Paul D. Tennis, Portland Cement Association
Although all materials play a role in high performance concrete (HPC) mixtures, cement is the essential component. The characteristics of HPC may include low permeability, high strength, low temperature rise, high durability, and combinations of these or other properties. Cement is the material largely responsible for these critical properties. It is important to note that most cements perform well in HPC applications. However, some cements have been developed with particular characteristics that lend themselves to use in HPC. Table 1 provides a list of the cement types described in the AASHTO and ASTM specifications classified according to their intended applications.
General Purpose Cements
General purpose cements are used where no special properties of the other cement types are necessary. HPC can be made with these “general purpose” cements if the concrete mixture is carefully chosen to provide the specified concrete properties. For example, high strength concrete can be made with Type I cement if low water to cement ratios and high cement contents are used. One benefit of using these cements is that they are usually locally available, whereas, some specialized cements may not be.
Low Heat Cements
All cements generate heat from the chemical reaction between cement and water. In massive concrete placements, this can be a concern. The rate that cements generate heat and the corresponding temperature rise in concrete can be controlled somewhat by the cement fineness, chemistry, and choice of supplementary cementitious materials (SCMs). Cement Types IV, P(LH), and LH have low heat of hydration requirements. However, these cements are rarely produced because there is not a large demand for them.
Cement types developed to provide moderate heat generation are more readily available. For Type II cements and cements under AASHTO M 240 (ASTM C 595), a moderate heat (MH) option must be requested. Under ASTM C 1157, Type MH can be specified. SCMs can be used in the concrete mixture to reduce the temperature rise in HPC concretes; but the rate of strength development may be slower.
High Early Strength Cements
Cement Types III and HE are used when high early strength is required. Later age strengths may be lower than with other cements, but the tradeoff is worthwhile in some applications. These cements are often used in fast-track and repair applications where concrete strengths of about 4000 psi (28 MPa) are needed in just a few hours.
Sulfate-Resistant Cements
If concrete is placed in an environment where exposure to sulfate is a concern, a water-cementitious materials (w/cm) ratio of about 0.40 is essential to minimize permeability. If w/cm ratios up to 0.50 must be used, cements with moderate or high sulfate resistance are available. Type II, C 1157 Type MS, and M 240 (C 595) cements with (MS) suffix are moderately sulfate resistant. Types V and HS are highly sulfate resistant. These designations are applied based on expansion in standardized tests. Sulfate resistant cements will not control sulfate attack in concretes with high w/cm ratios.
ASR-Resistant Cements
When aggregates that are susceptible to alkali-silica reaction (ASR) must be used in the concrete, cements resistant to ASR may help durability. All three cement specifications have optional requirements for ASR resistance. AASHTO M 85 (ASTM C 150) has a low alkali option. AASHTO M 240 (ASTM C 595) and ASTM C 1157 use expansion in a mortar bar test to predict performance. Testing to confirm that the job material combinations are sufficient to control deleterious expansions should also be performed as a check, if sufficient history on the use of specific mixtures is not available.
Conclusions
Although HPC may be obtained with the proper use of SCMs, chemical admixtures, w/cm ratios, and aggregate gradations, non-general purpose cements may enhance performance. Laboratory or field testing should always be performed to confirm that the chosen concrete mixture meets project requirements.
Further Information
More information on cements for HPC can be found in the following publication:
Portland, Blended and Other Hydraulic Cements, IS004, Portland Cement Association, Skokie, Illinois, 2001.
Editor’s Note
This article is the third in a series that addresses the benefits of specific materials used in HPC. The benefits of silica fume and lightweight aggregate were discussed in previous issues of HPC Bridge Views.