Table of Contents
Limitations of Schmidt Hammer Test
Limitations of Schmidt Hammer Test: While the rebound hammer is an efficient and economical method of testing the uniformity of concrete, it does have significant disadvantages that must be understood. The Schmidt rebound hammer’s findings are influenced by:
Smoothness of the Tested Surface
The texture of the surface has a major impact on the quality of the test findings. When a test is conducted on a rough textured surface, the plunger tip induces severe crushing, which results in a decreased rebound amount. More precise findings can be achieved by smoothing a rough surface with a carborundum stone. It has been shown that trowelled surfaces or surfaces formed against metal forms provide rebound values that are 5 to 25% greater than surfaces formed against wooden forms.
This implies that in order to use such surfaces, a unique correlation curve or correction map must be created. Additionally, trowelled surfaces provide a greater scatter of individual outcomes, implying a lower degree of confidence in approximate power.
Test Specimens’ Dimensions, Shape, and Rigidity
Where the concrete segment or test specimen is short, such as a thin beam, a wall, a 152-mm cube, or a 150 by 300-mm cylinder, any movement caused by the impact reduces the rebound readings. In these instances, the member must be held rigidly or backed up by a significant mass.
Mitchell and Hoagland demonstrated that the restraining load required to maintain a steady rebound number tends to differ with the individual specimen. The successful restraining load, on the other hand, tends to be approximately 15% of the ultimate strength of 152 by 305-mm cylinders (Figure 1).
Zoldners, Greene, and Grieb recorded effective stresses of 1, 1.7, and 2.0 MPa, respectively, which are significantly less than Mitchell and Hoagland’s 15% value.
The Test Specimen’s Age
According to Kolek, the rate in gain of surface hardness in concrete is rapid up to the age of seven days, after which there is little or no gain in surface hardness; however, for properly cured concrete, there is considerable strength gain beyond seven days.
Zoldners and Victor confirmed that for equal strength, 7-day-old concrete has a higher rebound value than 28-day-old concrete. It is stressed that when testing old concrete, direct correlations between the rebound values recorded on the structure and the compressive strength of cores extracted from the structure are required.
The Schmidt hammer should not be used to measure low-strength concrete at an early age or when the concrete strength is less than 7 MPa, as the rebound values are too low for precise readings and the test hammer does significant damage to the concrete surface. Figure 2 illustrates blemishes produced by rebound testing on the surfaces of concrete cylinders aged 8 hours and 3 days.
Concrete’s Surface and Internal Moisture Conditions
The saturation level of the concrete and the presence of surface moisture have a significant impact on how the test hammer effects are evaluated. Zoldners showed that when well-cured, air-dried specimens are immersed in water and measured in the saturated surface-dried state, they exhibit rebound readings that are five points lower than when tested dry.
When the same specimens were left in a room set to 70 degrees Fahrenheit (21.1 degrees Celsius) and allowed to air dry, they recovered three points in three days and five points in seven days. Klieger demonstrated that for a 3-year-old concrete, variations in rebound numbers of up to 10 to 12 points occurred between specimens preserved in a wet state and laboratory-dry samples. This variation in rebound values corresponds to a compressive strength difference of approximately 14 MPa.
When the current status of the field concrete or specimens is unknown, it is recommended that the surface be presaturated several hours prior to testing and the correlation used with experiments on saturated surface-dried specimens.
Type of Coarse Aggregate
It is well accepted that the rebound number is influenced by the aggregate type used. According to Klieger, concretes made with crushed limestone coarse aggregate have a rebound value roughly 7 points lower than concretes made with gravel coarse aggregate, corresponding to a compressive strength differential of approximately 7 MPa. Grieb demonstrated that even though the coarse aggregate is the same form, if it is derived from different sources, distinct correlation curves are needed.
Figure 3 illustrates the findings of one such analysis, in which four distinct types of gravel were used to construct the concrete cylinders under evaluation. The spread of compressive strength between the correlation curves at similar rebound numbers ranged between 1.7 and 3.9 MPa. Greene discovered that when the test hammer was used on specimens and structures made of lightweight concrete, the effects were highly variable.
For example, lightweight concrete made with expanded shale aggregate had a different rebound number than concrete made with pumice aggregate at the same compressive strength. However, the rebound values for any particular kind of lightweight aggregate concrete were found to be proportional to the compressive power.
The type of concrete used has a major effect on the rebound readings. Real strengths of high alumina cement concrete can be 100 percent greater than those achieved using a correlation curve dependent on ordinary portland cement concrete. Additionally, super sulfated cement concrete may have a 50% lower strength than the correlation curves for ordinary portland cement concrete.
Type of Mold
Mitchell and Hoagland conducted research to ascertain the impact of concrete mold type on the rebound number. As cylinders cast in steel, tin can, and paper carton molds were examined, no noticeable variation in rebound readings was observed between those cast in steel and tin can molds, but paper carton-molded specimens had a higher rebound value.
This is likely because paper molds absorb moisture from fresh concrete, reducing the water-cement ratio at the surface and resulting in increased strength. Due to the hammer’s use as a surface hardness tester, it is possible for the hammer to show an unrealistically strong strength in such situations.
Therefore, it is proposed that if paper carton molds are used in the area, the hammer’s strength can be compared to that of test cylinders cast in identical molds.
Concrete Surface Carbonation
Carbonation on the surface of concrete has a noticeable effect on the Schmidt rebound hammer test results. Carbonation effects are more severe in older concretes, where the carbonated layer can be several millimeters thick, or even up to 20 millimeters thick in extreme cases.
The rebound values obtained in these situations can be up to 50% greater than those obtained on an uncarbonated concrete floor. Appropriate corrective factors should be identified in such situations, otherwise the concrete strength would be overestimated.