Non-Destructive Testing of Masonry Structures

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Non-destructive testing of masonry structures is not common in practice. However methods of NDT testing of masonry structures are available which can be used for evaluation of existing masonry structures when visual inspection is not sufficient or is not reliable. Appearance of cracks in masonry structure is a clear indication of distress and deterioration of masonry structures which can be evaluated through visual inspection. But, deterioration of masonry materials inside the structural element, internal cavities, weak bond etc. can cause the safety concern for the structural masonry elements. Thus to ensure it becomes necessary for regular monitoring an inspection of masonry structure by the use of non-destructive testing methods available. Use of NDT methods for testing existing masonry structures time to time ensures the quality and strength requirements for intended use. Sometimes, the masonry structure has to be used for purpose other than intended use when designed, in that case, it is essential that the structure is evaluated for new loads or load combinations or additional loads based on existing capacity and conditions. Identification of ongoing distress and damages in early stage is essential to prevent further damages and to select suitable repair and rehabilitation methods for masonry structures.

Methods of Non-Destructive Testing of Masonry Structures

1. Flat jack testing method
2. Impact echo testing method
3. Impulse radar testing method
4. Infrared thermography
5. Boroscope testing method

Flat Jack Testing of Masonry Structures

Flat jack NDT testing method for masonry structures is used to determine engineering properties such as compressive strength and in-situ stress of older structures for structural evaluation.

A flat jack testing equipment consists of flexible steel envelope which is very thin and fits within the masonry mortar joint. This flat jack is then hydraulically pressurized to apply stress on the surrounding masonry structures. This method directly measures the existing compressive stress present within the masonry structure. It is useful for determination of stress gradient present within masonry structural elements such as walls, columns. Flat jack NDT testing works on the principle of stress relief within the masonry. The testing procedure involves removal of mortar from a joint. When mortar is removed from the joint, the compressive stresses in the masonry element forces the slot to close by a small amount. A flat jack is then inserted into the slot and pressurized to restore the slot to its original opening size. The pressure which is required to restore the original slot dimension is a measure of compressive stress in masonry element. The compressive stress value is obtained by comparing the pressure with calibration constant of flat jack equipment. The other tests done by using the flat jack equipment is in situ deformability test. This test is used for direct measurement of masonry deformability properties and to estimate the masonry compressive strength. For conducting in-situ deformability test, two parallel flat jacks are used. Then masonry between them is subjected to compressive stress. The stress strain curve which is obtained during the test is used for obtaining both compressive modulus and an estimate of compressive strength.

Impact Echo Testing of Masonry Structures

Impact echo NDT testing method is used for detection of stress wave propagation and flaw in masonry structures as well as concrete structures. An impact-echo test system is consists of following three components:

• An impact source.
• A displacement transducer.
• A computer or waveform analyzer.

The principle of this test is inducing stress pulses on the surface of masonry elements by a mechanical impact and then measure the stress propagation at the other end of the masonry element. It measures the time of travel and speed of the wave propagation from the originating source to the source at which it is measured. All the data obtained from this test is analyzed with the help of a computer.

Impulse Radar Testing of Masonry Structures

The impulse radar testing method is used for detection of delamination in masonry structures. This is the electromagnetic analog of sonic & ultrasonic pulse echo methods. In this method, the electromagnetic waves propagate through materials of different dielectric constants. The reflection of energy from various interfaces is measured in this NDT method for determination of various defects.

Infrared Thermographic Method of Masonry Testing

This NDT test is conducted to determine subsurface delamination in masonry structures. This test is also used to detect the internal voids, cracks in masonry structures. Infrared thermographic method gives fairly accurate picture about the condition of masonry elements inside and can be effectively applied for larger surfaces.

Boroscope Method of Masonry Structure NDT

Boroscope method is used for commonly used for concrete and masonry structure, and also used for steel structural elements. A boroscope is used to look inside inaccessible or small voids.

For masonry structures the boroscope can be used to gain information of the depth of the outer layer of bricks or natural stones and it can provide information of the filling material in between the arches. It may also be used to examine the mortar between bricks or natural stone.