Rebound hammer test
The rebound hammer or Schmidt hammer (Figure 43) gives a comparative measure of concrete quality as indicated by surface hardness, and can enable an approximate estimate to be made of concrete strength provided that users have prepared their own calibration charts by recording, as part of their quality control routine, the results of regular tests with the hammer on cubes and units made from the same concrete.
To standardize readings as far as possible it is recommended that they should be taken on cubes held in the testing machine under a stress of about 7 N/mm2. Since readings taken on a trowelled face are often more variable than those on a moulded face, it is recommended that the hammer should be used on a vertical face of the cube as cast.
To standardize readings as far as possible it is recommended that they should be taken on cubes held in the testing machine under a stress of about 7 N/mm2. Since readings taken on a trowelled face are often more variable than those on a moulded face, it is recommended that the hammer should be used on a vertical face of the cube as cast.
A rebound hammer may also be useful for indicating the variability of concrete in a structure as a guide to the significance of a core test. The use of rebound hammers is described in BS 1881: Part 202 and BS EN 12504-2.
Ultrasonic test
Ultrasonic tests give a comparative measure of concrete quality as indicated by the time taken by an ultrasonic pulse to travel through a section of concrete. Ultrasonic equipment, such as the Pundit (Figure 44) is portable and consistent in its behaviour. Its use is described in BS 1881: Part 203 and BS EN 12504-1.
The test consists of measuring the velocity of an ultrasonic pulse through the concrete. A transducer placed in close contact with the surface of the concrete transmits vibrations into the concrete that are picked up by another transducer on the opposite face of the specimen or member under test. The time taken by the pulse to travel through the concrete is accurately measured by the apparatus, and the velocity calculated knowing the distance between the transducers (thickness of member).
This method of test is essentially comparative and, because so many factors affect the pulse-velocity/strength relationship, it will not generally be possible to relate pulse-velocity values and strength without knowledge of the site conditions and constituents of the concrete under test. Interpretation of the results requires care and experience.
Pull-out test
This test can determine the strength of concrete that is less than three days old. It uses pull-out inserts that are cast into the slab or column, and the load is applied through a manually operated jack (Figure 45). The peak tensile force is recorded and correlated against the equivalent concrete cube strength. For more information see BCA publication Early age strength assessment of concrete on site, and CIRIA Report 1 36.
Electromagnetic covermeter
The covermeter (Figure 46) described in BS 1881: Part 204, is a non- destructive method of locating the depth, size and direction of reinforcement in hardened concrete. It is a portable electromagnetic instrument. Recent models have a working range of up to 100 mm cover and operate from rechargeable batteries. A typical instrument consists of a search head connected to the main unit containing the battery circuits and display panel. When the instrument is switched on, or switched from one scale to another, it is necessary to place the search head well away from any steel and to calibrate the instrument by adjusting the 'zero set'. The search head is then positioned on the surface of the concrete and, if any steel exists below the surface within a depth of 100 mm, the depth will be shown, usually by digital display.
The search head is rotated and moved methodically across the surface until the minimum reading is obtained: this reading indicates the depth of cover and the direction of the steel. Some covermeters also give a strong audible signal that assists in locating bars by rising when approaching steel.
The accuracy likely to be obtained on the average site can be within about ±10% or ±2 mm, whichever is the greater, when used to detect normal reinforcement. Calibration by breaking out some concrete to reveal the reinforcement is recommended in all cases and is essential when very small bars or stainless steel are being detected. Calibration methods are given in the standard.
Analysis of fresh concrete
The Rapid Analysis Machine (RAM) (Figure 47) is a floor-mounted unit which enables the cement content of a sample of fresh concrete to be easily and rapidly determined in accordance with BS 1881: Part 128. It provides a practical and accurate method of analysing a concrete for cement content in the short time available after mixing and before placing the concrete. The total time for carrying out a test from loading the RAM to reading off the cement content from a calibration graph is less than ten minutes.
The operating cycle of the RAM is fully automatic, and is controlled by an electronic timer that is started after the machine has been loaded with an 8 kg sample of fresh concrete. Water is pumped through the sample at a carefully controlled rate to separate the cement size particles and wash them up and over the top of the elutriation column. Three sampling channels at the top of the column remove 10% of the cement slurry, with the remainder going to waste.
The 10% sample passes through a 150 µm vibrating sieve, which removes all the small particles greater than cement size from the slurry. After these particles have been removed, the slurry passes into a conditioning vessel where chemical agents are stirred into the slurry causing the cement particles to cling together and drop out of suspension to the bottom of the vessel. Excess water is removed by siphons until a constant volume is obtained in the removable pot at the bottom of the conditioning vessel. After all the cement particles have settled, this vessel is removed from the RAM and weighed on a balance (to an accuracy of 1 g), and the cement content of the concrete sample is determined from a calibration graph.
Modules are available for determining the water/cement ratio of the concrete and for determining the quantities of any mineral additions.