Aly A. Hussein, South Carolina Department of Transportation
The South Carolina Department of Transportation (SCDOT)’s first use of high performance concrete (HPC) in bridge decks started in the early 1990s. A 1993 report(1) entitled “A Study of Microsilica Concrete,” was used as the basis for the development of SCDOT’s high performance concrete specifications. The primary purpose of the study was to find the general range of combinations of cementitious materials, aggregates, and various admixtures in a standard laboratory mixture that would yield concrete with improved durability and workability.
The resulting mixture was named Class E (later changed to Class 45) concrete and had the following ingredients per cu yd: 600 lb (356 kg/cu m) of Type I cement; 140 lb (83 kg/cu m) of Type F fly ash; 42 lb (25 kg/cu m) of silica fume; fine to coarse aggregate ratio of 35:65 for crushed stone and 36:64 for gravel; entrained air of 4.5 ±1.5 percent; and a water-cementitious materials ratio of 0.37; with a high-range water reducer (HRWR) and corrosion inhibitor added to the mix. Specified compressive strength for the Class E concrete was 4000 psi (30 MPa) at 28 days and 6500 psi (45 MPa) at 56 days.
Class E concrete was used on several bridge decks in the upstate region of South Carolina. Most of these bridge decks experienced problems with cracking occurring both before being opened to traffic and immediately thereafter. In an effort to determine the likely causes of cracking experienced in the new Class E HPC bridge decks, a study entitled “Review of Class E Concrete Bridge Decks in South Carolina” was conducted.(2)
From the nine bridges that were investigated, it was concluded that the observed cracking had two likely causes: poor curing practices and load-induced cracking. The load-induced cracking that appeared shortly after the spans were open to traffic may have resulted from the relatively stiff decks being placed on more flexible bridge superstructures. A summary of conclusions and key observations from the study were as follows:
1. Early age cracking is likely the result of improper curing techniques. Curing mats were not placed as soon as they should have been and on-site inspection and quality assurance related to curing may, at times, have been substandard.
2. All inspected bridges exhibited some degree of load-induced cracking. This cracking took the form of full-width transverse cracks. In all cases, these cracks were spaced uniformly along the bridge span and were always observed over the intermediate piers.
3. The Class E concrete mix in South Carolina was, in the opinion of the investigators, a very “rich” mix, which required very stringent quality control during placing and curing. A less “rich” mix was recommended.
4. The performance aspect of concrete most important in bridge decks is durability rather than strength.
The report recommended the following changes related to the use of Class E concrete:
- Provide improved on-site quality control/quality assurance in all aspects of mixing, placing, and curing when high performance concrete is used.
- Develop a new concrete mix with enhanced durability characteristics.
- Review placement sequence documentation and ensure the sequence does not lead to large tensile stresses in previously placed segments.
- Initiate the placement sequence in the positive moment regions prior to the negative moment regions.
- Adopt the FHWA parameter characterization for the specification of HPC mixes.
- Include likely vibration or deflection effects associated with more flexible bridge superstructures in the design of the bridge decks.
During the last few years, the SCDOT implemented some requirements and made changes regarding the Class E HPC mix design:
- The new mix has less cement. The new ingredients per cu yd are: 500 lb (297 kg/cu m) of Type I cement; 140 lb (83 kg/cu m) of Type F fly ash; 35 lb (21 kg/cu m) of silica fume maintained at 7 percent of the weight of cement; fine to coarse aggregate ratio of 37:63 for crushed stone and 38:62 for gravel; entrained air 4.5 ±1.5 percent; water cementitious materials ratio of 0.37; with a HRWR required and corrosion inhibitor added to the mix.
- Trial mixes for HPC must be performed and approved by the SCDOT.
- All bridge decks, regardless of the class of the mix, must be wet cured for 7 full days.
- Wind barricades and foggers must be used during the placement of all bridge deck concrete.
The new mix resulted in a concrete compressive strength of 4000 psi (28 MPa) at 28 days. Rapid chloride permeability values ranging between 1000 and 3000 coulombs were recommended as a means of acceptance and quality control for performance mix designs. Since the implementation of the new requirements and the changes that were incorporated, a few projects with HPC bridge decks were constructed without cracking. It appears that the rich mix was a major factor in causing the early age cracking. Additionally, during the placement of bridge decks, the operation went very smoothly due to the change of the ratio of fine aggregate to the coarse aggregate.
References
- Mou, C. H. and Hwang, S. Y., “A Study of Microsilica Concrete,” School of Engineering Technology, South Carolina State University, 1993.
- Petrou, M. F. and Harries, K. A., “Review of Class E Concrete Bridge Decks in South Carolina,” University of South Carolina, 1999.
Further Information
For further information, the author may be contacted at [email protected] or 803-737-6687.