Testing of Hardened Concrete
Many tests are used to evaluate the hardened concrete properties, either in the laboratory or in the field. Some of these tests are destructive, while others are nondestructive. Tests can be performed for different purposes; however, they are mostly conducted to control the quality of the concrete and to check specification compliance.
Probably the most common test performed on hardened concrete is the compressive strength test, since it is relatively easy to perform and since there is a strong correlation between the compressive strength and many desirable properties. Other tests include split tension, flexure strength, rebound hammer, penetration resistance, ultrasonic pulse velocity, and maturity tests.
Compressive Strength Test
The compressive strength test is the test most commonly performed on hardened concrete. Compressive strength is one of the main structural design requirements to ensure that the structure will be able to carry the intended load. As indicated earlier, compressive strength increases as the water–cement ratio decreases.
Since the water–cement ratio is directly related to the concrete quality, compressive strength is also used as a measure of quality, such as durability and resistance to weathering. Thus, in many cases, designers specify a high compressive strength of the concrete to ensure high quality, even if this strength is not needed for structural support. The compressive strength f′c of normal-weight concrete is between 20 and 40 MPa.
In the United States, the test is performed on cylindrical specimens and is standardized by ASTM C39. The specimen is prepared, either in the lab or in the field, according to ASTM C192 or C31, respectively. Cores could also be drilled from the structure following ASTM C42.
The standard specimen size is 0.15 m in diameter and 0.30 m high, although other sizes with a height–diameter ratio of two can also be used. The diameter of the specimen must be at least three times the nominal maximum size of the coarse aggregate in the concrete.
Specimens are prepared in three equal layers and are rodded 25 times per layer. After the surface is finished, specimens are kept in the mold for the first 24 ± 8 hours. Specimens are then removed from the mold and cured at 23 ± 1.7°C, either in saturated-lime water or in a moist cabinet having a relative humidity of 95% or higher, until the time of testing. Before testing, specimens are capped at the two bases to ensure parallel surfaces. High-strength gypsum plaster, sulfur mortar, or a special capping compound can be used for capping and is applied with a special alignment device (ASTM C617).
The specimens are tested by applying axial compressive load with a specified rate of loading until failure (Figure 1). The compressive strength of the specimen is determined by dividing the maximum load carried by the specimen during the test by the average cross-sectional area. The number of specimens and the number of test batches depend on established practice and the nature of the test program.
Usually three or more specimens are tested for each test age and test condition. Test ages often used are 7 days and 28 days for normal concrete.
Note that the test specimen must have a height–diameter ratio of two. The main reason for this requirement is to eliminate the end effect due to the friction between the loading heads and the specimen.
Thus, producing a zone of uniaxial compression within the specimen. If the height–diameter ratio is less than two, a correction factor can be applied to the results as indicated in ASTM C39. The compressive strength of the specimen is affected by the specimen size.
Increasing the specimen size reduces the strength because there is a greater probability of weak elements where failure starts in large specimens than in small specimens. In general, large specimens have less variability and better representation of the actual strength of the concrete than small specimens. Therefore, the 0.15-m by 0.30-m size is the most suitable specimen size for determining the compressive strength. However, some agencies use 0.10-m diameter by 0.20-m high specimens.
The advantages of using smaller specimens are the ease of handling, less possibility of accidental damage, less concrete needed, the ability to use a low-capacity testing machine, and less space needed for curing and storage.
Because of the strength variability of small specimens, more specimens should be tested for smaller specimens than are tested for standard-sized specimens.
In some cases, five 0.10-m by 0.20-m replicate specimens are used instead of the three replicates commonly used for the standard-sized specimens. Also, when small-sized specimens are used, the engineer should understand the limitations of the test and consider these limitations in interpreting the results. For example research by the
Missouri Department of Transportation recommended a strength correction factor of 0.94 when the smaller samples are used. This recommendation was for a specific application of one type of concrete and it should not be applied to other situations.
The interface between the hardened cement paste and aggregate particles is typically the weakest location within the concrete material. When concrete is stressed beyond the elastic range, microcracks develop at the cement paste–aggregate interface and continuously grow until failure.
Figure 2 shows a scanning electron microscope micrograph of the fractured surface of a hardened cement mortar cylinder at 500X. The figure shows that the cleavage fracture surfaces where sand particles were dislodged during loading. The figure also shows the microcracks around some sand particles developed during loading.
The split-tension test (ASTM C496) measures the tensile strength of concrete. In this test, a 0.15-m by 0.30-m concrete cylinder is subjected to a compressive load at a constant rate along the vertical diameter until failure, as shown in Figure 3.
Failure of the specimen occurs along its vertical diameter, due to tension developed in the transverse direction. The split tensile (indirect tensile) strength is computed as
T = 2P ÷ πLd (Equation 1)
Where T = tensile strength, MPa,
P = load at failure, N,
L = length of specimen, mm, and
d = diameter of specimen, mm
Typical indirect tensile strength of concrete varies from 2.5 to 3.1 MPa. The tensile strength of concrete is about 10% of its compressive strength.
Flexure Strength Test
The flexure strength test (ASTM C78) is important for design and construction of road and airport concrete pavements. The specimen is prepared either in the lab or in the field in accordance with ASTM C192 or C31, respectively. Several specimen sizes can be used.
However, the sample must have a square cross section and a span of three times the specimen depth. Typical dimensions are 0.15-m by 0.15-m cross section and 0.30-m span. After molding, specimens are kept in the mold for the first 24 ± 8 hours, then removed from the mold and cured at 23 ± 1.7°C, either in saturated-lime water or in a moist cabinet with a relative humidity of 95% or higher until testing.
The specimen is then turned on its side and centered in the third-point loading apparatus, as illustrated in Figure 4. The load is continuously applied at a specified rate until rupture.
If fracture initiates in the tension surface within the middle third of the span length, the flexure strength (modulus of rupture) is calculated as
R = Mc ÷ I (Equation 2)
Where R = flexure strength, MPa,
M = maximum bending moment = PL/6, N.mm,
c = d/2, mm,
I = moment of inertia = bd3 /12, mm4
P = maximum applied load, which is distributed evenly (1/2 to each) over the two loading points, N,
L = span length, mm,
b = average width of specimen, mm, and
d = average depth of specimen, mm
Note that third-point loading ensures a constant bending moment without any shear force applied in the middle third of the specimen.
Thus, Equation 2 is valid as long as fracture occurs in the middle third of the specimen. If fracture occurs slightly outside the middle third, the results can still be used with some corrections. Otherwise the results are discarded.
For normal-weight concrete, the flexure strength can be approximated as:
R = (0.62 to 0.83) √f′c MPa (Equation 3)
Rebound Hammer Test
The rebound hammer test, also known as the Schmidt hammer test, is a nondestructive test performed on hardened concrete to determine the hardness of the surface (Figure 5).
The hardness of the surface can be correlated, to some extent, with the concrete strength. The rebound hammer is commonly used to get an indication of the concrete strength. The device is about 0.3 m long and encloses a mass and a spring. The spring-loaded mass is released to hit the surface of the concrete. The mass rebounds, and the amount of rebound is read on a scale attached to the device. The larger the rebound, the harder is the concrete surface and, therefore, the greater is the strength.
The device usually comes with graphs prepared by the manufacturer to relate rebound to strength. The test can also be used to check uniformity of the concrete surface. The test is very simple to run and is standardized by ASTM C805.
To perform the test, the hammer must be perpendicular to a clean, smooth concrete surface. In some cases, it would be hard to satisfy this condition. Therefore, correlations, usually provided by the manufacturer, can be used to relate the strength to the amount of rebound at different angles.
Rebound hammer results are also affected by several other factors, such as local vibrations, the existence of coarse aggregate particles at the surface, and the existence of voids near the surface. To reduce the effect of these factors, it is desirable to average 10 to 12 readings from different points in the test area.
Penetration Resistance Test
The penetration resistance test, also known as the Windsor Probe test, is standardized by ASTM C803. The instrument (Figure 6) is a gun like device that shoots probes into the concrete surface in order to determine its strength.
The amount of penetration of the probe in the concrete is inversely related to the strength of concrete. The test is almost nondestructive since it creates small holes in the concrete surface. The device is equipped with a special template with three holes, which is placed on the concrete surface.
The test is performed in each of the holes. The average of the penetrations of the three probes through these holes is determined, using a scale and a special plate. Care should be exercised in handling the device to avoid injury. As a way of improving safety, the device cannot be operated without pushing hard on the concrete surface to prevent accidental shooting.
The penetration resistance test is expected to provide better strength estimation than the rebound hammer, since the penetration resistance measurement is made not just at the surface but also in the depth of the sample.
Ultrasonic Pulse Velocity Test
The ultrasonic pulse velocity test (ASTM C597) measures the velocity of an ultrasonic wave passing through the concrete (Figure 7). In this test, the path length between transducers is divided by the travel time to determine the average velocity of wave propagation. Attempts have been made to correlate pulse velocity data with concrete strength parameters.
No good correlations were found, since the relationship between pulse velocity and strength data is affected by a number of variables, such as age of concrete, aggregate–cement ratio, aggregate type, moisture condition, and location of reinforcement. This test is used to detect cracks, discontinuities, or internal deterioration in the structure of concrete.
Maturity of a concrete mixture is defined as the degree of cement hydration, which varies as a function of both time and temperature. Therefore, it is assumed that, for a particular concrete mixture, strength is a function of maturity.
Maturity meters have been developed to provide an estimate of concrete strength by monitoring the temperature of concrete with time. This test (ASTM C1074) is performed on fresh concrete and continued for several days. The maturity meter must be calibrated for each concrete mix.