A lot can be done with site visits and visual inspections. However, it is often laboratory work that tells the tale of the tape.
Petrographic exams use a branch of geology that deals with the descriptions and classifications of rocks. Hardened concrete is considered a synthetic sedimentary rock. Petrographic exams check for the following:
■ Aggregate condition
■ Pronounced cement-aggregate reactions
■ Deterioration of aggregate particles in place
■ Denseness of cement paste
■ Homogeneity of concrete
■ Settlement and bleeding of fresh concrete
■ Depth and extent of carbonation
■ Occurrence and distribution of fractures
■ Characteristics and distribution of voids
■ Presence of contaminating substances
Chemical analysis of hardened concrete can be used to estimate the cement content, original water –cement ratio, and the presence and amount of chloride and other admixtures. This is another form of testing that is part of the larger puzzle in determining the qualities of concrete.
Physical analysis is often done on core samples. This testing looks for nine elements:
■ Compressive strength
■ Modulus of elasticity
■ Poisson’s ratio
■ Pulse velocity
■ Direct shear strength of concrete bonded to foundation rock
■ Friction sliding of concrete on foundation rock
■ Resistance of concrete to deterioration caused by freezing and thawing
■ Air content and parameters of the air-void system
Nondestructive testing (NDT) is used to determine various relative properties of concrete. Strength, modulus of elasticity, homogeneity, and integrity of concrete can be calculated with NDT. There are many approaches to NDT, and they require inspectors to have expertise in the given approach to arrive at accurate data.
Earlier, we talked a little about rebound hammers. This is a form of NDT and a fast and simple way to test concrete. However, the test is imprecise and cannot accurately predict the strength of concrete. Some factors that can skew a test with a rebound hammer include the following:
■ Smoothness of a concrete surface
■ Moisture content
■ Type of course aggregate
■ Size, shape, and rigidity of specimen
Carbonation of concrete surface probes
Probes can be used to do NDT. The probe may use a powder cartridge to insert a high-strength steel probe into a section of concrete. The results of probe measurements can be converted to compressive strength values. There are reports, however, that probes can sometimes supply inaccurate data.
A probe is normally used to test density. A probe will embed deeper in concrete that is suffering from failure in density, subsurface hardness, and as the strength of concrete weakens. This type of testing is fine for on-site, general tests, but it is limited.
Precise measurements are not available from probe testing. The act of probing concrete will leave a hole in the concrete surface that must be repaired.
Ultrasonic pulse-velocity testing
|PUNDIT (Portable Ultrasonic Nondestructive Digital Indicating Tester)|
Image courtesy: University of Cyprus
Ultrasonic pulse-velocity testing is probably the most frequently used means of NDT. The results of this testing can be calculated. High velocities indicate good concrete, while low velocities reveal weak concrete. The system for this testing is portable and can penetrate about 35 linear feet of concrete. Testing of this type is fast. However, an inspector must have access to opposite sides of the section being tested, and this can present a problem.
Acoustic mapping provides comprehensive evaluation of the top surface wear of concrete in such structures as aprons, sills, lock chamber floors, and so forth. Fast, accurate evaluations of horizontal sections below water can be done with acoustic mapping. Dewatering is not needed. Accuracy falls off at depths greater than 30 feet.
Ultrasonic pulse-echo testing
Ultrasonic pulse-echo testing is good for fl at surfaces. It can detect steel and plastic pipe that is embedded in concrete. Resolution is good with this type of testing equipment. Improvements in this form of testing continue to develop.
Radar is an NDT. It does not require contact with concrete. Resolution and penetration is somewhat limited. Some opinions favor signal testing over radar, but radar is a growing element in concrete evaluation.
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