MITIGATION OF DETRIMENTAL ALKALI-SILICA REACTION IN FUTURE PORTLAND CEMENT CONCRETE OF THE INFRASTRUCTURE OF THE COUNTY OF ORANGE, CALIFORNIA

The 45-year-old County of Orange infrastructure contains numerous distressed concrete structures. Various physical and chemical processes external and internal to the concretes are responsible for the distress. Forms of this distress include map cracking (polygonal cracking), pop-outs and spalling. One of the most common causes of distress is from the internal chemical process known as alkali-silica reaction (ASR). The ASR has resulted in the life of some concrete structures being shortened far below their design life. ASR, first identified 65 years ago by Stanton (1940) and now recognized worldwide as a problem, involves a chemical reaction between amorphous- to cryptocrystalline-silica, highly metamorphic quartz, chert, optically strained quartz, micro cracked quartz, or glass in the aggregate, with alkalis and hydroxyl ions in the cement paste and pore waters. These varieties of silica are more reactive for ASR than others because they are less ordered molecularly, have weaker bonding, have less chemical stability and higher tendency for reaction. The reaction produces hydrous, calcium, alkali-silica gel that absorbs moisture, swells and cracks the concrete where the expansive pressure exceeds the tensile strength of the concrete. Reactive grain types, micro cracks and associated ASR gel are identifiable with a petrographic, polarizing microscope in thin-sections of distressed concretes. All of the County's current aggregate sources contain potentially reactive grain types. Aggregates proposed for County Public Works concretes are subjected to petrographic analyses, and those found with potentially reactive siliceous grains in excess of threshold percentages are required to contain 20% substitution of flyash for cement. New concrete specifications include use of low alkali cement. The process of petrographic analysis, flyash substitution, and use of low-alkali cement is expected to save the County hundreds of thousands of dollars or more for demolition, repair or replacement of prematurely deteriorated concrete structures.