Reid W. Castrodale and Kenneth S. Harmon, Carolina Stalite Company
High strength lightweight concrete (HSLWC) is gaining wider use for bridges, especially in pretensioned girders. HSLWC offers the structural advantage of longer spans, reduced cost of girder transportation, and reduced cost of the entire structure because the superstructure weighs less. An example is the bridge shown in the photo, which carries Route 33 over the Mattaponi River at West Point, VA. HSLWC was used for the girders to achieve 240-ft (73.2-m) long spans.
HSLWC is produced in the United States using lightweight aggregates (LWA) manufactured by processing shale, clay, or slate at high temperatures. This article introduces the topic of specifying HSLWC for bridges; a key issue that must be addressed if the material is to be used successfully.
The main characteristics that must be addressed in a specification for HSLWC are the lightweight aggregate properties, aggregate absorption, concrete compressive strength, concrete density, air content, and resistance to freezing and thawing.(1) Each of these is discussed below. In some situations, other properties such as modulus of elasticity, creep, and shrinkage may be specified, especially for more complex structures where deflections or time-dependent effects are significant. To avoid unnecessary complication and expense, only properties that are essential to accomplish the project should be specified.
Aggregate Properties
Structural lightweight aggregates are just another type of aggregate that, when used in concrete, result in lower density concretes. Therefore, lightweight aggregate and concrete must meet the same structural and durability requirements that are imposed on normal weight concrete. Lightweight aggregate must satisfy the minimum requirements given in AASHTO M 195 Standard Specification for Lightweight Aggregates for Structural Concrete.(2) This specification stipulates several characteristics and properties for LWA and LWC, including grading and bulk density requirements and minimum average tensile splitting and compressive strength requirements. A maximum limit on drying shrinkage of concrete is specified, but it is only intended as an acceptance test for the lightweight aggregate.
Other aggregate properties may also be specified, such as resistance to freezing and thawing of aggregate, abrasion resistance, soundness, reactivity, and silica content. Some owners also specify the type of raw material from which the lightweight aggregate is manufactured.
Aggregate Absorption
The absorption of lightweight aggregates is greater than the absorption of normal weight aggregates. The moisture content of lightweight aggregate prior to batching must be carefully monitored especially when the concrete is to be pumped. Therefore, requirements to ensure proper moisture conditioning of the LWA prior to batching and/or limits on absorption should be specified. It is suggested that the specifications require the concrete supplier to submit a quality control plan for the production of LWC so that issues related to moisture control can be properly addressed.
Concrete Compressive Strength
HSLWC with design compressive strengths up to 10,000 psi (69 MPa) and an equilibrium density of 120 lb/ft3 (1920 kg/m3) has been successfully used.(3) The minimum specified compressive strength must be compatible with the density specified. As with any type of aggregate, the compressive strength of lightweight concrete may be limited by the strength of the aggregate. Designers should consult concrete suppliers to confirm that the desired combination of concrete strength and density are achievable.
Concrete Density
In most cases, lightweight concrete is used in a structure to reduce the dead load. Therefore, the density must be specified. Contract documents should specify the “equilibrium density,” which is defined in ASTM C567(4) as the density after exposure to a relative humidity of 50% and a temperature of 73°F (23°C) for sufficient time to reach a constant mass. It generally takes at least 90 days to obtain equilibrium density.
The density of fresh LWC is measured at the point of placement and used for acceptance. Designers may specify the fresh density. Alternatively, they may require the concrete supplier to provide a fresh density corresponding to the specified equilibrium density. A concrete or lightweight aggregate supplier can assist the designer in selecting an appropriate pair of consistent values. It is recommended that maximum densities be specified rather than densities with a tolerance.
If the weight of a member for handling, transportation, or erection is a concern, the fresh density should be used to compute the member weight because the concrete will not lose much moisture before handling, transportation, or erection. Contract documents must also indicate the density of reinforced concrete used for dead load calculations. This includes an allowance for reinforcing steel that is usually taken as an extra 5 lb/ft3 (80 kg/m3).
Entrained Air
While entrained air is generally used to protect the paste in concrete from damage caused by freezing and thawing, it is also used in LWC to improve its workability and finishing characteristics and to further reduce the concrete density. Air content must be measured at the point of placement using the volumetric method per AASHTO T 196(5) and not the pressure method per AASHTO T 152.(6)
Freezing and Thawing Resistance
Where resistance to freezing and thawing is important, testing should be required according to AASHTO T 161 Procedure A.(7) The standard test method must be modified for lightweight concrete by allowing specimens to dry prior to testing as specified in AASHTO M 195.(2) This modification must be followed to obtain meaningful results.
References
1. Castrodale, R. W. and Harmon, K. S., “Specifying Lightweight Concrete for Bridges,” Proceedings, 2008 Concrete Bridge Conference, St. Louis, MO, May 16-19, 2008, 12 pp.
2. Standard Specification for Lightweight Aggregates for Structural Concrete, AASHTO M 195, American Association of State Highway and Transportation Officials, Washington, DC.
3. Georgia Department of Transportation, “Special Provisions: Section 500—Concrete Structures,” Contract Documents for Project No. MSL-003-00(161) & IM-85-1(356), Coweta & Meriwether Counties, August 23, 2006, 5 pp.
4. Standard Test Method for Determining Density of Structural Lightweight Concrete, ASTM C567, ASTM International, West Conshohocken, PA.
5. Standard Method of Test for Air Content of Freshly Mixed Concrete by the Volumetric Method, AASHTO T 196, American Association of State Highway and Transportation Officials, Washington, DC.
6. Standard Method of Test for Air Content of Freshly Mixed Concrete by the Pressure Method, AASHTO T 152, American Association of State Highway and Transportation Officials, Washington, DC.
7. Standard Method of Test for Resistance of Concrete to Rapid Freezing and Thawing, AASHTO T 161, American Association of State Highway and Transportation Officials, Washington, DC.