What is High Performance Concrete?
There are several definitions for high performance concrete. Most of these definitions describe concrete with enhanced properties not found in conventional concrete. Some of these properties include higher compressive and flexural strength, lower permeability, lower heat generation, greater resistance to alkali silica reaction and greater resistance to sulfate attack. Slag cement has been used effectively in high performance concrete in North America for over 20 years and is specified by many highway departments in their high performance concrete mixtures.
High Compressive Strength
Slag cement increases the strength of conventional concrete and is often a vital component in producing high strength concrete. The higher the percentage of slag cement used (up to 50 percent) in portland cement concrete, the higher the twenty-eight day strengths will be. Many projects have achieved consistent field concrete strengths in excess of 8,000 psi. Additionally, engineers have specified ternary or even quaternary mixtures con- taining slag cement, portland cement and pozzolans in projects requiring strengths greater than 12,000 psi, such as Reliant Stadium in Houston and Key Tower in Cleveland.
High Flexural Strength
Concrete flexural strengths in excess of 1,000 psi are common with mixes containing slag cement. Flexural concrete strength is increased due to a stronger bond between the cement paste and aggregate particles. This improved bond results from enhanced paste characteris- tics that also increase compressive strengths. Lambert Field in St. Louis is an example of a project that benefits from the high flexural strengths achievable with slag cement.
When slag cement is used as part of the cementitious material in a concrete mix- ture, it reacts with water and calcium hydroxide (Ca(OH)2) to form additional calcium silicate hydrate (CSH). CSH is the glue that provides strength and holds concrete together. The additional CSH produced modifies the pore structure of the paste resulting in lower permeability. The level of improvement is proportional to the percentage of slag cement in the mixture, normally between 25 and 65 percent. Lower permeability reduces chloride ion ingress and thus reduces the corrosion potential of the structure. Projects such as Wacker Drive in Chicago and Charenton Canal Bridge in Louisiana used slag cement to help reduce chloride ion penetra- bility to less than 2,000 couloumbs (classified as "low"), by the Rapid Chloride Permeability Test (ASTM C1202).
Low Heat for Mass Concrete
One of the most difficult challenges in designing mass concrete structures is limiting the concrete temperature differential between the center and the surface of the concrete. If this differential becomes too large, thermal cracks can develop in the concrete. Slag cement has been used successfully to
substantially reduce the temperature of mass concrete. When used at high replacement rates, slag cement will provide lower heat in mass concrete than concrete produced with low heat cement. Examples include 75 percent slag cement replacement in the I-895 Pocahontas Parkway footings in Richmond, VA and 70 percent slag cement replacement in the Creve Coeur Lake Memorial Bridge in St. Louis.
Alkali silica reaction (ASR) occurs when the alkalis in portland cement react with
certain reactive aggregates and water to form an expansive gel that causes concrete
to prematurely deteriorate. Slag cement mitigates ASR by combining with the
alkalis in portland cement and making them unavailable for the ASR reaction. It
also lowers the permeability of the concrete, limiting the amount of water that is
available to support the reaction. In some cases, it will lower the total alkali content
of the cement paste. Examples of mitigation for reactive aggregates using slag
cement include concrete pavements in El Paso, Texas and the Selfridge Air Force
Base airfield pavement near Detroit, Michigan.
Mitigation of Sulfate Attack
Sulfate attack occurs in hardened concrete when sulfates, found in seawater, in some
soils and in wastewater, react with the tricalcium aluminate (C3A) in portland cement
paste. The reaction causes a material called ettringite to form. Ettringite formation can
result in concrete expansion and premature deterioration. The higher the C3A of the
cement, the greater the potential for sulfate attack. Slag cement does not contain C3A
so the higher the percentage of slag cement used, the lower the C3A of the mixture and
the lower the potential for deleterious expansion. Additionally, slag cement
reduces the permeability of the concrete and limits the ability of sulfates to penetrate into
the concrete. Examples of mitigating sulfate attack with slag cement include William
Preston Memorial Bridge in Maryland and the Hap Cremeans water treatment plant in
Proportioning High Performance Concrete with Slag Cement
Engineers and concrete producers who have successfully incorporated slag cement in their HPC mixtures have used slag cement in ranges from 20 to 80 percent of cementitious material, depending on the application and desired results.