A Unified Approach to Concrete Mix Design Optimization for Durability Enhancement and Life-Cycle Cost Optimization

ii EXECUTIVE SUMMARY Concrete bridges in coastal locations, as are common in Florida, are susceptible to chloride induced reinforcing steel corrosion and to resultant concrete cracking and spalling. Design approaches adapted in the past decade by the Florida Department of Transportation to provide enhanced corrosion resistance include 1) use of high performance concretes; that is, ones with a) low water-to-cement ratio and b) pozzolanic and corrosion inhibiting admixtures, 2) 76 mm (3.0 in) and 102 mm (4.0 in) of cover over reinforcement for prestressed and cast-in place concretes, respectively, and 3) elevation of substructure components above four meters (12 feet), where feasible. At the same time, Florida concretes, for the most part, are formulated using native aggregates, the coarse type of which is a relatively porous limestone (a more dense Alabama limestone may be employed in the panhandle region of the State). As such, basic principles suggest that the structure and properties of Florida coarse aggregates act against the overall objective of achieving 1) relatively impermeable concretes and 2) the requisite longevity for coastal bridges, which is now 75 years. The present study was based upon prior micro-compositional analyses of cores taken from the upper splash zone region of the Long Key Bridge which showed that chlorides were located in the paste only and not in the coarse aggregate. Such a finding infers that the ingress path for this species (chlorides) circumvented coarse aggregate particles such that these aggregates were of benefit rather than being detrimental to durability enhancement. Accordingly, the possibility exists that mix designs could be formulated where, by optimized grading and blending of coarse and perhaps fine aggregates, enhanced diffusional path tortuosity and a reduced chloride ingress rate could be affected. The objective of the present study was to more comprehensively investigate the influence of native Florida limestone coarse aggregates in concrete upon chloride diffusion and, based upon the results, propose mix designs that focus specific attention upon aggregate properties such that corrosion related durability is enhanced. To accomplish this objective, a series of mortar and concrete specimens were fabricated and, subsequent to curing, exposed to cyclic ponding with a ten w/o NaCl solution. After approximately one year, the exposures were terminated and chloride concentration was measured as a function of depth below the exposed surface by a wet chemistry method. From this, the effective diffusion coefficient was calculated. Mix design variables for the mortar specimens included …