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Effect of different mixing time on mechanical properties of concrete

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  • Save International Journal of Materials Engineering 2014, 4(1): 1-4 DOI: 10.5923/j.ijme.20140401.01 Effect of Different Mixing Times on Mechanical Properties of Concrete Ali Ugur Öztürk*, R. Tugrul Erdem Celal Bayar University Engineering Faculty Department of Civil Engineering, 45140, Manisa, Turkey Abstract It is significant for a structure not to lose its durability and strength properties and to protect its performance during its service life. It is possible to satisfy the intended purposes as durability and strength and also properties of the structure during its service life by providing the proper production conditions of the structural members. In the scope of this study, effect of different mixing times on concrete which is widely used as construction material is studied. For this purpose, mechanical properties of various concrete samples are investigated. These samples are produced according to different mixing times (20 sec, 45 sec, 90 sec, and 120 sec). Mixing ratios of materials used in concrete samples are taken constant to see the nominative effect of mixing times. The study indicates that while strength values increase in certain circumstances, slump values decrease by the increase in mixing durations in concrete production. Keywords Concrete Production, Mixing Time, Mechanical Properties 1. Introduction Concrete is a conventional construction material which is widely used in the construction process. In addition to its easy workability capacity and placement advantages in production process, concrete which is produced according to proper production procedures has some other advantages such as high compressive strength and durability properties against environmental aggressive attacks during its service life[1]. However, desired strength and durability properties of structural concrete members which are produced without taking necessary precautions cannot be achieved. Therefore, required properties of concrete members produced without essential production conditions cannot be obtained even if proper and quality material selections are made. In this day and age, some results as delay in concrete mixture time may occur during concrete production and placement in construction business. For this reason, decreases in concrete workability and strength shall be anticipated. Concrete is supposed to service related to its production scope without losing its characteristics and being exposed to undesired cracks for many years. In accordance with this purpose, some certain criteria shall be taken into consideration in production process of concrete. One of the most important of these criteria is mixing time. Amounts in aggregate granulation, water/cement ratio * Corresponding author: (Ali Ugur Öztürk) Published online at Copyright © 2014 Scientific & Academic Publishing. All Rights Reserved and admixture if any, shall not change during both mixing time and mixing the components of concrete[2]. Some important applications affecting the structure of concrete; workability characteristic, strength and durability properties in negative ways shall be taken into consideration during mixing. Mixing time starts with the time where first part of the mixture water and other mixture components start mixing and finishes with the end of the mixture process[3]. There are also some other studies about the delay in pouring of fresh concrete and remixing process[4, 5]. While workability is decreased in retarder and super plasticizer aided and pure concrete, there isn’t any difference observed in compression strength when studies are investigated in the literature[6]. Also, the relationship between delay periods and initial set of concrete with steam cure are studied in another study[7]. Mixing process of fresh concrete for 3 hours straightly effects the strength and durability values of concrete negatively[8]. Also, initial setting time of concrete is a remarkable factor that shall be taken into consideration due to waiting period of fresh concrete and remixing processes [9-11]. These processes must be finished before initial set of concrete to place the fresh concrete into the molds. In this paper, effect of various mixing times on mechanical properties of fresh concrete are studied. According to this purpose, concrete samples with four different mixing durations (20 sec, 45 sec, 90 sec and 120 sec) are produced. Amounts of mixture components as aggregate, cement and water are taken constant in these samples. In addition to strength values, slump, weight per unit of volume, percentage of air and concrete temperature 2 Ali Ugur Öztürk et al.: Effect of Different Mixing Times on Mechanical Properties of Concrete values are also determined of concrete samples. 2. Materials and Experimental Study 2.1. Materials In the scope of this study, effect of mixing time of concrete batch on mechanical properties was investigated. For this purpose, change in slump and compressive strength values of different concrete samples are taken into consideration. Four different mixing times (20 sec, 45 sec, 90 sec, and 120 sec) are used. CEM I 42.5 cement type having 3,15 kg/dm3density value was used while producing concrete and material properties of the used aggregates are given in Table 1.While mixing time is taken as variable for each concrete set, other mixing components are taken constant. structural members produced by non-homogenously and irregularly placed concrete mixtures will have pore structure at later ages. This situation effects both strength and durability properties negatively. Because of these reasons, researchers modify the properties of fresh concrete and production processes and investigate the effects on ultimate properties. In this study, effect of mixing time on fresh concrete and strength properties are investigated. Concrete is produced with different mixing times (20 sec, 45 sec, 90 sec and 120 sec). Effects of different mixing times on mechanical properties of concrete samples are stated. Compressive strength, slump, weight per unit volume, percentage of air and temperature of concrete are taken as mixture characteristics. Values of characteristics at the fresh state of concrete are presented below. Table 1. Aggregate properties Properties Aggregate type Density (g/cm3) Weight per unit of volume(kg/m3) Fine aggregate 2,71 Coarse aggregate 2,62 1630 1350 Water absorption (%) 0,8 1,2 2.2. Experimental Study Concrete samples are produced according to various mixing times by using the materials whose properties are summarized above. Fresh concrete samples are subjected to 4 different tests. Concrete temperatures are determined for each set. 7 and 28 days compressive strength values of concrete samples are determined in laboratory conditions. In this way, concrete compressive strength tests are performed in laboratory conditions at 20±2ºC temperature and approximately 80% relative humidity rate for 12 samples according to each mixing time. While half of the 12 samples are used for 7 days strength values, the rest is evaluated for 28 days strength values for each set. Materials that are used for 1m3 concrete design and mixture properties are given in Table 2. On the other hand, slump, weight per unit volume, percentage of air and temperature of concrete are determined to see the effects of mixing times. Table 2. Mixture Properties Cement (kg) Water Fine aggregate (kg) (kg) Coarse Aggregate (kg) W/C rate 419 192 685 1161 0.46 3. Conclusions and Suggestions Some undesired results may occur in the ultimate properties of concrete produced without obeying proper production requirements in the process. Especially, Table 3. Test results of fresh concrete Property Unit weight (kg/m3) Air ratio (%) Concrete Temperature (0C) 20 sec 2.407 1.5 15.3 Mixing time 45 sec 90 sec 2.369 2.394 1.7 1.5 15.8 15.1 120 sec 2.396 1.5 15.1 On the other hand, 7 and 28 days of compressive strength values of concrete samples are obtained according to 4 different mixing times and given in Figure 1. In this presented paper, slump values for each mixing time of fresh concrete are seen in Figure 2. The various components of a mix are proportioned so that the resulting concrete has adequate strength, proper workability for placing and cost effective. To achieve such properties the mixing should such that it produces an intimate mixture of cement, water, fine and coarse aggregate and suitable admixture of uniform consistency throughout each batch. The average strength of concrete increases with an increase in mixing time as it improves uniformity of mix. After experimental studies, maximum slump values are obtained by the samples which are prepared with 45 and 90 sec. mixing times. Thus, minimum workability was obtained from the samples produced with minimum mixing time. Linear increasing in slump values is not observed due to increase in mixing duration. The reason of this situation may be chemical reactions of the cement, long mixing durations, evaporation of mixing water and decrease in workability. Temperature of fresh concrete is especially higher at 45 seconds mixing duration. This situation can be explained by the reactions of components increasing hydration temperature like C3A in the first minute[1]. Some little decreases are observed at longer mixing durations depending on reaction rate of hydration. Furthermore, there is compatibility between air ratio change and unit weight. Unit weights of the samples have higher values while air ratio values are lower. Strength properties are obtained as well as characteristics of concrete. The highest compressive strength value is determined from the concrete samples having 120 sec International Journal of Materials Engineering 2014, 4(1): 1-4 3 mixture duration. Homogenous structure that is provided by longer mixing duration may cause higher strength values. low temperature of fresh concrete, lower air content and 7 days 28 days 40 36.5 34.6 34.7 35 30 26.7 27.7 25.2 25.5 20 Compressive strength (MPa) 10 0 20 set 1 20 sec set 2 45 sec set 3 90 sec Figure 1. 7 and 28 days compressive strength values 18 18 15 15 set 4 120 sec 17 Slump (cm) 10 5 0 set 1 20 sec set 2 45 sec set 3 90 sec Figure 2. Slump values set 4 120 sec 4 Ali Ugur Öztürk et al.: Effect of Different Mixing Times on Mechanical Properties of Concrete 20.0 15.0 2 Concrete temperature (°C) Air ratio 1.5 Concrete temperature (°C) Air ratio (%) 10.0 1 5.0 0.5 0.0 set 1 20 sec slump 15 set 2 45 sec slump 18 set 3 90 sec slump 18 0 set 4 120 sec slump 17 Figure 3. Concrete temperature values [6] ASTM C 944 ‘Standart Test Method for Abrasion Resistance of Conrete or Mortar surface by the Rotating- Cutter Method REFERENCES [1] Neville, A.M.,Properties of Concrete, Third Edition, Longman Scientific& Technical, 1993. [2] Ravındrarajah, R.,S., CastingDelay on Workabilityand Strength of Concrete, International Journal of Cement CompositesandLightweightConcrete: 7(2), 109-113, 1985. [3] Erdem,T.K.,Turanli,L.,Erdoğan,T.Y.,Settingtime:An Important Criteria on to Determine The Length of The Delay Period Before Steam Curing Of Concrete, Cement Concrete Res 33 (5): 741-745, May., 2003. [4] Ping-Kun Chang, Yaw-Nan Peng, Influence of mixing techniques on properties of high performance concrete, Cement and Concrete Research, Volume 31, Issue 1, January 2001, pp. 87–95 [5] Diamond S., The patch microstructure in concrete: effect of mixing time, Cement and Concrete Research, Volume 35, Issue 5, May 2005, pp. 1014–1016 [7] Bilal El-Ariss, Mix Design of Self Compacting Concrete Civil&Environmental Engineering Dept, U.A.E. University, Al-A in. [8] Skarendahl, APetersson, O., 2000. Self Compacting Concrete, State of the Art Report of RILEM Technical Comittee 174-SCC [9] Julnipitawong, Y.,Tangtermsirikul, S., Effect of Curing Temperature on Early Age Compressive Strength of Fly Ash Concrete [10] J.-K Kim, Y.-H Moon, S.-H Eo, Compressive strength development of concrete with different curing time and temperature, Cement and Concrete Research, Volume 28, Issue 12, December 1998, pp. 1761–1773 [11] Su, Kung-ChungHsu, His-Wen Chai, A simple mix design method for self-compacting concrete, Cement and Concrete Research, Volume 31, Issue 12, December 2001, pp. 1799–1807.

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