Cracking under Pressure: How Moisture and Heat Affect ASR and DEF Structures

Chris Sasser — Tue, 01 Jun 2010 14:33:15 +0000

Texas weather runs the gamut — cold winters in some areas, occasionally heavy rains, times of drought and always baking summer heat. Concrete structures are built to last, but it’s the little things that get you sometimes. Little things like moisture and heat over time can trigger reactions in concrete structures that cause cracking and ultimately deterioration.

Two reactions in concrete that are most troubling are alkali-silica reactions (ASR) and delayed ettringite formation (DEF). ASR occurs when the alkalis in concrete react with the silica in some aggregates (the rocks included in concrete). Moisture can feed this reaction, which causes expansion with time and cracking. The other reaction, DEF, occurs between sulfate in gypsum in the cement and calcium aluminates when calcium hydroxides are present. Heat and moisture cause the growth of crystals, which can also cause the concrete to crack.

“It seems to be the cycle of wetting and drying that causes the most damage with ASR,” says Zachary Grasley, a research scientist with the Texas Transportation Institute (TTI) and an assistant professor in the Zachry Department of Civil Engineering at Texas A&M University, who is studying these processes. “ASR can cause a distributed cracking pattern, called map cracking because it looks like lines on a map when you look at the surface of the concrete. DEF, on the other hand, can form much larger cracks.” Both deterioration mechanisms tend to create cracking in areas of concrete members that are in tension.

Texas Department of Transportation (TxDOT) research has helped develop specifications to minimize the risk of ASR and DEF, but older structures built before the new specifications were in place can be affected by them. Current TTI research is evaluating the structural performance of lap splices (overlapped steel reinforcing bars) in deteriorated bridge columns (project 0-5722) and the structural performance of critical D-regions (areas of complex stress distribution) of concrete frames (project 0-5997) that are damaged by ASR and/or DEF.

The D-region of a concrete frame displays cracking that might be amplified by ASR/DEF deterioration. TTI research is investigating the effect this cracking has on structural performance.

“TxDOT‘s job is to manage infrastructure, and in order to spend money wisely, we need to quantify the problem by seeing how much the structural integrity is affected,” says John Vogel, a senior bridge design engineer with TxDOT‘s Houston District. “Just because a column looks bad doesn’t mean it really is. This project will give us the tools we need to manage the problem.”

The first phase of the project, constructing and deteriorating test specimens with a range of crack sizes from these reactions, is underway at TTI‘s Riverside Campus. The second phase of the project will be to load these specimens to failure and compare their performance to that of specimens with undamaged concrete. Researchers will then be able to correlate the structures’ physical appearance with their structural strength and reliability.

“The testing we’re doing right now at Riverside involves inducing ASR and DEF in concrete structures,” says the principal investigator, Joe Bracci, TTI assistant research engineer and professor in Texas A&M’s Zachry Department of Civil Engineering. “The concrete specimens get 15 minutes of water four times a day using a sprinkler system and get exposed to Texas heat to simulate the cycle of wetting and drying. Internal strain gages take measurements and surface expansion data every two weeks.”

Cracking from ASR or DEF may reduce the bond between the concrete and the reinforcing steel. Structures can face significant demands at several locations where bond is critical. Severe deterioration in these locations could ultimately lead to a failure in overload scenarios — and a disaster for the entire structure. TxDOT‘s primary concern is safety; TTI research should identify if the deterioration conditions create any structural performance issues that need to be dealt with.

Concrete specimens are exposed to wetting and drying cycles by a sprinkler system at the TTI Riverside Campus to accelerate deterioration from ASR/DEF.

“The results of this research could have a huge financial impact,” says Vogel. “The only ‘cure’ for ASR and DEF is waterproofing, which slows down expansion. Not having to waterproof structures would save TxDOT funds.”

Vogel points to a waterproofing project TxDOT performed in November 2007. TxDOT paid $10,000 to $20,000 per column for waterproofing. Other ongoing research should reduce the cost to half what TxDOT paid in 2007, but that’s still a considerable sum.

“The affected concrete in the Houston area alone is on the order of tens of millions of square feet,” says Vogel. “I hope and believe that the current research will show that waterproofing is not necessary at all, letting us spend our limited resources wisely elsewhere.”

“The testing we’re doing right now at Riverside involves inducing ASR and DEF in concrete structures. The concrete specimens get 15 minutes of water four times a day using a sprinkler system and get exposed to Texas heat to simulate the cycle of wetting and drying. Internal strain gages take measurements and surface expansion data every two weeks.” Joe Bracci, TTI Assistant Research Scientist

“It seems to be the cycle of wetting and drying that causes the most damage with ASR. ASR can cause a distributed cracking pattern, called map cracking because it looks like lines on a map when you look at the surface of the concrete. DEF, on the other hand, can form much larger cracks.”Zachary Grasley, TTI Research Scientist

For more information:

Joe Bracci
(979) 845-3750
j-bracci@tamu.edu

Caring for concrete

Debbie Murillo — Sun, 01 Mar 2009 16:23:27 +0000

It’s easy enough to fix a pothole, but how easy is it to actually prevent them from appearing at all? Given the number of cracks and potholes that plague Texas roadways, there is no question about the value that a successful preventive maintenance could have in delaying more serious problems.

Various methods exist to identify distresses in concrete and asphalt concrete overlaid pavement, and simple patching is usually a good temporary fix. But less-experienced engineers may miss the signs of early serious structural damage to the pavement because of a lack of experience or a decision-making process based only on visual distress surveys.

“By the time a visual inspection shows distress, it’s time for a major overhaul,” says Texas Transportation Institute’s (TTI‘s) Tom Freeman, engineering research associate. “In this project, we collected the last 30 years of field experience and developed tools that engineers can fit in their pockets.”

The research team developed a pocket field manual, a field survey and field guidelines. As engineers retire from TxDOT, too often their expertise goes with them. Freeman’s work has captured some of that corporate knowledge, making the maintenance process more efficient overall.

The field manual provides a standard for maintenance crews to use in determining and describing the problems they may encounter; it also includes pictures, possible causes and recommended lab tests. The team also used a survey to evaluate the effectiveness of full-depth repair, various joint repair techniques, joint resealing and dowel bar retrofit with diamond grinding.

The guidelines include non-destructive test procedures for subsurface layers, such as the falling weight deflectometer and ground penetrating radar (a technique developed at TTI). The guidelines also provide discussion on each maintenance stage, promoting the best standard practices and specifications used by state departments of transportation across the country. A step-by-step repair method and decision-making process gives engineers the options of preventative maintenance, functional or structural concrete pavement repair or full resurfacing.

“Routine maintenance at the right time can extend pavement life in a cost-effective manner,” says Freeman. “It takes longer to repair a section of very poor pavement, while with the same amount of resources, an agency can maintain far more sections of pavement in good and fair condition.”

These tools are timely since there is a renewed federal interest in improving transportation infrastructure, which is good news to state departments of transportation. Freeman’s next project will work with TxDOT to develop a class that will teach engineers around the state the decision-making processes found in these products.

“This research defines another set of tools for TxDOT engineers to economically maintain pavements on an aging transportation system with decreasing funding,” says TxDOT‘s Paul Montgomery, director of maintenance. “These tools help us along the path of progress in pavement management.”

The pocket field manual, field survey and field guidelines developed by TTI researchers will aid maintenance crews in identifying concrete distresses like the ones shown above.

Texas A&M Transportation Institute» concrete

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