Fundamentals of Concrete - Design and Control of Concrete Mixtures - page 18

cracking. Only concretes of particular chemical makeup are
affected when they have achieved high temperatures,
usually after the first few hours of placement (between
70°C and 100°C [158°F to 212°F] depending on the concrete
ingredients and the time the temperature is achieved after
casting). This can occur because the high temperature
decomposes any initial ettringite formed and holds the
sulfate and alumina tightly in the calcium silicate hydrate
(C-S-H) gel of the cement paste. The normal formation of
ettringite is thus impeded.
In the presence of moisture, sulfate desorbs from the
confines of the C-S-H and reacts with calcium monosulfo-
aluminate to form ettringite in cooled and hardened
concrete. After months or years of desorption, ettringite
forms in confined locations within the paste. Such ettringite
can exert crystallization pressures because it forms in a
limited space under supersaturation. One theory: since
concrete is rigid and if there are insufficient voids to accom-
modate the ettringite volume increase, expansion and
cracks can occur. In addition, some of the initial (primary)
ettringite may be converted to monosulfoaluminate at high
temperatures and upon cooling revert back to ettringite.
Because ettringite takes up more space than monosulfoalu-
minate from which it forms, the transformation is an expan-
sive reaction. The mechanism causing expansion in the
paste is not fully understood at this time; the true influence
of ettringite formation on this expansion is still being inves-
tigated. Some research indicates that there is little relation-
ship between ettringite formation and expansion.
As a result of an increase in paste volume, separation
of the paste from the aggregates is usually observed with
heat induced delayed expansion. It is characterized by the
development of rims of ettringite around the aggregates
(Fig. 1-36). At early stages of heat induced delayed expan-
sion, the voids between paste and aggregate are empty (no
ettringite present). It should be noted that concrete can
sustain a small amount of this expansion without harm.
Only extreme cases result in cracking, and often heat
induced delayed expansion is associated with other deteri-
oration mechanisms, especially alkali-silica reactivity.
Only concretes in massive elements that retain the heat
of hydration or elements exposed to very high tempera-
tures at an early age are at risk of HIDE; and of these only
a few have the chemical makeup or temperature profile to
cause detrimental expansion. Normal sized concrete
elements cast and maintained near ambient temperatures
cannot experience HIDE when sound materials are used.
Fly ash and slag may help control heat induced
delayed expansion, along with control over early-age
temperature development. For more information, see Lerch
(1945) , Day (1992) , Klemm and Miller (1997) , Thomas
(1998) , and Famy (1999) .

Fig. 1-36. Heat induced delayed expansion is characterized
by expanding paste that becomes detached from non-
expansive components, such as aggregates, creating gaps
at the paste-aggregate interface. The gap can subsequently
be filled with larger opportunistic ettringite crystals as
shown here. Photo courtesy of Z. Zhang and J. Olek. (69154)

REFERENCES

Abrams, D. A., Design of Concrete Mixtures, Lewis Institute,
Structural Materials Research Laboratory, Bulletin No. 1,
PCA LS001 , http://www.portcement.org/pdf_files/LS001
.pdf , 1918, 20 pages.

Abrams, M. S., and Orals, D. L., Concrete Drying Methods
and Their Effect on Fire Resistance, Research Department
Bulletin RX181 , Portland Cement Association, http://
www.portcement.org/pdf_files/RX181.pdf , 1965.

ACI Committee 201, Guide to Durable Concrete, ACI 201.2R-
92, American Concrete Institute, Farmington Hills,
Michigan, 1992.

ACI Committee 318, Building Code Requirements for
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Concrete Institute, Farmington Hills, Michigan, 2002.

ACI Manual of Concrete Practice, American Concrete
Institute, Farmington Hills, Michigan, 2001.

Backstrom, J. E.; Burrow, R. W.; and Witte, L. P., Investigation
into the Effect of Water-Cement Ratio on the Freezing-Thawing
Resistance of Non-Air- and Air-Entrained Concrete, Concrete
Laboratory Report No. C-810, Engineering Laboratories
Division, U.S. Department of the Interior, Bureau of
Reclamation, Denver, November 1955.

Brinkerhoff, C. H., “Report to ASTM C-9 Subcommittee III-
M (Testing Concrete for Abrasion) Cooperative Abrasion
Test Program,” University of California and Portland
Cement Association, 1970.