UFC 3-260-02
30 June 2001
third-point flexural strength from the concrete compressive strength). However, the variation of the data
upon which such relations are based is quite large and the results too inaccurate to allow the use of such
relations reliably for military airfield pavement design. The different tests respond differently to changes
in the concrete mixture. For example, flexural tests are much more sensitive to inclusion of crushed
aggregates in the mixture than are compressive strength tests. It is possible to develop very good
correlations between the different tests if the correlation is based on tests on the specific concrete
mixture and the same materials are used in the laboratory as will be used in the field mixture. However,
simply changing an aggregate source can change the correlation. Correlations are allowed for quality
control testing of military concrete pavements during construction, but the correlations must be
developed for the specific concrete mixture being used on the project, and the mixture constituents used
during construction must be the same as used to develop the correlation in the laboratory.
(3) Selection of Design Strength. The designer should base the pavement thickness design on
a strength that is readily achievable with local materials. Design strengths on past projects at the base
or discussions with local producers should allow selection of a design strength that is readily achievable
with local materials. If no such information is available, some trial laboratory mixtures should be
prepared to evaluate local aggregate sources. Traditionally, pavement thickness design for military
airfields is based on the 90-day strength of laboratory-cured specimens. This lengthy cure time takes
maximum advantage of the long-term gradual strength gain characteristic of conventional portland-
cement concrete. On many rehabilitation projects today, pavements are returned to the user after much
shorter periods. Consequently, design strengths are often specified based on these shorter periods
when the pavement is returned to the user. Fly ash and GGBF slag gain strength more slowly than
portland cement, so the designer must be aware that strength tests at early ages for concrete mixtures
containing these materials may not reflect the ultimate long-term strength well at all. Specifying very
high strengths, particularly at early ages, usually requires very rich mixtures with liberal use of
admixtures. This may introduce workability and construction problems, excessive shrinkage, or other
undesirable characteristics that negate the economies of higher strength. In general, design ASTM C 78
flexural strengths of 414 to 448 MPa (600 to 650 psi) are readily achievable with most local materials,
and the designer should use higher design strengths only with caution.
f. Special Airfield Exposure Conditions. Properly proportioned, placed, and cured portland-cement
concrete requires no surface sealers, coatings, or treatments to withstand normal military aircraft
operations such as startup, warmup, taxiing, takeoff, and landing.
(1) Heat Effects on Portland-Cement Concrete. Rapid heating of moist concrete can vaporize
water in the concrete capillaries and cause explosive spalling. As the concrete temperature begins to
rise above about 149 oC (300 oF), the progressive cement paste dehydration, thermal incompatibilities
between paste and aggregate, and aggregate deterioration lead to irreversible damage and progressive
loss of strength that is more pronounced as the temperature rises. Aggregates have a major impact on
the thermal behavior of concrete and in decreasing order of desirability for thermal resistance they are
lightweight aggregates (e.g., expanded slags, clays, and shales or natural pumice or scoria), fine-
grained igneous rocks such as basalt or diabase, calcareous aggregates, and siliceous aggregates.
Including slag cements in the concrete mixture also seems to enhance thermal resistance. Heat
resistant conventional concrete can be achieved by proper mixture proportioning, use of appropriate
aggregates, inclusion of slag cement, and high-quality concrete placement, finishing, and curing.
However, if the concrete temperature will reach 204 oC (400 oF), conventional concrete probably will not
be sufficient, and thermal cycling at lower temperatures can cause damage. HQUSACE (CEMP-ET),
appropriate Air Force MAJCOM pavements engineer, or Naval Facilities Engineering Service Center
should be consulted for guidance for concrete that will be exposed to high temperatures or that will be
exposed to repeated cycles of high thermal exposure. Concrete is a moderately good insulator so there
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