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1.4.3 \x05Age of Test Specimen
Kolek10 has indicated that the rate of gain of surface hardness of concrete is rapid up to the age of 7 days,following which there is little or no gain in the surface hardness; however,for a properly cured concrete,there is significant strength gain beyond 7 days.It has been confirmed by Zoldners14 and Victor16 that for equal strength,higher rebound values are obtained on 7-day-old concrete than on 28-day-old concrete.It is emphasized that when old concrete is to be tested,direct correlations are necessary between the rebound numbers taken on the structure and the compressive strength of cores taken from the structure.
\x05The use of the Schmidt hammer for testing low-strength concrete at early ages,or where concrete strength is less than 7 MPa,is not recommended because rebound numbers are too low for accurate reading and the test hammer badly damages the concrete surface.14 Figure 1.10 shows blemishes caused by rebound tests on surfaces of 8-h-old and 3-day-old concrete cylinders.
FIGURE 1.10 Eight-hour-old (left) and the three-day-old (right) specimens showing surface blemishes after Schmidt hammer impact.(Adapted from Reference 14.)
Surface Hardness Methods\x051-9
1.4.4\x05Surface and Internal Moisture Condition of the Concrete
The degree of saturation of the concrete and the presence of surface moisture have a decisive effect on the evaluation of test hammer results.11,16,17 Zoldners13 has demonstrated that well-cured,air-dried specimens,when soaked in water and tested in the saturated surface-dried condition,show rebound readings 5 points lower than when tested dry.When the same specimens were left in a room at 70˚F (21.1˚C) and air dried,they recovered 3 points in 3 days and 5 points in 7 days.Klieger et al.18 have shown that for a 3-year-old concrete differences up to 10 to 12 points in rebound numbers existed between specimens stored in a wet condition and laboratory-dry samples.This difference in rebound numbers represents approximately 14 MPa difference in compressive strength.
\x05It is suggested that,whenever the actual condition of the field concrete or specimens is not known,it would be desirable to presaturate the surface several hours prior to testing and use the correlation for tests on saturated surface-dried specimens.
1.4.3 \x05Age of Test Specimen
Kolek10 has indicated that the rate of gain of surface hardness of concrete is rapid up to the age of 7 days,following which there is little or no gain in the surface hardness; however,for a properly cured concrete,there is significant strength gain beyond 7 days.It has been confirmed by Zoldners14 and Victor16 that for equal strength,higher rebound values are obtained on 7-day-old concrete than on 28-day-old concrete.It is emphasized that when old concrete is to be tested,direct correlations are necessary between the rebound numbers taken on the structure and the compressive strength of cores taken from the structure.
\x05The use of the Schmidt hammer for testing low-strength concrete at early ages,or where concrete strength is less than 7 MPa,is not recommended because rebound numbers are too low for accurate reading and the test hammer badly damages the concrete surface.14 Figure 1.10 shows blemishes caused by rebound tests on surfaces of 8-h-old and 3-day-old concrete cylinders.
FIGURE 1.10 Eight-hour-old (left) and the three-day-old (right) specimens showing surface blemishes after Schmidt hammer impact.(Adapted from Reference 14.)
Surface Hardness Methods\x051-9
1.4.4\x05Surface and Internal Moisture Condition of the Concrete
The degree of saturation of the concrete and the presence of surface moisture have a decisive effect on the evaluation of test hammer results.11,16,17 Zoldners13 has demonstrated that well-cured,air-dried specimens,when soaked in water and tested in the saturated surface-dried condition,show rebound readings 5 points lower than when tested dry.When the same specimens were left in a room at 70˚F (21.1˚C) and air dried,they recovered 3 points in 3 days and 5 points in 7 days.Klieger et al.18 have shown that for a 3-year-old concrete differences up to 10 to 12 points in rebound numbers existed between specimens stored in a wet condition and laboratory-dry samples.This difference in rebound numbers represents approximately 14 MPa difference in compressive strength.
\x05It is suggested that,whenever the actual condition of the field concrete or specimens is not known,it would be desirable to presaturate the surface several hours prior to testing and use the correlation for tests on saturated surface-dried specimens.
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