Peter C. Taylor, Construction Technology Laboratories, Inc. and Shri Bhidé, Portland Cement Association
To assist specifiers in selecting important criteria for HPC in bridges, the Portland Cement Association, in conjunction with the concrete industry, has developed a Guide Specification for High Performance Concrete for Bridge Elements. This document provides mandatory language that the specifier can cut and paste into project specifications. It also includes guidance on the characteristics to be specified in a given case and the performance limits needed to ensure satisfactory performance for a given element or environment. In cases where two performance criteria are in conflict, the commentary advises the user how to balance these conflicting requirements. Using the guide, specifiers should be able to select all criteria necessary for their structures, and then, using the commentary, apply appropriate performance limits for each element.
Specifiers are often tempted to select the highest grade for every parameter with the intention of achieving “high performance concrete.” This practice is undesirable and, in some cases, produces mutually incompatible requirements and can lead to unnecessarily excessive costs. For instance, low permeability is normally achieved by using a high cementitious materials content and low water-cementitious materials ratio. This, however, will increase the modulus of elasticity and heat of hydration and thus increase the risk of thermally induced cracking. It is, therefore, not advisable to specify extremely low permeability for concrete in a massive element that is not exposed to an aggressive environment.
Criteria
Durability- and strength-related criteria that may need to be specified are as follows:
- Abrasion Resistance. For bridge decks and perhaps for piers exposed to waterborne abrasion.
- Chloride Ion Penetration. For bridge decks and other structural elements exposed to deicing salts or seawater.
- Compressive Strength. For structural requirements. Strength should not be used as a control parameter for other criteria unless a correlation has been established for the specific concrete mix.
- Creep and Modulus of Elasticity. May be necessary for structural elements, particularly long-span members.
- Freeze-Thaw Durability. For concrete exposed to freezing and thawing in saturated or near-saturated conditions.
- Scaling Resistance. For bridge decks, and possibly other elements, exposed to deicing salts.
- Drying Shrinkage. For control of deflection and shrinkage-related cracking.
- Sulfate Resistance. For foundations and substructures in areas where sulfates are present in the soil or groundwater.
- Consistency. To allow the contractor to select an appropriate value with limits on variability.
- Alkali-Silica Reactivity. In areas where aggregates are potentially reactive.
The guide specification covers submissions that should be considered part of the pre-construction verification program such as mix designs, certifications, material sample retention, and plans for temperature monitoring, curing, and crack control. It also addresses and defines quality management issues, assigning responsibility for quality control and quality assurance tasks, and spelling out particular steps to be taken at each stage of construction. The guide specification lists production-related issues that can increase the likelihood of acceptable performance in the finished concrete such as equipment quality, mixing procedures and timing, temperature limits, trial batches, site addition of water or chemicals, delivery tickets and records, and measurement methods and tolerances.
Bridge Deck Example
A bridge deck exposed to deicing salts needs to resist chloride ion penetration in order to delay the onset of chloride induced corrosion. Both freeze/thaw durability and scaling resistance are also necessary if the bridge is in a cold region. Depending on structural requirements, the concrete may need to have some minimum compressive strength; however, a strength that is too high with a correspondingly high modulus of elasticity will increase the tendency of the deck to crack. Cracking is detrimental to durability, particularly in an environment conducive to corrosion. In such a case, the specifier might elect to include only the minimum strength requirement. The concrete specification would then be as follows:
Abrasion Resistance. The coarse aggregate shall be tested according to AASHTO T 96. The loss shall not exceed 40 percent. For bridge decks or surface courses, aggregates known to polish shall not be used.
Chloride Ion Penetration. The concrete shall have a charge passed in six hours of 1500 coulombs or less when tested according to AASHTO T 277 at age 56 days. The specimens shall be moist-cured to age 7 days, after which they shall be stored at 73.4 ± 3°F and 50 ± 4 percent relative humidity until the time of test.
Compressive Strength. The concrete shall have a compressive strength of at least 4000 psi when tested according to AASHTO T 22 at age 28 days. The specimens shall be moist-cured to age 7 days, after which they shall be stored at 73.4 ± 3°F and 50 ± 4 percent relative humidity until the time of test. Either 4×8-in. or 6×12-in. cylinders may be used.
Freeze/Thaw Durability. The concrete shall have a durability factor of 90 percent or greater when tested according to AASHTO T 161, Procedure A.
Scaling Resistance. The concrete shall have a visual rating of 1 or less when tested in accordance with ASTM C 672, except that the concrete shall be moist-cured to an age of 28 days, after which it shall be stored in air for 14 days at 73.4 ± 3°F and 50 ± 4 percent relative humidity, before being exposed to deicing chemicals.
Further Information
The guide specification will be published by the Portland Cement Association later this year. For more information, the second author may be contacted at [email protected] or 847-972-9100.
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