ASSESSMENT OF THE CONCRETE COMPRESSIVE STRENGTH USING NON-DESTRUCTIVE METHODS

Transcription

1 BULETINUL INSTITUTULUI POLITEHNIC DIN IAŞI Publiat de Universitatea Tehniă Gheorghe Asahi din Iaşi Volumul 63 (67), Numărul 2, 2017 Seţia CONSTRUCŢII. ARHITECTURĂ ASSESSMENT OF THE CONCRETE COMPRESSIVE STRENGTH USING NON-DESTRUCTIVE METHODS BY COSTEL CHINGĂLATĂ *, MIHAI BUDESCU, RADU LUPĂŞTEANU, VLAD LUPĂŞTEANU and MARIA-CRISTINA SCUTARU Reeived: April 19, 2017 Aepted for publiation: May 31, 2017 Gheorghe Asahi Tehnial University of Iaşi, Faulty of Civil Engineering and Building Servies, Abstrat. The ultrasoni pulse veloity (UPV) and the rebound hammer (RH) are the most utilized non-destrutive methods in determining the ompressive strength of onrete. In most of the ases, if only one method is used, the results that are obtained are not aurate enough. Thus, the experimental programs that have been onduted in this filed, have proved that the optimum approah onsists in applying both methods and orrelating the results in order to obtain reliable values for the ompressive strengths. The ombined method that onsists in both UPV and RH tests, is known as SonReb approah. The non-destrutive test results obtained by UPV and RH are applied in the ompressive strength assessment for onrete elements, using empirial mathematial equations based on linear regression models. The first part of this paper fouses on presenting the general priniples of UPV and RH methods, detailing their partiularities and limitations. In the seond part of the paper, a ase study is presented, aiming to verify nine mathematial models whih are ommonly applied in the assessment of the ompressive strength of onrete. The results that were alulated based on the equations of the mathematial models have been ompared to the real values of the orresponding ompressive strengths, obtained through laboratory * Corresponding author: ostel.hingalata@tuiasi.ro

2 44 Costel Chingălată et al. ompressive destrutive tests. The omparative analysis onluded that the SonReb method provides the most aurate results, when ompared to the single methods. Keywords: onrete ompressive strength; ultrasoni pulse veloity; rebound hammer; mathematial models; omparative study. 1. Introdution The ompressive strength assessment of onrete elements is frequently neessary for this type of strutures during their life yle, even from the early stages of eretion, when doubts may arise onerning the quality of the exeution or, even more frequently, during the servie stage, when some of the designed performane requirements are not fulfilled anymore (Breysse, 2012). The available methods, whih an be applied for obtaining this parameter, an be first divided, based on their impat upon the investigated element. Thus, destrutive and non-destrutive methods are available. The destrutive methods involve either loal damaging the onstrution element or oring samples of onrete whih are later tested under laboratory onditions. The most important disadvantage whih haraterizes the destrutive methods refers to the neessity of losing down the speifi ativities whih are undertaken inside the building and in repairing the elements that have been damaged during the testing stage. Under these irumstanes, the non-destrutive methods represent a viable alternative, espeially for those onstrutions in whih the interruption of their speifi ativities is not possible. Obtaining the ompressive strength for onrete elements based on the results delivered by non-destrutive methods is a omplex proess whih an be affeted by different fators. For this reason, a very important step in designing a feasible investigation plan onsists in evaluating the five harateristi stages (MCann & Forde, 2001): visual inspetion, damage identifiation and analysis (if it exists), obtaining information from the tehnial douments, seletion of the suitable non-destrutive method and testing approah. Also, the most important fators that strongly influene the seletion of the appropriate nondestrutive testing method are (MCann & Forde, 2001): the required depth of penetration into the struture, the vertial and lateral resolution for the investigated element, the ontrast in physial properties between the target and its surroundings, the signal to noise ratio and the historial information onerning the methods used in the onstrution of the struture. The evaluation of eah fator, improves the proess of seleting the suitable non-destrutive method and, onsequently, inreases the auray of the results (Puinotti, 2015). The most important advantages implied by the use of non-destrutive methods refer to the short duration of investigation, lower osts when ompared

3 Bul. Inst. Polit. Iaşi, Vol. 63 (67), Nr. 2, to those speifi to the destrutive tests and the possibility of applying the testing methods without losing down the speifi ativities (Malhotra, 1976). The ultrasoni pulse veloity (UPV) and the rebound hammer (RH) are among the most utilized non-destrutive test methods whih are both used for quality evaluation and onrete mehanial harateristi assessment (Malhotra and Carino, 2004). The studies arried out in this field (Breysse, 2012; Kheder, 1999; Huang et al., 2011; Qasrawi, 2000; Erdal, 2009) onluded that the use of a single non-destrutive method for determining the ompressive strength of onrete is not providing results with a reliable auray. Thus, at least two methods should be used and the results shall be ombined by applying suitable mathematial models (Faaoaru, 1961; Breysse, 2012). 2. NDT Methods for Assessing the Compressive Strength of Conrete The ultrasoni pulse veloity (UPV) and the rebound hammer (RH) are the most utilized non-destrutive methods in determining the ompressive strength of onrete. The degree of appliability and the auray of the results that are obtained by applying eah non-destrutive method are influened by a set of fators. Usually, these fators refer to the environmental onditions during the testing proess, to the physial harateristis of the onrete whih is investigated and to the mathematial model that is applied in order to alulate the values of the ompressive strengths Ultrasoni Pulse Veloity (UPV) Method The ultrasoni pulse veloity is a non-destrutive method generally used for heking the quality of the onrete elements (existene of voids, raks, honey ombs), but also for the assessment of its ompressive strength. The method is desribed in the Romanian standards C26:1985, SR EN :2004 and in the international standards respetively, ASTM C597:2009 and BS :1986. The devie that is used for this method is the ultrasoni pulse veloity tester. The testing proedure is based on measuring the ultrasoni pulse propagation time between two transduers (from a transmitter to a reeiver), whih are plaed at a previously established distane. One the propagation time is measured, the veloity an be alulated by applying eq. 1. Based on the values of this physial parameter, important appreiations an be formulated with respet to the quality, uniformity, damage extent and to the ompressive strength of the investigated onrete element. L v, (1) T

4 46 Costel Chingălată et al. where: v is the ultrasoni pulse veloity, [m/s] or [km/s]; L the distane between the transmitting and reeiving transduers, [m]; T the ultrasoni pulse propagation time, [s]. The quality and the auray of the data whih is reorded during the UPV test an be influened by some fators whih are presented in Table 1 (Breysse, 2012). By analyzing the impat of eah fator, it has been onluded that the most important ones refer to the onstituents and to the damage extent of the onrete element. Based on the experimental investigation onduted by Whitehurst (1951), Agunwamba & Adagba (2012) have developed a orrelation between the ultrasoni pulse veloity and the quality of the onrete element whih is investigated (having the density of 2,400 kg/m 3 ). The orrelation is presented in Table 2. Table 1 Influening Fators for UPV Method Aggregate Size Average influene Type High influene Constituents Perentage Moderate influene Cement of onrete Type of ement Moderate influene Other Fly ash ontent Average influene onstituents Water/ement ratio High influene Humidity degree / Moisture ontent Average influene Reinforements Moderate influene Other fators Age of onrete Moderate influene Voids, raks High influene Table 2 Ultrasoni Pulse Veloity an Index for Conrete Quality Assessment Conrete quality Ultrasoni pulse veloity (m/s) Exellent Over 4500 Good Doubtful Low Very low Under Rebound Hammer (RH) The rebound hammer method represents one of the most utilized nondestrutive proedures and it onsists in measuring the surfae hardness of the onrete element whih is investigated. This testing method is desribed in the Romanian standards C26:1985, SR EN :2012 and, also, Amerian

5 Bul. Inst. Polit. Iaşi, Vol. 63 (67), Nr. 2, standard ASTM C805:2008. This proedure is ommonly applied for onrete quality evaluation (uniformity, voids) and also, for evaluating the onrete ompressive strength. The Indian standard IS :1992 provides a orrelation between the rebound number (RN) and the investigated onrete quality, presented in Table 3. The method is simple to apply, assume low osts and provides quik results. Table 3 Rebound Number an Index for Conrete Quality Assessment Average Rebound Number Conrete quality Above 40 Very good onrete Good onrete Fair onrete Below 20 Poor onrete This type of test is performed by using a rebound hammer whih triggers a mobile mass to the surfae of the onrete element. After the impat, a part of the indued energy is onsumed as deformation, while the remaining energy produes the rebound effet. The latter, haraterizes the hardness of the onrete surfae and gives the rebound number, whih is reorded by the devie. The orrelation between the rebound number and the onrete ompressive strength is influened by several fators, eah of them having speifi influene. These fators are presented in Table 4 (Breysse, 2012). Table 4 Influening fators for RH method Size Average influene Aggregate Constituents Type High influene of onrete Perentage Moderate influene Cement Type of ement Moderate influene Humidity degree / Moisture ontent Average influene Carbonation degree High influene Smoothness degree Average influene Contat surfae properties Formwork type and Average influene uring onditions Temperature Moderate influene Other fators Voids High influene 2.3. The Combined Method - Sonreb The ombination of the UPV and RH methods was developed by Faaoaru (1961). The main advantage derived from ombining the results,

6 48 Costel Chingălată et al. onsists in obtaining a omplete haraterization of the material. Thus, the ompressive strength of the onrete is obtained by ombining the values at the exterior side of element, on a depth of about 2,...,3 m, obtained by using the RH method, with the ones haraterizing the inner part of the element, provided by applying the UPV method. By taking into aount the reordings of both UPV and RH tests, the outomes of the ombined method an be influened by all before mentioned fators (Arioz et al., 2009). Nevertheless, the results of the ombined method, an be differently influened under the ation of the same fator, whih will finally lead to an improved auray in the assessment of the onrete ompressive strength, based on the development of a self-orretion mehanism for the indued error. However, the researh programs that have been onduted in the area of ombined non-destrutive test methods for onrete, have onluded that, when ertain speifi harateristis of the element / material are known (waterement and aggregate-ement ratios, age, density), the auray of the results an be improved (Kheder, 1999; Huang et al., 2011; Tanigawa et al., 1984). Also, when using the ultrasoni pulse veloity method, the assessment of the onrete ompressive strength is strongly influened by the type and dimension of the aggregates used in the onrete mix. Thus, it was suggested that orretion fators should be developed (Trtnik et al., 2009). 3. Regression Models Proposed for Assessing the Compressive Strength of Conrete Many researh groups have foused on identifying the most appropriate losed-form mathematial models whih an be applied in the assessment of the ompressive strength of onrete, by using the results of the UPV or/and RH methods. The experimental programs that were onduted in this filed also onsisted in destrutive tests, aiming to alibrate the mathematial models whih were previously proposed for the non-destrutive results. The mathematial models proposed so far an be lassified based on the number of the variables that are used in the losed-form equations. Thus, single and multiple variable models have been proposed. The single variable models use one set of reordings given by a non-destrutive test, while the multiple variable ones involve the use of at least two sets of non-destrutive results and also, in some ases, knowing some properties that refer to the onstituents of the onrete (water/ement/aggregate ratio, onrete age, arbonation degree, density and type and aggregate dimension).

7 Bul. Inst. Polit. Iaşi, Vol. 63 (67), Nr. 2, For most of the onrete elements that are investigated, the information regarding the properties of the onstituents are limited or non-existent. Thus, the most effiient mathematial models are the ones that are only based on the variables resulted from the non-destrutive tests. Table 5 presents some of the most ommon mathematial models, based on the linear regression priniple, that are used in the assessment of the onrete ompressive strength. The variables that are onsidered in these models are only based on the results of the UPV and RH testing methods. Eq. no. (Code) 1 (K1) 2 (K2) Table 5 Mathematial Models for Conrete Strength Assessment f Proposed equations Author, year UPV Kheder, f RN Kheder, (Q1) f UPV Qasrawi 1, (Q2) f RN Qasrawi 2, (E1) f RN RN Erdal, (T) f RN UPV Tanigawa et al., (K3) f UPV RN Kheder, (E2) f 0.42 RN UPV Erdal, (H) f RN UPV Huang, Comparative Case Study The omparative ase study whih is presented in this paper fouses on determining the validity of the nine mathematial models that are listed in Table 5. The ompressive strength results obtained by applying the mathematial models are ompared with those resulted from destrutive testing, on a number of 20 ubi laboratory prepared speimens, having the dimensions of mm. The numerial values that are used in this omparative study are taken from the experimental program that has been onduted by Nikhil and his ollaborators (Nikhil et al., 2015). Thus, in Table 6 the first three olumns present the results of the nondestrutive methods and the values of the ompressive strengths obtained by laboratory destrutive tests, while the following 9 olumns present the ompressive strengths obtained by applying the mathematial models.

8 50 Costel Chingălată et al. Table 6 Input Data and Predited Compressive Strengths Input data (Nikhil et al., 2015) Predited ompressive strengths RN UPV (km/s) Effetive ompressive strength (MPa) UPV RN UPV RN RN SonReb (UPV + RN) Single-Variable eq. Multi-Variable eq. K1 K2 Q1 Q2 E1 T K3 E2 H

9 Bul. Inst. Polit. Iaşi, Vol. 63 (67), Nr. 2, For eah mathematial model, the values of the ompressive strengths have been statistially interpreted. Hene, for eah set of results, the root-meansquare error (RMSE), the mean-absolute perentage error (MAPE) and oeffiient of determination have been omputed. The statistial results are presented in Table 7. Statistial interpretation Table 7 Statistial interpretation of the predited ompressive strengths UPV RN UPV RN RN SonReb (UPV + RN) Single-Variable eq. Multi-Variable eq. K1 K2 Q1 Q2 E1 T K3 E2 H RMSE, [MPa] MAPE, [%] R For eah type of method, the predited values of the ompressive strengths have been ompared with the effetive ones. The results are presented in Figs. 1,...,3. In the first two graphial representations (Figs. 1 and 2) are illustrated the variation of the ompressive strengths based on applying the single variable equations. The third graph (Fig. 3) presents the variation of the ompressive strengths obtained based on the multiple variable mathematial models. Fig. 1 UPV vs onrete ompressive strength.

10 52 Costel Chingălată et al. Fig. 2 RH vs onrete ompressive strength. Fig. 3 SonReb vs onrete ompressive strength.

11 Bul. Inst. Polit. Iaşi, Vol. 63 (67), Nr. 2, Conlusions This paper presents two of the most used non-destrutive testing methods for the assessment of onrete ompressive strength, the ultrasoni pulse veloity (UPV) and the rebound hammer (RH). Eah of the two methods have a high degree of appliability, delivering results lose to the real ones. However, the auray of the predited ompressive strength an be affeted by a set of fators that depend on the inherent properties of the material and on the servie onditions. A onsistent number of researh groups have foused on identifying and proposing suitable mathematial models that an be used for assessing the ompressive strength of the onrete. Thus, until now, a number of mathematial equations have been proposed, based on the linear regression priniple, whih involve the use of non-destrutive testing results (ultrasoni pulse veloity and rebound number). These models an be lassified in single and multiple variable. This paper presents 9 mathematial models, ommonly applied in the assessment of the ompressive strength of onrete, that are only using the results of the UPV and RH non-destrutive methods. The ase study fouses on heking the validity and the auray of eah mathematial model by omparing the predited ompressive strengths with the effetive ones, previously obtained by destrutive laboratory tests. For eah losed-form equation, the predited values have been statistially interpreted and graphially ompared to the effetive ones. By analyzing the graphs presented in Fig. 1 and 2 it an be onluded that, for the single variable mathematial models, the losest ompressive strength results are obtained applying the equations proposed by E1 and K2. The higher degree of auray of these models, E1 and K2, is also onfirmed by the values of the oeffiient of determination, R 2. The model K1 offers results lose to the real ones only for the speimens with the ompressive strengths ranging between MPa, the model Q1 onsiderably under-estimates the entire set of values, while the model Q2 provides results lose to the real ones only for the range between MPa. By analyzing the graph presented in Fig. 3, whih presents the results obtained by applying the multiple variable mathematial models (provided by the SonReb method), it an be onluded that equations T, K3 and E2 lead to good results, with ompressive strength values lose to the experimental ones. Also in this ase, the higher degree of auray is onfirmed by the values of the oeffiient of determination, R 2. The model H is an exeption for this ase

12 54 Costel Chingălată et al. beause it under-estimates the results for speimens having the ompressive strength higher than 20 MPa. Based on the omparative study of the 9 proposed mathematial models, it has been shown that losest values of the predited ompressive strengths are obtained by applying the multi-variable equations. REFERENCES Agunwamba J.C., Adagba T., A Comparative Analysis of the Rebound Hammer and Ultrasoni Pulse Veloity in Testing Conrete, Nigerian Journal of Tehnology (NIJOTECH), 31, 1, (2012). Arioz O., Tunan A., Tunan M., Kavas T., Ramyar K., Kilin K., Karasu B., Use of Combined Non-Destrutive Methods to Assess the Strength of Conrete in Strutures, Afyon Koatepe University, Journal of Siene, (2009). Breysse D., Nondestrutive Evaluation of Conrete Strength: An Historial Review and a New Perspetive by Combining NDT Methods, Constrution and Building Materials, 33, (2012). Erdal M., Predition of the Compressive Strength of Vauum Proessed Conretes Using Artifiial Neural Network and Regression Tehniques, Sientifi Researh and Essay, 4, 10, (2009). Faaoaru I., Contribution à l étude de la relation entre la résistane du béton à la ompression et la vitesse de propagation longitudinale des ultra-sons, RILEM Bull., 12, (1961). Huang Q., Gardoni P., Hurlebaus S., Prediting Conrete Compressive Strength Using Ultrasoni Pulse Veloity and Rebound Hammer, ACI Materials Journal, 108, 4 (2011). Kheder G.F., A Two Stage Proedure for Assessment of in situ Conrete Strength Using Combined Non-Destrutive Testing, Materials and Strutures, 32, (1999). Malhotra V.M., Carino N.J., Handbook on Nondestrutive Testing of Conrete, ASTM 2 nd edition, CRC Press, Malhotra V.M., Testing Hardened Conrete: Non-destrutive Methods, ACI, monograph no. 9, Detroit, USA (1976). MCann D.M., Forde M.C., Review of NDT Methods in the Assessment of Conrete and Masonry Strutures, NDT&R International, 34, (2001). Nikhil M.V., Minal B.R, Deep C.S., Vijay G.D., Vishal T.S., Shweta P., The Use of Combined Non Destrutive Testing in the Conrete Strength Assessment from Laboratory Speimens and Existing Buildings, ISSN (PRINT): , (ONLINE): , 2, 5 (2015). Puinotti R., Reinfored Conrete Struture: Non-Destrutive in situ Strength Assessment of Conrete, Constrution and Building Materials, 75, (2015).

13 Bul. Inst. Polit. Iaşi, Vol. 63 (67), Nr. 2, Qasrawi H., Conrete Strength Combined Nondestrutive Testing Methods Simply and Reliably Predited, Cement and Conrete Researh, 30, (2000). Tanigawa Y., Baba K., Mori H., Estimation of Conrete Strength by Combined Nondestrutive Testing Method, In situ non-destrutive testing of onrete, Amerian Conrete Institute, Detroit, , Trtnik G., Kavi F., Turk G., Predition of Conrete Strength Using Ultrasoni Pulse Veloity and Artifiial Neural Networks, Ultrasonis, 49, (2009). Whitehurst E.A., Sonisope Tests Conrete Strutures, Journal Amerian Conrete Institute, 47, (1951). * * * Standard Test Method for Pulse Veloity through Conrete, Amerian Standard, ASTM International, West Conshohoken, USA, ASTM C597/2009. * * * Standard Test Method for Rebound Number of Hardened Conrete, Amerian Standard, ASTM International, West Conshohoken, USA, ASTM C805/2008. * * * Reommendations for Measurement of Veloity of Ultrasoni Pulses in Conrete, British Standard, BSI, U.K., BS :1986. * * * Non-Destrutive Testing of Conrete Methods of Test. Part 2: Rebound Hammer, Indian Standard, IS /1992. * * * Non-Destrutive Methods for Conrete Testing, Romanian Code, C26/1985. * * * Conrete Testing. Part 2: Determination of the Ultrasoni Pulse Veloity, Romanian Standard, SR EN /2004. * * * Testing Conrete in Strutures. Part 4: Non-Destrutive Testing. Determination of the Rebound Index, Romanian Standard, SR EN /2012. EVALUAREA REZISTENŢEI LA COMPRESIUNE A BETONULUI UTILIZÂND METODE NEDISTRUCTIVE (Rezumat) Metoda impulsului ultrasoni (UPV) şi ea bazată pe reul (RH) sunt ele mai des utilizate metode nedistrutive pentru determinarea rezistenţei la ompresiune a betonului. De ele mai multe ori, rezultatele obţinute prin utilizarea unei singure metode non-invazive nu sunt araterizate de un grad ridiat de aurateţe. Din aest motiv, eretările efetuate în aest domeniu au demonstrat ă abordarea optimă onstă în apliarea simultană a elor două metode şi orelarea rezultatelor pentru a obţine valori ale rezistenţei la ompresiune apropiate de ele reale. Combinarea elor două metode, viteza impulsului ultrasoni şi ea bazată pe reul, este unosută în literatura de speialitate sub denumirea de SonReb. Rezultatele înerărilor nedistrutive de tip UPV şi RH sunt utilizate la determinarea valorilor rezistenţelor la ompresiune a elementelor din beton, u ajutorul unor modele matematie empirie bazate pe prinipiul regresiei liniare. În prima parte a aestei lurări sunt prezentate prinipiile generale ale metodelor UPV şi RH, detaliindu-se partiularităţile şi limitările speifie. În partea a doua a lurării este prezentat un studiu de az e onstă în verifiarea a nouă modele matematie utilizate la aproximarea rezistenţei la ompresiune. Rezultatele alulate

14 56 Costel Chingălată et al. prin apliarea modelelor matematie au fost omparate u ele obţinute prin înerări distrutive. Studiul omparativ demonstrează ă metoda SonReb furnizează rezultate u un grad mai ridiat de aurateţe, omparativ u ele rezultate din apliarea unei singure metode nedistrutive.