Specification - Concrete Mix Design, Quality Control and Specification - page 119

is still not air entrained. ASTM C666 (especially Method A—freezing
and thawing in water) has been criticised for being too severe, resulting
in the rejection of concrete mixes that would have given acceptable field
performance. ASTM C666 may be a good test procedure for particularly
severe freeze-thaw conditions where concrete may be subject to freezing
and thawing in a saturated condition. However, rapid freeze-thaw cycles
from all six surfaces of a saturated prism is clearly not representative of the
exposure condition of most concrete elements in the field and alternative
procedures should be considered.
6.4 EARLY-AGE STRENGTH
Specifiers can use the maturity/equivalent age concept discussed in
Section 7.4 together with a thermocouple in the element of interest to esti-
mate the in situ strength as early age. Another good procedure is the use
of pullout tests on the actual structure, such as the Lok test that can be
used on high-strength concrete. The pullout strength of the hardened in
situ concrete can be measured in accordance with ASTM C900 using a sys-
tem with proven ability to assess concrete with cylinder/cube strength of at
least higher than the expected maximum. Systems based on high-strength
embedded inserts (such as L-43 inserts and the Lok test) or postinstalled
pullout systems (such as the Capo test) can be used. Where the reliability of
compliance sampling and testing is in question, the in situ strength of the
structural elements should be determined.
6.5 DRYING SHRINKAGE
Specifying a particular drying shrinkage requirement for concrete is quite
common. Indeed the Australian standard requires the drying shrinkage at
56 days to be less than 1000 microstrain for all concrete. However, there is
trend for specifiers to place more onerous shrinkage limits on concrete. While
the drying shrinkage test is an expensive and time-consuming procedure,
it is one of nonstrength tests that is regularly used during construction.
But how useful is standard shrinkage testing to reduce cracking in con-
crete structures? As with freeze-thaw testing, the standard shrinkage test
is far more severe than most field applications with concrete specimens
with a 75 mm × 75 mm cross section drying from all sides into a well-
ventilated 50% relative humidity environment. Increasing the dimensions
of a concrete element and restricting drying to one or two faces greatly
reduces the amount of shrinkage and more important the rate allowing
creep effects to reduce the strain (ACI 209, CEB Model Code 1990). A more
serious problem with the shrinkage test is the small test specimens that