There are several methods for testing the strength of hardened concrete. The two methods described here involve testing cylinders and beams. Some test methods are completely nondestructive and some are slightly destructive, leaving holes that are relatively small and easily patched. The individuals performing these tests should be qualified and have the appropriate certifications called for in the ASTM standard test methods.
The easiest and most common test to perform is making a cylinder of fresh, consolidated concrete, and then testing the hardened specimen at the specified age. In the February issue, we discussed the differences between field-cured and standard-cured cylinders.
One of the common requirements for casting strength specimens is that molds be cylindrical and have a height-to-diameter ratio of 2:1. Molds for forming concrete test cylinders vertically must conform to ASTM C470. Cylinders made in accordance with ASTM C31 can be tested to failure in the vertical position for compressive strength (ASTM C39) or instrumented and tested for Modulus of Elasticity and Poisson’s Ratio (ASTM C469). Cylinders can be placed horizontally and tested for splitting tensile strength (ASTM C496). Cylinders also can be weighed and tested for unit weight, which often is done for lightweight concrete (ASTM C567).
Cylinder molds can have diameters from 2 to 36 inches, but the most common sizes are 2-, 3-, 4-, and 6-inch diameters. The 6x12-inch cylinder has been the standard of the industry for many years, but after extensive testing, 4x8-inch cylinders now are recognized as suitable by ACI to be used for compressive strength acceptance testing. Keep in mind that the diameter of the mold must be at least three times the nominal maximum size of the coarse aggregate. The 3x6-inch molds are not commonly used, but the 2x4-inch molds can be used for mortar strength testing (ASTM C780).
One caution—never use impervious (plastic) molds for making masonry grout specimens. For instance, on one project all the grout tests were failing, even though the ready-mix supplier had a proven history of satisfactory results with that mix. But the testing lab for the job used 3x6-inch plastic molds. When the test specimens were cast in molds with absorbent sides in accordance with ASTM C1019, the grout passed. Absorbent molds let some of the mix water leave the grout, which is similar to what happens when grout is placed in a masonry block cavity.
Beam specimens of concrete, cast per ASTM C31, and hardened in the horizontal position, are used for flexural strength testing. The most common size of beam mold is 6x6 inches and 20 to 21 inches long. As with cylinders, the smallest cross-sectional dimension must be at least three times the nominal maximum size of the coarse aggregate. Beams are most commonly used for slabs and pavements, especially airport pavements. Flexural strength is most often tested with third-point loading (ASTM C78) and is specified by the FAA for airport pavements. Center-point loading (ASTM C293) is performed less often, and yields significantly higher results than third-point loading.
Cast-in-place pop-out cylinders
Cast-in-place pop-out cylinders (CIPPOC, ASTM C873) is a field-cured cylinder that is used infrequently. CIPPOCs are used in slab construction for estimating the load-carrying capacity of slabs, determining the time of shore and form removal, estimating in-place compressive strength before stressing post-tensioned concrete, and determining the effectiveness of curing and protection. CIPPOC molds are attached to the plywood or bottom deck form. The concrete in the CIPPOCs is cast, consolidated, and cured at the same time as the concrete for the rest of the slab. The specimens can be removed at any time, taken to the lab, and tested for compressive strength.
The molds are made of four separate plastic pieces. The center mold that forms the test specimen typically has an inside diameter of 4 inches, and is either 4 or 6 inches deep. The fixed outer member is attached by a flange to the bottom slab form. Between the outer member and the specimen holder is a threaded adjustable sleeve, which allows the specimen height to be raised or lowered so that it is at the same elevation as the top of slab. CIPPOCs also have a bottom plug. Once the bottom slab support is removed, the bottom plug can be unscrewed, leaving a 4-inch pipe chase. Otherwise, the plug can be left in, and the cavity filled with concrete.
Cores taken from a concrete structure (ASTM C42) can give reliable results, as long as the test specimens are not taken until the concrete is strong enough to permit sample removal without disturbing the bond between the mortar and the coarse aggregate. ASTM C843 provides guidance for developing a plan to examine and obtain samples of hardened concrete from a structure.
Cores often are used to obtain strength information on older concrete structures. Core samples also are requested when questions arise as to the in-place concrete quality due to either low-strength test results during construction or to signs of distress in the structure.
Coring concrete is not exactly nondestructive, but cores can be taken in less critical locations of the member with the help of the structural engineer. Concrete imaging radar can be used to avoid rebar, conduits, and post-tensioned cables, and the core holes can be filled so they are nearly unnoticeable.
Many times, if the 28-day compressive strength from lab results is 100 or 200 psi below the required f´c (specified compressive strength), the architect or structural engineer will demand that cores be taken. ACI 318, Section 126.96.36.199 states: “Strength level of an individual class of concrete shall be considered satisfactory if both of the following requirements are met: (a) Every arithmetic average of any three consecutive strength tests equals or exceeds f´c; (b) No individual strength test (average of two cylinders) falls below f´c by more than 500 psi when f´c is 5000 psi or less; or by more than 0.10 f´c when f´c is more than 5000 psi.”
This means, for instance, if the structural engineer chose a safe f´c that is good for all of the columns in the structure, he should check his calculations to see if an individual column or columns with low compressive strength test results are adequate at the lower strength. There is no reason to be dogmatic in insisting that every test meets f´c if it is not really needed.
ACI 318, Section 5.6.5 then states that if either of the provisions of 188.8.131.52 is not met, then steps should be taken to assure that the load-carrying capacity of the structure is not jeopardized. The Code Commentary states that “the building official should apply judgment as to the significance of low strength results and whether they indicate need for concern. If further investigation is deemed necessary, such investigation may include nondestructive tests, or in extreme cases, strength tests of cores taken from the structure” (emphasis added).
In next month’s column, maturity testing and techniques for in-place strength evaluation will be covered.