Rex Gilley, PB
Lightweight concrete was specified for use in three units of the Route 33 bridge over the Mattaponi River in Virginia. One unit consists of three spans of 145 ft (44.2 m) each that utilize prestressed concrete bulb-tee beams made continuous for live load. The two others are twin units and consist of four spans of 200, 240, 240, and 200 ft (61, 73, 73, and 61 m) that utilize spliced post-tensioned haunched pier segments with drop-in midspan segments. Each unit utilizes bulb-tee beams that are 93.5 in (2.37 m) deep with a top flange width of 60 in. (1.52 m), web thickness of 8 in. (200 mm), and a bottom flange depth of 9 in. (230 mm). This section was one of many developed by the Mid-Atlantic States Prestressed Concrete Committee for Economic Fabrication. Lightweight high performance concrete was specified for the precast, prestressed beams as well as the cast-in-place deck slabs to achieve longer span lengths while reducing foundation loads.
Selection of Material Properties
During the design phase of this project, review of available lightweight concrete material properties was undertaken to determine appropriate design values. Test data from other projects using similar materials was provided by the Virginia Department of Transportation (VDOT). Material properties that were investigated were density, modulus of elasticity, shrinkage, and ultimate creep strain. The design values from this review were then used in the material specifications to provide the following target values for the lightweight concrete material for both the deck and beam concretes.
Target Values for the Lightweight Aggregate Hydraulic Cement Concrete
1. Based on a concrete age of 93 days with loading at 3 days.
2. Based on 93 days of drying.
3. Includes the weight of the reinforcing and prestressing steel.
The above table lists separate target values for compressive strength, creep, shrinkage, density, air content, and modulus of elasticity for the concrete used in the beams and decks.
Also, the lightweight concrete for the decks and beams was specified to have a maximum chloride permeability of 1500 coulombs at 28 days to provide a durable structure. In addition to the literature review, a sensitivity analysis was made to determine the effect that the variability of each of these parameters would have on the completed structure. This analysis resulted in acceptance criteria for the contractor’s concrete mix. In the case of compressive strength and modulus of elasticity, minimum acceptable values were given. For density, shrinkage strain, and creep strain, maximum acceptable limits were specified.
Specification Requirements for Acceptance of Lightweight Aggregate Hydraulic Cement Concrete
1. Based on a concrete age of 93 days with loading at 3 days.
2. Based on 93 days of drying.
1 millionth = 10-6 in/in.The above table lists separate values for creep, shrinkage, density, and modulus of elasticity for the concrete used in the beams and decks.
The sensitivity analysis showed that the variation allowed in the acceptance criteria for modulus of elasticity, creep, and shrinkage resulted in changes in stress at the outer fibers of the beams of about 100 psi (700 kPa). Thus, a contingency was included in the design, while maintaining a reasonable range for the requirements of the materials.
Testing Requirements
The specifications required the following testing during construction:
- Prior to production, test mix designs for the beam concrete and the deck concrete were submitted for approval. From each test mix design, two samples were tested, and the results were used to determine values for the modulus of elasticity, creep, and shrinkage.
- During production of the beam and deck concretes, five samples from the beam mix and three samples from the deck mix were tested to determine the modulus of elasticity, creep, shrinkage, and density.
- During production, one compressive strength cylinder for each concrete type was required on each sublot, defined as 1 day’s placement up to a maximum of 100 cu yd (76 cu m). For each compressive strength cylinder, two cylinders were required for the permeability test.
- During production, air content and temperature were monitored on the first two loads per sublot and every five loads thereafter.
For the deck concrete, additional discussion was held during construction by the VDOT’s construction administration and materials division staff. While the specifications had testing requirements, further clarification was needed to assure that the concrete being placed met the intent of the specifications. This resulted in sampling and testing for air content, temperature, plastic density, and slump at the concrete plant on the first three trucks and every third truck thereafter, for each placement.
Testing during construction indicated that all parameters except the drying shrinkage for the deck concrete were within the acceptable limits as stated in the tables. The drying shrinkage of 560 millionths for the deck concrete was not considered to be enough outside the limit to warrant concern in the final product.
Lessons Learned
One issue addressed during construction was the definition of density. As shown in the tables, a specified density was given. This seemed straightforward enough as it was intended to be the oven dry density. For hydraulic cement concrete, however, there are several other “densities” to consider. The primary densities are equilibrium density and plastic or fresh density.(1) The issue was twofold: to ensure that the density of the concrete does not exceed the value used in determining the structure’s dead load during design; to provide a rational means of acceptance for the concrete prior to placement. The fresh or plastic density was used as the means of acceptance. The fresh density of the concrete for the beams was allowed to vary from 117 to 123 lb/cu ft (1875 to 1970 kg/cu m) compared to an oven dry density of 115 lb/cu ft (1840 kg/cu m). The fresh density of the concrete for the deck was 116 lb/cu ft (1860 kg/cu m) compared to an oven dry density of 110 lb/cu ft (1760 kg/cu m). So clarification and coordination of the specified density is critical.
Limitations on pumping the deck concrete also required discussion. Due to the nature of the lightweight concrete mix, the distance over which the concrete mix could be pumped was limited to 500 ft (152 m). There were no requirements in the specifications pertaining to this issue. So it would be prudent to add a restriction in the specifications to preclude the contractor from potentially placing concrete that may be adversely affected during the pumping operation. Materials such as the ones used on this project will continue to be used on the bridges of the future as owners strive to make more durable bridges in order to stretch their funding. When the envelope is pushed, it requires good leadership and a team effort. Ultimately, that was the key to the success of this project.
Reference
1. Castrodale, R.W. and Harmon, K.S., “Specifying Lightweight Concrete for Long Span Bridges,” Proceedings, First International Conference on Recent Advances in Concrete Technology, Washington, D.C., September 19-21, 2007.