Building Design and Construction Handbook - Merritt - Ventech!

CONCRETE CONSTRUCTION 9.19
Conventional Tests. The strength tests performed after various periods of field
curing are typically specified to determine curing adequacy. For lightweightaggregate
concretes only, the same type of laboratory-cured test specimen is tested
for tensile splitting strength ƒ ct to establish design values for deflection, development
of reinforcing steel, and shear. Applicable ASTM specifications for these tests
are
C31, ‘‘Making and Curing Concrete Test Specimens in the Field.’’
C39, ‘‘Test for Compressive Strength of Cylindrical Concrete Specimens.’’
C496, ‘‘Test for Splitting Tensile Strength of Cylindrical Concrete Specimens.’’
The specifications for standard methods and procedures of testing give general
directions within which the field procedures can be adjusted to jobsite conditions.
One difficulty arises when the specimens are made in the field from samples taken
at the jobsite. During the first 48 h after molding, the specimens are very sensitive
to damage and variations from standard laboratory curing conditions, which can
significantly reduce the strength-test results. Yet, jobsite conditions may preclude
sampling, molding, and field storage on the same spot.
If the fresh-concrete sample must be transported more than about 100 ft to the
point of molding cylinders, some segregation occurs. Consequently, the concrete
sample should be remixed to restore its original condition. After the molds for test
cylinders have been filled, if the specimens are moved, high-slump specimens segregate
in the molds; low-slump specimens in the usual paper or plastic mold are
often squeezed out of shape or separated into starting cracks. Such accidental damage
varies with slump, temperature, time of set and molding, and degree of carelessness.
If the specimen cylinders are left on the jobsite, they must be protected against
drying and accidental impact from construction traffic. If a worker stumbles over a
specimen less than 3 days old, it should be inspected for damage. The best practice
is to provide a small, insulated, dampproofed, locked box on the site in which
specimens can be cast, covered, and provided with 60 to 80�F temperature and
100% humidity for 24 to 72 h. Then, they can be transported and subjected to
standard laboratory curing conditions at the testing laboratory. When transported,
the cylinders should be packed and handled like fresh eggs, since loose rattling will
have about an equivalent effect in starting incipient cracks.
Similarly, conditions for field-cured cylinders must be created as nearly like
those of the concrete in place as possible. Also, absolute protection against impact
or other damage must be provided. Because most concrete in place will be in much
larger elements than a test cylinder, most of the in-place concrete will benefit more
from retained heat of hydration (Fig. 9.5). This effect decreases rapidly, because
the rate of heat development is greatest initially. To ensure similar curing conditions,
field-cured test cylinders should be stored for the first 24 h in the field curing box
with the companion cylinders for laboratory curing. After this initial curing, the
field-cured cylinders should be stored near the concrete they represent and cured
under the same conditions.
Exceptions to this initial curing practice arise when the elements cast are of
dimensions comparable to those of the cylinders, or the elements cast are not protected
from drying or low temperatures, including freezing, or test cylinders are
cured inside the elements they represent (patented system).
These simple, seemingly overmeticulous precautions will eliminate most of the
unnecessary, expensive, project-delaying controversies over low tests. Both con-