- Author
- Stutzman, P. E.
- Title
- Petrographic Examination of Four Texas Department of Transportation Hardened Concrete Cores. Letter Report Submitted to the Porgland Cement Association and the Texas Department of Transportation.
- Coporate
- National Institute of Standards and Technology, Gaithersburg, MD
- Report
- Letter Report, 1998, 34 p.
- Keywords
- concretes | transportation
- Abstract
- Four concrete cores were received from the Texas Department of Transportation for characterization and evaluation. Two of the cores (Nos. 11 and 27) exhibited cracking visible to the unaided eye, the others (Nos. 5 and 29) visually appeared to be sound. Our analyses indicate that cracking in the damaged cores is the result of expansion due to alkali-silica reaction involving both the coarse aggregate (a limestone containing clay, quartz, and feldspar), and portions of the fine aggregate (chert and quartz). Cracks in the coarse aggregate that extend into the paste are visivle in hand specimen, while cracking associated with the chert and quartz in the sand and the reaction gel can be observed using a stereo microscope. Cores 5 and 29 do not exhibit alkali-silica reaction products and are not cracked. Increased abundance of ettringite and hemicarbonate with lesser amounts of monosulfoaluminate are found in the damaged cores. Although ettringite is found within the hardened cement pastes of all these cores as a crack-filling phase, it does not appear to have caused expansion. Evidence of paste expansion, uniform gaps in the paste/aggregate interface proportional to aggregate size, is not present. Moisture availability afforded by the fracture network has probably facilitated dissolution, migration and subsequent precipitation of secondary ettringite in the pre-existing fractures. Zones within the cement paste adjacent to the microcracks which are found depleted in monosulfoaluminate and ettringite, which supports this conclusion. Additionally, the occurrence of ettringite cross-cutting alkali-silica reaction gel indicates that formation of ettringite is subsequent to the alkali-silica reaction, the associated expansion, and cracking. Conditions necessary for alkali-silica reaction include water, a reactive phase within the aggregate, and cement alkali oxide levels greater than 0.6% (expressed as equivalent Na₂O). TDOT may consider using a different aggregate or cement, or a pozzolanic admixture such as silica fume, fly ash, ground blast furnace slag, or a natural pozzolan that has been shown to reduce expansion due to alkali-silica reaction. Finally, none of these specimens are air-entrained and the entrapped air present is not adequate, considering both bubble spacing and volume, to provide freeze-thaw cycle protection when saturated. The lack of air entrainment will also adversely affect concrete workability.