The principal properties of concrete with which designers are concerned and symbols commonly used for some of these properties are:
Other properties, frequently important for particular conditions are: durability to resist freezing and thawing when wet and with deicers, color, surface hardness, impact hardness, abrasion resistance, shrinkage, behavior at high temperatures (about 500F), insulation value at ordinary ambient temperatures, insulation at the high temperatures of a standard fire test, fatigue resistance, and for arctic construction, behavior at cold temperatures (-60 to -75F). For most of the research on these properties, specially devised tests were employed, usually to duplicate or simulate the conditions of service anticipated. (See ‘‘Index to Proceedings of the American Concrete Institute.’’)
In addition to the formal testing procedures specified by ASTM and the special procedures described in the research references, some practical auxiliary tests, precautions in evaluating tests, and observations that may aid the user in practical applications follow.
Compressive Strength, . The standard test (ASTM C39) is used to establish the ƒc quality of concrete, as delivered, for conformance to specifications. Tests of companion field-cured cylinders measure the effectiveness of the curing (Art. 9.14).
Core tests (ASTM C42) of the hardened concrete in place, if they give strengths higher than the specified ƒ’c or an agreed-on percentage of ƒ’c (often 85%), can be used for acceptance of material, placing, consolidation, and curing. If the cores
taken for these tests show unsatisfactory strength but companion cores given accelerated additional curing show strengths above the specified , these tests estab- ƒ’c lish acceptance of the material, placing, and consolidation, and indicate the remedy, more curing, for the low in-place strengths.
For high-strength concretes, say above 5000 psi, care should be taken that the capping material is also high strength. Better still, the ends of the cylinders should be ground to plane.
Indirect testing for compressive strength includes surface-hardness tests (impact hammer). Properly calibrated, these tests can be employed to evaluate field curing.
(See also Art. 9.14.)
Modulus of Elasticity Ec. This property is used in all design, but it is seldom determined by test, and almost never as a regular routine test. For important projects, it is best to secure this information at least once, during the tests on the trial batches at the various curing ages. An accurate value will be useful in prescribing camber or avoiding unusual deflections. An exact value of Ec is invaluable for longspan, thin-shell construction, where deflections can be large and must be predicted accurately for proper construction and timing removal of forms.
Tensile Strength. The standard splitting test is a measure of almost pure uniform tension ƒct. The beam test (Fig. 9.4a) measures bending tension ƒr on extreme surfaces (Fig. 9.4b), calculated for an assumed perfectly elastic, triangular stress distribution.
The split-cylinder test (Fig. 9.4c) is used for structural design. It is not sensitive to minor flaws or the surface condition of the specimen. The most important application of the splitting test is in establishment of design values for reinforcingsteel development length, shear in concrete, and deflection of structural lightweight aggregate concretes.
The values of ƒct (Fig. 9.4d) and ƒr bear some relationship to each other, but are not interchangeable. The beam test is very sensitive, especially to flaws on the surface of maximum tension and to the effect of drying-shrinkage differentials, even between the first and last of a group of specimens tested on the same day. The value ƒr is widely used in pavement design, where all testing is performed in the same laboratory and results are then comparable.
Special Properties. Frequently, concrete may be used for some special purpose for which special properties are more important than those commonly considered.
Sometimes, it may be of great importance to enhance one of the ordinary properties.
These special applications often become apparent as new developments using new materials or as improvements using the basic materials. The partial list of special properties is constantly expanding—abrasion and impact resistance (heavy-duty floor surfacings), heat resistance (chimney stacks and jet engine dynamometer cells), light weight (concrete canoes), super-high-compressive strength, over ksi (high-rise columns), waterproof concrete, resistance to chemical attack (bridge decks, chemical industry floors, etc.), increased tensile strength (highway resurfacing, precast products, etc.), shrinkage-compensating concrete (grouting under base plates), etc. Some of these special properties are achieved with admixtures (see Art. 9.9). Some utilize special cements (high-alumina cement for heat resistance or expansive cement for shrinkage-compensating concrete). Some utilize special aggregates (lightweight aggregate, steel fiber, plastic fiber, glass fiber, and special heavy aggregate). (See ‘‘State-of-the-Art Report on Fiber Reinforced Concrete,’’ ACI 544.1R). Some special properties—increased compressive and tensile strength, waterproofing, and improved chemical resistance are achieved with polymers, either as admixtures or surface treatment of hardened concrete. (See ‘‘Guide for the Use of Polymers in Concrete,’’ ACI 548.1R.)