Detection of damage to structures has recently received considerable attention from the viewpoint of maintenance and safety assessment. In this respect, the vibration characteristics of buildings have been applied consistently to obtain a damage index of the whole building, but it has not been established as a practical method until now. It is reasoned that this is perhaps due to restrictions on the experiment, use of an improper method, and lack of inspection opportunity for the structures. In addition, in the case of large-scale structures such as buildings, many variables to be considered for the analysis contribute to a large number of degrees of freedom, and this can also be a considerable problem for the analysis. A practical method for the detection of structural damage using the first natural frequency and mode shape of the building is proposed in this paper. The effectiveness of the proposed method is verified by numerical analysis and experimental tests. From the results, it is observed that the severity and location of the damage can be estimated with a relatively small error by using modal properties of the building.
Damage Assessment can be carried out broadly based on two techniques namely Destructive and Non-destructive Testing Analysis.
Destructive Testing or Destructive Physical Analysis:
Destructive testing (or destructive physical analysis, DPA) tests are carried out to the specimen’s failure, in order to understand a specimen’s performance or material behaviour under different loads. These tests are generally much easier to carry out, yield more information, and are easier to interpret than nondestructive testing. Destructive testing is most suitable, and economical, for objects which will be mass-produced, as the cost of destroying a small number of specimens is negligible.
Types of Destructive Testing:
- Crash Tests
- Shake Table Test
Non Destructive testing:
There is no strength test, which provides the requisite information on concrete in-situ without damaging the concrete. These and other drawbacks of destructive test methods have led to the development of nondestructive methods of testing. Non-destructive methods are quick and can be performed both in the laboratory and in-situ with convenience.
Types of Non-Destructive Testing:
The Windsor probe is generally considered to be the best means of testing penetration. It consists of powder-actuated gun or driver, hardened alloy probes, loaded cartridges, a depth gauge for measuring the penetration of probes and other related equipment. A probe of diameter 6.5 mm and length of 80 mm, is driven into the concrete by means of a precision powder charge. Depth of penetration provides an indication of the compressive strength of the concrete. Although calibration charts are provided by the manufacturer, the instrument should be calibrated for the type of concrete and the type and size of aggregate used.
Rebound Tests: Schmidt Test Hammer:
The rebound hammer is a surface hardness tester for which an empirical correlation has been established between strength and rebound number. The only known instrument to make use of the rebound principle for concrete testing is the Schmidt hammer, which weighs about 1.8 kg and is suitable for both laboratory and fieldwork. It consists of a spring-controlled hammer mass that slides on a plunger within a tubular housing. The hammer is forced against the surface of the concrete by the spring and the distance of rebound is measured on a scale. The test’s surface can be horizontal, vertical, or at any angle but the instrument must be calibrated in the position.
Is more authentic than the concrete core test. A specially shaped steel rod with one end enlarged is embedded in concrete in the form-work. After the concrete hardens the rod is pulled out and in so doing it comes out with a block of concrete. The pullout force determined by a hollow tension ram is related to the compressive strength of concrete.
Concrete Core Test:
Concrete cores are drilled from the structure and are tested in a compression testing machine. The average equivalent cube strength of the cores is equal to at least 85% of the cube strength of the concrete specified for the corresponding age.
Concrete absorbs X-rays and -rays passing through it and the degree of absorption depends on the density of concrete. These rays, while passing through concrete, are partly absorbed and partly scattered. The scattered radiation can be shielded from the measuring device and the density of concrete is determined by the degree of absorption of the rays traversing a direct path of known length. Radium and radio-cobalt are used as a source of rays. Radium has the advantage that its activity can be regarded as constant since it takes 1000 to 2000 years for its activity to be reduced by half. However, radio-cobalt whose activity reduces to half in just five years is preferred because it is quite cheap.
Maturity Concept (Test on Fresh Concrete):
Is based on the principle that concretes having equal maturities will have equal compressive strengths. The maturity of the in-situ concrete at the early ages can be determined with the aid of an instrument known as maturity meter. This is used to determine the earliest safe time for removal of formwork. The results are authentic provided the concretes have an initial temperature between 15-26°C and there is no loss of moisture during the period of curing.
Ultrasonic Test: The ultrasonic pulse velocity method as described for green concrete can also be used to determine the strength of hardened concrete. The flaws, quality of concrete, reinforcement, moisture content, the temperature of concrete materials, etc. affect the pulse velocity and suitable adjustments should be made in evaluating the concrete strength.
Selection of Test Method:
- The availability and reliability of the calibration charts
- The effects and acceptability of surface damage
- The accuracy desired
- Economic consideration
- Practical limitations such as member size and type, surface conditions and access to test points.
Non-destructive methods have following distinct advantages over the prevalent destructive methods of testing.
- The measurement can be done on concrete in-situ and thus representative samples are not required. In the destructive method of testing the change in the quality of concrete has to be studied on a long-term basis with respect to curing or deterioration due to certain causes. A large number of specimens are required which could be tested to destruction, at various ages. Since it cannot be guaranteed that all specimens are of the same quality, the results obtained may not be very reliable.
- Non-destructive testing makes its possible to study the variation in the quality of concrete with time and external influences.
- In N.D.T. method the concrete is not loaded to destruction. Its quality is judged by measuring certain of its physical properties, which are related to its quality.
- In N.D.T. there is no wastage of material as in destructive methods of testing.