Non destructive testing for determining the strength of concrete

Dr. Sanjeev Kumar Verma, Professor and Head, Department of Civil Engg., TIT, Bhopal

Civil engineering infrastructures are designed to operate for long periods of time, such as 50 to 100 years. However, several unpredictable and uncontrollable factors reduce their expected performance and life cycle. Lack of performance of these systems may significantly impact a nation’s economy. Hence, regular in-situ inspection of structures is required for assessing its condition. Monitoring and evaluating compressive strength of deteriorating structures is required for tracking the condition. Here, in this paper two most significant non destructive testing (NDT) methods for determining strength of concrete were discussed. Henceforth, an experimental study performed in Bhopal for determining relation between results of these tests and crushing strength of concrete has been presented.

Introduction

Compressive strength is the capacity of a material or structure to withstand loads tending to reduce size. It can be measured by plotting applied force against deformation in a testing machine. Some materials fracture at their compressive strength limit; others deform irreversibly, so a given amount of deformation may be considered as the limit for compressive load. Compressive strength is a key value for design of structures. There are two methods for finding compressive strength of concrete - Destructive testing method and Non-destructive testing method. The standard method of evaluating the quality of concrete in buildings or structures is to test specimens cast simultaneously for compressive, flexural and tensile strengths (Feldman, 1977) [1]. As the name suggests, destructive testing (DT) includes methods where your material is broken down in order to determine mechanical properties, such as strength, toughness and hardness. However, NDT are performed in a manner that does not affect the future usefulness of the reinforced concrete (RC) structures.

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 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. The process of SHM involves monitoring a structure over a period of time using appropriate sensors, extracting damage sensitive features from the measurements given by the sensors, and analyzing these features to determine the current state of the structure.

Objective and scope of present work

1. To study most significant NDT methods for determining the strength of concrete – Rebound hammer and Ultrasonic pulse velocity. To collect and study relevant literature for performing above studies.
2. This study provides useful data regarding structural health monitoring for the new researchers and engineers interested in this topic.
3. Presents a relation for evaluating the strength of concrete based on the results of NDT survey.

Literature Review

Several researchers applied NDT for evaluating strength and condition for RC structures. Refurbishment design of existing structures as well as to the design of new structures has been related using probabilistic modelling Grosso. (2013) [2]. Sanayei et al. (2012) [3] performed static truck load test on a newly constructed bridge, to capture the response of bridge when a truck travelled across it.

Amini and Tehrani (2011) [4] applied experimentally four sets of exposure conditions, weight and compressive strength of the samples had been measured before and after the freeze thaw cycles, and the results were analyzed. Ultrasonic guided waves have been utilized by Sharma and Mukherje in 2011 [5] for monitoring progression of rebar corrosion in chloride and oxide environment.

Wang and Liu (2010) [6] determined change in bond strength, loss of concrete cover in tensile zone and/or reduction of concrete cover in compressive zone as the major cause of deterioration of RC structures. Mitra et al. (2010) [7] discussed that repair and maintenance planning of concrete structures mainly depends on following conditions - rusting and cracking, delaminating, loss in steel section, workmanship, carbonation and chloride contents. Chen and Wimsatt (2010) [8] used 400MHzground-coupled penetrating radar (GCPR) to evaluate the subsurface conditions of roadway pavements. Terzic and Pavlovic (2010) [9] applied NDT methods that are Image Pro plus (IPP) and Ultrasonic Pulse Velocity (UPV), on the corundum and bauxite-based refractory concretes. Shah and Hirose (2010) [10] presented an experimental investigation of the concrete applying nonlinear ultrasonic testing technique. Bagchi et al. (2010) [11] applied cost effective and easy to implement, vibration-based damage identification (VBDI) techniques for Structural Health Monitoring of a bridge, based on changes in the dynamic characteristics of a structure to determine the location and extent of damage in the structure.

Ervin et al. (2009) [12] created an ultrasonic sensing network to assess reinforcement deterioration. Guided ultrasonic waves had been used to monitor reinforced mortar specimens under accelerated uniform and localized corrosion. Stergiopoulou et al. (2008) [13] presented a procedure for NDT of urban concrete infrastructures using UPV measurements and applied to assessment of concrete properties by concrete garages. UPV has been used as an indicator of concrete quality.

Rens and Kim (2007) [14] inspected a steel bridge using several NDT methods such as visual inspection, hammer sounding, Schmidt hammer, and UPV testing including tomography imaging; results of NDT had been used to determine areas, to be tested with local destructive tests such as compressive strength, chloride testing, and petro graphic testing. Dilek (2006) [15] discussed the use of pulse velocity, Young’s modulus of elasticity, and air permeability of concrete to evaluate the extent of damage of a concrete. Parthiban et al. (2006) [16] carried out potential surveys on the concrete structures. Among all electrochemical methods potential measurement is the mostly used field technique for detecting corrosion activity in steel.

Amleh and Mirza (2004) [17] performed concrete cover test, Half cell Potential, corrosion rate, electrical resistivity, chloride content at steel level (%), steel bar mass loss (%), absorption, pulse velocity, compressive strength, carbonation depth, petro graphic examination, and permeability test. Akuthota et al.(2004) [18] presented the experimental results of using near field microwave NDT techniques for detecting disbond in a specially prepared carbon fiber reinforced polymer (CFRP) reinforced mortar sample. Pascale et al. (2003) [19] carried out an experimental program involving both destructive and non destructive methods applied to different concrete mixtures, with cube strength varying from 30 to 150MPa, to define a relation between strength and parameters. Tests performed are pulse velocity, rebound hammer, pull out, and probe penetration, microcoring and combined methods.

Determination of compressive strength through NDT

Several methods are developed and reviewed by researchers to determine the compressive strength of concrete through NDT. Few methods are listed in table 1.

Table 1 – NDT methods for determining compressive strength (Verma et al. 2013)[20]
NDT method	Advantages	Limitations	Principle
Rebound hammer	Simple, quick and inexpensive	Not so reliable, smoothness, age of concrete , carbonation, moisture contain can affect results	Rebound of plunger when strucked with concrete indicates strength
Ultrasonic pulse velocity                                                      (UPV)	quick, Portland, large penetration depth simple interpretation, and moderate cost	not very reliable, moisture variation and presence of reinforcement can affect results	Ultrasonic wave velocity and its attenuation
Pull of test	Fast result, evaluate adhesion, and tensile strength which can be converted to compressive strength	Damage to the surface	A disc is bonded to the testing surface , and when disc is pulled off,  force required is used to obtain pull of strength
CAPO TEST	Correlation between pull out force and compressive strength is relianble	Damage to the surface	Expanded ring in the cored hole is pulled out

Rebound hammer and UPV are the two most significant methods for determining strength of concrete. These methods are described here-

Rebound hammer test

The rebound hammer test is basically a surface hardness test to find out the compressive strength of concrete by using rebound hammer as per the second part of IS 13311 (1992) [21] with little apparent theoretical relationship between the strength of concrete and the rebound number of the hammer. Rebound hammer test the surface hardness of concrete, which cannot be converted directly to compressive strength.

The Schmidt rebound hammer or rebound hammer is a simple and handy tool, which can be used to provide a convenient and rapid indication of the compressive strength of concrete. It consists of a spring controlled mass that slides on a plunger within a tubular housing. The Schmidt rebound hammer is shown in figure 1.

The rebound hammer method could be used for –

1. Assessing the compressive strength of concrete with the help of suitable co-relations between rebound index and compressive strength.
2. Assessing the uniformity of concrete.
3. Assessing the quality of concrete in relation to standard requirements.
4. Assessing the quality of one element of concrete in relation to another.

The method is based on the principle that the rebound of an elastic mass depends on the hardness of the surface against which mass strikes. When the plunger of rebound hammer is pressed against the surface of the concrete, the spring controlled mass rebounds and the extent of such rebound depends upon the surface hardness of concrete. The rebound is taken to be related to the compressive strength of the concrete. The rebound value is read off along a graduated scale and is designated as the rebound number or rebound index.


The compressive strength can be read directly from the graph having rebound number on the x-axis and compressive strength on the y-axis. The rebound scale and graph are graduated on the body of rebound hammer. The rebound scale and graph are shown in figure 2.

Ultrasonic Pulse Velocity Test

The UPV test is performed for strength determination of concrete, in accordance with IS 13311 (1991) [22]. It involves measuring the velocity of sound travelling through the concrete. Since, concrete is a multiphase material; velocity of sound in concrete depends on the relative concentration of its constituent materials, degree of compacting, moisture content and the amount of discontinuities present. This technique is applied for measurements of composition (e.g. to monitor the mixing materials during construction and to estimate the depth of damage caused by fire), strength estimation, homogeneity, elastic modulus and age, to check presence of defects, crack depth and thickness measurement. Generally, high pulse velocity readings in concrete are indicative of concrete of good quality. The drawback is that this test requires large and expensive transducers. In addition, ultrasonic waves cannot be induced at right angle to the surface. Hence, they cannot detect transverse cracks.

The ultrasonic pulse velocity method could be used to establish –

1. The homogeneity of the concrete.
2. The presence of cracks, voids and other imperfections.
3. The change in structure of the concrete which may occur with time.
4. The quality of concrete in relation to standard requirement.
5. The quality of one element of concrete in relation to another.
6. The values of dynamic elastic modulus of the concrete.

The apparatus for ultrasonic pulse velocity measurement is shown in figure 3. It is a handy and portable instrument. The apparatus consists of the following –

1. Electrical pulse generator
2. Transducer
3. Amplifier
4. Electronic timing device

High pulse velocity readings are generally indicative of good quality concrete. Qualities of concrete for different values of UPV are presented in table 2.

Table 2– Concrete quality for different values of UPV
Pulse velocity (km/sec)	Concrete quality
>4.0	Very good
3.5 to 4.0	Good, but may be porous
3.0 and 3.5	Poor
2.5 and 3.0	Very poor
2.0 and 2.5	Very poor and low integrity
2.0 and reading fluctuating	No integrity, large voids suspected

Results of experimental survey

Following steps are adopted for performing the required study –

15 cubes had been casted targeting different mean strengths. Further, the cubes were cured for different number of days to ensure availability of a wide range of compressive strength attained by these cubes. Size of each cube was 150×150×150 mm. then each is tested for rebound number, ultrasonic pulse velocity and crushing strength as per Indian codes. Table 3, presents the experimental data.

Cube no.	Rebound Number (N)	Velocity (V) from UPV (m/s)	Crushing strength (f) of cube (Mpa)
1	17	3660	13.0
2	17	3520	12.5
3	20	4120	14.6
4	21	4290	15.8
5	17	3580	12.7
6	18	3810	13.5
7	12	2590	9.2
8	13	2710	9.6
9	20	4150	14.7
10	22	4520	16.5
11	23	4570	17.6
12	17	3670	13.0
13	17	3530	12.5
14	16	3490	12.4
15	16	3320	11.8

To determine the relation between rebound number and crushing a graph is plotted as shown in fig. 4 -

Relation obtained between the Rebound number (N) and the actual Crushing strength of concrete is shown in eqn. 1

f = 0.7475* (N) + 0.0376 ---(1)

To validate the above eqn. 1, values of compressive strength obtained from eqn. 1 were compared with values of actual crushing strength for each cube in fig. 5. It has been observed that results of eqn. 1 are almost similar to the actual crushing strength of concrete cubes.


Similarly to obtain a relation for determining the compressive strength of concrete through UPV a graph shown in fig. 6 has been plotted between the Velocity (V) from UPV (m/s) and the Actual Compressive Strength (f) –

Relation obtained between the UPV (V) and the actual Crushing strength (f) of concrete is shown in eqn. 2

y = 0.004x - 1.3516 ---(2)

To validate the above eqn. 2, values of compressive strength obtained from eqn. 2 were compared with values of actual crushing strength for each cube in fig. 7. It has been observed that results of eqn. 1 are almost similar to the actual crushing strength of concrete cubes.

Conclusions

Following are the conclusions of the present research

1. From literature review it has been concluded that Rebound hammer and Ultrasonic Pulse velocity are the most significant NDT methods for evaluating the strength of concrete.
2. Relation between the Rebound Number and Actual Crushing strength of concrete structures is determined as - f = 0.7475 (N) + 0.0376 m, where ‘f’ is the compressive strength and N is the rebound number.
3. Relation between the UPV and Actual Crushing strength of concrete structures is determined as - f =0 .004(V) -1.3516, where ‘f’ is the compressive strength and V is the velocity.
4. Above relation are useful for the researchers interested in field of NDT.

Refrences:

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21. IS: 13311 (Part 1): 1992 Non-Destructive Testing of Concrete-Method of Test; Part 1-Ultrasonic Pulse Velocity.
22. IS: 13311 (Part 2): 1992 Non-Destructive Testing of Concrete-Method of Test; Part 2-Rebound Hammer.