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The Investigation into the Effect of Hydrostatic Pressure on the Engineering Properties of Hardened Concrete

The Investigation into the Effect of Hydrostatic Pressure on the Engineering Properties of... The aim of this study is to investigate the effect of hydrostatic pressure on the engineering properties of hardened concrete. To this end, a concrete column with dimensions of 100 cm width, 25 cm depth and 250 cm height was produced using C20 class concrete. While pouring the concrete, 15 cm reference cube samples were taken from the fresh concrete. After 28 days, 8 from the cube samples and 128 from different hydrostatic heights, in total 136 pieces of core samples with Ø100 mm diameter were taken and their compressive strength was determined. The average compressive strength of the reference core samples was 2 2 2 36.95 N/mm and the compressive strength of other samples changed between 37.3 N/mm and 43.0 N/mm according to the hydrostatic pressure. It was concluded that compressive strength changed between 0.95% and 16.37% according to the reference sample. Statistical analysis was conducted based on the experimental results. The compressive strength of the core samples related to its hydrostatic height and physical properties were predicted with a high reliability. A model equation was formed to convert the compressive strength of the core samples into reference compressive strength according to hydrostatic height and the convertibility coefficients were ascertained. Keywords: concrete; hydrostatic pressure; core sample; compressive strength 1. Introduction to withstand the load from the concrete cast above Due to the plastic consistency and viscosity of it without increasing the lateral stress against the concrete, hydrostatic pressure in structural elements formwork. This conclusion was drawn by observing such as columns and reinforced walls emerges. Over the decrease in pressure in the first few hours after the years, various factors, which affect the lateral casting even though the hydration process had not yet 3) and 4) pressure of fresh concrete on vertical forms, have been started , the lateral stress on the formwork steadily investigated. These factors include rate of placing decreased. The only phenomenon that can occur after the concrete, temperature of the concrete, proportion casting at this time in fresh concrete is the build-up of 5) and 6) of the concrete mix, consistency of the concrete, the internal structure at rest . As concrete hardens, consolidation method of the concrete, impact during the hydrostatic pressure decreases. Thus, the formwork placing, size and shape of the formwork, amount and can be removed after the concrete hardens. However, distribution of the reinforcing steel, unit weight of the it must be taken into consideration that in hardened concrete, hydrostatic height of the concrete, ambient concrete, hydrostatic pressure affects the compressive temperature, smoothness and permeability of the strength of the concrete. 1) formwork, pore water pressure and type of cement This situation is especially significant in preparing and 2) . During placing, the material indeed behaves as the reinforcement projects for buildings that are a fluid but, if it is cast slowly enough or if it is at rest, da m a ge d by e a rt hqua ke s. On t he ot he r ha nd, i n it builds up an internal structure and has the ability situations where fresh samples of concrete are not taken, fresh concrete pressure test results are not compatible with the standards, or compressive strength is doubtful, it is crucial to determine the compressive *Contact Author: Ercan Özgan, Chair, Professor, Dr., 7) strength of the concrete . A common way of Faculty of Art Design Architecture, Department of Architecture, determining the in-situ strength of concrete is to drill University of Düzce, Konuralp, Düzce Turkey 8), 9), 10), 11), 12), 13), 14), 15), 16), and 17) and test cores . Although Tel: +90-380-5421264 Fax: +90-380-5421297 the method consists of expensive and time-consuming E-mail: ercanozgan@gmail.com operations, cores give reliable and useful results since ( Received March 20, 2017 ; accepted July 23, 2018 ) 9) they are mechanically tested on destruction . However, DOI http://doi.org/10.3130/jaabe.17.565 Journal of Asian Architecture and Building Engineering/September 2018/571 565 the test results should be carefully interpreted because concrete was poured into the formwork to prevent core strengths are affected by a number of factors such segregation. as diameter, l/d ratio, and moisture conditions of the 2.1.3 Casting, Curing, and Instrumentation of the core specimen, the direction of drilling, presence of Columns reinforcement steel bars in the specimen, and even the The specimens were cast vertically in a specially 18), 19), 20), 21), 22) and 23) strength level of the concrete . In core fabricated stand and concrete was compacted in layers. samples, some mechanical and physical properties such During the casting of columns, concrete was poured as the compressive strength of the concrete, density, vertically—similar to the direction of loading. The water absorption capacity, split t ensile strength, concrete was vibrated using a portable poker vibrator expansion due to alkali-silica reaction, the space ratio, with a diameter of 3 cm and was manually poured. and saturation degree can be determined as well. The Control specimens in the form of cubes were also studies including the future standards related to this cast for concrete. The samples and control specimens subject were published in several countries such as were remolded after 24 hours and cured in a curing 24) 25) 7) England , America and Turkey . tank for 28 days. Then, the core samples were cured at In t he pr e se nt st udy, t he e ffe c t of hydro st a t i c room temperature until testing. The details of concrete pressure on the compressive strength of concrete was mixtures including water to cement ratio (W/C) are investigated. With this aim in mind, using C20 class presented in Table 1. The concrete properties of the concrete, a concrete column was produced. By taking columns are also presented in Table 1. samples from different hydrostatic heights of the During this process, in order to use as a reference, column, the compressive strength was determined. In 8 cube samples measuring 15 cm in dimension were order to predict the compressive strength according taken according to Turkish Standards TS EN 12350-1 to hydrostatic height and the physical properties of and were kept in a curing tank for 28 days. the samples, statistical analyses were conducted and 2.2 Method prediction models were established. 2.2.1 Taking Core Samples Three days after pouring the concrete, the formwork 2. Material and Method was removed. A wet cure was applied for 28 days 2.1 Material in the morning, in the afternoon, and in the evening 2.1.1 Formworks to the construction site. After 28 days of pouring In this study, 100 x 25 cm cross-sections and 250 the concrete, in order to take the core sample, the cm high column formwork was produced. While position of the column was changed from vertical to producing the block, a 3 cm wide metal framed horizontal. 16 in accordance with the column height wooden panel with 50 x 100 x 2.5 cm dimensions was and 8 by the width; a total of 128 core samples were used on the front and back surfaces. 25 x 250 x 2.5 cm taken. The internal diameter of the core samples was pine timber was used on the sides after this process chosen as 100 mm and since the length of the samples and sika concentrated block liquid was applied on the was 25 cm, h/d=1.5 was chosen and a total of 128 interior surfaces of the block so the formwork would core samples were taken. The core samples were taken be prepared for pouring the concrete. similarly with the previous 8 pieces of the 15 cm cube 2.1.2 Concrete reference ones (Fig.1.). In this study, a column block in a cross-section with 2.2.2 Determination of the Physical Properties of 100 x 25 cm and 250 cm height was produced. While the Samples producing the block, a metal-framed wooden panel and All the samples taken were kept in the laboratory for pine timber were used on the sides. C20 ready mixed 48 hours and their air-dry weights were measured with Table 1. Concrete Mix Design Class of the concrete C 20/25 Dmax: 22 Consistency: 8.3 Water/Cement: 0,63 The goal of the Moisture The goal of the Weighted Aggregates Amount Total prescription content % quantity system 0-5K 546 kg 2.730 kg 5,6 2.880 kg 2.861 kg 2.861 kg 15-25 470 kg 2.350 kg 1,0 2.372 kg 2.251 kg 2.251 kg 5-15 486 kg 2.430 kg 1,0 2.452 kg 2.417 kg 2.417 kg Stone dust 347 kg 1.735 kg 5,5 1.851 kg 1.851 kg 1.828 kg 9.245 kg 288 kg 9.380 kg 9.532 kg CEM2 42.5R 290 kg 1.450 kg 1.448 kg 1.448 kg 1.450 kg Super plasticizers 2,32 kg 11,6 kg 11,6 kg 11,497 kg 11,497 kg Water 183 kg 915 kg 626 kg 623 kg 623 kg 2.377 kg 2.088 kg 2.082 kg 2.324 kg Unit Weight (for each m ) 11.462 kg 2.293 kg 566 JAABE vol.17 no.3 September 2018 Metin M. Uzunoğlu 0.1 gr sensitivity. Afterward, these samples were kept in 20 ±2 C water until their weight became constant and saturated with water. Water saturated weights and their weights in water were measured. Samples were kept in a drying oven at 105 ±2 C until their weights were constant. 2.2.3 Determination of the Compressive Strength of the Samples In order to conduct the compressive strength test, headings with 70% sulphur and 30% graphite were produced. Samples were kept in the laboratory setting for 24 hours and a compressive strength test was conducted according to TS EN 12390-4. As a result of the tests, samples were broken in accordance with TS EN 12390-3. The physical properties and compressive strength values of reference samples and others are provided in Table 2. 3. Analysis of the Findings The descriptive statistics of the experiment results in a total of 128 core samples were conducted. The correlation coefficient between the fundamental physical properties, the height of hydrostatic pressure, and the compressive strength of core samples was determined. Multi-Linear Regression analysis (MLR) was carried out to predict the compressive strength of core samples. Variance analysis was conducted in order to test the significance of the model. Fig.1. A Schematic Showing of the Core Samples on the Column Table 2. The Compressive Strength, Physical Properties, and Hydrostatic Height of the Concrete Core Samples Natural unit Saturated Core row numbers Unit volume Compressive volume unit volume Amount of water Volume of Hydrostatic for core samples on weight for strength weight weight voids cm height cm 3 absorption % 2 the column dry air gr/cm N/mm 3 3 gr/cm gr/cm 1 2,552 2,486 2,574 3,514 8,030 235 37,30 2 2,596 2,512 2,618 4,151 9,365 224 38,40 3 2,550 2,489 2,570 3,245 7,437 213 37,70 4 2,899 2,541 2,655 4,491 10,213 191 38,60 5 2,808 2,513 2,608 3,760 8,595 180 39,80 6 2,590 2,517 2,614 3,830 8,744 147 40,20 7 2,604 2,524 2,625 3,979 9,092 136 39,60 8 2,606 2,528 2,627 3,890 8,912 114 41,00 9 2,635 2,545 2,658 4,409 10,000 103 40,80 10 2,585 2,516 2,606 3,582 8,229 92 39,90 11 2,573 2,505 2,594 3,546 8,151 70 40,30 12 2,587 2,516 2,609 3,662 8,386 59 41,00 13 2,538 2,491 2,557 2,648 6,177 48 42,00 14 2,573 2,512 2,592 3,179 7,379 35 41,90 15 2,554 2,500 2,573 2,892 6,717 23 42,50 16 2,547 2,493 2,566 2,893 6,713 11 43,00 Reference 2,322 2,343 2,368 1,074 2,518 - 36,95 JAABE vol.17 no.3 September 2018 Metin M. Uzunoğlu 567 3.1 Descriptive Statistics 3.3 Multiple Linear Regression Analysis The physical properties and descriptive statistics In order to predict the compressive strength of values of core samples for compressive strength are samples according to the physical properties and the shown in Table 3. hydrostatic height, multiple linear regression analysis 3.2 Correlation Analysis was carried out on all data and also only according to In order to determine the level of significance in the the hydrostatic height (Table 5.). relationship between fundamental physical properties, hydrostatic height, and compressive strength of core samples, correlation analysis was carried out (Table 4.). Table 3. Descriptive Statistics Properties investigated N Range Min. Max. Mean Std. Error Std. Deviation Variance Compressive strength 16 5.70 37.30 43.00 40.235 .393 1.620 2.625 Natural unit volume weight 16 .36 2.54 2.90 2.609 .023 .097 .009 Dry unit volume weight 16 .06 2.49 2.54 2.510 .004 .018 .000 Saturated unit volume weight 16 .10 2.56 2.66 2.601 .007 .030 .001 Water absorption percentage 16 1.84 2.65 4.49 3.592 .126 .519 .270 Volume of voids 16 4.03 6.18 10.21 8.229 .274 1.129 1.274 Hydrostatic height 16 224.0 15.0 239.0 127.47 18.186 74.983 5623.0 Table 4. Correlation Analyses Compressive Natural unit Dry unit volu- Saturated unit Water absorpti- Volume of Hydrostatic Correlation coefficients strength volume weight me weight volume weight on percentage voids height Compressive strength 1.00 -.273 .001 -.219 -.504 -.491 .916 Natural unit volume weight -.273 1.00 .605 .638 .621 .628 -.327 Dry unit volume weight .001 .605 1.00 .966 .813 .825 .020 Saturated unit volume weight -.219 .638 .966 1.00 .935 .942 -.211 Water absorption percentage -.504 .621 .813 .935 1.00 1.000 -.508 Volume of voids -.491 .628 .825 .942 1.000 1.00 -.492 Hydrostatic height .916 -.327 .020 -.211 -.508 -.492 1.00 Table 5. Model Summary R Adjusted R Change Statistics Std. Error of the Estimate Square Square R Square Change F Change df1 df2 Sig. FChange .932 .869 .809 .707 .869 14.597 5 11 .000 a. Predictors: (Constant), Hydrostatic height, Unit volume weight for dry air, Natural unit volume weight, Amount of water absorption, Volume of voids. Table 6. ANOVA Resolution Sum of Squares df Mean Square F Sig. Regression 36.498 5 7.300 14.597 .000 Residual 5.501 11 .500 Total 41.999 16 a. Predictors: (Constant), Natural unit volume weight (x ), Unit volume weight for dry air (x ), Amount of water absorption (x ), 1 2 3 Volume of void (x ), Hydrostatic height (x ). 4 5 b. Dependent Variable: Compressive strength (y). Table 7. Coefficients Resolution Standardized 95% Confidence Unstandardized Coefficients Coefficients Interval for B Model Sig. B Std. Error Beta Lower Bound Upper Bound (Constant) -53.940 93.040 - .574 -258.719 150.839 Natural unit volume weight (x1) 1.764 2.754 .105 .535 -4.299 7.827 Unit volume weight for dry air (x2) 39.078 40.641 .430 .357 -50.373 128.529 Amount of the water 24.817 20.108 7.963 .243 -19.440 69.074 absorption (x3) The volume of voids (x4) -12.146 9.428 -8.463 .224 -32.896 8.604 Hydrostatic height (x5) .018 .006 .829 .013 .005 .031 a. Dependent variable: Compressive strength (y). 568 JAABE vol.17 no.3 September 2018 Metin M. Uzunoğlu b ANOVA 3.3.1 Physical Properties and Regression Analysis Sum of Mean According to the Hydrostatic Height Squares df Square F Sig. In order to predict the compressive strength of Regression 37.261 1 37.261 111.497 .000 samples according to the physical properties and the Residual 4.679 14 .334 hydrostatic height, multiple regression analysis was Total 41.940 15 carried out. The results of the analysis can be seen below (Table 6. and Table 7.). Coefficients The model equation is shown below. According to this: y  53.94  1.764x  39.078x  24.817x  12.146x  0.018x 1 2 3 4 5 Sig. In the equation; Std. Lower Upper y: Compressive strength (N/mm ) B Beta Error Bound Bound x : Natural unit volume weight, (Constant) 42.737 .276 .000 42.144 43.330 x : Dry unit volume weight, Hydrostatic -.021 .002 -.943 .000 -.025 -.017 x : Percentage of water absorption 3 height x : Volume of voids x : Hydrostatic height 5 a. Dependent Variable: Compressive strength The relationship between the estimated and real 3.3.2 Regression Analysis According to Hydrostatic compres sive strength according to the physical Height properties [Natural unit volume weight (x ), Dry unit Regression analysis was conducted and a prediction volume weight (x ), Percentage of water absorption model was established to predict the compressive (x ), Space volume (x ), Hydrostatic height (x )] and 3 4 5 strength of core samples according to hydrostatic height. compressive strength of core samples with multiple The model obtained from the regression analysis, linear regression can be seen in Fig.2. which aims to predict the compressive strength related to hydrostatic height specifically, can be seen in Fig.3. B y u si n g t h i s m o d e l , d i ff e r e n t c o r e c o m p r e ssi v e strength values taken from different hydrostatic heights can be converted into core values taken from reference R = 0,8 samples using the coefficients in Table 8. Com pres s ive s trenght 37 38 39 40 41 42 43 44 Experimental result (N/mm ) Fig.2. The Relationship between Estimated Compressive Strength and Experimental Compressive Strength Descriptive Statistics Mean Std. Deviation N Compressive strength 40.2500 1.67212 16 Hydrostatic height 117.56 74.49829 16 Fig.3. The Estimated Compressive Strength Value According to the Hydrostatic Height Model Summary Std. Error Change Statistics R Adjusted R of the R Square F Sig. F Square R Square Estimate Change Change df1 df2 Change .943 .888 .880 .578 .888 111.497 1 14 .000 a. Predictors: (Constant), hydrostatic height JAABE vol.17 no.3 September 2018 Metin M. Uzunoğlu 569 P red ic te d v a lu es (N /m m ) Unstandardi- zed Coefficients Standardized Coefficients 95% Confi - dence Interval for B Table 8. The Coefficients of Converting the Compressive • Core samples are generally taken from the Strength of the Core Samples According to Hydrostatic Height buildings that would be reinforced to determine the Upper border Coefficient for converting to compressive strength of the concrete. However, in distance (cm) reference values this study, it was seen that the compressive strength 0 1.00 (reference value) of the core samples changed between 0.95% and 15 0,98 26 0,97 16.37%, according to hydrostatic height. For this 37 0,97 reason, the compressive strength of the core samples 50 0,96 can be used if they are converted into reference 59 0,95 compressive strength according to the suggested 70 0,95 100 0,93 coefficient below (Table 5.). In order to convert the 103 0,93 compressive strength of the concrete into a reference 114 0,93 value, the hydrostatic height must be measured while 136 0,92 taking the core samples. 147 0,91 • Besides, for different hydrostatic height values not 150 0,91 158 0,91 included in the table, the model equation obtained 180 0,90 from regression analysis can be used to predict the 191 0,89 compressive strength of the core samples related to 200 0,89 hydrostatic height. 202 0,89 • The model equation obtained through multiple 215 0,88 227 0,87 linear regression analysis can be used to predict 235 0,87 the compressive strength related to the hydrostatic 239 0,87 height and physical properties of the core samples (R =0.932). 4. Results and Discussion In the case of using a different concrete class, In this study, the effect of hydrostatic height in mixture design, and different types and amounts of hardened concrete on the compressive strength was plasticizers, investigating the effects of hydrostatic investigated experimentally and statistically. The height in structural elements such as columns and results are indicated below. reinforced walls on the compressive strength of • The average compressive strength of the reference hardened concrete can be useful. samples was 36.95 N/mm and the compressive strength of other samples changed between 37.3 N/mm References and 43.0 N/mm . 1) Gardner, N.J., (1980) Pressure of concrete on formwork, ACI • The compressive strength of the concrete increased Mater J 77 (4), pp.279-286. when the hydrostatic height increased as well. This 2) Rodin, S., (1952) Pressure of concrete on formwork, Proc Inst increase ratio according to the reference sample was Civil Eng I (4), pp.709-746. 3) Andriamanantsilavo N.R. and Amziane S., (2004) Maturation of 0.95% for 0.15 m, 4.22% for 0.5 m, 7.09% for 1.0 m, fresh cement pastes within 1- to 10-m-large formworks, Cem. 9.95% for 2 m, 12.82% for 2.35 m, and 16.37% for Concr. Res. 34 (11), pp.2141-2152. 2.39 m. 4) Assaad J. and Khayat K.H., (2004) Variations of lateral and pore • It was determined that the natural unit volume water pressure of self-consolidating concrete at early age, ACI weight of core samples changed between 2.54 gr/ Mater. J. 101 (4), pp.310-317. 3 3 5) Assaad J., Khayat K.H. and Mesbah H., (2003) Assessment of cm and 2.90 gr/cm , dry volume unit weight between 3 3 thixotropy of flowable and self-consolidating concrete, ACI Mater. 2.49 gr/cm and 2.54 gr/cm , water saturated unit 3 3 J. 100 (2), pp.99-107. volume weight between 2.56 gr/cm and 2.66 gr/cm , 6) Khayat K.H., Assad, J. Mesbah H. and Lessard M., (2005) Effect absorption amount between 2.65 and 4.49, and volume of section width and casting rate on variations of formwork 3 3 of voids between 6.18 cm and 10.21 cm . pressure of self-consolidating concrete, RILEM Mat. Struct. 38, pp.73-78. • The coefficient between the compressive strength 7) Ts En 12504-1 (Nisan 2002) Karot Numuneler - Karot Alma, and saturated unit volume weight of the core sample Muayene ve Basinç Dayaniminin Tayini, Türk Standartlar was -0.273 and measured at 0.01 for dry unit Enstitüsü, Ankara, Türkiye. volume weight, 0.219 for saturated unit volume 8) Troxell G.E., Davis H.E. and Kelly, J.W. (1968) Composition and weight, -0.504 for water absorption amount, -0.491 properties of concrete, McGraw-Hill Book Company, New York. 9) Neville A.M., (1995) Properties of concrete, Addison-Wesley, UK. for the volume of voids, and 0.916 for hydrostatic 10) Mindess S. and Young J.F., (1981) Concrete, Prentice-Hall, Inc., height. According to this finding, the relationship New Jersey. between hydrostatic height and compressive strength 11) Arioglu E. and Arioglu N., (1998) Testing of concrete core is quite strong, on the other hand, the relationship samples and evaluations, Evrim Publisher, Istanbul [in Turkish]. between compressive strength, water absorption 12) Erdoğan T.Y., (2003) Concrete, METU Publisher, Ankara [in Turkish]. amount, and volume of voids is nearly the same or at 13) Sullivan P.J.E., (1991) Testing and evaluation of concrete strength mid-level but negative. The relationship between dry in structures, ACI Mater J 88 (5) pp.530-535. unit volume weight and the compressive strength of the concrete is not significant. 570 JAABE vol.17 no.3 September 2018 Metin M. Uzunoğlu 14) Haque M.N. and Al-Khaiat H., (1997) Carbonation of concrete structures in hot dry coastal regions, Cement Concr Compos 19 pp.123-129. 15) Miao B., Aitcin P.C., W.D. (1993) Mitchell, Cook and D., Influence of concrete strength on in situ properties of large columns, ACI Mater J 90 (3) pp.214-219. 16) Price W.F. and Hynes J.P., (1996) In-situ strength testing of high strength concrete, Mag Concr Res 48 (176) pp.189-197. 17) Khayat K.H., Manai K. and Trudel A., (1997) In-situ mechanical properties of wall elements cast using self-consolidating concrete, ACI Mater J 94 (6), pp.491-500. View Record in Scopus | Cited By in Scopus. 18) Bungey J.H., (1979) Determining concrete strength by using small diameter cores, Mag Concr Res 31 (107) pp.91-98. 19) Bartlett F.M. and MacGregor J.G., (1994) Effect of core diameter on concrete core strengths, ACI Mater J 91 (5) pp.460-470. 20) Bartlett F.M. and MacGregor J.G., (1994) Effect of core length- to-diameter ratio on concrete core strengths, ACI Mater J 91 (4) pp.339-348. 21) Bartlett F.M. and MacGregor J.G., (1993) Effect of moisture condition on concrete core strengths, ACI Mater J 91 (3) pp.227- 22) Bartlett, F.M., (1997) Precision of in-place concrete strengths predicted using core strength correction factors obtained by weighed regression analysis, Struct Saf 19 (4) pp.397-410. 23) Bartlett F.M. and MacGregor J.G., (1993) Effect of moisture condition on concrete core strengths, ACI Mater J 91 (3) pp.227- 24) BS 1881: Part 122 (July 2011) Testing concrete. Method for determination of water absorption, British Standarts. 25) ASTM C42 (1.10.2016) Standard Test Method for Obtaining and Testing Drilled Cores and Sawed Beams of Concrete, American Society for Testing and Materials. JAABE vol.17 no.3 September 2018 Metin M. Uzunoğlu 571 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Journal of Asian Architecture and Building Engineering Taylor & Francis

The Investigation into the Effect of Hydrostatic Pressure on the Engineering Properties of Hardened Concrete

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Abstract

The aim of this study is to investigate the effect of hydrostatic pressure on the engineering properties of hardened concrete. To this end, a concrete column with dimensions of 100 cm width, 25 cm depth and 250 cm height was produced using C20 class concrete. While pouring the concrete, 15 cm reference cube samples were taken from the fresh concrete. After 28 days, 8 from the cube samples and 128 from different hydrostatic heights, in total 136 pieces of core samples with Ø100 mm diameter were taken and their compressive strength was determined. The average compressive strength of the reference core samples was 2 2 2 36.95 N/mm and the compressive strength of other samples changed between 37.3 N/mm and 43.0 N/mm according to the hydrostatic pressure. It was concluded that compressive strength changed between 0.95% and 16.37% according to the reference sample. Statistical analysis was conducted based on the experimental results. The compressive strength of the core samples related to its hydrostatic height and physical properties were predicted with a high reliability. A model equation was formed to convert the compressive strength of the core samples into reference compressive strength according to hydrostatic height and the convertibility coefficients were ascertained. Keywords: concrete; hydrostatic pressure; core sample; compressive strength 1. Introduction to withstand the load from the concrete cast above Due to the plastic consistency and viscosity of it without increasing the lateral stress against the concrete, hydrostatic pressure in structural elements formwork. This conclusion was drawn by observing such as columns and reinforced walls emerges. Over the decrease in pressure in the first few hours after the years, various factors, which affect the lateral casting even though the hydration process had not yet 3) and 4) pressure of fresh concrete on vertical forms, have been started , the lateral stress on the formwork steadily investigated. These factors include rate of placing decreased. The only phenomenon that can occur after the concrete, temperature of the concrete, proportion casting at this time in fresh concrete is the build-up of 5) and 6) of the concrete mix, consistency of the concrete, the internal structure at rest . As concrete hardens, consolidation method of the concrete, impact during the hydrostatic pressure decreases. Thus, the formwork placing, size and shape of the formwork, amount and can be removed after the concrete hardens. However, distribution of the reinforcing steel, unit weight of the it must be taken into consideration that in hardened concrete, hydrostatic height of the concrete, ambient concrete, hydrostatic pressure affects the compressive temperature, smoothness and permeability of the strength of the concrete. 1) formwork, pore water pressure and type of cement This situation is especially significant in preparing and 2) . During placing, the material indeed behaves as the reinforcement projects for buildings that are a fluid but, if it is cast slowly enough or if it is at rest, da m a ge d by e a rt hqua ke s. On t he ot he r ha nd, i n it builds up an internal structure and has the ability situations where fresh samples of concrete are not taken, fresh concrete pressure test results are not compatible with the standards, or compressive strength is doubtful, it is crucial to determine the compressive *Contact Author: Ercan Özgan, Chair, Professor, Dr., 7) strength of the concrete . A common way of Faculty of Art Design Architecture, Department of Architecture, determining the in-situ strength of concrete is to drill University of Düzce, Konuralp, Düzce Turkey 8), 9), 10), 11), 12), 13), 14), 15), 16), and 17) and test cores . Although Tel: +90-380-5421264 Fax: +90-380-5421297 the method consists of expensive and time-consuming E-mail: ercanozgan@gmail.com operations, cores give reliable and useful results since ( Received March 20, 2017 ; accepted July 23, 2018 ) 9) they are mechanically tested on destruction . However, DOI http://doi.org/10.3130/jaabe.17.565 Journal of Asian Architecture and Building Engineering/September 2018/571 565 the test results should be carefully interpreted because concrete was poured into the formwork to prevent core strengths are affected by a number of factors such segregation. as diameter, l/d ratio, and moisture conditions of the 2.1.3 Casting, Curing, and Instrumentation of the core specimen, the direction of drilling, presence of Columns reinforcement steel bars in the specimen, and even the The specimens were cast vertically in a specially 18), 19), 20), 21), 22) and 23) strength level of the concrete . In core fabricated stand and concrete was compacted in layers. samples, some mechanical and physical properties such During the casting of columns, concrete was poured as the compressive strength of the concrete, density, vertically—similar to the direction of loading. The water absorption capacity, split t ensile strength, concrete was vibrated using a portable poker vibrator expansion due to alkali-silica reaction, the space ratio, with a diameter of 3 cm and was manually poured. and saturation degree can be determined as well. The Control specimens in the form of cubes were also studies including the future standards related to this cast for concrete. The samples and control specimens subject were published in several countries such as were remolded after 24 hours and cured in a curing 24) 25) 7) England , America and Turkey . tank for 28 days. Then, the core samples were cured at In t he pr e se nt st udy, t he e ffe c t of hydro st a t i c room temperature until testing. The details of concrete pressure on the compressive strength of concrete was mixtures including water to cement ratio (W/C) are investigated. With this aim in mind, using C20 class presented in Table 1. The concrete properties of the concrete, a concrete column was produced. By taking columns are also presented in Table 1. samples from different hydrostatic heights of the During this process, in order to use as a reference, column, the compressive strength was determined. In 8 cube samples measuring 15 cm in dimension were order to predict the compressive strength according taken according to Turkish Standards TS EN 12350-1 to hydrostatic height and the physical properties of and were kept in a curing tank for 28 days. the samples, statistical analyses were conducted and 2.2 Method prediction models were established. 2.2.1 Taking Core Samples Three days after pouring the concrete, the formwork 2. Material and Method was removed. A wet cure was applied for 28 days 2.1 Material in the morning, in the afternoon, and in the evening 2.1.1 Formworks to the construction site. After 28 days of pouring In this study, 100 x 25 cm cross-sections and 250 the concrete, in order to take the core sample, the cm high column formwork was produced. While position of the column was changed from vertical to producing the block, a 3 cm wide metal framed horizontal. 16 in accordance with the column height wooden panel with 50 x 100 x 2.5 cm dimensions was and 8 by the width; a total of 128 core samples were used on the front and back surfaces. 25 x 250 x 2.5 cm taken. The internal diameter of the core samples was pine timber was used on the sides after this process chosen as 100 mm and since the length of the samples and sika concentrated block liquid was applied on the was 25 cm, h/d=1.5 was chosen and a total of 128 interior surfaces of the block so the formwork would core samples were taken. The core samples were taken be prepared for pouring the concrete. similarly with the previous 8 pieces of the 15 cm cube 2.1.2 Concrete reference ones (Fig.1.). In this study, a column block in a cross-section with 2.2.2 Determination of the Physical Properties of 100 x 25 cm and 250 cm height was produced. While the Samples producing the block, a metal-framed wooden panel and All the samples taken were kept in the laboratory for pine timber were used on the sides. C20 ready mixed 48 hours and their air-dry weights were measured with Table 1. Concrete Mix Design Class of the concrete C 20/25 Dmax: 22 Consistency: 8.3 Water/Cement: 0,63 The goal of the Moisture The goal of the Weighted Aggregates Amount Total prescription content % quantity system 0-5K 546 kg 2.730 kg 5,6 2.880 kg 2.861 kg 2.861 kg 15-25 470 kg 2.350 kg 1,0 2.372 kg 2.251 kg 2.251 kg 5-15 486 kg 2.430 kg 1,0 2.452 kg 2.417 kg 2.417 kg Stone dust 347 kg 1.735 kg 5,5 1.851 kg 1.851 kg 1.828 kg 9.245 kg 288 kg 9.380 kg 9.532 kg CEM2 42.5R 290 kg 1.450 kg 1.448 kg 1.448 kg 1.450 kg Super plasticizers 2,32 kg 11,6 kg 11,6 kg 11,497 kg 11,497 kg Water 183 kg 915 kg 626 kg 623 kg 623 kg 2.377 kg 2.088 kg 2.082 kg 2.324 kg Unit Weight (for each m ) 11.462 kg 2.293 kg 566 JAABE vol.17 no.3 September 2018 Metin M. Uzunoğlu 0.1 gr sensitivity. Afterward, these samples were kept in 20 ±2 C water until their weight became constant and saturated with water. Water saturated weights and their weights in water were measured. Samples were kept in a drying oven at 105 ±2 C until their weights were constant. 2.2.3 Determination of the Compressive Strength of the Samples In order to conduct the compressive strength test, headings with 70% sulphur and 30% graphite were produced. Samples were kept in the laboratory setting for 24 hours and a compressive strength test was conducted according to TS EN 12390-4. As a result of the tests, samples were broken in accordance with TS EN 12390-3. The physical properties and compressive strength values of reference samples and others are provided in Table 2. 3. Analysis of the Findings The descriptive statistics of the experiment results in a total of 128 core samples were conducted. The correlation coefficient between the fundamental physical properties, the height of hydrostatic pressure, and the compressive strength of core samples was determined. Multi-Linear Regression analysis (MLR) was carried out to predict the compressive strength of core samples. Variance analysis was conducted in order to test the significance of the model. Fig.1. A Schematic Showing of the Core Samples on the Column Table 2. The Compressive Strength, Physical Properties, and Hydrostatic Height of the Concrete Core Samples Natural unit Saturated Core row numbers Unit volume Compressive volume unit volume Amount of water Volume of Hydrostatic for core samples on weight for strength weight weight voids cm height cm 3 absorption % 2 the column dry air gr/cm N/mm 3 3 gr/cm gr/cm 1 2,552 2,486 2,574 3,514 8,030 235 37,30 2 2,596 2,512 2,618 4,151 9,365 224 38,40 3 2,550 2,489 2,570 3,245 7,437 213 37,70 4 2,899 2,541 2,655 4,491 10,213 191 38,60 5 2,808 2,513 2,608 3,760 8,595 180 39,80 6 2,590 2,517 2,614 3,830 8,744 147 40,20 7 2,604 2,524 2,625 3,979 9,092 136 39,60 8 2,606 2,528 2,627 3,890 8,912 114 41,00 9 2,635 2,545 2,658 4,409 10,000 103 40,80 10 2,585 2,516 2,606 3,582 8,229 92 39,90 11 2,573 2,505 2,594 3,546 8,151 70 40,30 12 2,587 2,516 2,609 3,662 8,386 59 41,00 13 2,538 2,491 2,557 2,648 6,177 48 42,00 14 2,573 2,512 2,592 3,179 7,379 35 41,90 15 2,554 2,500 2,573 2,892 6,717 23 42,50 16 2,547 2,493 2,566 2,893 6,713 11 43,00 Reference 2,322 2,343 2,368 1,074 2,518 - 36,95 JAABE vol.17 no.3 September 2018 Metin M. Uzunoğlu 567 3.1 Descriptive Statistics 3.3 Multiple Linear Regression Analysis The physical properties and descriptive statistics In order to predict the compressive strength of values of core samples for compressive strength are samples according to the physical properties and the shown in Table 3. hydrostatic height, multiple linear regression analysis 3.2 Correlation Analysis was carried out on all data and also only according to In order to determine the level of significance in the the hydrostatic height (Table 5.). relationship between fundamental physical properties, hydrostatic height, and compressive strength of core samples, correlation analysis was carried out (Table 4.). Table 3. Descriptive Statistics Properties investigated N Range Min. Max. Mean Std. Error Std. Deviation Variance Compressive strength 16 5.70 37.30 43.00 40.235 .393 1.620 2.625 Natural unit volume weight 16 .36 2.54 2.90 2.609 .023 .097 .009 Dry unit volume weight 16 .06 2.49 2.54 2.510 .004 .018 .000 Saturated unit volume weight 16 .10 2.56 2.66 2.601 .007 .030 .001 Water absorption percentage 16 1.84 2.65 4.49 3.592 .126 .519 .270 Volume of voids 16 4.03 6.18 10.21 8.229 .274 1.129 1.274 Hydrostatic height 16 224.0 15.0 239.0 127.47 18.186 74.983 5623.0 Table 4. Correlation Analyses Compressive Natural unit Dry unit volu- Saturated unit Water absorpti- Volume of Hydrostatic Correlation coefficients strength volume weight me weight volume weight on percentage voids height Compressive strength 1.00 -.273 .001 -.219 -.504 -.491 .916 Natural unit volume weight -.273 1.00 .605 .638 .621 .628 -.327 Dry unit volume weight .001 .605 1.00 .966 .813 .825 .020 Saturated unit volume weight -.219 .638 .966 1.00 .935 .942 -.211 Water absorption percentage -.504 .621 .813 .935 1.00 1.000 -.508 Volume of voids -.491 .628 .825 .942 1.000 1.00 -.492 Hydrostatic height .916 -.327 .020 -.211 -.508 -.492 1.00 Table 5. Model Summary R Adjusted R Change Statistics Std. Error of the Estimate Square Square R Square Change F Change df1 df2 Sig. FChange .932 .869 .809 .707 .869 14.597 5 11 .000 a. Predictors: (Constant), Hydrostatic height, Unit volume weight for dry air, Natural unit volume weight, Amount of water absorption, Volume of voids. Table 6. ANOVA Resolution Sum of Squares df Mean Square F Sig. Regression 36.498 5 7.300 14.597 .000 Residual 5.501 11 .500 Total 41.999 16 a. Predictors: (Constant), Natural unit volume weight (x ), Unit volume weight for dry air (x ), Amount of water absorption (x ), 1 2 3 Volume of void (x ), Hydrostatic height (x ). 4 5 b. Dependent Variable: Compressive strength (y). Table 7. Coefficients Resolution Standardized 95% Confidence Unstandardized Coefficients Coefficients Interval for B Model Sig. B Std. Error Beta Lower Bound Upper Bound (Constant) -53.940 93.040 - .574 -258.719 150.839 Natural unit volume weight (x1) 1.764 2.754 .105 .535 -4.299 7.827 Unit volume weight for dry air (x2) 39.078 40.641 .430 .357 -50.373 128.529 Amount of the water 24.817 20.108 7.963 .243 -19.440 69.074 absorption (x3) The volume of voids (x4) -12.146 9.428 -8.463 .224 -32.896 8.604 Hydrostatic height (x5) .018 .006 .829 .013 .005 .031 a. Dependent variable: Compressive strength (y). 568 JAABE vol.17 no.3 September 2018 Metin M. Uzunoğlu b ANOVA 3.3.1 Physical Properties and Regression Analysis Sum of Mean According to the Hydrostatic Height Squares df Square F Sig. In order to predict the compressive strength of Regression 37.261 1 37.261 111.497 .000 samples according to the physical properties and the Residual 4.679 14 .334 hydrostatic height, multiple regression analysis was Total 41.940 15 carried out. The results of the analysis can be seen below (Table 6. and Table 7.). Coefficients The model equation is shown below. According to this: y  53.94  1.764x  39.078x  24.817x  12.146x  0.018x 1 2 3 4 5 Sig. In the equation; Std. Lower Upper y: Compressive strength (N/mm ) B Beta Error Bound Bound x : Natural unit volume weight, (Constant) 42.737 .276 .000 42.144 43.330 x : Dry unit volume weight, Hydrostatic -.021 .002 -.943 .000 -.025 -.017 x : Percentage of water absorption 3 height x : Volume of voids x : Hydrostatic height 5 a. Dependent Variable: Compressive strength The relationship between the estimated and real 3.3.2 Regression Analysis According to Hydrostatic compres sive strength according to the physical Height properties [Natural unit volume weight (x ), Dry unit Regression analysis was conducted and a prediction volume weight (x ), Percentage of water absorption model was established to predict the compressive (x ), Space volume (x ), Hydrostatic height (x )] and 3 4 5 strength of core samples according to hydrostatic height. compressive strength of core samples with multiple The model obtained from the regression analysis, linear regression can be seen in Fig.2. which aims to predict the compressive strength related to hydrostatic height specifically, can be seen in Fig.3. B y u si n g t h i s m o d e l , d i ff e r e n t c o r e c o m p r e ssi v e strength values taken from different hydrostatic heights can be converted into core values taken from reference R = 0,8 samples using the coefficients in Table 8. Com pres s ive s trenght 37 38 39 40 41 42 43 44 Experimental result (N/mm ) Fig.2. The Relationship between Estimated Compressive Strength and Experimental Compressive Strength Descriptive Statistics Mean Std. Deviation N Compressive strength 40.2500 1.67212 16 Hydrostatic height 117.56 74.49829 16 Fig.3. The Estimated Compressive Strength Value According to the Hydrostatic Height Model Summary Std. Error Change Statistics R Adjusted R of the R Square F Sig. F Square R Square Estimate Change Change df1 df2 Change .943 .888 .880 .578 .888 111.497 1 14 .000 a. Predictors: (Constant), hydrostatic height JAABE vol.17 no.3 September 2018 Metin M. Uzunoğlu 569 P red ic te d v a lu es (N /m m ) Unstandardi- zed Coefficients Standardized Coefficients 95% Confi - dence Interval for B Table 8. The Coefficients of Converting the Compressive • Core samples are generally taken from the Strength of the Core Samples According to Hydrostatic Height buildings that would be reinforced to determine the Upper border Coefficient for converting to compressive strength of the concrete. However, in distance (cm) reference values this study, it was seen that the compressive strength 0 1.00 (reference value) of the core samples changed between 0.95% and 15 0,98 26 0,97 16.37%, according to hydrostatic height. For this 37 0,97 reason, the compressive strength of the core samples 50 0,96 can be used if they are converted into reference 59 0,95 compressive strength according to the suggested 70 0,95 100 0,93 coefficient below (Table 5.). In order to convert the 103 0,93 compressive strength of the concrete into a reference 114 0,93 value, the hydrostatic height must be measured while 136 0,92 taking the core samples. 147 0,91 • Besides, for different hydrostatic height values not 150 0,91 158 0,91 included in the table, the model equation obtained 180 0,90 from regression analysis can be used to predict the 191 0,89 compressive strength of the core samples related to 200 0,89 hydrostatic height. 202 0,89 • The model equation obtained through multiple 215 0,88 227 0,87 linear regression analysis can be used to predict 235 0,87 the compressive strength related to the hydrostatic 239 0,87 height and physical properties of the core samples (R =0.932). 4. Results and Discussion In the case of using a different concrete class, In this study, the effect of hydrostatic height in mixture design, and different types and amounts of hardened concrete on the compressive strength was plasticizers, investigating the effects of hydrostatic investigated experimentally and statistically. The height in structural elements such as columns and results are indicated below. reinforced walls on the compressive strength of • The average compressive strength of the reference hardened concrete can be useful. samples was 36.95 N/mm and the compressive strength of other samples changed between 37.3 N/mm References and 43.0 N/mm . 1) Gardner, N.J., (1980) Pressure of concrete on formwork, ACI • The compressive strength of the concrete increased Mater J 77 (4), pp.279-286. when the hydrostatic height increased as well. This 2) Rodin, S., (1952) Pressure of concrete on formwork, Proc Inst increase ratio according to the reference sample was Civil Eng I (4), pp.709-746. 3) Andriamanantsilavo N.R. and Amziane S., (2004) Maturation of 0.95% for 0.15 m, 4.22% for 0.5 m, 7.09% for 1.0 m, fresh cement pastes within 1- to 10-m-large formworks, Cem. 9.95% for 2 m, 12.82% for 2.35 m, and 16.37% for Concr. Res. 34 (11), pp.2141-2152. 2.39 m. 4) Assaad J. and Khayat K.H., (2004) Variations of lateral and pore • It was determined that the natural unit volume water pressure of self-consolidating concrete at early age, ACI weight of core samples changed between 2.54 gr/ Mater. J. 101 (4), pp.310-317. 3 3 5) Assaad J., Khayat K.H. and Mesbah H., (2003) Assessment of cm and 2.90 gr/cm , dry volume unit weight between 3 3 thixotropy of flowable and self-consolidating concrete, ACI Mater. 2.49 gr/cm and 2.54 gr/cm , water saturated unit 3 3 J. 100 (2), pp.99-107. volume weight between 2.56 gr/cm and 2.66 gr/cm , 6) Khayat K.H., Assad, J. Mesbah H. and Lessard M., (2005) Effect absorption amount between 2.65 and 4.49, and volume of section width and casting rate on variations of formwork 3 3 of voids between 6.18 cm and 10.21 cm . pressure of self-consolidating concrete, RILEM Mat. Struct. 38, pp.73-78. • The coefficient between the compressive strength 7) Ts En 12504-1 (Nisan 2002) Karot Numuneler - Karot Alma, and saturated unit volume weight of the core sample Muayene ve Basinç Dayaniminin Tayini, Türk Standartlar was -0.273 and measured at 0.01 for dry unit Enstitüsü, Ankara, Türkiye. volume weight, 0.219 for saturated unit volume 8) Troxell G.E., Davis H.E. and Kelly, J.W. (1968) Composition and weight, -0.504 for water absorption amount, -0.491 properties of concrete, McGraw-Hill Book Company, New York. 9) Neville A.M., (1995) Properties of concrete, Addison-Wesley, UK. for the volume of voids, and 0.916 for hydrostatic 10) Mindess S. and Young J.F., (1981) Concrete, Prentice-Hall, Inc., height. According to this finding, the relationship New Jersey. between hydrostatic height and compressive strength 11) Arioglu E. and Arioglu N., (1998) Testing of concrete core is quite strong, on the other hand, the relationship samples and evaluations, Evrim Publisher, Istanbul [in Turkish]. between compressive strength, water absorption 12) Erdoğan T.Y., (2003) Concrete, METU Publisher, Ankara [in Turkish]. amount, and volume of voids is nearly the same or at 13) Sullivan P.J.E., (1991) Testing and evaluation of concrete strength mid-level but negative. The relationship between dry in structures, ACI Mater J 88 (5) pp.530-535. unit volume weight and the compressive strength of the concrete is not significant. 570 JAABE vol.17 no.3 September 2018 Metin M. Uzunoğlu 14) Haque M.N. and Al-Khaiat H., (1997) Carbonation of concrete structures in hot dry coastal regions, Cement Concr Compos 19 pp.123-129. 15) Miao B., Aitcin P.C., W.D. (1993) Mitchell, Cook and D., Influence of concrete strength on in situ properties of large columns, ACI Mater J 90 (3) pp.214-219. 16) Price W.F. and Hynes J.P., (1996) In-situ strength testing of high strength concrete, Mag Concr Res 48 (176) pp.189-197. 17) Khayat K.H., Manai K. and Trudel A., (1997) In-situ mechanical properties of wall elements cast using self-consolidating concrete, ACI Mater J 94 (6), pp.491-500. View Record in Scopus | Cited By in Scopus. 18) Bungey J.H., (1979) Determining concrete strength by using small diameter cores, Mag Concr Res 31 (107) pp.91-98. 19) Bartlett F.M. and MacGregor J.G., (1994) Effect of core diameter on concrete core strengths, ACI Mater J 91 (5) pp.460-470. 20) Bartlett F.M. and MacGregor J.G., (1994) Effect of core length- to-diameter ratio on concrete core strengths, ACI Mater J 91 (4) pp.339-348. 21) Bartlett F.M. and MacGregor J.G., (1993) Effect of moisture condition on concrete core strengths, ACI Mater J 91 (3) pp.227- 22) Bartlett, F.M., (1997) Precision of in-place concrete strengths predicted using core strength correction factors obtained by weighed regression analysis, Struct Saf 19 (4) pp.397-410. 23) Bartlett F.M. and MacGregor J.G., (1993) Effect of moisture condition on concrete core strengths, ACI Mater J 91 (3) pp.227- 24) BS 1881: Part 122 (July 2011) Testing concrete. Method for determination of water absorption, British Standarts. 25) ASTM C42 (1.10.2016) Standard Test Method for Obtaining and Testing Drilled Cores and Sawed Beams of Concrete, American Society for Testing and Materials. JAABE vol.17 no.3 September 2018 Metin M. Uzunoğlu 571

Journal

Journal of Asian Architecture and Building Engineering – Taylor & Francis

Published: Sep 1, 2018

Keywords: concrete; hydrostatic pressure; core sample; compressive strength

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