In-situ Concrete Strength Assessment based on Ultrasonic (UPV), Rebound, Cores and the SONREB Method

Transcription

1 In-situ Concrete Strength Assessment based on Ultrasonic (UPV), Rebound, Cores and the SONREB Method Frank Papworth 1, David Corbett 2, Reuben Barnes 3, Joseph Wyche 4 and Jonathon Dyson 5 1 BCRC (WA); 2 Proceq; 3 PCTE; 4 Wyche Consulting; 5 BCRC (NSW). Synopsis: The strength of concrete in a new structure is sometimes called into question. This may be due to cylinders not being taken, poor cylinder production, transport or testing, the actual concrete strength being low or the suspicion that in-situ cylinders are not representative of in-situ concrete. Whatever the reason testing of the in-situ concrete is generally called for. This paper gives a brief outline of the key aspects of strength assessment including a review of concrete supply and testing records, extent of testing required, assessment by cores to AS (1) and AS 3600 (2), ultrasonic (direct and indirect methods) and rebound hammer testing and analysis of in-situ strength using cores and NDT results (including published methods such as EN (3), EN (4), BS 6089 (5) and those under consideration by RILEM). If the concrete strength is found to be low, a structural analysis has to be undertaken. The paper includes information of six projects assessed. In two, strengths were low across the whole mine site but in each case only one structure required strengthening. In another, very low strength was identified at an early stage and a risk assessment identified the structure should be replaced. In the others, strengths were found to be adequate. Keywords: Concrete, compressive strength, ultrasonics, UPV, rebound, cores, structural assessment, strengthening. 1. Introduction For many years CSTR 11 (7) was the revered document on assessment of concrete compressive strength from cores. This was updated in 1987 (8). In 2002 CIA Z11 (9) was published and became the Australian reference for assessment of core strengths as a supporting document for AS (1). AS provides performance requirements on sampling, conditioning, and reporting on cores and calls on AS (6) as the method of testing concrete samples for compressive strength. However, in 2004 a project report on core strengths by the Concrete Society (10) provided updated information on tests from a range of samples and findings were subsequently incorporated in an updated BS 6089 (5) which is complimentary to BS EN (4), the European standard on testing concrete strength in structures. Consequently CSTR 11 (7,8), and AS (1) and CIA Z11 (9) which are at least partly based on them should be used with caution as some of the analysis methods have been superseded. The Concrete Society issued Advice Note 43 (11) in 2013 which summarises some of the approaches in the new European standards. In this paper some of the differences between European Standards and the methods used in Australia based on CSTR 11 are given. BS EN now provides clear guidance on the use of ultrasonic pulse velocity and rebound hammer testing. These methods enable rapid scanning of the concrete to detect variations in strength and BS EN provides guidance on how their use can be incorporated into reducing the number of cores required. In this paper the methods are outlined with specific reference to direct and indirect UPV measurements and combined use of rebound, UPV and core strengths using the SonReb method. An outline of the strength assessment on seven projects is given in the paper to show the variety of approaches that may be appropriate. For each project a durability assessment was also undertaken for each structure but this is not included here as the paper s focus is strength and structural assessment. The methods employed for strength assessment range from reliance on 28 day strengths to the use of cores, UPV and rebound testing. The subsequent analysis of the structures included a full structural analysis to determine structural reliability and risk assessment if there is a reduced reliability.

2 2. Methods of Testing 2.1 Cores To maximise the number of cores meeting AS requirements and to minimise damage to the core and structure, the engineer should ensure that the coring contractor has adequate experience in using his equipment to take cores for the specified purpose. On one project in remote Botswana the contractor arrived with a brand new coring machine, no hold down anchors and 5 people. His first core failed the requirements of AS because the machine was not properly secured and because of stop start coring. Being a remote location it took two days to get the team working. Before coring the reinforcement locations are carefully mapped and marked with due allowance for bar diameter. The upper reinforcement layer can generally be identified using a covermeter but this may miss saddles which may be identified using ground penetrating radar. Assessment of core strength can be based on BS EN and BS A comparison of these with current Australian standards is given in Table 1. Table 1 : Australian and European Standard Requirements for Assessing In-situ Concrete Strength Not Yet Harmonised with European Practice (based on CSTR 11) State of the art documents on in-situ strength assessment AS CIA Z11 BS EN 13791/BS6089 Core diameter 75mm 75min or 2.5 x agg size Core length:diameter ratio Close to 2:1 1 to 2 (ideally 1.9 to 2) Actual Strength Corrections Correction l:d k1 = 1 for l/d =2 reducing in steps to 0.87 for l/d=1 100mm (increase number of cores if using diameters down to 50mm) 1 to 2 Kcyl=2.0/(1.5+1/(l/d)) Correction rebar present - k2 by formula Avoid rebar Correction core axis - k3= 1 perp. to casting direction and 1.08 if parallel Core locations limits Vertical pours - Not from top 20% up to 300mm Face of concrete - Not within 50mm of face Number of cores where suspect batch - 3 from each suspect batch Conditioning Wet 3d or air 7d Dry (7d) as AS 3600 End preparation AS Sulphur cap or ground ends Influence noted but UK Annex NA says no allowance to be made Exclude top 300mm. Core top third Not within 50mm of face 15 with no NDT, 9 with NDT. Move to reduce the number with NDT Air 3d but allowance can be made for wet cure Ground ends recommended Provision for outliers - No Yes NDT assessment - - UPV, pullout and rebound accepted as means of reducing core numbers 2.2 Rebound The original rebound hammer measured the hardness of a material by the degree of rebound. The instrument worked by firing a known mass using a standard spring loading to impact on a rod held in contact with the surface being tested. When the mass hits the rod it rebounds to an extent that depends on the hardness of the surface in contact with the rod. The original rebound hammer measured the rebound value R mechanically as a distance that the mass rebounded from the concrete surface. Extensive testing carried out in the late 1950 s gave correlation curves between rebound value and concrete compressive strength.

3 In 2007 an electronic version of the original rebound hammer was introduced. This instrument measures rebound value (now Q) as the quotient between the velocity of the hammer mass just before and after impacting the rod. The validity of the new measurement principle has been recognized by the major standards bodies. It should be noted that Q-values and R-values are not interchangeable. Correlation curves developed for the original rebound hammers cannot be used with Q-value and correlation curves developed for the new hammer cannot be used with the classical hammer. The manufacturer provide a strength relationship between Q values and compressive strength based on a lower 10 th percentile curve. They do not provide direct relationships between Q values and R values, as testing on various types of concrete have shown that the relationship is not constant. When assessing in-situ compressive strength using cores, EN requires at least 15 cores to be taken to establish the insitu concrete strength. The number of cores to be taken may be reduced to 9 when used in combination with NDT tests such as rebound hammer or ultrasonic pulse velocity. In the German national annex to EN there is also the possibility to assign a compressive strength class based on rebound hammer testing alone, as in many cases, it is not allowed to take cores. EN is currently under review and there are proposals for reducing the number of cores further when used in combination with NDT testing and also to generally accept the method described in the German national annex. Testing should be undertaken in accordance with EN A test location should be a minimum 100m thick, 300x300mm test area, minimum of nine readings, impact points >25mm apart, surface clean and smooth. Nine such test locations are required for a test region as described in EN A core should also be taken at each test location to establish the correlation. The method then uses the core correlation to shift a base correlation curve upwards. The same method may be used with ultrasonic pulse velocity. 2.3 Ultrasonic Pulse Velocity UPV can be determined in number of ways direct, indirect and semi-direct. Only the first two are discussed here. Direct UPV is the most reliable as the velocity is measured across the entire element and gives an average velocity for a large thickness of concrete. Direct UPV can also be measured on cores to give a direct correlation between UPV and core strength. Pulse Time ( S) y = x R² = y = x R² = Head Specing (mm)) Figure 1 : Indirect Pulse Velocity Using Yaman's Five Point Method B2 B3 However in cases where access to only one face is possible a direct velocity measurement is not possible. Yaman (16) developed a method of measuring the indirect velocity over 4 head spacings (200, 250, 300 and 350mm). This is very similar to the surface velocity method described in Annex A of EN The slope of a straight line plot of time vs head spacing (Figure 1) gives an indirect velocity that is very close to the direct velocity for homogeneous specimens. The principle difference between direct and indirect UPV s is that direct measurements are largely through the bulk concrete and indirect measurements are largely through the near surface but Yaman s five point method avoids direct surface effects such as carbonation and finishing. This is particularly useful for assessing slabs. It should be noted however, that typically concrete is an inhomogeneous material and the difference between indirect and direct pulse velocities can vary significantly if surface effects are deep.

4 In the indirect mode a particular issue is the low signal level compared with direct measurements. The first pulse may not trigger the timer unless the gain is suitably increased. For such measurements, it is advisable to use a waveform display to be certain of correct triggering. This may or may not be apparent from the best fit line through the four data points. Testing should otherwise be undertaken in accordance with EN SonReb One issue with combined use of NDT and cores for strength assessment is the number of core correlations required according to EN BS By reducing uncertainty by combining two NDT measurements the number of correlation points decreases. Breysse (15) describes the SonReb method of combined UPV and Rebound measurement as discussed by RILEM Technical Committee TC 207-INR as follows: This combination has received the name of SonReb, for Sonic and Rebound. Rebound and ultrasonic pulse velocity measurements can be carried out quickly and easily. The underlying concept is that if the two methods are influenced in different ways by the same factor, their combined use can cancel the effect of this factor and improve the accuracy of the estimated strength. Breysse describes two approaches to the combined assessment based on best fit of data but the multivariate approach A is preferred, i.e.: Approach A : fc=av b R c where a,b and c are constants (1.15x10-10 ; 2.6 and 1.3 respectively); V is UPV; R rebound number. Although standard values for a, b and c are given project specific values are determined. Generally, the SonReb method provides an increase in correlation accuracy when compared with using either the rebound method or UPV method in isolation. The data, presented in Figure 2 was collected by the rebound hammer manufacturer to establish a SonReb curve using the Q-value. It illustrates the benefit of using the combined method. The SonReb method has been established as a national standard in several countries including Italy and China in particular and is currently being considered by the RILEM TC-249-ISC committee dealing with in-situ compressive strength estimation. Compressive Strength (MPa) ISI13311 Concrete Classification <3000 m/s doubtful m/s medium m/s good >4500 m/s excellent Compressive Strength EN13791 Base Expon. (Compressive Strength) y = e x R 2 = Compressive Strength Expon. (Compressive Strength) y = e x R 2 = y = 1.001x R 2 = UPV (m/sec) Q Value SonReb Compressive Strength a) UPV correlation to compressive strength b) Q-value correlation to compressive strength c) SonReb correlation to compressive strength Figure 2 : Data from 240 Cubes to Establish SonRep Calibration Curves for Q Value 3. Structural Assessment of New Concrete Structures in a Ghana Mine Shortly after construction deterioration of the primary crusher approach slab led to a preliminary investigation of the strength of concrete. This indicated that there was cause to suspect that the strength of concrete could be lower than design. Subsequently a detailed investigation of the concrete to identify if there are any significant deficiencies in construction was undertaken. For cores testing SANS standards were followed but as there were no SANS standards for non-destructive tests international standards were followed. The specified strengths varied and are shown together with core strength results in Table 2.

5 Table 2 : Core Test Results From Ghana Mine Element Orientation Vertical Horizontal Specified Strength 40MPa 25MPa 40MPa 25MPa No of Cores No of Compressive Strength Tests Average Strength (MPa) Standard Deviation (MPa) % Failing Individual Result 0% 57% - 54% % Failing Average Result 0% 100% - 71% The large fraction of cores that failed the individual and average strength requirements, and the margin by which some cores failed, was a serious concern. Rebound hammer and core testing results from the same location are shown in Figure 3. The best fit relationship is achieved at compressive strength = 2.161e 0.043x where x is the Q value. The rebound results are from the surface of slabs and the finishing may account for why a high Q value is achieved for a given compressive strength but more likely was that carbonation had occurred hardening the surface strength. This effect means rebound results on the top of the slab are not an effective method of assessing bulk strength. Consequently the assessment was based on core strengths. Figure 3 : Core Strength Vs Rebound Hammer Based on the core strength tests an equivalent characteristic cylinder strength of 15MPa was determined for the 25MPa concrete and 25MPa for 40MPa concrete. The structural assessment was undertaken in conjunction with the original designers with free and unfettered access to the design calculations. Jointly with the designer critical elements were identified and checked based on the strength of 15MPa and 10 year design life. The structural review identified all structures behaved acceptably at the actual in-situ strengths except for one tall conveyor support. Additional cores from this structure indicated there were still some concerns and hence strengthening was instigated. 4. Structural Assessment of New Concrete Structures in a Botswana Mine Plant 25MPa and 32MPa concrete was specified for footings/slabs and walls/columns respectively but review of the cube test results showed a much lower strength was achieved in practice. Cores were taken from various locations and results were consistent with the cube results. Multiple cube results from the same batch tested at the same age were quite consistent and 7 day results were reasonably consistent at 67% of the 28 day results. This indicates that, failing an unexpected consistent error in testing, the results are a realistic and able to be used as a true indication of the strength of concrete supplied. The results are summarised in Table 3.

6 Table 3 : Summary of 28day Compressive Strength Test Results Period 4 th -9 th May Remainder f c cube (MPa) f c cylinder (MPa) 9 15 Compressive strengths from 2 cores taken from each of three different bases gave a characteristic in-situ cylinder strength of 17.5MPa. This was marginally higher than the 28day cube results and may have an improvement due to the aging. 28 rebound hammer results provided had an average cube strength of 28MPa and a standard deviation of 5.6MPa to give a characteristic cube strength of 18.8MPa. A review of the mix indicated low coarse aggregate volume and a high proportion of crushed sand. The mix would have had a high water demand and it was concluded that to achieve a workable concrete extra water would have likely been added. Hence for the project it was determined that: a) No further NDT testing was generally required as adequate data was available to give a reasonable estimate of the concrete strength. Originally the client had asked for more detailed NDT testing to show which elements had low strength concrete. b) A 28 day characteristic compressive cylinder strength of 15MPa was to be used for structural assessment of all bases except those produced between 4-9 th May. c) For the concrete poured 4-9th May the location of the concrete placed was to be identified and design undertaken for strengthening to take the full design loads. The structural assessment was undertaken of all structures by review of the original designs. This showed that in general the 15MPa cylinder strength was adequate. The exceptions were: a) Primary Crusher Base Slab Development lengths up to 400mm to short due to low strength and low cover. Strengthen to reduce stresses or effectively increase development lengths b) Primary Crusher Rear Wall Will crack as applied moments are 3 times the moment capacity 5. Structural Assessment of New Concrete in an Australian Mine Plant On this project the concrete strength was called into question when the supervisor recorded that after taking cylinders for strength assessment the concrete water was added to the mix for placement. Cores and NDT were used to establish the actual characteristic strength of 18MPa for most elements, well below the minimum specified strength of 32MPa. Structural analysis was undertaken to show if these low strengths might be acceptable. A foundation slab was considered as 1m wide strips with actual load points imposed and resisted by soil spring nodes. Although the concrete strength was only 18MPa the authors believe that it is reasonable to extrapolate outside the range of characteristic compressive strengths of 20 MPa to 100 MPa, specified in Clause of AS3600, provided that a reliable coherent set of core results can be obtained. Of note is the anchorage bond length where requirements will increase beyond those in AS3600 in highly stressed elements. The analysis undertaken indicated that although capacities were significantly reduced most parameters were predicted to be acceptable except for shear and hence a risk assessment was undertaken. Risk is conventionally assessed by ISO (12) based on a combination of consequence and likelihood of failure. For some failure modes full probabilistic modelling can be undertaken to assess failure likelihood quite accurately. Where these methods are not possible a qualitative approach can be followed. The consequence of structural failure can vary significantly depending. Discussions with the owner identified that the increased likelihood of failure was unacceptable because when combined with the consequence of failure the risks during operations became unacceptable. Consequently the concrete was replaced.

7 6. Strength Assessment of an Old Australian Shopping Centre Floor Slab The strength of a concrete slabs on grade had been called into question. The strength of the surface layer is highly affected by finishing and curing and hence assessment using rebound hammer testing is not recommend as the results are very dependent on near surface properties. It was agreed to undertake widespread testing using indirect ultrasonic pulse velocity to show potential variations in strength. Cores were undertake for calibration of the UPV. Rebound hammer results were also taken over a wide area to determine if it gave any further insight into concrete performance. UPV results were taken using the five point indirect method. Results were recorded on a spreadsheet which automated a linear regression analysis to check the four results gave adequate correlation (Table 4). The velocities, and the rebound results, were plotted in a spreadsheet using conditional formatting to highlight variations (Table 5). Table 4 : Indirect UPV Data for Shopping Centre Floor Slab. D is the Transmitter/Receiver Separation and T the Corresponding Wave Transit Time Table 5 : Colour Coded Plot of Rebound and UPV Test Results In the case of the rebound results there appears to be two distinct areas where the 10 percentile strengths are 24.9 and 38.2MPa. These variations are not seen in the UPV results. The Rebound variations may be due to variations in finishing and curing of the two pours and be limited to surface effects. The UPV results were correlated with strength and gave the following relationship: Strength (MPa) = e v where v is ultrasonic pulse velocity in m/s. Converting the UPV to strength results gave a plot of strength as shown in Table 6. The colour coding is based on highlighting areas where the strength is less than the required 32 MPa. Table 6 : Plot of Compressive Strength estimated using UPV measurements

8 7. Strength Assessment of New Columns and Walls of an Australian Office Tower S65 concrete was poured in columns and stair well walls. Cylinder results were marginal and hence strength assessment was requested. An assessment of in-situ strength was undertaken using the SonReb method with UPV and rebound measurements calibrated against core strength tests. The process used for the SonReb method was: 1) Mark out reinforcement grid on the concrete surface using GPR so that UPV test results avoided the influence of reinforcement as far as possible. 2) Mark out sixteen measurement points for each element. These comprised eight pairs of measurement points precisely on opposite faces of the element. Typically four measurement points were at 0.5m, 1m, 1.5m and 2.0m above base level so that strength with height could be assessed. No significant variation was found (Figure 4) Figure 4 : Plot of UPV and Rebound With Height for a Shaft Wall 3) Assess the path length for the measurement points. 4) Take direct UPV measurements using digital UPV equipment with the transmitter on one face and receiver on the other for each of the eight pairs of measurement points. The transmitter and receiver were then swapped at each pair of points to give sixteen direct measurements. The UPV equipment s built in measuring process takes multiple measurements to give an average of several reading for each result. Results were also verified by the strength of the received signal and only highly reliable results were recorded. 5) Prepare the concrete surface using a grinding stone at each of the sixteen points and take rebound measurements using the digital rebound hammer. Ten rebounds were used to give one Q value. 6) Enter the data in the results spreadsheet prepared for the project to verify results were sensible and consistent. The formula for strength generally used is fck =a.v b.q c where a, b and c are constants V is the ultrasonic pulse velocity in m/s. Q is the rebound value as given in RILEM (14) 8. Strength Assessment of 50 Year Old Basement Columns in an Australian Building On this city centre project the building height was to be increased adding several new floors of office space. This translated to additional load in the basement columns and diaphragm wall. The existing structure had reached its 50 year design life and hence a durability and strength assessment was undertaken to confirm that the load capacity was adequate and that the structure would provide an additional 50 year life commensurate with the requirements for the new structure. The strength assessment was in two parts. Cores were taken from walls, columns and slabs. For the more critical columns a wider assessment of strength was required. This was achieved by correlating direct UPV results with core strengths and then using UPV results to give an indication of in-situ strengths. The correlation was based on compressive strength =1.0828e v where v is UPV.

9 Table 7 : UPV Correlation with Core Strengths Col Pulse Velocity (m/s) UPV Strength (MPa) Core Strength (MPa) Bottom Middle Top Bottom Middle Top B G Broke Piles were tested using a force vibration test as developed by Davis and Dunn (17). Testing was undertaken by measuring the response from a series of hammer blows on the concrete surface using a geophone held on the concrete surface adjacent to the hammer. Where there was no pile response the test location was moved along until a response was found. Having located the pile the test results were recorded. The received signal was put through a fast Fourier transform to give Mechanical Admittance and Frequency. Mechanical Admittance gives the load deflection curve based on wave theory. Frequency gives parameters of the pile model. Information obtained includes pile length, minimum pile diameter, presence of an end bulb, concrete modulus and safe pile load. 9. Strength Assessment of Old Shopping Centre Columns in Australia Testing was undertaken on ground floor columns of a shopping centre in Queensland testing to assess the capacity for planned extensions. No coring was allowed in the structure and so the completely nondestructive NDT method was proposed. NDT results depend on the materials used and so cylinders made using local materials were used to create a calibration between NDT (UPV and rebound) and cylinder strengths. Sixteen cylinders were tested. Direct UPV measurements were taken on the cylinders and then once the cylinder has been compressed to 3.5MPa the rebound values were taken using an original rebound hammer. The cylinders were then crushed. The calibration co-efficient were assed using the equation proposed by Samirin (18), i.e. f c = ar +bv 4. Samin s approach was an early form of combined ultrasonic pulse velocity and rebound for strength assessment and the SonReb equation would be recommended today.excel s Solver function was used to give the values for a and b of and 4.96x10-15 respectively. The correlation achieved was Ten columns were tested. Two rebound hammer tests were carried out in accordance with BS 1881:202 on each face. Two UPV measurements (BS1881:203) were taken between each of the two sets of opposing faces. Care was taken to avoid any reinforcing steel by locating this first using a covermeter. Column Rebound Number UPV (m/se) Calculated Strength (MPa) All of the columns were found to have strengths in excess of the 40MPa required for the extension. 10. Conclusions & Recommendations The paper identifies recent changes in the state of the art for assessing the in-situ strength of structures and outlines some of the principles used on various structures. The principles include core testing, rebound hammer and ultrasonic pulse velocity use to support in-situ strength assessment, methods of structural assessment and use of risk assessment to determine acceptability of reduced reliability due to low strength concrete. AS 3600, AS and CIA Z11 represent the current recommendations for structural assessment in Australia. These are now inconsistent with overseas documents on which they were based due to recent developments with the overseas documents. They do not incorporate the use of NDT and do not give structural assessment guidelines. It is recommended that AS and CIA Z11 are updated to reflect the current state of the art and be expanded to give guidance from testing to structural assessment.

10 Acknowledgements The authors wish to thank their clients for involvement in their projects. References 1. Standards Australia AS Method of testing concrete. Methods for securing and testing cores from hardened concrete for compressive strength Standards Association of Australia, 1991, Homebush, Australia. 2. Standards Australia AS Concrete Structures. Standards Association of Australia, 2009 Homebush, Australia. 3. BS EN Testing concrete in structures., Part 1 Cored specimens - taking, examining and testing in compression, (2009) Part 2 Non-destructive testing. Determination of rebound number (2012). Part 3 Determination of pull out force (2005). Part 4 Determination of ultrasonic pulse velocity (2004). BSI, London, UK. 4. BS EN Assessment of compressive strength in structures and precast concrete component British Standards Institute, 2007, London, UK. 5. British Standards BS6089 Assessment of in-situ compressive strength in structures and precast concrete components complementary guidance to BS EN British Standards Institute, 2010, London, UK. 6. Standards Australia AS Method of testing concrete. Methods for the determination of the compressive strength of concrete specimens Standards Association of Australia, 1999, Homebush, Australia. 7. Concrete Society Concrete Testing for Strength Technical Report 11, Concrete Society, 1976, Camberly, UK. 8. Concrete Society Concrete Testing for Strength Technical Report 11, Concrete Society, 1987, Camberly, UK. 9. CIA Z11 The Evaluation of Concrete Strength by Testing Cores Recomme, nded Practice Z11, Concrete Institute of Australia, 2002, Sydney, Australia. 10. Concrete Society In situ concrete strength. An investigation into the relationship between core strength and standard cube strength. Concrete Society. Project Report 3, 2004, Camberly, UK. 11. Crook N. Assessment of in-situ concrete strength using data obtained from core testing. Advice Note 47, Concrete Society, 2013, Camberly, UK. 12. ISO ISO 31000:2009 Risk Management International Organization for Standardization, 2009, Geneva, Switzerland. 13. fib The fib Model Code for Concrete Structures 2010 Fédération internationale du béton, 2010 Lausanne, Switzerland. 14. RILEM NDT4 Recommendation for in situ concrete strength determination by combined nondestructive methods, Réunion Internationale des Laboratoires et Experts des Matériaux, systèmes de construction et ouvrages. 1993, Bagneux, France, 15. Breysse D. Main challenges of non-destructive evaluation of on-site concrete strength Concrete Repair, Rehabilitation and Retrofitting III 2012 Taylor Franicis Group, 2012, London, UK 16. Yaman I.S., Inci G., Yesiler N. and Aktan H. Ultrasonic Pulse Velocity in Concrete Using Direct and Indirect Transmission ACI Materials Journal Nov/Dec 2001 pp Davis A.G. and Dunn C.S. From theory to field experience with non-destructive vibration testing of piles. Proc. Inst. Civ. Eng. Part 2, No 59, pp , Samarin A., and Meynink, P. Use of combined ultrasonic and rebound hammer method for determining strength of concrete structural members, Concrete International, vol. 3, no. 3, pp , 1981