in fine aggregate as the cause of blistering of the flooring in
the Texas building.
Generally, our investigations revealed a reactive particle
(rhyolite or chert) below a blister (Fig. 2(b), 3, and 4), and the
size of a surface blister generally correlated with the size and
depth of the reactive aggregate particle. Microcracks filled
with ASR gel emanated from the affected particles and
extended to the concrete surface (shown in Fig. 2(b) and 4).
ASR gel was frequently observed at the blister area between
the base coat or adhesive of the flooring system and the
concrete surface. The near-surface reactive particle
occasionally exhibited material loss along the internal cracks
due to consumption caused by reactions. Microcracks in the
concrete were usually more abundant below and near the
blistered area than away from the blister.
In the two cases involving rhyolite, ASR gel and associated
microcracking were also observed in the body of the concrete.
Exposed rhyolite particles and nearby paste frequently
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below the concrete. In the third case, the floor slabs were
elevated. In each case, blistering was observed in the flooring
from a few months to about 2 years after installation.
Two cases occurred in Colorado, 120 miles (193 km) apart.
In one of these cases, the flooring comprised vinyl
composition tile (VCT) bonded directly to the concrete slab.
In the second case, a continuous flooring material was bonded
to a moisture-reduction system on the concrete surface.
Another case occurred in San Antonio, TX. In this and in one
of the Colorado cases, the blistering problem had reportedly
recurred repeatedly after repair or replacement of the flooring.
In the other Colorado case, blistering occurred only in areas of
floor slabs where concrete had been removed and replaced.
Our investigations revealed that the blistering of the
flooring in each building was caused by ASR in the nearsurface
concrete. The ASR is located near the surface because
an impermeable flooring acts as a vapor retarder, allowing
condensation to form at the interface with the concrete.
Combined with the presence of high alkalinity near the
surface of a slab, the liquid water supplied by the
condensation will affect potentially ASR-reactive particles in
We identified ASR of rhyolite, a volcanic rock with high
amorphous and microcrystalline silica, to be the culprit of
blistering in both cases in Colorado. Rhyolite is an extrusive
rock, equivalent to plutonic granite, that is frequently present
in the gravel used by producers as coarse aggregate. In the
Colorado building that exhibited blistering only in repaired
floor areas, we determined that the coarse aggregate in the
repair concrete contained rhyolite. We identified ASR of chert
Fig. 3: Cross sections of floor coating showing blisters and
underlying ASR-reactive chert particles (red arrows). Two flooring
layers are observed. The base layer is a polymer layer (up to 10 mil
0.3 mm thick) with white portland cement and small amounts of
finely ground limestone. The top layer is a 120 mil (3 mm) thick layer
consisting of an organic matrix, portland cement, and sand aggregate
Fig. 4: Rhyolite aggregate: (a) close-up view showing a cracked
particle below a blister with emanating cracks (yellow arrows); and
(b) thin-section photomicrograph showing ASR gel lines in
microcracks (red arrows) (Note: 100 mil = 2.5 mm)