Civil Engineering Reference
In-Depth Information
Alkali silica reaction (ASR or AAR)
Alkali silica reaction (ASR) is the most common form of alkali aggregate
reaction (AAR) and can occur in concretes made with aggregates containing
reactive silica, as long as there is a sufficient supply of alkali (usually provided
by the cement) and a supply of moisture.
The reaction product is a hygroscopic gel which takes up water and
swells. This may create internal stresses sufficient to crack the concrete. If
core samples are washed and wrapped in cling film, when taken, concrete
suffering from ASR often develops dark, sweaty patches under the cling film
surface, as gel oozes out of the concrete. This is a very good diagnostic
indication that ASR is occurring, but does not necessarily indicate that
damaging expansion either has or will result. This can only be shown by
petrography and expansion testing.
One of the most frequently found aggregates in affected concrete is chert.
This is a common constituent of many gravel aggregates, but a number of
other geological types may be reactive, such as strained quartz in sands
and some quartzites. Some Irish aggregates, notably greywackes, have been
found to be susceptible to ASR. These tend to be quite slow reacting and
damage can take 20-50 years to become serious. Greywackes have also
caused problems in Wales.
Figure 1.11
illustrates cracking to the Beauharnois Dam on the St.
Lawrence Seaway. The expansion caused by the ASR on this structure caused
it to grow several cm in length and distorted the turbines providing hydro-
electric power for the region.
Guidance on diagnosis of ASR is given in a BCA report (BCA, 1992). A
photomicrograph of ASR in petrographic thin section is given in
Figure 2.5
in
Chapter 2.
Other rarer forms of reaction include alkali carbonate reaction and alkali
silicate reaction. Discussion of these is beyond the scope of this chapter.
Freeze-thaw damage
Concrete of inadequate durability, if subjected to a wet environment and
freezing, can be disrupted by freeze-thaw attack. Water enclosed in the pores
of the wet concrete will expand on freezing and the high internal stresses
so created can disrupt the surface. The effects are intensified by subsequent
freeze-thaw action as minute cracks develop which, in turn, become filled
with water. Again, concrete with a high water to cement ratio is especially
vulnerable. Addition of an air-entraining agent to the concrete reduces the risk
of frost damage by entraining minute, closely spaced air voids in the concrete,
which provide a release mechanism for the expansive disruptive forces.
The problem is often characterised by parallel lines of cracking as freeze-
thaw damage penetrates deeper into the concrete. Leaching of calcium
hydroxide from the concrete also often occurs as water passes through the
cracked concrete.
