Strength Tests

Generally, concrete quality is measured by the specified compressive strength of Æ’’c 6- x12-in cylinders after 28 days of laboratory curing.

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 Æ’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 80F 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 tractor and owner are justifiably annoyed when costly later tests on hardened concrete, after an even more costly project delay, indicate that the original freshconcrete test specimens were defective and not the building concrete.

Special Tests. Many other strength tests or tests for special qualities are occasionally employed for special purposes. Those most often encountered in concrete building construction are strength tests on drilled cores and sawed beams (ASTM C42); impact tests (ASTM C805), e.g., Schmidt hammer; pullout tests (ASTM C900); penetration tests (ASTM C803); determination of modulus of elasticity during the standard compression test; and deflection measurements on a finished building element under load (Chap. 20, ACI 318-99). (See also Commentary on ACI 318-99 and the Manual of Concrete Inspection, (ACI SP-2.)
Newer methods for evaluating in-situ strength of concrete include the following:
Methods, such as the one in which test cylinders are field-cured inside the in-situ concrete, measure compressive strength directly, refined even to measuring it in a desired direction. Others actually measure other properties, such as penetration, impact, or pullout, which are indirect measures of compressive strength, but may be employed because the property they measure is itself important. For example, in cantilevered form construction where forms for each new lift are bolted into the previous lift, pullout results may be more meaningful than standard compression tests. (See Testing Hardened Concrete, ACI Monograph No. 9, 1976.) Most of the in-situ tests may also be classified as accelerated tests, although not all accelerated tests are performed in situ.
Because construction time is continually becoming a more important factor in overall construction economy, the standard 28-day strength becomes less significant.

For example, the final strength at completion of a high-rise project requiring highstrength concrete in lower-story columns is often specified 90-days. At the other extreme, a floor system may be loaded by the forms and concrete for the floor above in as little as 2 days. These conditions demand accelerated testing. (See  Standard Specifications for Structural Concrete, ACI 301; and ASTM C684, Standard Test Method for Making, Accelerated Curing, and Testing Concrete Compression Test Specimens.)

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