ASR IN CONCRETE

The Nuts and Bolts that Cause the Damage

INTRODUCTION

Durability is far more important to long performing concrete than strength.

Durability can be broken up into different issues: Freeze/Thaw Cycles Salt and/or De-Icer Exposure Sulfate Exposure ASR

Damages from ASR

Many problems related to ASR have been attributed to other mechanisms.

A survey of the United States and Canada taken in 1999 showed that only about 20% of the states recognized that they had ASR problems.

In New Mexico, damages from ASR were so extreme that corrosion never had a chance to become an issue.

History of ASR

First recognized in early ’40’s by Stanton in California pavements as the specific damaging mechanism found that problems were less

noticeable if alkali levels in cement were less than 0.6%;

Suitable additions of pozzolanic material prevented excessive expansions.

The “Bad” Cations

ASR gel is formed when the positively charged “cations” from the available alkalis combine with the hydroxide ions.

Sodium and Potassium, which are the normal constituents of the available alkalis, form expansive gels.

When the relative humidity of the pore space exceeds 80%, the reaction will proceed (This is most of the time).

“Bad Guy” Aggregates

Opal – Colorless to pale gray or brown.

Chalcedony – a fibrous, micro-crystalline form of silica.

Quartz – Microcrystalline and highly fractured quartz in cherts, quartzite, gneisses and strained quartz. Coarse, megascopically-crystalline quartz is normally not reactive.

“Bad Guy” Aggregates

Cristobalite – Normally found in small square crystals or aggregates in the cavities of obsidian, rhyolite, andesite and basalt. Has also been found as a component of some slag materials.

Tridymite – small euhedral (well formed) crystals as cavity linings in volcanic rocks such as obsidian, rhyolite, andesite and as a porous crystalline aggregate.

The Next Steps

ASR gel forms at the surface of the aggregates, and begins to in-fill around and through the aggregate particles.

After the developing gel has filled all available space in and around the aggregate, any additional gel fills air voids within the cement paste.

The “Gel that Broke the Concrete’s Back”

When the gel gets wet, any sodium or potassium rich gel swells. The swelling can not be accommodated within the aggregate and/or paste structure.

Tension cracks are created in the concrete.

As the cracks grow, more water gets in, more gel forms, and more expansion takes place.

Fighting the ASR Menace

In most locations, it is not cost-effective to prohibit the use of reactive aggregates.

Current research indicates that with proper mix modifications, the risk of ASR can be minimized or even eliminated.