Prediction of Some Properties of Retempered Concrete in Hot Weather

This paper present statistical study to proposed empirical equations to predicting some properties of the fresh concrete (additional water for first and second retempering, final slump and dry unit weight), also some properties of the hardened concrete (compressive, flexural, split tensile strength and modulus of elasticity) depending on the simple properties of the retempered concrete mix (water cement ratio, temperature, air content, humidity, mix proportion and unit weight). Theoretical results obtained from these proposed equations found to be in good agreement with the experimental data found in literature. *Corresponding author: Ayad Zeki Saber Agha, Department of Civil Engineering, Erbil Technical Engineering College, Erbil Polytechnic University, Erbil-44002, Iraq, Tel: 009647704454107; E-mail: agha_ayad@epu.edu.krd Received December 02, 2017; Accepted October 18, 2018; Published October 24, 2018 Citation: Rashid MHF, Agha AZS (2018) Prediction of Some Properties of Retempered Concrete in Hot Weather. J Civil Environ Eng 8: 324. doi: 10.4172/2165784X.1000324 Copyright: © 2018 Rashid MHF, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.


Introduction
Using and placing concrete during the hot summer months present far different challenges than use and placement during cold weather. The summer month effects of temperature, wind, and air humidity can all have a negative impact on the performance of concrete. For purposes of concrete use and placement, "hot weather" can be defined as any period of high temperature during which special precautions need to be taken to ensure proper handling, placing, finishing and curing of concrete. Hot weather problems are most frequently encountered in the summer, but critical drying factors such as high winds and dry air can occur at any time, especially in arid or tropical climates.
Higher temperatures cause water to evaporate from the surface of the concrete at a much faster rate and cement hydration occurs more quickly, causing the concrete to stiffen earlier and improving the chances of plastic cracking occurring. Concrete cracking may result from rapid drops in the temperature of the concrete. This occurs when a concrete slab or wall is placed on a very hot day and which is immediately followed by a cool night. High temperature also accelerates cement hydration and contributes to the potential for cracking in massive concrete structures. Higher relative humidity tends to reduce the effects of high temperature.
Other hot weather problems include increased water demand, which raises the water-cement ratio and yield lower potential strength, accelerated slump loss that can cause loss of entrained air, fast setting times requiring more rapid finishing or just lost productivity. Environmental conditions and delays in the placement of concrete may cause loss in the workability of super-plasticized concrete. The loss can be restored by retempering with water or super-plasticizers. The use of water usually results in a reduction in strength. There is divergence of opinion on the engineering properties of the tempered concrete. The practice of retempering in hot-dry environments is frequently performed to increase slump beyond typical specification's limits (of 100 ± 25 mm) in order to cope with the need for expediting the casting operations and reducing the consolidation effort.
The strength of concrete of given mix proportions are very seriously affected by the degree of its compaction. Therefore that the consistence of the mix be such that the concrete can be transported, placed and finished sufficiently easily and without segregation. The concrete mix satisfying these conditions is said to be workable. However workability is measured by slump, compacting factor or flow test. Freshly mixed concrete stiffened with time, some of the water of the mix absorbed by the aggregate and some is lost by evaporation. Particularly if the concrete is exposed to sun or wind and some is removed by the initial chemical reactions. This loss of mix water lead to reduction in slump or compacting factor therefore reduction in workability of the concrete. Generally workability reduces with the time and increasing of temperature. Therefore it is apparent that on a hot day water content of the mix would have be increased for constant workability [1,2].
Loss of slump and the consequent reduction in workability with time is an inherent property of fresh concrete, this loss in workability is accelerated in hot climates. A delay in the discharge of concrete from a truck mixer, or a delay in the placement of concrete due to other reasons could cause stiffening to the point of unworkability. Therefore in actual field application in hot climate, it may be necessary to retemper the concrete to maintain the required workability. The process or procedure adopted in achieving the desired consistency of a given fresh concrete, already mixed to the specified consistency, by the addition of water, a super plasticizer or cement paste or any combination of these, is known as retempering [3].
Hanayneh and Itani [4] attempted to study the engineering properties of retempered concrete having different normal strengths. The slump, compressive strength, modulus of rupture, splitting strength, Poisson's ratio and the modulus of elasticity were determined. Alhozaimy [5], investigated the effect of retempering on the workability and strength of ready-mixed concrete (RMC) in hot-dry environments was investigated. This study covered 12 construction sites with concrete delivered by 11 different RMC suppliers. The results indicate that the reduction in strength due to water addition is proportional to the associated increase in slump. In cases where water was added to restore the slump to the specifications limits (100 ± 25 mm), the reduction of strength was below 10%. However, when water was added to increase slump beyond these limits, the reduction of strength may be as high as 35%. The study shows the change in slump can be used to predict reduction of strength due to jobsite water additions when practical considerations preclude accurate determination of the w/c ratio.
Erdogdu [6][7][8] used the super-plasticizer of ASTM C (494) Type F for retempering concrete to restore its initial slump. Concrete mixes having an initial slump of about 19 cm were prepared and subjected to prolonged mixing with different mixing duration such as 30 min, 60 min, 90 min, 120 min and 150 min following an initial mixing of 5 min to ensure homogeneity. At the end of each mixing period, cube specimens of 15 cm were cast from concrete retempered to its initial slump level and tested at the age of 28 days for compressive strength. Results revealed that compared to the concrete retempered with water, those retempered with a superplasticizer admixture have yielded significantly higher strength regardless of the mixing duration. This paper present a statistical study to proposed empirical equations to predicting some properties of the fresh and hardened concrete in term of simple properties of the retempered concrete mixes, theoretical results obtained from these proposed equations found to be in good agreement with the experimental data found in literature [3].

Analysis and Results
Method of multi-linear regression analysis is used to proposed different expressions to predicting some properties of the fresh and hardened concrete depending on the simple properties of the concrete mix, the general proposed equation take the following form: where: ( ) Values of these coefficients are determined by incorporation of experimental data and using computer program. Depending on the least square principle: where: S = Sum of square of differences between calculated and experimental results. y = Experimental value of the dependent variable.
Y = Calculated value of the dependent variable.
To determine value of the coefficients, the error function (S) is minimized with respect to the coefficients: These equations lead to generate a set of simultaneous equations as shown below, solved by using computer programs and incorporation of experimental data to determine the coefficients value for different cases.
Where [A] is the coefficient matrix.
Properties of the fresh and hardened concrete determined for the mixes measured or tested before addition of water for retempering this stage is known as an initial stage (or first retempering), but approximately (30) minutes after the initial mixing additional water added to the concrete mix to increase the workability, this stage is known as a first retempering. Approximately one hour after the initial mixing. The second additional water is added to the concrete mix to keep workability constant; this stage is known as a second retempering. For all these stages properties of the fresh & hardened concrete are measured or tested.
All these properties of the fresh and hardened concrete relating with the selected independent variables, which represent the simple properties of the concrete mix. The independent variables ( ) are taken as the following form to represent the correct mix properties: where: C, w, S and G are cement, water, sand and gravel proportions respectively. X 4 = Air content % (A) X 5 = Humidity % (H) A computer program for developing such multi-linear regression analysis was adopted which yield the different equations by incorporation of experimental data found in literature [3]. The fresh and hardened concrete properties relating with the selected independent variables are shown in Tables 1a and 1b.

Fresh concrete
The following properties of the fresh concrete are taken as dependent variables to evaluate general empirical equations relating these properties with the independent variables ( ) 1  2  3  4  5  6 , , , , & X X X X X X 1 R Wd = Water dosage for first retempering (kg).      Using computer programs and incorporation of experimental data lead to formulation general equations, predicting previous dependent variables (Y) in term of the dependent variables (X) as following:  Table 2.
Theoretical results obtained from the proposed equations found to be in a good agreement with the experimental data. Figures 1-6 show the relationship between theoretical and experimental results.    The following equations are obtained: (8) where:  Table 3.
Theoretical results obtained from proposed equations found to be in good agreement with the experimental data. Figures 7-22 show the relationship between theoretical and experimental results.

Discussion
The experimental data given in literature [3] are used to predict empirical equation to estimate the concrete properties (fresh and hardened) depending on the mix properties and retempering of the concrete mix in hot weather. Value of the equation coeff.                       2. This estimation is useful to predicting some properties of the fresh and hardened concrete after first and second retempering easily without needing measuring or casting and testing control specimens to get these properties.

3.
Theoretical results obtained from these equations found to be in good agreement with the experimental data found in literature.