Bijan Khaleghi, PhD, PE, SE, Washington State DOT
Bridge decks are not typically classified as mass concrete, but as is the case with mass concrete, the need to reduce or prevent cracking is a high priority. Temperature control is one of many criteria used to achieve this. The Washington State Department of Transportation (WSDOT) revised its concrete specification for bridge decks in 2011 to be more performance based with the desired effect of having less early-age shrinkage cracking.
A report comparing bridges constructed with the traditional WSDOT specification to those constructed with the performance based specification showed much less early-age shrinkage cracking for the performance based approach.
The WSDOT bridge deck concrete specifications were revised to eliminate or reduce early-age restraint cracking in bridge decks, caused by length changes due to shrinkage coupled with the restraint by girders and internal reinforcement and show up primarily as transverse through-cracks. Many of the revisions came from recommendations from a WSDOT report “Mitigation Strategies for Early-Age Shrinkage Cracking in Bridge Decks.” The study showed that cracking of bridge decks is variable within the same bridge and even within the same concrete placement, indicating that there are many factors that affect the cracking performance of a bridge deck that change during the construction of the bridge.
The performance based specification was first implemented in mid-2011 and is now included in the WSDOT Standard Specifications.
To evaluate the effectiveness of the revised concrete specification, a sample of bridges constructed with the performance based specification and the traditional specification were compared for cracking severity.
WSDOT Traditional and Contractor Performance-Based Mix Design
The revisions to the “Contractor Mix Design” remove some of the prescriptive requirements and replace them with performance based requirements. The most significant change was removal of the prescriptive requirement for a minimum cementitious content for the Class 4000D concrete. The performance based requirement for minimum concrete compressive strength at 28 days remains in the specification as 4,000 psi. Added were performance limits on permeability, length change (“shrinkage”) and scaling (as well as an optional requirement for freeze-thaw durability to reduce prescribed air content). In addition to the performance limits, modulus of elasticity and density are required to be provided (but no limits attached).
Another significant change resulting from recommendations of WA-RD Report 747.1 was to increase the aggregate size. The nominal maximum aggregate size increased from 1 in. to 1½ in.
The overall intent of the changes to the Class 4000D mix design is to focus on the behavior (or performance) of the concrete rather than providing a set “recipe.” This puts more burden on the Contractor and concrete supplier but allows for more flexibility and provides more information on the needed properties of the concrete being placed. The comparison of class 4000D traditional and Contractor performance-based mix designs is shown in Table 1.
(*Drilled shaft elements have a 6′ minimum dimension requirement)
Concrete Placement, Texturing and Curing
In addition to revisions to the mix design, changes were made to the placement, finishing and texturing portions of the specification. The ultimate goal of these revisions is to begin adequate wet curing as soon as possible. The original specifications for placing and texturing typically resulted in a delay of application of wet burlap to the surface of the bridge deck, but now, revisions to the curing portion of the specification require fogging (Figure 1) of the deck immediately after the finishing machine passes, eliminating tining, and applying presoaked burlap almost immediately. The use of curing compound is explicitly forbidden. Fogging continues until the concrete has achieved initial set when soaker hoses are added. The wet burlap and soaker hoses remain in place for 14 consecutive days as shown in Figure 2.
After the concrete has hardened, the bridge deck is textured using diamond-tipped saw blades that create longitudinal grooves (Figure 3).
Bridge Deck Temperature
Another change to the (Class 4000D) specification requires the concrete temperature at the time of placement to be between 55°F and 75°F, a tighter tolerance than the original specification of between 55°F and 90°F, intended to reduce the peak temperature of the concrete during placement and curing. Concrete typically heats up as it sets and hardens (Figure 4). If concrete temperature is much higher than ambient temperature when it achieves initial set, stresses will be locked in which can lead to cracking.
Additionally, requirements were added to monitor both ambient temperature and temperature of the bridge deck concrete for 7 days after placement. The Contractor is then required to submit this data to WSDOT; however, no other contractual limits are placed on this information.
Recommendations regarding temperature:
To achieve reduced early-age cracks in bridge decks:
- Indicate locations for embedded temperature monitors in the contract plans. Use multiple temperature monitors for each deck placement, a minimum of one at each end and one mid-span. The embedded monitors should be located as close to mid-slab thickness as possible.
- Require that temperature monitor data received from the Contractor include, at a minimum, the following elements: date and time which concrete placement started, where concrete placement started, location of monitor, temperature measurements at hour max intervals.
- Establish peak temperature and maximum temperature limits. This may provide a tool to reject a deck that performs very poorly due to extreme temperature or temperature differences. While no evidence of type of this behavior was seen in this study, adding contractual limit requirements may result in better temperature data.
- During the mix design phase, evaluate temperature changes over time for a specific concrete mix. This helps compare one mix to the other and possibly aids in developing performance based limits that can be added to the concrete mix design requirements.
The Evaluation of Performance Based Concrete for Bridge Decks in Washington State report was prepared by Eric Ferluga and Patrick Glassford