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INFLUENCE OF PERMEABILITY REDUCING AND CORROSION INHIBITING ADMIXTURES IN CONCRETE UPON INITIATION OF SALT INDUCED EMBEDDED METAL CORROSION

The research reported in this report was performed to determine threshold chloride concentrations for high performance concretes under exposure conditions relevant to coastal bridge substructures in Florida. The experiments were based upon a series of reinforced and non-reinforced concrete specimens that contained: (1) 20%, 35%, and 50% cement replacement by fly ash; (2) 3%, 8%, and 15% cement replacement by silica fume; and (3) control specimens (no pozzolanic admixtures). In addition, the utility of calcium nitrite as a corrosion inhibitor was evaluated based upon slabs that contained: (1) 35% cement replacement with fly ash and 4 gal/cu yd (19.8 liters/cu m) calcium nitrite; (2) 8% cement replacement with silica fume and 2 gal/cu yd (9.9 liters/cu m) calcium nitrite; and (3) controls - no pozzolanic admixture with 4 gal/cu yd (19.8 liters/cu m) calcium nitrite. The specimens were exposed to a one week wet - one week dry ponding cycle with natural sea water. Potential and macro-cell current of bars in the reinforced concrete slabs were monitored with time in order to determine when active corrosion commenced. At the same time, cores were taken from the non-reinforced slabs such that the extent of chloride ingress was determined without modification or destruction of the reinforced ones. From the chloride concentration profile, the effective diffusion coefficient of the different concrete mixes was determined. In addition, a unique procedure was developed for determining pore water pH which, unlike the expression method, is readily applicable to high performance concretes. After four-plus years of testing, active corrosion has not yet commenced in any of the specimens, although there are indications that passive film breakdown is underway in the control and 3% cement replacement silica fume cases. From the chloride analyses, an effective diffusion coefficient of 3.48 x 10 to the -8 power sq cm/s was calculated for the control mix specimens; and this was reduced by as much as a factor of six in the fly ash case (35% cement replacement) and 3.5 for silica fume (15% cement replacement). Thus, the beneficial effect of these admixtures in reducing concrete permeability to chlorides was confirmed. The pore water pH in the control mix specimens was 13.84, and this decreased with increasing pozzolan addition in both the fly ash and silica fume cases. An analysis of time-to-corrosion initiation was performed based upon the assumption that active corrosion of the control specimens had commenced and that the chloride-to-hydroxide ratio for this case (0.61) applied also to the other mixes. However, inconsistencies in the data and analysis results based upon this assumption were apparent which, instead, indicated that the critical chloride-to-hydroxide ratio was greater the higher the concrete quality. On this basis, the chloride concentration at which corrosion commences in the high performance mixes can only be determined experimentally or by development of a quantitative expression for the critical chloride-to-hydroxide--concrete quality interrelationship.

  • Corporate Authors:

    Florida Atlantic University, Dania Beach

    Department of Ocean Engineering, 101 North Beach Road
    Dania Beach, FL  United States  33004

    Florida Department of Transportation

    Haydon Burns Building, 605 Suwanee Street
    Tallahassee, FL  United States  32301
  • Authors:
    • Hartt, W H
    • Charvin, S
    • Lee, S K
  • Publication Date: 1999-6-18

Language

  • English

Media Info

  • Features: Figures; References; Tables;
  • Pagination: 72 p.

Subject/Index Terms

Filing Info

  • Accession Number: 00765977
  • Record Type: Publication
  • Report/Paper Numbers: WPI 0510716,, Final Report
  • Files: TRIS, STATEDOT
  • Created Date: Jul 21 2001 12:00AM