The following letter was received from Pierre-Claude Aitcin of the University of Sherbrooke, Quebec, Canada, concerning Issue No. 15 of HPC Bridge Views.
This issue of Bridge Views pleased me very much. Actual problems related to the implementation of HPC by the construction industry are well addressed. For too long, too many agencies have applied conventional concrete technology to HPC. This has been a serious mistake. In spite of the fact that HPC is still a concrete made with the same ingredients as a conventional concrete, the compactness of the matrix in the fresh and harden state makes HPC a different material. HPC mixes are very sensitive to the compatibility and robustness of the cement/superplasticizer combination. Non airentrained HPC is sticky. HPC can experience slump loss. It can be difficult to entrain an air bubble system with a low spacing factor in HPC. Pumping affects the spacing factor. HPC does not bleed as much as conventional concrete and is more susceptible to plastic shrinkage. When HPC is not water cured before hydration begins, HPC develops a very rapid and significant autogenous shrinkage at a time when the concrete has not developed any tensile strength.
The use of HPC necessitates the implementation of precise placing and curing specifications. This can be done successfully. The City of Montreal is paying contractors specifically to cure concrete. The contractors are now zealous because they see concrete curing as an easy source of profit. The cost premium for the implementation of a particular water-curing program has been calculated to be between 0.1 and 0.5 percent for an HPC bridge. This is much less than the 0 to 20 percent premium cited by Hannah Schell in her article. The Department of Transportation of Quebec has found that an HPC bridge costs 8 percent less than its counterpart in conventional concrete.
The kind of technological information contained in HPC Bridge Views will help the U.S. Departments of Transportation and the construction industry take advantage of the full benefits of HPC. In Quebec, we are now using air entrained HPC with a 0.35 water-cementitious materials ratio to build our infrastructure. This concrete is made using a blended silica fume cement or a ternary cement that includes 5 percent silica fume and about 20 percent fly ash or slag.
CORRECTION
In Issue No. 17 of HPC Bridge Views, it was stated that the HPC bridge compilation was available for viewing and downloading at the NCBC web site. Due to technical difficulties, it is only available for downloading.
NCHRP PROJECT
The National Cooperative Highway Research Program has announced the award of Project No. 12-56, entitled “Application of the LRFD Bridge Design Specifications to High-Strength Structural Concrete: Shear Provisions,” to the University of Illinois. The Principal Investigator is Dr. Neil Hawkins. For further information about the project, go to www4.nas.edu/trb/crp.nsf/all+projects/nchrp+12-56 or www.ce.uiuc.edu/nchrp.