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142 The Masterbuilder - November 2012 • www.masterbuilder.co.in

‘Equipment for Evaluation, Testing and Maintenance of Concrete Structures’

It is often necessary to test the concrete structure after the concrete has hardened to determine whether the structure is suitable for its designed use. It is important

to carryout tests without damaging the concrete. The tests available for testing of concrete range from the completely non-destructive, where there is no damage to the concrete, through those where the concrete surface is slightly damaged, to partially destructive tests, such as core tests and pullout and pull off tests, where the surface has to be repaired after the test. Another purely non-destructive way of estimating the in-situ strength of concrete is by “Maturity method”. The maturity method is a technique that allows the in-place concrete strength to be estimated using the time and temperature history of freshly placed concrete.

The range of properties that can be assessed using non-destructive tests and partially destructive tests is quite large and includes such fundamental parameters such as density, elastic modulus and strength as well as surface hardness and surface absorption, and reinforcement location, size and distance from the surface. In some cases it is also possible to check the quality of workmanship and structural integrity by the ability to detect voids, cracking and delamination. Non-destructive testing can be applied to both old and new structures. For new structures, the principal applications are likely to be for quality control or the resolution of doubts about the quality of materials or construction. The testing of existing structures is usually related to an assessment of structural integrity or adequacy. In either case, if partially destructive testing alone is used, for instance, by removing cores for compression testing, the cost of coring and testing may only allow a relatively small number of tests to be carried out on a large structure which may be misleading. Non-destructive testing can be used in those situations as a preliminary to subsequent coring.

Typical situations where non-destructive testing may be useful are, as follows:

- Quality control of pre-cast units or construction in situ- Removing uncertainties about the acceptability of the

material supplied owing to apparent non-compliance with specification

- Confirming or negating doubt concerning the workmanship involved in batching, mixing, placing, compacting or curing of concrete

- Monitoring of strength development in relation to formwork removal, cessation of curing, prestressing, load application or similar purpose

- Location and determination of the extent of cracks, voids, honeycombing and similar defects within a concrete structure

- Determining the concrete uniformity, possibly preliminary to core cutting, load testing or other more expensive or disruptive tests

- Determining the position, quantity or condition of re-inforcement

- Increasing the confidence level of a smaller number of partially destructive tests

- Determining the extent of concrete variability in order to help in the selection of sample locations representative of the quality to be assessed

- Confirming or locating suspected deterioration of concrete resulting from such factors as overloading, fatigue, external or internal chemical attack or change, fire, explosion, environmental effects

- Assessing the potential durability of the concrete- Monitoring long term changes in concrete properties- Providing information for any proposed change of use

of a structure for insurance or for change of ownership

NDT Equipments

A wide range of Non-Destructive Testing (NDT) equipments are available for testing of concrete structures. They are classified as follows:

Dr. R NagendraTechnical Director, Civil-Aid Technoclinic Pvt. Ltd.

Non-Destructive Testing

www.masterbuilder.co.in • The Masterbuilder - November 2012 143

- Concrete rebound hammers- Ultrasonic Pulse Velocity testers- Profometers or Cover meters- Corrosion analyser- Permeability apparatus- Impact echo testers- Semi/partial destructive testing- Core extraction Pull-

out tester, Pull-off tester- Devices for Non-destructive testing of piles- Concrete Maturity meter- Penetration resistance testing- Windsor probe- Instrumentation of structures

Concrete Rebound Hammer

Rebound hammer test is a quick method for assessing the quality of concrete based on surface hardness indicated by the rebound number. A higher rebound value indicates higher strength / surface hardness of concrete.

Technical Reference

IS 13311-(part-II)-1992-(reaffirmed in 2004), ASTM C 805

The equipment consists of a pair of transducers (probes) of same frequencies, electrical pulse generator, electrical timing device and cables.

Cover meters

Cover meter is used for assessing the cover and mapping of rebars. It can also be used for estimating the size / diameter of rebars most popular equipment used is pro-fometers from M/s. Proceq, Switzerland.

The equipment works on electromagnetic principles. The equipment consists of a display unit coupled with universal probe

Technical reference: BS :1881 (part 204)

a) Original Schmidt Concrete Rebound Test Hammer

b) Digi-Schmdt Hammer c) Schmdt Rebound Hammer – P, L, L9 Types

Ultrasonic Pulse Velocity testers

Ultrasonic pulse velocity test method is extensively used to assess the quality and strength of in-situ concrete in members. This test is generally used to check the compaction, uniformity of concrete, determination of cracks, presence of honeycombs, and also strength estimation (qualitatively). Most popular equipment used for ultrasonic pulse velocity test is ‘PUNDIT’ (portable ultrasonic non-destructive digital indicating tester) from U.K, ust (ultrasonic tester) from U.K and ‘TICO’ meter from Switzerland.

Photos of Ultrasonic Pulse Velocity Tester

Profometer-5 (Cover Meter) Profometer-5+ (Cover Meter)

Profoscope Ferroscan

Corrosion analyser

Corrosion of embedded steel is the major cause of deterioration of concrete structures. This may lead to structural weakening due to loss of steel cross-section, surface staining and cracking or spalling.

Half-cell potential measurement test the half-cell potential measurement test essentially consists of measurement of absolute potential at the concrete surface with a reference electrode.

The measured absolute potential is considered to be the best criterion for assessing the corrosion status of the embedded rebars.

The most popular equipment used for the above test is



‘CANIN corrosion analyser’ from Proceq, Switzerland. The equipment consists of digital mili - voltmeter and copper - copper sulphate half-cell.

Impact-Echo can also be used to determine the location and extent of flaws such as cracks, delaminations, voids, honeycombing and debonding in plain, reinforced and post-tensioned concrete structures. It can locate voids in the subgrade directly beneath slabs and pavements, measure the depth of surface-opening cracks, and determine thickness or locate cracks, voids and other defects in masonry structures where the brick or block units are bonded together with mortar. Impact-echo is not adversely affected by the presence of steel reinforcing bars.

Semi / Partial Destructive Test:

The most common semi / partial destructive testing methods adopted to evaluate structural members are:

a.Extraction of cores & carrying out compressive strength test.

- Pull out test (lok test).

Photos of Corrosion Analyser

Permeability Tester

In-situ permeability test is conducted on the concrete surface. Gas / water under pressure is allowed to diffuse into the concrete media. The reduction in pressure with time is an indication of porosity in concrete.

Based on the rate of reduction in pressure permeability can be calculated. This serves as a measure to evaluate concrete quality.

Photos of Water Permeability Test Set-Up

Initial Surface Absorption Test

Initial surface absorption is defined as the rate of flow of water into concrete per unit area at a stated interval from the start of the test at a constant applied head and temperature. Results will be expressed as ml/ml2/s at a stated time from the start of test. This method is detailed in BS 1881: Part 5 (207)

Impact Echo Tester

Impact-Echo is a method for nondestructive evaluation of concrete and masonry, based on the use of impact-generated stress (sound) waves that propagate through the structure and are reflected by internal flaws and external surfaces. Impact-Echo can be used to make accurate, nondestructive, measurements of thickness in concrete slabs and plates, following an ASTM C 1383-98a Standard.

Torrent: Permeability Tester

Photos of Initial Surface Absorption Test



- Capo test.- Pull-off test - Load test on structures.

the strength and homogenity of in-situ concrete. Most popular equipments used for core extraction are HILTI from Germany, ELE from U.K., Bosch from Germany. The equipment consists of diamond tipped core barrels fitted to the electrically driven motor with water as a coolant.

Echo Impact Tester

Pistol Grip Transducer

Intrument Components

a.Extraction of cores & carrying out compressive strength test.

Core test is one of the most appropriate method to assess

Photo of Core Cutting Machine

Pull out test

Pull out test is also known as lok test. A specially shaped steel insert with enlarged end will be cast while concreting at the proposed test location. The extended end of the steel insert above concrete is pulled through a pulling device and insert is pulled out with a cone of concrete. The force required to pull the insert is measured. Pull out tests fall into two basic categories; those which involve an insert which is cast into the concrete, and those which offer the greater flexibility of an insert fixed into a hole drilled into the hardened concrete. Cast-in methods must be preplanned and will thus be of value only in testing for specification compliance, whereas drilled hole methods will be more appropriate for field surveys of mature concrete. In both cases, the value of the test depends upon the ability to relate pull-out forces to concrete strengths. Although the results will relate to the surface zone only, the approach offers the advantage of providing a more direct measure of strength and at a greater depth than surface hardness testing by rebound methods, but still requires only one exposed surface.

An appropriate calibration chart shall be established in the laboratory to correlate the pull out force with estimated compressive strength of concrete. Most popular equipment used for the above test is pull out tester from German equipments. The equipment consists of specially designed inserts, pulling device with a load indicator.


www.masterbuilder.co.in • The Masterbuilder - November 2012 147

Capo Test

Capo test is an improved version of pull out test. At the test location, special inserts are introduced in the driven hole and pulled out. The required force to pull the inserts along with concrete is measured and co-related with the calibration chart developed for the above test in the laboratory.

to the concrete surface with an epoxy resin and jacked off to measure the force necessary to pull a piece of concrete away from the surface.

Lok Test Apparatus

Pull-off Test

This approach has been developed to measure the in-situ tensile strength of concrete by applying a direct tensile force. The method may also be useful for measuring bonding of surface repairs and a wide selection of equipment is commercially available with disk diameters typically 50 mm or 75 mm. Procedures are covered by BS 1881: Part 207 and it should be noted that the fracture surface will be below the concrete surface and will thus leave some surface damage that must be made good A disk is glued

Load Testing of Structures

Load test is conducted on the identified members to check the behaviour of members under design loads. Dial gauges / deflectometers are placed under the member to measure the deflection & deflection recovery during load test. If the measured deflection and deflection recovery are within the permissible limits then the tested members can be accepted.

Technical reference IS 456-2000

Devices for Non-Destructive Testing of Piles

The quality of pile can be tested using low strain Pile Integrity Tester (PIT). The equipment comprising of hammer, accelerometer and data collector. This equipment is used to find out change in cross section of pile (bulging or necking), quality of concrete and length of pile.



High strain equipment: This consists of heavy hammer, strain gauges, accelerometers and data collector. This equipment is meant for arriving at load carrying capacity of pile, settlement of pile and integrity of pile.

Accoustics Concrete Tester

The Acoustic Concrete Tester (ACT) is a concrete thickness gauge that measures the concrete thickness of pavements, slabs, retaining walls, foundation footings and tunnel linings. This nondestructive testing instrument also identifies flaws such as delaminations, voids, and spalls.

The ACT is precise, light, battery operated and rugged for field use in all weather conditions. It works for concrete structures from 75 to 900 mm thick.

The ACT uses Ultrasonic Echo Technology. It electronically generates a broadband pulse that includes all frequencies required to accurately determine the thickness of the structure. Measuring thickness with the ACT consists of placing two small probes on the structure to be tested and touching the ACT screen. A telescoping pole helps test hard to reach structures. As the structure responds to its natural frequency, its thickness is displayed on the high visibility ACT screen. The ACT also calculates the wave speed of structures of unknown thickness.

The ACT eliminates coring or excavation, and easily self calibrates the concrete wave speed prior to the test. It only takes seconds to measure the thickness of the concrete.

Concrete Maturity systems

The Maturity Method of estimating in-place concrete strength has been studied for more than 50 years, and as a result, the science of concrete curing is well understood. Since 1987, it has been an ASTM Standard Practice (C1074). When concrete hardens, it gives off heat proportional to its curing rate. By learning how much heat is released, an accurate estimate of the strength can be determined. Generally, concrete in a structure cures at a much faster rate than concrete in a test cube. This is due to the much larger mass of the structure, and better hydration which aids curing.

When determining the early-age strength of cast-in-place concrete, reliance on test cubes can lead to problems. For example, if test cubes are cured at a lower temperature than the structure, the cubes would underestimate the strength of the slab, which means that critical construction operations are delayed unnecessarily. Or conversely, if the deck is cooler than the cubes, the cubes would overestimate the strength, a clear safety concern. Maturity testing monitors the curing of the structure and compares it to the cube, to more accurately track strength gain in the structure, improving both safety and construction operations.

Photo of Pile driving Analyzer Photo of Sensors for Pile driving analyzer

Photo of Accoustic Concrete Tester

Allows you live access to your data from any computer, 24hrs a day

Penetration Resistance Testing

The technique of firing steel nails or bolts into a concrete surface to provide fixings is well established, and it is known that the depth of penetration is influenced by the



strength of the concrete. A strength determination method based on this approach, using a specially designed bolt and standardized explosive cartridge, was developed in the USA during the mid 1960s and is known as the Windsor probe test. It has gained popularity in the USA and Canada, especially for monitoring strength development on site, and is the subject of ASTM C803.

Although it is difficult to relate theoretically the depth of penetration of the bolt to the concrete strength, consistent empirical relationships can be found that are virtually unaffected by operator technique. The method is a form of hardness testing and the measurements will relate only to the quality of concrete near the surface, but it is claimed that it is the zone between approximately 25 and 75 mm below the surface which influences the penetration. The depth is considerably greater than for rebound or any other established ‘surface zone’ tests.

Instrumentation of structures for stresses and strains:

Strain Gauges for Steel: Weldable strain gauges measure strain in steel. Typical applications include:

Photo of Windsor HP Probe - ASTM C-803

Testing Being Conducted

- Monitoring stresses in structural members of buildings, bridges, tunnel linings and supports during and after construction.

- Monitoring the performance of wall anchors and other post-tensioned support systems.

- Monitoring loads in strutting systems for deep excavations.

- Measuring strain in tunnel linings and supports.

- Monitoring areas of concentrated stress in pipelines.

- Monitoring distribution of load in pile tests.

Strain Gauges for Concrete: Embedment strain gauges measure strain in concrete. Typical applications include:

- Measuring strains in reinforced concrete and mass concrete.

- Measuring curing strains.- Monitoring for changes in load.- Measuring strain in tunnel linings and supports.

Conclusion

It can be said that wide range of NDT equipments are available for testing, evaluation and analysis of concrete structures. It is the expertise available to interpret the test results plays a very important role in deciding acceptance or otherwise of a structural member from the point of view of quality control or in resolving a dispute.

References

- Bungey J.H., Millard S.G., Testing of Concrete in Structures, Third Edition, 1996, Blackie Academic & Professional, an imprint of Chapman & Hall,

- ACI 228.1R-03, In-Place Methods to Estimate Concrete Strength


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