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SPECIFIED DENSITY CONCRETE -A TRANSITION

Thomas A. Holm and John P. ~Expanded Shale, Clay & Slate Institute (ESCSI) -United States

SUMMARY

Applications of Specified Density Concrete (SDC) are increasing in the US, Canada and Europe.The use of SDC is driven by engineers' decisions to optimize concrete density to improve structuralefficiency (the strength to density ratio), reduce transportation cost, and to enhance the hydration ofhigh cementitious concrete mixtures with very low w/c ratios <.40. Specific projects includebridges, marine structures, precast elements and consumer products with strength ranging from 20-70 Mpa (2900-10150 psi) and densities from 1200 to 2200 kg/m3 (75 to 1381b/ft3). SDC is achievedby customizing the mixture proportion by replacing part of the ordinary Normal Density Aggregates(NDA) (>.2600 kg/m3, sa >2.60) with either coarse or fine Low Density Aggregates (LDA) (gener-ally <1600 kg/m3, sa <1.60).

Examples of optimizing the design by using SDC include more structurally efficient members inbridges and buildings, improved buoyancy in marine structures, and reduced transportation costs ofconsumer products like wallboard, imitation stone, precast element, masonry, etc. SDC is defined asconcrete with a range of density less than what is generally associated with Normal Density Con-crete (NDC) and greater than the lowest density possible when using all LDA. This paper will focuson the 1800-2200 kg/m3 (112-137lb/f3) density range.

The American Concrete Institute Standard Building Code (ACI 318) provides structural engineerswith adequate guidance when designing with structural LDC over the strength range of 20-35 Mpa(2900-5080 psi). ACI 318 precisely defines the differing engineering properties of NDC and LDCincluding reduced elastic modulus, reduced tensile shear and torsion capacities, increaseddevelopment length...etc. The increased use of SDC is creating an urgent need for comprehensive,industry wide investigations into the physical properties and engineering characteristics of concreteswith strength/density combinations outside of traditional ranges. Future code revisions shouldinclude a seamless transition of engineering criteria for concrete properties of all practical achievablestrengths with density ranges from 1200-2500 kg/m3 (75-1561b/ft3).

STRUCTURAL EFFICIENCY

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There is a paradigm shift taking place in the way engineers design and specify concrete characteris-tics for structural efficient projects. No longer content to use the suggested "off-the-shelf' concretemixtures routinely proposed for conventional applications, engineers now require "optimized"concrete performance to satisfy specific needs. These "custom tailored" concretes, often referred toas SDC, have been developed through the combined efforts of design professionals, material suppli-ers and concrete producers.

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planning stages. The hull of the floating platform, approximately 70,000 m3 (91,000 yd3) is con-structed entirely of HSLDC with a maximum density of 2000 kg/m3 (125 Ib/ft3 ). Heidron was builtin Norway and towed to the North Sea.

Hibemia Oil Platform -1998: Another significant application of SDC is in the Mobil Oil Hibemiaoffshore gravity based structure. To improve buoyancy of the largest floating structure built in NorthAmerica, LDA replaced approximately 50% of the NDA (coarse fraction) in the high strength con-crete (HSC) used. The resulting density was 2170 kg/m3 (135 lb/ft3)). Hibemia was built in New-foundland, Canada, where the structure was floated out of dry dock and towed to a near by deepwater harbor area where construction continued. When finished, the more than one-million tonstructure was towed to the Hibemia North Sea oil field site and set in place on the ocean floor. Acomprehensive testing program was reported by Hoff et al. [1].

Bridges: Numerous bridges in North America and Europe have utilized SDC. Examples are theShelby Creek Bridge, Kentucky, density 2080 kg/m3 (130 lb/ft3) [2] as well as numerous long spanprecast bulb tee bridge girders placed in Ohio and Indiana, 2000-2160 kg/m3 (125-135lb/ft3). Aseries of major long-span Norwegian prestressed box-girder bridges [3,4], also incorporatedHSLDC, for example, the entire bridge span (Bokriasundet, 1990), the central part of long mainspans (Raftsundet, 1998) and side spans that balanced NDC in main spans, (Sandhomia,1989). TheRaftsundet Bridge, located north of the Arctic Circle in Norway, is of box girder construction utiliz-ing HSLDC for 220M (721 ft) of the main span length of 298m (fe2'3 ft). At the time of construc-tion this bridge was the longest span bridge of this type in the world. q 7-c:

REDUCED TRANSPORTATION COST

The concept of specified density concrete is not new. Almost 20 years ago a precast manufacturerevaluated the trade-offs between the physical properties and the transportation costs. Mixes includeda typically used limestone "control" concrete paralleled by other mixtures in which 25, 50, 75 and100% of the ND limestone coarse aggregate was replaced by an equal absolute volume of an LDA.This resulted in 5,11,15 and 21 percent reductions in density respectively. Results of the testingprograms are shown in Table 1 and Fig. 2. Figure 3 demonstrates that for the particular LDA andlimestone tested, these replacement levels had little effect on early or 28 day compressive strengths.

Because of weight limits on roads, this precast producer developed the lower density SDC mixturesthat reduced the weights of members allowing an increased number of precast elements per truck.By adjusting the density of the concrete, precasters are able to maximize the number of concreteelements on a truck without exceeding highway load limits. This reduces the number of truck loadswhich lowers project cost. Opportunities for increased trucking efficiency are greater when trans-porting smaller concrete products ( e.g. hollow core plank, wallboard, precast steps, imitationstone...etc.

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Figure 3. Compressive Strength 1520 x 304 mm (6" x 12") Cylinderswith varying replacements of Limestone Coarse Aggregate with LDA

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Concretes containing LDA have lower modulus of elasticity at both early and later ages. Sinceexact modulus data at release (18 hrs. ::I::) is crucial to strand location, camber and deflection control,it is essential to determine the properties directly from the proposed concrete mixture. It is alsoimportant to realize that even with NDA's at the same density, the modulus of elasticity can varyconsiderably. Table 1 reveals that for the "control" limestone NDC, the tested elastic moduluscorrelated with the computed value using the ACI 318 formula Ec=33w1.5"'Fc. For LDC at earlierages and with compressive strengths over 35 MPa (5080 psi), the ACI formula clearly over estimatesthe value of the elastic modulus.

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In a series of papers addressing long term service performance of LDC, Holm and Bremner[6, 7, 8, 9] repeatedly cited the improved integrity of the LDAfmatrix interface, attributing the im-proved quality to internal curing, pozzolanic activity at the contact zone, and reduction in stressconcentrations resulting from elastic compatibility of the concrete phases.

It appears that Philleo [10] in 1991 was the first to recognize the potential benefits to high perfor-mance NDC possible with the addition of LDA containing high volumes of absorbed moisture.Weber and Reinhardt [11] have also conclusively demonstrated reduced sensitivity to poor curingconditions in HSNDC containing an adequate volume of high moisture content LDA.

The benefits of "internal curing" are increasingly important when pozzolans (silica fume, fly ash,metokaolin, calcined shales, clays and the fines of LDA) are included in the mixture. It is wellknown that the pozzolanic reaction of finely divided alumina-silicates with calcium hydroxideliberated as cement hydrates is contingent upon the availability of moisture. Additionally, "internalcuring" provided by absorbed water minimizes the "plastic" (early) shrinkage due to rapid drying ofconcretes exposed to unfavorable drying conditions.

The benefits of "internal curing" go far beyond the improvements in long-term strength gain. In theopinion of the authors, the principal contribution of "internal curing" rests in the reduction of perme-ability that develops from a significant extension in the time of curing. Powers [12] showed thatextending time of curing increased the volume of cementitious products formed which caused thecapillaries to become segmented and discontinuous.

One interesting demonstration of the practicality of providing internal curing with LDA was in theconstruction of secondary containment slab-on-grade. In this application, high performance, lowpermeability concrete containing high moisture content LDA and high volumes of silica fume wereused successfully. The containment slabs were under steel storage tanks that contained hazardouswaste fuels at production plants in the US. To lower concrete permeability to values acceptable tothe permitting authorities, high volume additions of silica fume were essential. To minimize sensi-tivity to curing for concretes placed in hot weather, special precautions were taken to provide LowDensity Coarse Aggregate (LDCA) with a relatively high degree of saturation. Despite high ambienttemperatures, concrete construction proceeded without incident. No visible surface cracking hasbeen observed after several years of exposure. This concrete provides an industrial grade barrier thatresists deterioration due to freezing and thawing, chemical attack, and ultraviolet rays that attacktroweled-on thin mortar alternatives. These secondary containment slabs can stand up to forklifttraffic, movement of pallets and heavy duty commercial exposure, and require minimum mainte-nance while meeting environmental containment standards.

Internal curing is typically provided by LDCA in high performance concrete applications. However,Low Density Fine Aggregate (LDFA) is more effective in distributing available moisture for internalcuring. As Bentz [13) has pointed out, a much more efficient spatial distribution could be accom-plished by partial replacement of the sand fraction with LDFA.

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RECOMMENDATIONS

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Recognizing the fact that concrete is heavy, and because construction generally involvestransportation, design professionals may consider SDC to improve structural efficiency andlower transportation costs.

To assure an adequate curing regime that improves internal integrity and performance,designers may consider the addition of LDA containing high moisture content to NDC toenhance hydration through internal curing.

There is now an urgent need for comprehensive, industry wide investigations into thephysical and engineering properties of structural concretes with strength/densitycombinations outside of traditional ranges.

Future code revisions should develop a seamless transition of criteria for engineeringproperties for concretes of all practical combinations of achievable strengths with densityranges from 1200-2500 kg/m3 (75-156lb/ft3)

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