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Conclusion of assessing concrete strength using rebound hammer only

Rebound hammer technique is the most simple, rapid and cheapest nondestructive method for assessing the concrete strength both in-situ and in laboratory. Identifying a theoretical correlation between the concrete compressive strength and the rebound value is difficult so alternatively empirical correlation is commonly used. However, the value derived from this test, rebound value, is affected by several mix and member characteristics in addition to the concrete strength. Consequently, using a general model between concrete strength and rebound number for assessing a real structure leads to unreliable estimations and calibration is mandatory. Therefore, rebound hammer technique cannot be used alone in assessing the concrete strength and it should be combined with the destructive tests in order to derive/calibrate a model for the case under consideration.

Ultrasonic pulse velocity method

This technique is one of the stress wave propagation methods that is based on the determination of the longitudinal wave pulse velocity where pulses are generated in concrete by an electro-acoustical transducer. After the Second World War, the earlier devices (Soniscope in Canada and Ultrasonic Concrete Tester in England) were developed (Carino, 1994). Since that, the method and its device have been progressively developed to become a widespread use nondestructive method for testing concrete in-situ as well as in laboratory.

Physical principle of the ultrasonic pulse velocity method

The application of an impulse to a large solid elastic media generates propagating stress waves that are classified, according to the particles motion with respect to propagation direction, in three main types: longitudinal waves (P-waves), transverse waves (S-waves) and Rayleigh waves (R-waves) (Naik, et al., 2004). These waves travel at different velocities with the longitudinal waves being the quicker.
The principle of ultrasonic pulse velocity method is to introduce generated pulses into concrete using transmitting transducer which is held on the concrete surface and to measure the arrival time, transit time t, of the first waves (longitudinal waves) to a receiving transducer. A schematic diagram of the device is shown in Figure 2.6. After measuring the distance between the two transducers, path length l, the ultrasonic pulse velocity of longitudinal waves, Vp, can be simply calculated using Equation (2.4): 43 Ch.2 Concrete strength assessment in the existing structures: Literature review = / (2.4).

Testing procedure

Many countries have standardized procedures for testing concrete using ultrasonic pulse velocity. (Komlos, et al., 1996) listed more than 20 standards published between 1972 and 1994. The most popular standards are the European standard (EN 12504-4, 2004) and the American standard (ASTM C597, 2002).
The testing procedure should start by verifying that the device measures the transit time, t, properly. A reference bar is usually provided by the manufacturer for this purpose. At the selected test location, it is necessary to ensure a perfect coupling between the transducers and the concrete surface because the presence of air pockets leads to misleading reading of the transit time. Many existing viscous materials can be used as a coupling agent like: grease, petroleum jelly, soft soap and oil. A thin layer of coupling agent should be applied to the transducer face or the concrete surface. For rough surfaces, the preferred coupling agent is thick grease. However, for very rough surface, a surface preparation by grinding or using a quick setting mortar is recommended (Bungey, et al., 2006).
After the application of the coupling agent, the transducers are pressed firmly against the concrete surface and the minimum transit time reading is recorded from several repeated readings (transducers are removed then re-applied at the same points) in order to avoid reading resulted from insufficient coupling. The distance between the transducers should be accurately measured and consequently the ultrasonic pulse velocity is calculated.
At each test location, in order to reduce the measurement uncertainty, the test is repeated several times (replicates) i.e. at different points within the small area of test location and consequently the test result for this test location is the mean value of these replicates. According to (ACI 228.1R, 2003) five replicates are required for the case of existing construction while three are sufficient for new construction. On contrary, the (EN 12504-4, 2004) and (ASTM C597, 2002) standards say nothings about this important issue and this is one of their shortcomings.
Most standards describe three possible ways of transducers arrangements (Komlos, et al., 1996), these are: direct transmission, semi-direct (or diagonal) transmission and indirect (or surface) transmission, see Figure 2.7. The direct transmission is the most accurate and should be used when it is possible (Garnier, 2012). When the access to opposite faces of concrete member is impossible or when there is reinforcement congestion, semi-direct transmission can be used. However, when only one surface is accessible, the transducers are applied on the same surface (indirect transmission). Due to uncertain measured path length and the fact that the calculated velocity represents the surface layer, the indirect configuration should be avoided when it is possible (ASTM C597, 2002).
In order to carry out the testing procedure efficiently and to produce accurate test results, an experienced operator is necessary.

Factors affecting the test results

The calculated pulse velocity at each test location is affected by several factors:
– coarse aggregate: since the wave velocity is larger in the aggregates than in the cement paste, the aggregate-to-cement ratio affects significantly the pulse velocity versus concrete compressive strength relationship. For a given strength, the pulse velocity increases as the aggregate-to-cement ratio increases (Wheen, 1974).
– moisture content: for a given strength, wet concrete shows higher pulse velocity as compared with dry concrete (Bungey, 1980).
– water-to-cement ratio: as the water-to-cement ratio decreases, the compressive strength and the pulse velocity increase (Kaplan, 1959). However, the ratio of increases in compressive strength and pulse velocity are not the same (Lin, et al., 2007).
– concrete age: the velocity increases with the concrete age but with a decreasing rate (Popovics, et al., 1990). This effect is significant at earlier ages, therefore it can be neglected in the case of existing old structures.
– concrete temperature: when the concrete temperature varies within the interval (10-30)°C, no significant changes take place in the pulse velocity unless the occurrence of changes in elastic properties or strength (EN 12504-4, 2004).
– cracks and voids: the presence of cracks and voids leads to longer travel path of the propagated wave and as a result the transit time increases (Carino, 2008).
– path length: generally, the path length has no effect on the pulse velocity, however for small path lengths, the pulse velocity may be significantly affected by the heterogeneous nature of concrete (Jones & Fącąoaru , 1969). Therefore, (EN 12504-4, 2004) recommends that the minimum path length is 100 mm for concrete having maximum aggregate size ≤ 20 mm. While for maximum aggregate size within the interval (20-40) mm, the minimum path length should be 150 mm.
– transducer frequency: the frequency of the commonly used transducer is 54 kHz. However, for laboratory specimens or in-situ concrete members that have small lateral dimensions, the frequency should be selected carefully in order to be sure that the lateral dimension (path length) is equal or greater than the wavelength (wavelength = pulse velocity/frequency) (Bungey, et al., 2006).
– reinforcing bars: the pulse velocity of reinforced concrete in the vicinity of reinforcement is higher as compared with that of plain concrete. Consequently, wherever possible, reinforcing bars parallel and close to the path between transducers (or transverse bars that intersect this path) should be avoided when selecting the transducers testing positions (BS 1881: Part 203, 1986).

Conclusion of assessing concrete strength using cores and single nondestructive technique

a) From the basics of the existing approaches (regression and calibration) illustrated above, it is obvious that none of these approaches has the objective to capture the concrete variability although the standards recommend the estimation of the concrete strength variability since it is an essential parameter in the calculation of the characteristic strength of concrete.
b) The minimum number of cores required by standards (18 for regression approach and 9 for calibration approach according to (EN 13791, 2007), and (ACI 228.1R, 2003) requires at least 12 cores) is variable and generally high. A consequence is the slow development of strength assessment using NDT and cores.
c) The assessment methodology is affected by all factors that influence the core strengths and NDT measurements. In the existing structures, influencing factors (like concrete mix characteristics, concrete moisture condition, concrete surface carbonation, concrete temperature, and voids) are difficult to assess accurately. Therefore, these factors are usually not included in the conversion model and consequently they are considered as uncontrolled factors. Therefore, these factors will be considered as a source of uncertainty in the assessment methodology.
d) The methodology inherent characteristics (number of test locations for cores used to identify the conversion model, number of test locations for NDT measurements, the way of selecting the test locations, the quality of measurements, the type of NDT technique, using single or combination of NDT techniques, the model identification approach and the model type) can also affect the uncertainty of assessment. Since the methodology inherent characteristics factors can be controlled in the assessment methodology, therefore these factors deserve a more comprehensive analysis. e) The quality of assessment of concrete strength using NDT measurements remains low even after the model identification/calibration with the core strengths. Consequently the quality of assessment remains an open question and needs to be improved or at least really known. The only way to do this is by controlling the sources of uncertainty (in-situ concrete strength variability, sampling uncertainty, measurement uncertainty, model uncertainty). To this end, it is necessary to correlate the uncertainty with the methodology inherent characteristics (controlled factors) in order to study how the quality of assessment can vary with any change in these factors. Therefore this issue needs farther studies.

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Strength assessment using cores and combination of nondestructive techniques

Instead of using a single NDT technique with cores for assessing the concrete, the NDT techniques can be used in combination (in addition to the cores). The theoretical principle of combination is that when two or more NDT techniques are affected inversely by an influencing factor, combining these techniques can reduce or eliminate this effect and as a result improve the reliability of strength estimation (Soutsos, et al., 2012; Sbartaï , et al., 2012). As an example the effect of the concrete moisture condition which produces an increase in pulse velocity and decrease in rebound number when it increases. However, the benefit of this improvement in reliability resulted from using the combination of NDT techniques should be assessed against the additional time, cost, and complexity of using this combination (Samarin, 2004).
Combining the ultrasonic pulse velocity and rebound hammer techniques is the most popular combination which is known as SONREB. RILEM Technical Committee (TC 43) played a major role in the development of the SONREB method. Its recommendation (RILEM TC 43-CND, 1993) provided a procedure to establish iso-strength curves for a reference concrete (concrete has the materials and composition from a particular region or country for which the curves are devoted), as an example of these curves, see Figure 2.11. For different concrete compositions, correction factors are used for this purpose. When the composition is unknown (as it is the case for old structures), the correction factor should be estimated using cores extracted from the structure under investigation (RILEM TC 43-CND, 1993). In fact, the iso-strength curves represent specific conversion models that correlate the concrete strength with NDT values (pulse velocity and rebound number) and the correction factor looks like the calibration factor that was illustrated in Subsection The nomogram shown in Figure 2.11 is not unique and many other versions were developed by researchers all around the world [see examples (Cianfrone & Facaoaru, 1979; Knaze & Beno, 1984; Schickert, 1984) for iso-strength curves, (Qasrawi, 2000; IAEA, 2002) for iso-rebound number curves, and (Galan, 1984) for iso-pulse velocity curves].

Table of contents :

1.1 Problem statement
1.2 Scope of thesis
1.3 Objective and general research methodology
2.1 Introduction
2.2 Strength assessment using cores only
2.2.1 Planning an investigation program: number and location of cores
2.2.2 Drilling cores
2.2.3 Testing cores
2.2.4 Interpreting the core strengths
2.2.5 Conclusion of the assessment methodology when using cores only
2.3 Strength assessment using nondestructive tests only
2.3.1 Rebound hammer method
2.3.2 Ultrasonic pulse velocity method
2.4 Strength assessment using cores and single nondestructive technique
2.4.1 Model identification approaches: Regression approach
2.4.2 Model identification approaches: Calibration approach
2.4.3 Types of models
2.4.4 Factors affecting the quality of assessment
2.4.5 Sources of uncertainty
2.4.6 Quality of assessment
2.4.7 Conclusion of assessing concrete strength using cores and single nondestructive technique
2.5 Strength assessment using cores and combination of nondestructive techniques
2.5.1 Model identification approaches
2.5.2 Types of models
2.5.3 Factors affecting the quality of assessment
2.5.4 Sources of uncertainty
2.5.5 Quality of assessment
2.5.6 Efficiency of combination
2.5.7 Conclusion of assessing concrete strength using cores and combination of nondestructive techniques
2.6 Conclusions
3.1 Introduction
3.2 Definition of the assessment strategy
3.3 Sources of data
3.3.1 In-situ on structure and laboratory studies data
3.3.2 Synthetic data
3.4 The simulator developed in the present thesis
4.1 Introduction
4.2 Studying the effect of several key influencing factors
4.2.1 Effect of number of test locations for cores
4.2.2 Effect of quality of measurements
4.2.3 Effect of in-situ concrete strength variability
4.2.4 Effect of type of model (linear or nonlinear)
4.2.5 Effect of combining NDT techniques
4.2.6 Conclusions
4.3 Analyzing several assessment strategies presented in an international benchmark
4.3.1 The benchmark in brief
4.3.2 Simulation of the assessment strategies
4.3.3 Analysis of simulation results
4.3.4 Conclusions
4.4 Analyzing the existing model identification approaches
4.4.1 Comparing the prediction capacity of the existing model identification approaches
4.4.2 Conclusions
5.1 Introduction
5.2 Development of the bi-objective approach
5.2.1 Derivation of the model parameters for the case of linear model
5.2.2 Derivation of the model parameters for the case of nonlinear model
5.3 Validation of the bi-objective approach
5.3.1 Case of linear model
5.3.2 Case of power model
5.3.3 Failure of the model identified by each approach
5.4 Conclusions
6.1 Introduction
6.2 Analysis of quality of assessment using synthetic datasets
6.2.1 Datasets
6.2.2 Assessing mean strength and strength standard deviation (concrete variability) and developing the cumulative distribution functions (CDF)
6.2.3 Assessing the quality of estimation by developing “Risk Curves”
6.2.4 Studying the effect of the quality of measurements on the quality of assessment
6.2.5 Studying the effect of the way of selection the NC test locations on the quality of assessment
6.2.6 What is the minimum number of test locations for cores that can ensure a specific quality of assessment?
6.3 Analysis of quality of assessment using real datasets
6.4 Conclusions
6.5 Recommendations for better practice


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