Study on estimation of compressive strength of concrete in structure using ultrasonic method

Transcription

1 Architectural Institute of Japan Translated Paper Study on estimation of compressive strength of concrete in structure using ultrasonic method Satoshi Watanabe, 1 Kazumi Hishikawa, 2 Kengo Kamae 2 and Satoru Namiki 3 1 Technology Center, Taisei Corporation, Yokohama, Japan; 2 Construction Department, The General Environmental Technos Co., Ltd., Osaka, Japan; 3 Building Construction Div., Taisei Corporation, Tokyo, Japan Correspondence Satoshi Watanabe, Technology Center, Taisei Corporation, Yokohama, Japan. wtnsts1@pub.taisei.co.jp Funding information No funding information is provided. The Japanese version of this paper was published in Volume 81, Number 72, pages 1 198, /aijs.81.1 of Journal of Structural and Construction Engineering (Transactions of AIJ). The authors have obtained permission for secondary publication of the English version in another journal from the Editor of Journal of Structural and Construction Engineering (Transactions of AIJ). This paper is based on the translation of the Japanese version with some slight modifications. Abstract In this study, for improving the accuracy of estimating the compressive strength of concrete using the ultrasonic method, a method for measuring the ultrasonic velocity and a method for estimating the strength of concrete were investigated. In an experiment with reinforced concrete members of actual size, the precision of the proposed method for measuring the ultrasonic velocity of inner concrete was confirmed to be high. Furthermore, the estimated value of the compressive strength of concrete in structure using the proposed strength estimation method was close to the actual measurement value. Keywords compressive strength, concrete, nondestructive testing, ultrasonic velocity, Young s modulus Received July 1, 217; Accepted October 5, 217 doi: 1.12/ Introduction In general, the higher the compressive strength, the higher is the velocity of propagation of ultrasonic longitudinal waves ( ultrasonic velocity ) in concrete, and a high correlation can be determined between these two when the materials used or other conditions are limited. By utilizing this property, a nondestructive testing method for estimating the strength of concrete in structure was proposed based on the results of measurement of the ultrasonic velocity. This property involves using a strength estimation equation written using the relation between ultrasonic velocity and compressive strength that was confirmed in advance with specimens fabricated based on conditions similar to the concrete in the target structure. 1 The ultrasonic velocity is known to be strongly affected by the water content of concrete. If a concrete specimen is dried, by reducing its water content, the ultrasonic velocity decreases, 2 but conversely the compressive strength increases. 3,4 For this reason, if the water content of concrete is different, the relation between the ultrasonic velocity and compressive strength will also differ. Accordingly, the ultrasonic velocity used to estimate the strength of concrete in structure should be measured under the same water content conditions as the data that were used to prepare the strength estimation equation. The water content in the surface sections of concrete in structure will differ considerably depending on the environmental conditions, but the water content of the inner sections that are little affected by drying is thought to be similar to the water content of a seal-cured specimen. Accordingly, the authors developed a method ( proposed method ), in which 2 ultrasonic probes are inserted into 2 holes drilled into the concrete in structure in order to measure the ultrasonic velocity in the inner sections that are largely unaffected by drying. The accuracy of the ultrasonic velocity measurement determined using the proposed method was verified using a square column specimen ( mm) with 2 measurement holes. 5 In the case of existing structures, as it is difficult to identify the material used for the concrete at the time of construction and to acquire a similar material, it is impossible in many cases to determine the relation between the ultrasonic velocity and compressive strength of a specimen fabricated under conditions close to the concrete in the target structure. A method for such cases was proposed, in which the strength estimation equation is developed using core specimens taken from the concrete in the target structure. 6,7 Even in this case, however, unless a sufficient quantity and breadth of compressive strength data are obtained, it is difficult to accurately determine the trends in the relation between the ultrasonic velocity and compressive This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made. 218 The Authors. Japan Architectural Review published by John Wiley & Sons Australia, Ltd on behalf of Architectural Institute of Japan. Jpn Archit Rev January 218 vol. 1 no

2 WATANABE et al. wileyonlinelibrary.com/journal/jar3 strength, and an equation for estimating strength with sufficient accuracy cannot be obtained. Accordingly, the authors combined the Young s modulus estimation equation based on elastic theory with other elements such as the New RC equation 8 that shows the general relation between Young s modulus and the compressive strength of concrete in order to propose basic forms in a strength estimation equation. 5 In the previously reported study 5 at the specimen level, the ultrasonic velocity measurement accuracy using the proposed method and the applicability of the proposed strength estimation method were confirmed. Unlike the specimen, however, in the case of an actual structure, considerable differences in the compressive strength of concrete may be produced at various member locations owing to the effect of bleeding and the like. 9,1 For this reason, in order to confirm the applicability to actual structures, it is necessary to confirm that it is possible to assess the strength of concrete in structure using the same ultrasonic velocity measurement method and strength estimation method at various locations of members, in which a distribution of concrete strengths has been produced. This paper reports the results of the study conducted to determine the applicability of the ultrasonic velocity measurement method that uses the proposed method as well as the proposed strength estimation method using actual-size specimens that simulate reinforced concrete members. 2. Experiment outline 2.1 Mix proportion of concrete Tables 1 and 2 show the materials and mix proportions of the concrete used for specimen fabrication. The source location for limestone crushed stone is different from that in the previous study, 5 but the type and source locations for the other materials are the same. The water-cement ratio ( W/C ) was the same range as in the previous study 5 and comprised 3 levels: 4%, 55%, and 65%. Of these levels, the mix proportions with W/C = 55% were conducted using 2 types of coarse aggregate. The values for slump and air content at placing were the same as in the previous study. 5 Out of consideration for a decrease Table 1. Materials of concrete Cement C Ordinary Portland cement (density: 3.16 g/cm 3 ) Fine aggregate S Kimitsu sand (density (SSD): 2.62 g/cm 3, absorption: 1.45%) Coarse aggregate Chemical admixture G1 G2 AD AE Otsuki andesite crushed stone (density (SSD): 2.62 g/cm 3, absorption: 2.57%) Chichibu limestone crushed stone (density (SSD): 2.7 g/cm 3, absorption:.6%) Air-entraining and water-reducing Air-entraining in slump and air content due to transport from the ready-mixed concrete plant, the unit water content was increased by 5 kg/ m 3 compared to the value in the previous study, 5 and the admixture quantity was also increased. For W/C = 55 and 65%, the sand-total aggregate ratio (s/a) was decreased by 1%. 2.2 Preparation of specimens When the concrete was received, a fresh concrete test was conducted to confirm that no material separation occurred and that the slump and air content were in the range of cm and %, respectively. For each mix proportion, 15 (3 each for 5 ages) cylindrical specimens with ø1 9 2 mm were prepared, and 1 wall and 1 slab specimen, as shown in Figure 1, were also fabricated. The wall and slab specimens were fabricated with a double arrangement of deformed bars D13 at 2 mm intervals in 2 directions and then provided with a cover depth of 3 mm. The placement of the concrete for the specimens used a free-fall height of 1 m or less to simulate the actual construction of wall and slab members. In general, the concrete placement surfaces of the specimens were moist-cured by covering them with a waterproof sheet up through an age of 6 days. However, the sheet was temporarily removed from the slab specimen at an age of 3 days in order to take core samples and measurements, and for this reason after the measurement, the surface was sprinkled with water and covered with the sheet again. The specimens were removed from the molds at an age of 6 days, and their sides were covered with aluminum tapes to prevent drying. The specimens were stored in an outdoor environment where they would not be exposed to rainwater or direct sunlight. After the concrete hardened, the effect of bleeding, compaction, etc., was thought to be minimal, so the wall specimen was turned on its side at an age of 6 days and stored flat in the same manner as the slab specimen. The cylindrical specimens were sealcured in the same location as the wall and slab specimens. 2.3 Measurement Figure 1 and Table 3 show the measurement locations and quantities. The measurements were conducted at an age of 3, 7,,, and days using the following procedure. The ultrasonic velocity was measured using the 2 probes that were used in the previous study 5 (an insertion section diameter of 2 mm, an oscillator diameter of 18 mm, and a nominal frequency of 1 khz), using the same method as in the previous study 5 (see Figure 2). 1. The ø1 mm core specimens were extracted from the large circled sections shown in Figure 1. The specimens were sampled on the same day in the case of an age of 3 days, on the previous day in the case of an age of 7 days, and 4 or less days before in the case of the other Table 2. Mix proportions of concrete Mix no. Slump (cm) Air (%) W/C (%) s/a (%) Unit content (kg/m 3 ) AD AE W C S G1 G2 (C9%) (C9%) W/C is water-cement ratio, s/a is sand-total aggregate ratio, W is unit content of water. Jpn Archit Rev January 218 vol. 1 no. 1 88

3 wileyonlinelibrary.com/journal/jar3 WATANABE et al. 7 : Sampling core (Numbers indicate age) 7 : Measuring by usual transmission method (Numbers indicate age) 7 : φ25 mm holes for measuring ultrasonic (Drilling at 3 or 7 days) 7 : φ25 mm holes for measuring ultrasonic (Drilling at days) : Interval distance (35 mm) of measuring ultrasonic after 3 or 7 days : Interval distance (35 mm) of measuring ultrasonic after days : Reinforcing bar (D13) Wall specimen (W18 H3 t2 mm, D13-@2 mm double) Slab specimen (18 12 t2 mm, D13-@2 mm double) Figure 1. Outline of specimens (elevation view of wall specimen, plan view of slab specimen) Table 3. Specimen Measurement quantity per mix proportion Seal-cured specimen Slab specimen Age All 3,,, days Wall specimen Core Structure Core Structure 7,,, days Usual method Proposed - - 4[2] - 4[2] 9 2 method Compressive strength Static modulus of elasticity 1 {1} - {1 9 2} - [ ] and {}indicate quantity at 3 or 7 days and days; measurement targets for static modulus of elasticity were the each middle core of the higher and lower parts in walls. ages. Both ends were polished along with the seal-cured specimens. 2. The ultrasonic velocity was measured between both ends of the seal-cured specimens and the core specimens (the distance between the measurements was approximately 2 mm), using the usual transmission method ( usual method ). 3. The compressive strength and some static modulus of elasticity (using a 6 mm strain gauge) of the seal-cured specimens and the core specimens were measured. 4. At the locations shown by the and circles in Figure 1, holes were drilled in the slab specimen at an age of 3 and days and in the wall specimen at an age of 7 and days, with a diameter of 25 mm and a depth of approximately 15 mm. As there was no impact on the results of the measurement with the proposed method resulting from differences in the times when the holes were drilled, the ultrasonic velocity was determined by averaging the measurement results for all paths for each location. 5. The ultrasonic velocity in the member was measured using the usual method in the member thickness direction (the distance between the measurements was approximately 2 mm) at the locations in Figure 1 (15 mm from the end) and using the proposed method between the holes (the distance between the measurements was approximately 35 mm), as shown in Figure 1. For the measurements using the proposed method, the oscillator center position was placed at a depth of 8 mm, so the oscillator would be located deeper than the reinforcing bar. 6. Following the measurement, the core holes were filled with nonshrink mortar. The drilled holes were stuffed with wet rags that were thoroughly wrung out and then covered with aluminum tape. 3. Experiment results and discussion 3.1 Physical properties of concrete in structure Figure 3 shows the distribution of the physical properties of the concrete in structure. Each graph shows the distribution in Jpn Archit Rev January 218 vol. 1 no. 1 89

4 WATANABE et al. wileyonlinelibrary.com/journal/jar3 Figure 2. Measuring ultrasonic (core-usual method, structure-usual method, and structure-proposed method) the height direction as the ratio of the physical properties of the concrete in structure to the seal-cured specimen with the same mix proportion and age. The height from the top surface indicates the height of the center position of the core specimens ( 1 mm for the slab specimen and 4, 6, 8, 22, 24, and 26 mm for the wall specimen). For the ultrasonic velocity, the results of not only the core specimens but also the measurement using the usual method in the member thickness direction at the locations (height 15 mm and 5 mm from the top) are shown in Figure 1. Because of the effect of bleeding, compaction, etc., the strength of the concrete in structure is known to decrease the closer the location is to the concrete placement surface. 9,1 The same is true for the density and ultrasonic velocity. In this test as well, with the exception of Mix No.1, the density, compressive strength, and ultrasonic velocity tended to be lower the closer the location was to the top. In this way, the distribution of the physical properties in the height direction was confirmed in the wall specimen. Accordingly, the consideration of the results of the measurement using the proposed method as covered in the subsequent sections is based on the results of the measurement of 2 core specimens, whose heights were similar to the measurement holes in the proposed method for each age and location. As the W/C for Mix No.1 was low at 4%, the effect of bleeding and the like is thought to be small. The density of the core specimens decreased with the age (drying time), and the rate of the density reduction owing to drying from the time of moist curing (the age was 3 days in the case of the slab specimen and 7 days in the case of the wall specimen) was approximately 1% or less up to an age of days and approximately 1% 4% at an age of days or more. In addition, the ratio of the ultrasonic velocity of the concrete in structure with respect to the seal-cured specimen decreased as the age increased. However, there was no clear change in the compressive strength ratio as a result of age. 3.2 Comparison of usual and proposed methods Figure 4 shows the relation between the ultrasonic velocity of the core specimens determined using the usual method and the ultrasonic velocity determined using the proposed method. The 4 mix proportions (Mix No.1 4) are plotted in the figure for each age and location. Based on the 4 points plotted for the slab specimen and the 8 points plotted for the higher and lower parts of the wall specimen, the regression lines passing through the origins are depicted for each age and specimen (a broken line in the case of the slab specimen and a solid line in the case of the wall specimen). The regression expressions are presented in the same order used in the legend. As in the previous study, 5 there was no clear difference between the results of the measurement using the usual method and the results of the measurement using the proposed method for ages up to days. However, at an age of days or higher, the measurement values determined using the proposed method were larger. Figure 5 shows the influence of differences in drying status on the results of the measurement obtained using the usual method and the proposed method. The horizontal axis shows the rate of decrease in density due to drying after moist curing. The vertical axis shows the ratio of the ultrasonic velocity obtained using the proposed method to the ultrasonic velocity obtained using the usual method. As the rate of density decrease of the core specimen increased, the ratio of the ultrasonic velocity obtained using the proposed method to the ultrasonic velocity obtained using the usual method tended to be greater as well. This is thought to be because the proposed method tends not to be affected by drying in contrast to the usual method, in which the surface sections are affected by drying. Based on these findings, the proposed method is thought to be suitable for measuring the ultrasonic velocity of internal sections. 3.3 Relation between ultrasonic velocity and compressive strength Figure 6 shows the relation between the ultrasonic velocity and compressive strength in the seal-cured specimens. The results in the figure are categorized by the type of coarse aggregate and are presented along with the exponential trend lines. As in the previous study, 5 there was a high correlation between the ultrasonic velocity and compressive strength in the seal-cured specimens when the results are viewed by the type of coarse aggregate. Figure 7 shows the relation between the ultrasonic velocity and compressive strength of the concrete in structure. The horizontal axes of the top graph and the bottom graph show the ultrasonic velocity of the core specimens derived using the usual method and the ultrasonic velocity derived using the proposed method, respectively. Both graphs also show the trend lines for the seal-cured specimens from Figure 6. In the top graph, there is a high correlation between the ultrasonic velocity and compressive strength in the seal-cured specimens up to an age of days (during which the effects of drying are comparatively minor), but at an age of days and larger, when the effects of drying are great, the ultrasonic velocity is lower as compared to the seal-cured specimens for the same compressive strength. This is thought to be because the ultrasonic velocity for the core specimens determined using the usual Jpn Archit Rev January 218 vol. 1 no. 1 9

5 wileyonlinelibrary.com/journal/jar3 WATANABE et al. Height from the top surface (mm) Height from the top surface (mm) Height from the top surface (mm) Height from the top surface (mm) Density ratio (core/seal-cured) Compressive strength ratio (core/seal-cured) Ultrasonic velocity ratio (core/seal-cured) Figure 3. Physical properties of concrete in structure (density, compressive strength, and ultrasonic velocity) method was lower owing to the drying of the surface sections. For this reason, the bottom graph that shows the ultrasonic velocity derived using the proposed method (which tends to not be affected by drying) shows less of an effect owing to the age than the top graph. Even in that case, however, there is some deviation in terms of the relation with the seal-cured specimens. This study proposes a method, in which the static modulus of elasticity of concrete is estimated based on the ultrasonic velocity, and this estimate of the static modulus of elasticity is used to estimate the compressive strength. Accordingly, the remainder of this study will focus on determining whether the differences in relation between the ultrasonic velocity and the compressive strength in these types of seal-cured specimens and other specimens depend on the differences in the relation between the ultrasonic velocity and the static modulus of elasticity or on the differences in the relation between the static modulus of elasticity and the compressive strength. Jpn Archit Rev January 218 vol. 1 no. 1

6 WATANABE et al. wileyonlinelibrary.com/journal/jar3 Proposed method velocity (m/s) y =.99 x y =.99 x y = 1. x y = 1. x y = 1.1 x y = 1.2 x y = 1.1 x y = 1.2 x Seal-cured strength (N/mm 2 ) Andesite Limestone 3 d 3 d 7 d 7 d d d d d d d Usual method velocity (m/s) Seal-cured velocity (m/s) Figure 4. Ultrasonic velocity by usual and proposed methods Figure 6. specimen Ultrasonic velocity and compressive strength of seal-cured Velocity ratio (Proposed/Usual) Figure 5. y =.1 x + 1. Density reduction rate by drying (%) Influence of dry condition on ultrasonic velocity The commonly used value of m =.2 was used for Poisson s ratio. ð1 2vÞð1 þ vþ E ¼ q V 2 ð1þ 1 v where E is Young s modulus (dynamic modulus of elasticity) [kn/mm 2 ], q is density [g/cm 3 ], V is ultrasonic velocity [km/s], and m is Poisson s ratio [-]. Figure 8 shows the relation between the values for the dynamic modulus of elasticity and static modulus of elasticity. The relation between the estimate of the dynamic modulus of elasticity and the measurement of the static modulus of elasticity for the seal-cured specimen (solid line in the figure) was generally the same as in the previous study 5 (broken line in figure). Moreover, the relations for both values at each location on the specimens were generally the same. Accordingly, the following consideration was conducted based on Equation (2) as proposed in the previous study. 5 E c ¼ 1:4 ð1 2v cþð1 þ v c Þ q 1 v c Vc 2 12: ð2þ c 3.4 Relation between ultrasonic velocity and static modulus of elasticity First, a study was conducted to study the relation between the measurement of static modulus of elasticity and the estimate of Young s modulus (dynamic modulus of elasticity) determined using Equation (1) based on the ultrasonic velocity, etc. This study used the data of 1 (in the wall specimen, the height of each location is in the middle) of the 3 core specimens at each location where the static modulus of elasticity test was performed for an age of days. To assess the dynamic modulus of elasticity in the interior where the effect of drying is minimal, the density of the core specimens that were not affected by drying (an age of 3 days for the slab specimen and an age of 7 days for the wall specimen) and the ultrasonic velocity measured using the proposed method were used for calculating the estimate for the dynamic modulus of elasticity. where E c is the static modulus of elasticity [kn/mm 2 ] of concrete, q c is density [g/cm 3 ] of concrete, V c is ultrasonic velocity [km/s] in concrete, and m c is Poisson s ratio [-] of concrete (=.2). 3.5 Relation between static modulus of elasticity and compressive strength The previous study 5 proposed a method for estimating the compressive strength from Equation (3), based on the estimate of the static modulus of elasticity of the concrete. However, if the coefficient k in this equation differed, the relation between the estimate of the static modulus of elasticity and the compressive strength would differ as well. Accordingly, a study was conducted on the coefficient k calculated back based on the static modulus of elasticity and the density for the sealcured specimens and the core specimens. The values of the Jpn Archit Rev January 218 vol. 1 no. 1 92

7 wileyonlinelibrary.com/journal/jar3 WATANABE et al. Core strength (N/mm 2 ) Andesite Limestone 3 d 3 d 7 d 7 d d d d d d d Sta c modulus of elas city (kn/mm 2 ) Seal-cured 3 d Seal-cured d Seal-cured d Wall higher d Seal-cured Seal-cured 7 d Seal-cured d Wall lower d Seal-cured 1 15 Core velocity (m/s) Dynamic modulus of elas city (kn/mm 2 ) Core strength (N/mm 2 ) Andesite 3 d Limestone 3 d 7 d 7 d d d d d d d Figure 8. Coefficient Dynamic and static moduli of elasticity Andesite(Seal-cured) Limestone(Seal-cured) Andesite(Structure) Limestone(Structure) Andesite(Slab) Limestone(Slab) Andesite(Wall higher) Limestone(Wall higher) Andesite(Wall lower) Limestone(Wall lower) 1 1. Proposed method velocity (m/s).9 Figure 7. Ultrasonic velocity and compressive strength of concrete in structure (obtained using the usual and proposed methods) density and ultrasonic velocity of the core specimens used for the calculation were the same as the values used in the preceding section. E c 2:4 2 3 F c ¼ 6 k 33:5 q 2 ð3þ c where F c is the compressive strength [N/mm 2 ] of concrete and k is the coefficient [-] determined by the material type. Figure 9 shows the value of k determined through back-calculation from Equations (2) and (3). The values in the figure are categorized according to the type of coarse aggregate for the seal-cured specimens and the concrete in structure, and the trend in the average value of k is depicted for each aggregate type using broken lines and solid lines. A look at the values for each location on the specimens reveals that the values of k for the higher parts of the wall specimen are smaller than the values for the lower parts of that specimen. This suggests that the static modulus of elasticity Age (d) Figure 9. Coefficient calculated back from Equations (2) and (3) for the higher parts is small despite the compressive strength, indicating that there is some difference in the relation between the compressive strength and static modulus of elasticity for each location as a result of the effect of bleeding and the like. However, the value of k at each location is generally within the range of.5 of the average value, and it is thought that a certain degree of the strength estimation accuracy can be secured even if different values of k are not set for each location. Accordingly, the average values of k for the seal-cured specimens and for all locations of the concrete in structure are considered below. In both the seal-cured specimens and the concrete in structure, k is low for an age of 3 days. This is consistent with the knowledge 11 that, in specimens with a small age, the static modulus of elasticity tends to be low despite the compressive strength. The value of k increased from an age of 7 days and peaked at an age of days, and declined thereafter, but the change after an age of days was comparatively small. In Jpn Archit Rev January 218 vol. 1 no. 1 93

8 WATANABE et al. addition, the value of k was smaller in the case of the concrete in structure than in the case of the seal-cured specimens. No clear conclusion was made as to what caused this, but it suggests that the concrete in structure had a lower static modulus of elasticity despite the compressive strength, and it is thought that the differences, which were described in Section 3.3, in the relation between the ultrasonic velocity and compressive strength between the seal-cured specimens and the concrete in structure, occurred. Accordingly, it is thought that the value of k in the strength estimation equation should be set based on the core specimen data rather than the specimens fabricated using the same materials as the concrete in structure. 3.6 Precision of strength estimation In the proposed strength estimation method, some specimens are fabricated with using the same materials as the concrete in the target structure, or some core specimens are sampled from the concrete in the target structure; a value of k is calculated back with Equations (2) and (3) based on the compressive strength, ultrasonic velocity, density, and other values of the specimens above. Then the compressive strength is estimated based on the ultrasonic velocity measurement. In this experiment, k can be set based on the data for the seal-cured specimens or the core specimens. In the following discussion, the compressive strength of Estimated core strength (N/mm 2 ) Estimated core strength (N/mm 2 ) Figure 1. specimen 1% estimated with coefficient for seal-cured specimen 15% estimated with coefficient for structure Measured core strength (N/mm 2 ) 35% 15% Measured and estimated compressive strength of core wileyonlinelibrary.com/journal/jar3 the core specimens was estimated with Equations (2) and (3), using the average value of k for an age of days (as there is a little change thereafter) in the case of the seal-cured specimens and using the average value of k for the same age as the target of estimation in the case of the concrete in structure. Figure 1 shows the comparison between the measured and estimated values for the compressive strength of the core specimens. The top graph shows the results of a consideration of the estimates derived using the average values of k (1.5 when andesite crushed stone was used and 1.24 when limestone crushed stone was used) at an age of days for the seal-cured specimens in Figure 9. The bottom graph shows the results of a consideration of the estimates derived using the average values of k at various ages for the concrete in structure in Figure 9. The estimate of the compressive strength when k was set based on the data for the seal-cured specimens was in the range of approximately 35% to +1% as compared to the actual measurements and tended to be lower than the measurements overall. As noted in the preceding section, this is because the value of k is higher for the seal-cured specimens than for the concrete in structure. However, the estimate of the compressive strength when k was set based on the data for the concrete in structure was in the range of approximately 15% as compared to the actual measurements. The error of 15% in the compressive strength estimates corresponds to the error of approximately.5 in the coefficient. Accordingly, to secure the accuracy of the strength estimates, it is essential to consider thoroughly the specimen sampling method and the quantity of samples, etc., in the preliminary survey and to set a possible appropriate k value. 4. Conclusion A study was conducted regarding the applicability of the methods for measuring the velocity of propagation of ultrasonic longitudinal waves ( ultrasonic velocity ) inside the concrete in structure ( proposed method ) and for estimating the compressive strength as proposed in the previous study, 5 using actual-size specimens that simulate the wall and slab members in a reinforced concrete structure. The findings of the study were as follows. 1. The larger the density reduction rate of the core specimens (the degree of drying of the specimens), the larger the ratio of the ultrasonic velocity measured using the proposed method as compared to the value determined using the usual transmission method tended to be. This is thought to be because the proposed method tends to not be affected by dryness in contrast to the usual method, which is affected by the dryness of the surface sections. Accordingly, the proposed method is thought to be appropriate for measuring the ultrasonic velocity inside the concrete. 2. In the core specimens that were greatly affected by drying, the ultrasonic velocity determined using the usual method tended to be lower than that of the seal-cured specimens when the compressive strength was the same. The effect of drying on the ultrasonic velocity determined using the proposed method is small, but even in this case, there was some deviation in the relation between the ultrasonic velocity and the compressive strength from that of the seal-cured specimens. 3. The relation between the estimates of the dynamic modulus of elasticity and the measurements of the static modulus of elasticity in the seal-cured specimens and the Jpn Archit Rev January 218 vol. 1 no. 1 94

9 wileyonlinelibrary.com/journal/jar3 WATANABE et al. individual locations on the wall and slab specimens was generally the same as in the previous study The coefficient k determined through back-calculation with the proposed strength estimation equation was smaller for the top of the wall specimen than the bottom of that specimen. The value of k at each location was generally within the range of.5 with respect to the average value. 5. As the value of k was smaller in the case of the seal-cured specimens than that of the concrete in structure, it is thought that the value of k in the strength estimation equation should be set based on the core specimen data rather than the specimens fabricated using the same materials as the concrete in structure. 6. In the proposed strength estimation equation, the compressive strength estimates when k is set based on the data for the seal-cured specimens tended to be lower overall than the measurements. However, the compressive strength estimates when k is set based on the data for the concrete in structure was generally within the range of 15% with respect to the measurements. In order to ensure the measurement accuracy when the proposed method is used in actual examinations, as noted in the previous study, 5 a hole drilling method, etc. that improves the precision of the hole surface is needed. In the case of existing structures, it was proposed that a strength estimation equation by means of sampling core specimens should be established, but a study is also needed for methods that can determine strength estimation equations for new structures during construction without sampling core specimens. Moreover, in terms of strength estimation methods as well, Equation (2) is an empirical equation derived under limited conditions, and Equation (3) is an equation that was developed by referring to the New RC equation for different purposes from the original. Accordingly, the authors intend to continue to collect data in order to confirm the applicable range and validity of the strength estimation equation. Disclosure The authors have no conflict of interest to declare. References 1 Architectural Institute of Japan. Test Methods for Quality Control and Maintenance of Reinforced Concrete Buildings. Tokyo: Architectural Institute of Japan; 27: (In Japanese). 2 Nozaki Y. Effects of internal structure of concrete upon propagation of ultrasonics. J Struct Constr Eng. 19;425:9-17 (In Japanese). 3 Miura T. Neville s Concrete Bible. Tokyo: Gihodoshuppan; 24: (In Japanese). 4 Bartlett FM, MacGregor JG. Effect of moisture condition on concrete core strengths. ACI Mater J. 1994;: Watanabe S, Hishikawa K, Sonoda S, Namiki S. Study on estimation for compressive strength of concrete by ultrasonic method. J Struct Constr Eng. 215;715: (In Japanese). 6 Architectural Institute of Japan. Manual of Nondestructive Testing Method for Estimating Concrete Strength. Tokyo: Architectural Institute of Japan; 1983:-41 (In Japanese) 7 Phoon KK, Wee TH, Loi CS. Development of statistical quality assurance criterion for concrete using ultrasonic pulse velocity method. ACI Mater J. 1999;96: Architectural Institute of Japan. Japanese Architectural Standard Specification JASS5 Reinforced Concrete Work. Tokyo: Architectural Institute of Japan; 215: (In Japanese). 9 Miura T. Neville s Concrete Bible. Tokyo: Gihodoshuppan; 24: (In Japanese). 1 Petersons N. Should standard cube test specimens be replaced by test specimens taken from structures? Mater Struct. 1968;1: Nakata Y, Sawamoto T, Otsuka S, Haruyama N. Effect of strength development of high-strength concrete using different sorts of cements on elastic modulus of elasticity. J Struct Constr Eng. 27;622:17-23 (In Japanese). How to cite this article: Watanabe S, Hishikawa K, Kamae K, Namiki S. Study on estimation of compressive strength of concrete in structure using ultrasonic method. Jpn Archit Rev. 218;1: Jpn Archit Rev January 218 vol. 1 no. 1 95