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Hardened Concrete Tests to Consider

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Coring in progress. An R-meter was previously used to locate embedded reinforcing steel (shown marked in chalk). Core samples may be located to avoid or in some instances intercept steel.

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Core diameters for compressive strength testing should be at least 3.7 inches in diameter. Preferred core length is two times the diameter. Core lengths less than 95% of a core diameter should not be tested for compressive strength.

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Cores can be located to avoid rebar or intercept rebar location to gain insight on consolidation of concrete around steel and to confirm rebar size and position within the thickness of member.

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The rebound hammer is essentially a surface-hardness tester used to provide a quick, simple means of checking concrete uniformity (ASTM C 805). It measures the rebound of a spring-loaded plunger after it has struck a smooth concrete surface. The rebound number gives an indication of concrete compressive strength and stiffness. (Photo courtesy of the Portland Cement Association)

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The view a petrographer sees when microscopically examining the air-void system of concrete. The examination will provide total air content, air-void spacing, and specific surface of air voids in concrete. This information is compared to parameters known to be durable in freeze-thaw environments (ASTM C 457).

ASTM C 42, Obtaining and Testing Drilled Cores and Sawed Beams of Concrete.
This standard test method provides procedures for obtaining and testing specimens to determine compressive, splitting tensile, and flexural strength of in-place concrete. Common core diameters submitted for testing are 4 inches (actual diameter of 3.75 inches matching the inner diameter of a diamond-tipped core barrel). Core diameters should be a minimum of two times the maximum aggregate size. The preferred core diameter for a compressive strength specimen is three times the maximum aggregate size of the concrete (see section 7 of ASTM C 42). Length-to-diameter ratios are ideally 2:1, but this test method provides correction factors for ratios as low as 1:1. Note that for compressive strength to be considered structurally adequate, an average of 3 cores should be 85% of specified strength with no core falling below 75% of specified strength. Cores also allow a visible examination for general concrete characteristics, such as thickness of a slab if core is full depth, general degree of consolidation, aggregate distribution or signs of segregation.

ASTM C 174, Measuring Thickness of Concrete Elements Using Drilled Concrete Cores.
When slab thickness or member thickness is in question, measurement of core dimensions as described in this test method should be followed. When a test lab also has capabilities to conduct nondestructive testing, a good indication of slab thickness may also be determined with the use of ground penetrating radar (GPR).

ASTM C 805, Rebound Number of Hardened Concrete.
This is a frequently employed nondestructive test to assess concrete uniformity and estimate in-place compressive strength based on rebound numbers. Rebound numbers on tested concrete surfaces are correlated to compressive strength according to the vertical, horizontal or inclined direction of travel of the spring -loaded plunger. Calibration of rebound values with the actual project concrete test cylinders provides the most useful data. This test method is not intended as a basis for acceptance or rejection of concrete because of the inherent uncertainty in the estimated strength.

ASTM C 856, Petrographic Examination of Hardened Concrete.
This is probably one of the best examinations to consider since it provides information on the overall quality of a concrete. For example, it identifies the use of supplementary cementitious materials, such as fly ash or slag. Although not a mandatory parameter, many experienced petrographers can provide an estimate of the water-to-cement ratio of the concrete and estimate air content in the mix and relative distribution of air voids. Paste-aggregate bond, depth of carbonation, overall consolidation and many other concrete characteristics are also identified. Since the practice incorporates a microscopic examination of the concrete, it may identify concrete aspects not expected after the field observations of the problem and can redirect a test program to focus on a specific issue.

ASTM C 457, Air-Void Parameters in Hardened Concrete.
Issues of freeze-thaw durability are directly dependent on sufficient air content and a proper air-void system in the concrete. Recommended air content in concrete is correlated to the maximum aggregate size used in a mix. This test examines a cross-section of a concrete core, measuring air content and spacing factors of the microscopic air voids. In-place air-void characteristics are compared to established parameters known to provide durable concrete performance in a freeze-thaw environment. This is an important concrete characteristic to check, since having a sufficient amount of entrained air that is evenly distributed throughout the concrete plays a significant role in the durability of the material. ACI 318, "Building Code Requirements for Structural Concrete", Chapter 4 (available from the American Concrete Institute) provides recommended air contents based on the aggregate size used in a mix and the exposure class of the concrete.

ASTM C 1218, Water-Soluble Chloride Content in Concrete.
This test provides data concerning the water-soluble chloride content of concrete at the depth the concrete powder sample is taken. It is a frequently used test when premature freeze-thaw or corrosion problems are being examined. It may be useful to conduct tests near the concrete surface and toward mid-thickness of a slab. This helps indicate if chlorides were externally applied or originally added to the mix. ACI 318 provides chloride limits for new construction based on type of construction and test method selected to determine the percentage of chloride by mass of sample.

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