse of 6,000 psi (41 MPa) compressivestrength concrete in the lower columns of a23-story commercial building requires a 34-in. (865-mm) square column at a cost of$0.92/ft2 ($9.90/m2). The use of 12,000 psi

(83 MPa) concrete allows a reduction in col-umn size to 24-in. (610 mm) square at a costof $0.52/ft2 ($5.60/m2). In addition to the

reduction in initial cost, a smaller columnsize results in less intrusion in the lower sto-ries of commercial space and, thereby, morerentable floor space. Yet the use of high-strength concrete in columns has not beenlimited to tall buildings: Parking garageshave also used the material to reduce col-umn sizes. Since columns intrude into thelayout for parking spaces, a small column isadvantageous.

In addition to specifying concrete com-pressive strength, modulus of elasticity hasbeen specified for the concrete in severalhigh-rise buildings. The most notable build-ing is Two Union Square in Seattle where amodulus of elasticity of 7.2 million psi (50GPa) was required in addition to a com-pressive strength of 14,000 psi (97 MPa). Toachieve this modulus of elasticity, a com-pressive strength of 19,000 psi (131 MPa)

was required. A higher modulus of elastici-ty provides a stiffer structure which has lesslateral deflection under wind loads.

Bridges

In 1993, the Federal Highway Administra-tion (FHWA) initiated a national programto implement the greater use of HPC inbridges. Applications include bridge decks,girders, piers and abutments. Nine bridgeshad been completed under the nationalprogram by the end of 1998. In addition, anumber of other states are using HPC undertheir own programs. The use of high-strength concrete in prestressed concretegirders allows for longer span lengths (Fig-ure 1), wider girder spacings or shallowersections (Figure 2).

The use of concrete with a specified com-pressive strength of 14,700 psi (101 MPa) at56 days permitted the use of AASHTO Type

IV girders for a span of 157 ft. (47.9 m) onthe North Concho River, U.S. 87, and SouthOrient Railroad Overpass in San Angelo,Texas. A simple span length of 157 ft. is im-possible to achieve with normal strengthconcretes and a 54-in. (1,372-mm) deep gird-er. On multi-span bridges, the use of longergirders results in fewer spans and fewer sub-structures.

Henry Russell is an engineering consultant based in Glenview, Ill. He is a member of American Concrete Institute, American Societyfor Testing and Materials, Precast/Prestressed Concrete Institute and American Segmental Bridge Institute. He is currently thechairman of ACIs subcommittee on High-Performance Concrete.

With column mixes carrying design

strengths in excess of 10,000 psi,

Chicagos 225 West Wacker Drive

was among a wave of late 1980s-early 1990s buildings to incorporate

high-strength concrete.

Favorable experience with high-

strength concrete prompted

Washington DOT engineers tospecify 7,000 psi compressive

strength mixes for the cable-stay

bearing piers of Tacomas Theo FossWaterway Bridge.

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121March 1999 ConcreteProducts

technical talk

High-performance concrete was used on a bridge onS.R. 516 near Auburn, Wash. The use of concrete with aspecified strength of 10,000 psi (69 MPa) for the prestressedconcrete girders resulted in five lines of girders comparedto seven lines that would have been required with normalstrength concrete. In addition to strength, the concrete

was required to have a rapid chloride permeability not ex-ceeding 2,000 coulombs and a freeze-thaw resistancegreater than 80 percent.

In Colorado, an HPC bridge was used to replace a pre-vious structure that carried Interstate 25 over Yale Avenuein Denver. The previous structure consisted of a four-span,cast-in-place T-girder bridge with piers located in the me-dian of Yale Avenue and at each side of th e roadway. TheHPC bridge used 10,000 psi (69 MPa) concrete and con-sisted of two spans in place of the original four spans. Theuse of HPC, in combination with adjacent box beams, metthe requirements for longer spans while maintaining ashallow superstructure depth.

High-performance concrete is alsobeing used in bridge decks wheredurability is far more important thancompressive strength. Consequent-ly, performance requirements otherthan strength are being specified. For

durability, performance can be mea-sured using freeze-thaw resistance,deicer scaling resistance, abrasion re-sistance or chloride permeability. Atthe present time, most states arespecifying a limit on chloride per-meability for their HPC decks.

The goal is to specify quantifiableperformance to match the intendedapplication. However, selecting dura-bility performance requirements canbe difficult, and specifications areusually a combination of prescriptiveand performance requirements.

In Virginia, moist curing of HPCbridge decks is required for a mini-mum of seven days and until 70 per-cent of the specified 28-day strengthis obtained. Protection by fogging toprevent rapid drying of the concretesurface prior to application of wetburlap and plastic sheeting is required.

Aft er moist curing , a cur ing com -pound is applied to the deck surface.Most states with HPC bridge decks re-quire at least seven days of wet curingfor the concrete. At least one state re-quires 14 days of wet curing.

Unlike high-strength concrete inbuildings, the use of high-perfor-mance concrete in bridges is moredifficult to justify based on initialcosts only. The economic advantageof HPC varies depending of the pre-mium cost for the HPC product. In

many of the HPC bridges built to date, the premium costsfor the HPC have not been entirely offset by savings in ma-terials. However, as more contractors and producers devel-op experience with HPC, it is anticipated that the premium

will decrease and initial costs will compare more closely withcosts for conventional concrete bridges.

With bridges, there are additional costs associated withmaintenance and repair. The use of HPC with its greater dura-bility is likely to result in less maintenance and longer life. Withthe introduction of life-cycle costing, the long-term econom-ic benefits are likely to more than offset the premium costs forinitial construction. Further information about the use of HPCin bridges is provided at the FHWAs web site: http://hpc.fhwa.dot.gov.

Figure 1 Longer Span Lengths with High-Strength Concrete

(72, 63, 54-ft. girder depths)

Figure 2 Girder Depths for 100-ft. Span

References

1. Moreno, J., High-Performance Concrete: EconomicConsiderations, Concrete International, Vol. 20, No. 3,Mar