High-performance concrete today: nothing routine: initially used in bridge decks, today's HPC applications are diverse—requiring demanding performance from materials and construction

Concrete Construction, June, 2002 by Susan C. McCraven

"All high-strength concrete is high-performance concrete, but not all high-performance concrete is high-strength concrete," says Henry G. Russell, consulting engineer and former chairman of the American Concrete Institute's high-performance concrete committee. High-performance concrete (HPC) is not one product but includes a range of materials with special properties beyond conventional concrete and routine construction methods.

Terence C. Holland, materials consultant and ACI president, says that HPC is an "umbrella term for many exacting specifications for concrete construction."

HPC "found its widest and earliest use in the bridge field," explains John A. Bickley, principal investigator with Concrete Canada. Consequently, many contractors and engineers do not have a good understanding of its broader applications. How can we get a handle on the demands and benefits of HPC today? This article looks at the materials, properties, and construction challenges of high-performance concrete in three successful projects.

High-performance concrete is relatively new technology. HPC began in France in 1980, followed by Canada in 1990. In 1989, under the direction of Paul Zia of North Carolina State University, a major effort in HPC technology began in the United States with the initiation of the Strategic Highway Research Program (SHRP). SHRP defined HPC in terms of strength, low w/cm, and freeze/thaw durability. These early efforts were in response to alarming deterioration rates of the nation's roads and bridges. According to Terry D. Halkyard of the Federal Highway Administration (FHWA), "To increase the durability of bridge decks, the FHWA encouraged state DOTs to develop new HPC mixes to reduce the permeability of concrete to the penetration of chlorides from de-icing salts."

HPC, according to ACI's committee on the subject, is concrete that meets a combination of special performance and uniformity requirements that cannot be routinely achieved with conventional materials and practice. These include:

* Ease of placement and compaction without segregation

* High early strength

* Impermeability and high density

* Durability (based on exposure) and toughness

* Long service life ([greater than or equal to] 75 years)

* Low heat of hydration

* Volume stability (minimal shrinkage or thermal expansion)

* Flowability and self-leveling capability

Characteristics of HPC mixtures

* Durability and extended service life--Above all, "HPC means structures of enhanced durability and service life," says Harold R. Sandberg, Alfred Benesch & Co., Chicago, and a member of ACI's HPC committee. Project specifications for HPC can require a service life of up to 100 years.

* Compressive strength and modulus of elasticity--While columns constructed of HPC may have concrete strengths of up to 15,000 psi, there is a growing awareness that specifications requiring high compressive strength make sense only when there are specific strength design advantages. Design has also moved away from an industry preoccupation with strength due to the need to improve constructibility of HPC containing silica fume. Zia reports that, "Often a higher modulus of elasticity, not compressive strength, is the controlling requirement in HPC construction." This was the challenge for the Cleveland Society Towers' contractor.

* Flash set and temperature--Early setting problems in HPC mixtures can often be solved by using retarding admixtures. Because HPC mixes have low w/cm values, the concrete placement temperature can be limited to 65[degrees]F in some projects.

* Fast tracking--State DOTs use paving concrete that reaches 4000 psi in 4 hours after placement to minimize traffic disruptions.

Material properties and cost

* Low w/cm--A maximum water-cementitious materials ratio of 0.40 or lower is specified on most HPC projects. High cementitious material content is typical in HPC mixtures, but that alone may not be enough to increase modulus of elasticity values.

* Admixtures--Obtaining finishability with mixes containing silica fume often requires superplasticizers. Typically, high-range water-reducing admixtures (HRWRA) are used. Concrete for bridge decks typically include water-reducing admixtures (WRA).

* Cementitious materials--Castin-place HPC today employs blended cements that include silica fume, fly ash, and ground granulated blast-furnace slag (GGBF slag or slag cement). These cementitious materials can exceed 25% of the total cement by weight. Typical HPC today can include 5% to 15% silica fume, 50% to 65% slag cement (as much as 80% in mass concrete), and up to 50% fly ash. Silica fume contributes to strength and durability; fly ash and slag cement result in better finishability, decreased permeability, and increased resistance to chemical attack. According to Jan R. Prusinski of the Slag Cement Association, "HPC mixtures are often proportioned to achieve low permeability. Lower concrete permeability provides corrosion resistance for reinforcing steel by reducing the rate of chloride ion migration into the concrete." More importantly for the contractor, Prusinski adds, "slag cement improves the workability, placeability, and consolidation of concrete, resulting in better finishing."