Relationship between cylinder and core compressive strength

Transcription

1 Journal of Science & Technology Vol. (15) No.(1) 2010 JST 26 Relationship between cylinder and core compressive Hamood Bisher + Abstract The main objective of this paper is to report the results of an experimental study conducted to obtain the relationship between the compressive s of concrete cylinders and cores made using four different water-cement ratios, two cement contents, and two different types of aggregates. The results of cast and cored cylindrical specimens were compared and correlated to each other. It was found that the linear relationship between the cylinder and core compressive is very significant and almost independent of the mixture parameters. 1. Introduction The most common and convincing technique used to determine actual compressive, the physical and chemical characteristics of existing concrete structures, is extracting cores and testing them. This method facilitates the visual inspection of the interior matrix of concrete members. Cores are always cut by means of a rotary coring machine equipped with diamond bits. The equipment is portable and heavy and must be firmly supported and braced against the concrete to prevent any relative movement, which may result in a distorted or broken core. Water supply is also necessary to lubricate and cool the cutter. While testing cores in compression, they have to be capped to ensure full contact between the pressure heads of the compression testing machine and the sample faces. A great deal of information can be obtained from visual examination of the core such as aggregate size, shape and distribution, porosity, presence of honeycombing, reinforcement and degree of compaction, etc. The core compressive may be influenced by several factors, such as: curing time and regime, voids, type, geometry and dimensions of structure, length to diameter ratio of the core, diameter of the core, and direction of drilling (1-4). However, the conventional method of determining the compressive of concrete structures has some limitations due to the inherent errors in sampling and the fact that concrete to be cast in structures is transported, placed, compacted and cured differently from that cast in typical laboratory cylinders or cubes. There is a need to correlate core to cylinder, as the core is known to be slightly less than cylinder. 2. Experimental Program A coarse-to-fine aggregate ratio of 1.60 by weight was kept invariant in all concrete mixtures and type 1 cement. Two cement contents of 300 and 350 kg/m3 were used. Two types of coarse aggregates (A and B) were used with a maximum size of 20 mm. The specific gravity and absorption of fine and coarse aggregates are presented in Table 1. Four water-tocement ratios of 0.0,, and were used. + Dept. of Civil Eng., Faculty of Science and Engineering, University of Science & Technology, Sana a, Yemen.

2 Journal of Science & Technology Vol. (15) No.(1) 2010 JST 2 Potable water was used in the mixing and curing of all concrete specimens. 15 cylinders, 100 mm in diameter and 200 mm in height, and slab panels, 1000*500*200 mm, were cast for each mix by filling the molds in approximately three equal layers and compacting them on a vibrating table. The specimens were cured by spraying with water twice a day for seven days followed by curing at the exposure site until testing. Following the prescribed curing, three cylindrical specimens and three cores were tested after,, and days in accordance with ASTM C Results and Discussion Tables 2 and 3 present the results of this study numerically. each data point is an average of triplicate specimens of the same concrete mixture which were cured and tested under similar conditions. This data follows the conventional behaviour of an increase in compressive when the curing period is increased. It is well known that actual in-situ concrete is often determined by testing cores taken from the structure. The conventional method is based on cylinder, but, it is beneficial to correlate core to the cylinder. For the purpose of correlating the cylinder to the insitu, three cores of 100 mm diameter were taken from each panel at,, and days. The average compressive was measured for cores at each age so as to compare it with the corresponding values of the cylinder. The ratio of core to cylinder is shown in Tables 2 and 3, for both A and B types of concrete respectively. Results showed that the ratio of core to cylinder varied from a maximum of 0.98 to a minimum of 0.82 for concrete A. However, this ratio varied from 0.9 to 0.81 for concrete B. Both results fell within a narrow band. The average value of core to cylinder is 0.89 for concrete A and 0.8 for concrete B which may indicate that this ratio is independent of the mixture design parameters. This result is in close agreement with ACI Committee 318 and ACI Committee 301(5, 6). The compressive data of the concrete cylinders and cores using aggregate A is presented in Tables 2a and 2b for the various water-to-cement ratios and made with cement contents of 300 and 350 kg/m3. Each data point was an average of triplicate specimens of the same concrete mixture cured and tested under similar conditions. The data in Table 2 follows the conventional behaviour of an increase in compressive with an extension in the curing period. The rate of increase in decreased marginally beyond the initial -day period of curing and markedly after days. Similar trends can also be observed for the other mixtures using 350 kg/m3 cement content as well as for those using type B aggregate. The data from Tables 2 and 3 shows the change in compressive with the water-tocement ratio at various curing periods. As expected, the compressive decreases with an increase in the ratios, and the decrease is more significant when the ratio is more than. The effects of aggregate types on the concrete can be obtained by comparing the previous data. For example, a water-to-cement ratio of and a cement content of 300 kg/m3 indicates that the of the cylinder increased when aggregate A was replaced by aggregate B. An increase of 10% in the -day cylinder was attained when aggregate B was used. A similar improvement can be observed for other mixtures. The main purpose of this study was to develop the best possible correlative model capable of evaluating the actual of concrete structures using the cylinder compressive data. The results of the statistical analyses are quantitatively presented in Table 4.

3 Journal of Science & Technology Vol. (15) No.(1) 2010 JST This table shows, respectively, the influence of cement content, curing period, and aggregate type on the cast and cored cylinder correlations. From the statistical test results, it seems that the best relationship between the cylinder and core is linear. Figure 1 shows that the solid line drawn through the data is a good representation of the relationship between the core and cylinder of the concrete. The prediction of core has taken the following form: Where, F cor = A + B. F cy.. (1) F cor is the core of concrete, F cy is the cylinder of concrete, A and B are regression coefficients (intercept and slope, respectively). This equation indicates that a plot of the core against the cylinder has a linear relationship. Thus, if the constants A and B are known for a specific concrete, the actual of concrete can be determined using equation (1) for any age. The degree of correlation between the cast and cored cylinders is excellent irrespective of the mixture design parameters. This is evidenced by the coefficient of multiple determination (R 2 ), which is invariably above 0.94, and the coefficient of variation which is in the range of 5.13 to 8.22 percent. Also, the constants A and B of Equation (1) vary slightly with each model, as shown in Table 4. Table (1): Absorption and specific gravity of aggregates. Aggregate type Specific gravity Absorption (%) Fine aggregate Type A aggregate Type B aggregate Table (2a): Experimental data for A aggregate concrete, cement content = 300 kg/m 3 Core Cylinder Core/Cylinder

4 Journal of Science & Technology Vol. (15) No.(1) 2010 JST 29 Table (2b): Experimental data for A aggregate concrete, cement content =350 kg/m Core Cylinder Core/Cylinder Table (3a): Experimental data for B aggregate concrete, cement content = 300 kg/m 3 Core Cylinder Core/Cylinder Table (3b): Experimental data for B aggregate concrete, cement content = 350 kg/m Core Cylinder Core/Cylinder

5 Journal of Science & Technology Vol. (15) No.(1) 2010 JST 30 Core Cylinder Core/Cylinder Table (4): Summary of the statistical analyses of the results Aggregate Cement conten kg/m 3 Equation Correlations R 2 A 350 F cor = 0.96 F cy A 300 F cor = F cy A 300 and 350 F cor = F cy B 350 F cor = 3 F cy B 300 F cor = F cy B 300 and 350 F cor = 0.8 F cy A+B 300 and 350 F cor = F cy Conclusions 1. It is clear that age, cement content, water to cement ratio and type of aggregate do not significantly affect the ratio between core and cylinder compressive. 2. A linear relationship exists between the of cast cylinders and cored cylinders with a high coefficient of correlation, almost above. 3. The ratio of cored cylinder to cast cylinder is 0.89 for A concrete and 0.8 for B concrete, which indicates that this ratio is independent of the mixture design parameters. 4. The within-test variations of core and cylinder are within the acceptable limits, where coefficients of variation are less than 10%. 5. The reduction of core related to cylinder may be due to the effect of cutting, extracting, and the difference in curing time and water supply during the coring process.

6 Journal of Science & Technology Vol. (15) No.(1) 2010 JST References [1]. Karim, W. N. and Akthem, A. M., "Comparison of Nondestructive Testers of Hardened Concrete", ACI Materials Journal, September- October 198, pp [2]. Stone, W. C. and Reeve, C. P., " A new Statistical Method for Prediction of Concrete Strength from In-place Tests", Cement, Concrete and Aggregates, V. 8, No. 1, Summer 1986, pp [3]. Di Maio, A. A., Traversa, L. P., and Giovambattista, A., Non-destructive Combined Methods Applied to Structural Concrete Members", American Society for Testing and Materials, 1985, pp [4]. Malhotra, V. M., " In-situ Non-destructive Testing of Concrete", ACI Sp-82, American Concrete Institute, Detroit, [5]. ACI Committee 318, "Building code Requirements for Reinforced Concrete (ACI 318-1)", American Concrete Institute, Detroit, 191, 8 pp. [6]. ACI Committee 301, "Specifications for Structural Concrete for Building (ACI 301-2)", American Concrete Institute, Detroit, 192, 36 pp. Appendix Relationship of core and cylinder for both aggregates y = 0.802x R 2 = Core, MPa Cylinder, MPa