Yunping Xi, University of Colorado at Boulder
The presence of cracks in newly constructed concrete bridge decks in Colorado prompted the Colorado Department of Transportation (CDOT) to initiate a study to determine the extent and causes of the cracking and to identify changes needed in the material specifications, construction processes, and design specifications to alleviate the problem.
Investigation
An analysis of field inspection results collected in 2002 for 72 bridges built by CDOT between 1993 and 2000 revealed that 82 percent of the decks had defects including 37 percent with spalling and delaminations, 37 percent with unsealed cracks of either moderate size or density, and 5 percent with unsealed cracks of both moderate size and density. The analysis was confirmed with more detailed field inspections on nine newly constructed bridge decks that showed excessive cracking. The cracks widths varied from 0.01 to 0.10 in. (0.25 to 2.5 mm). The cracking was described as severe, widespread, and uniformly spaced. Typically, the cracks were oriented in the transverse and longitudinal directions. Occasionally, the cracks formed in random orientations.
The 1999 CDOT materials and construction specifications lists Class D concrete for new bridge decks, Class DT for deck topping rehabilitation, and Class SF for bridge deck overlays. The specified requirements for the concretes are shown in Table 1. In addition, the concretes were required to use an approved water-reducing admixture. Class DT concrete was required to contain at least 50 percent AASHTO M 43 No. 7 or No. 8 coarse aggregate. Class D concrete had been used since about 1976, although the original specified water-cement ratio was 0.48 maximum.
For construction, the air temperature at the deck surface was required to be between 40 and 90°F (4 and 32°C). A minimum curing period of 5 days was required.
Implementation
As a result of the study, CDOT established two new classes of concrete for bridge decks without membranes—Class H concrete for exposed bridge decks and Class HT for overlays. The specified requirements for the new classes of concrete and their cementitious contents are shown in Tables 1 and 2, respectively. In addition, laboratory trial mixes for each class of concrete must have a rapid chloride permeability per AASHTO T 277 not exceeding 2000 coulombs at 56 days and must not exhibit a crack at or before 14 days in the cracking tendency test of AASHTO PP 34. The main difference in the two classes of concrete is the coarse aggregate content. Class H concrete contains a minimum of 55 percent AASHTO M 43 size No. 67 coarse aggregate, whereas, Class HT concrete contains a minimum of 50 percent size No. 7 or No. 8 coarse aggregate.
Both classes of concrete are to be placed only when the concrete temperature at time of delivery is between 50 and 80°F (10 and 27°C), the air temperature does not exceed 80°F (27°C), and the wind velocity does not exceed 10 mph (16 km/h). If it can be determined that the evaporation rate is less than 0.20 lb/sq ft/hr (1.0 kg/sq m/hr) in accordance with Fig. 2.1.5 of ACI 305, concrete placement is permitted.
During concrete placement and before final curing is started, the concrete surface is required to be kept moist at all times by fogging except fogging is not required from October 1 through April 30 if the evaporation rate is less than 0.10 lb/sq ft/hr (0.50 kg/sq m/hr). The new minimum curing period is 168 hours (7 days) and from May 1 through September 30, water curing must be used. Between November 1 and March 31, decks are to be cured by application of a membrane-forming curing compound followed by curing blankets. Decks placed in April or October may be cured by either of the above methods.
More Information
This article is based on the CDOT report entitled “Assessment of the Cracking Problem in Newly Constructed Bridge Decks in Colorado,” Report No. CDOT-DTD-R-2003-3.