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Crack induced in a prestressed concrete beam by alkali-silica reaction.
ASR Prevention in Texas
Brian D. Merrill, Texas Department of TransportationFor some states, alkali-silica reaction (ASR) in concrete has become just as big a concern as corrosion of reinforcing steel. Such is the case in Texas, where reinforcing steel corrosion mainly affects the far northern part of the state and along the coastline.
ASR is the result of the reaction between alkalies in the cement and certain siliceous aggregates. This reaction can result in excessive expansion and cracking of concrete exposed to moisture. Cracking of structures suffering from ASR is usually observed within 10 years of construction. For excessive expansion of concrete due to ASR to occur, four requirements must be met: the aggregate must be sufficiently reactive; the pH of the pore fluid must be high (high alkalinity); the amount of reaction product formed (ASR gel) must be large; and there must be sufficient water available in the concrete.
The Texas Department of Transportation (TxDOT) revised its structural concrete specifications in 1999 in an attempt to prevent ASR in new concrete by (1) limiting the total alkali contribution to the concrete mix; (2) using supplementary cementitious materials (SCMs) such as Class F fly ash, ground granulated blast-furnace slag (GGBFS), and silica fume; (3) using blended Type IP or IS cements; or (4) performance testing using ASTM C1260 or ASTM C441.
At the same time, TxDOT launched a massive ASR research project to study the issue. One of the studies was Project 0-4085, “Preventing Premature Concrete Deterioration due to ASR/DEF in New Concrete” conducted at the University of Texas’ Center for Transportation Research. This project used extensive laboratory testing along with a large exposure site to evaluate the effectiveness of various ASR mitigation methods. TxDOT used the results to confirm and expand the 1999 specifications to prevent ASR.
TxDOT’s current ASR specifications for structural concrete are largely prescriptive due to the high volume of concrete usage (> 60 million yd3 in 2006) and the time it takes to run tests on more than 150 commonly used aggregate sources. All aggregates are treated as if they are potentially reactive unless we have test data confirming otherwise. The following eight mix design options were developed with industry input to provide maximum flexibility:
Option 1. Replace 20 to 35% of the cement with Class F fly ash.Also, TxDOT generally uses prescriptive specifications when specifying HPC to keep construction costs down. Performance specifications for concrete in Texas have tended to increase bid prices because the high volume of concrete consumption introduces some level of risk to the contractors that the concrete may not meet the performance requirements. We have done enough testing of HPC mixes that we are comfortable prescribing mix designs that will meet our needs. TxDOT’s HPC specifications limit the mix design options that can be used to Options 1 through 5 (no testing required) and Option 8, if the permeability is less than 1500 coulombs at 56 days when tested in accordance with AASHTO T 277.
HPC mixes as specified by TxDOT have been shown through testing to mitigate ASR in two ways. The first is physical mitigation because the permeability of the concrete is much lower meaning less moisture can penetrate the concrete to form ASR gel. The second is chemical mitigation. SCMs react with calcium hydroxide and this reaction lowers the alkalinity of the concrete and ties up free calcium ions needed to form ASR gel.
For further information on TxDOT’s ASR efforts, please contact Brian D. Merrill at 512-416-2232 or email@example.com.
HPC Bridge Views, Issue 51, Sept/Oct 2008