Influence of accelerated curing on the compressive strength of polymer-modified concrete

Abstract

In recent building practice, rapid construction is one of the principal requisites. Furthermore, in designing concrete structures, compressive strength is the most significant of all parameters. While 3-d and 7-d compressive strength reflects the strengths at early phases, the ultimate strength is paramount. An effort has been made in this study to develop mathematical models for predicting compressive strength of concrete incorporating ethylene vinyl acetate (EVA) at the later phases. Kolmogorov-Smirnov (KS) goodness-of-fit test was used to examine distribution of the data. The compressive strength of EVA-modified concrete was studied by incorporating various concentrations of EVA as an admixture and by testing at ages of 28, 56, 90, 120, 210, and 365 d. An accelerated compressive strength at 3.5 hours was considered as a reference strength on the basis of which all the specified strengths were predicted by means of linear regression fit. Based on the results of KS goodness-of-fit test, it was concluded that KS test statistics value (D) in each case was lower than the critical value 0.521 for a significance level of 0.05, which demonstrated that the data was normally distributed. Based on the results of compressive strength test, it was concluded that the strength of EVA-modified specimens increased at all ages and the optimum dosage of EVA was achieved at 16% concentration. Furthermore, it was concluded that predicted compressive strength values lies within a 6% difference from the actual strength values for all the mixes, which indicates the practicability of the regression equations. This research work may help in understanding the role of EVA as a viable material in polymer-based cement composites.

Graphical abstract

Keywords

compressive strength prediction / polymer-modified concrete / linear regression fit / early age strength / ethylene vinyl acetate

1 Introduction

Compressive strength of concrete is one of its most important mechanical properties, addressed by most standards such as ACI 318 [1], ABNT NBR6118 [2] and Eurocode 2 [3]. Compressive strength of concrete (1−28 d) is essential for rapid construction as structural design depends upon this property. The traditional 28-d compressive strength test gives a general idea of the concrete quality and concrete acceptance. To further improve the concrete quality, supplementary cementitious materials such as fly ash, silica fume and metallurgical slag (ground granulated blast furnace slag and steel slag) are commonly incorporated in the concrete. Berndt [4] found that the overall performance of concrete is improved by using fly ash, slag and recycled aggregate in concrete. Similarly, the compressive strength of concrete is increased with the incorporation of micro-silica by filling the capillary pores [5]. Besides, polymers such as ethylene vinyl acetate (EVA), styrene butadiene rubber (SBR), polyvinyl acetate (PVA), may also be incorporated in concrete, resulting in the enhanced mechanical properties of polymer modified concrete (PMC). The study [6] reported that the compressive strength and flexural strength of concrete increase significantly with the incorporation of 16% EVA. Similarly, higher compressive strength of PMC containing EVA and ladle furnace slag in comparison with conventional concrete was reported [7]. Compared to conventional concrete, PMC shows excellent properties in both fresh and hardened state such as improved workability and enhanced compressive and flexural strength [6]. In several research studies [8–12], it was concluded that water-proofness and adhesive properties are significantly improved by incorporating polymers in concrete. Some researchers [13–15] concluded that there was a reduction in permeability related to penetration of chlorides. Similarly, many other characteristics, such as high freeze–thaw resistance [16], higher shrinkage resistance [17] and enhanced durability [18–21] were also reported for PMC. However, together with improvements in various characteristics of PMC, it has a limited tensile strength [6,22]. The main purpose of using polymers is to make the environment free of waste products, bring economy to the concrete industry and minimize the emission of CO₂ during manufacture of cement. For example, EVA is mostly found in the form of shoe soles as a waste [23]. Globally, an enormous quantity of waste is generated after its utilization by the consumer. The use of these wastes in construction may generate economic and environmental benefits. EVA was first utilized as a replacement material for leather in 1970s [24]. Worldwide, about 17 billion shoe pairs are manufactured every year and at the end of the useful life the material is converted into a waste which is then usually disposed of in land fill sites [25]. During the manufacture of shoes’ outer soles, waste generated consists of 25%–35% of the initial material, thus producing about 80 tons of waste per million shoe pairs. EVA presents approximately 14% of the aforementioned waste by mass. Such waste has longer biodegradation period and requires large areas of land for its disposal, with associated high costs. Therefore, it is desirable to use EVA in concrete which enhances mechanical properties of concrete and reduces waste in the environment.

Prediction of compressive strength of concrete at later age, especially if from the first few hours after concrete is poured, can be helpful for necessary remedy or modification. Several methods have been developed for predicting 28-d compressive strength. These methods include performing accelerated curing tests and correlation with strengths at early age under standard curing conditions. For instance, statistical regression analysis was adopted for predicting 28-d compressive strength of concrete using accelerated curing, warm water and autogenous curing methods [26]. Ozkul [27] performed linear regression analysis for compressive strength prediction of conventional concrete cured with warm and boiling water and reported that conventional Portland cement could be efficiently used for thermal curing. Murugan et al. [28] predicted the compressive strength of high-volume fly ash concrete (HVFAC) using polynomial regression analysis and adopted various curing methods such as accelerated, warm water, alternate wet and dry and normal curing methods. Pheeraphan et al. [29], predicted 28-d compressive strength of concrete using microwave curing process and linear regression equations. It was found that the predicted strength values were within 15% of experimentally measured values. Younis and Pilakoutas [30] predicted compressive strength of recycled aggregate concrete (RAC) using multi-linear and non-linear regression analysis. They used various fine materials such as fly ash, silica fume and ground granulated blast furnace slag for coating of recycled aggregate surface. Results indicated a good correlation between the strength of RAC and particle density, as confirmed by Kong et al. [31]. Similarly, Kim et al. [32] investigated strength prediction of concrete using two different curing temperatures of 5 and 40 °C. It was identified that the concrete specimens cured at the higher temperature resulted in high early age strength while concrete specimens cured at the lower temperature resulted in lower early age strength. Kim et al. [33] used experimental results in developing a strength prediction model for fly ash concrete and reported the same results as that obtained in Ref. [32] based on four different curing temperatures. Similar mathematical models were developed by Zain et al. [34] and used to predict compressive strength of concrete mixtures. Results showed a good correlation between the data and provided accurate estimation of compressive strength. From the literature, most of the studies related to prediction of later age compressive strength were performed on conventional concrete [29,33,35], Fly ash concrete [28,30,33,36,37], RAC [30,38], BFS concrete [32,37]. However, up till now there has been no single study related to prediction of later age compressive strength of PMC using accelerated curing methods. Therefore, this study was carried out to predict 28-d compressive strength of PMC using EVA as a polymer. Linear regression analysis models were developed to demonstrate the relationship between dependent and independent variables. The results of these models were tested against the experimental results. Regression equations were used to validate the compressive strength relationship between normal curing at 28, 56, 90, 120, 210 and 365 d and accelerated curing at 3.5 h for PMC.

2 Scope of the study

The most appropriate usage of tools and techniques is always needed for economical construction, operating within the defined constraints. Examples include reuse of discarded materials as raw materials for construction, reduction of total workforce, cost and time, careful utilization of the energy resources, and conservative use of natural materials. This study investigated use of discarded materials obtained from shoe industries; EVA may be derived by lopping off of flat solid waste sheets of the material followed by rendering to fine powder grains [39]. According to the study [25], EVA has been used in the shoe industry to replace leather. As a result, huge amount of shoe waste is produced that requires land for its dumping.

However, putting this amount of waste into useful service provides opportunity to reduce the cost of concrete production. Furthermore, this will help in preserving natural resources and controlling environmental pollution.

The compressive strength of concrete at 28 d is one of the input parameters during concrete mix design. During the construction, the required compressive strength is sometimes not achieved due to inadequacies in the construction or improper mix design. In such instances, the entire procedures for testing must be repeated to achieve the targeted compressive strength, which results in wastage of time and thus compromising the schedule as well as economy of the project. Therefore, early prediction of later age compressive strength is essential for the sake of economic and quality considerations. Early prediction of compressive strength is also critical to determine the required time of form work removal, total duration of the project and required quality of the concrete. Early prediction of compressive strength involves accelerated curing of concrete and correlations between empirical equations and experimental data.

This study focused on the following objectives.

1) Influence of EVA on the compressive strength of concrete.

2) Development of regression analysis models for prediction of later age compressive strength of EVA-modified concrete using accelerated curing approach.

3 Materials and methods

The detail of the materials and methods is described in the following subsections.

3.1 Mix design

Mix design was carried out according to absolute volume method in compliance with ACI 211.1 [40] as this method provides a complete set of procedures for estimating each component of concrete. Also, this method applies near-identical procedure to that required when using angular aggregate; in this study angular aggregates were incorporated from the local quarry with maximum aggregate size less than 19 mm. However, other methods of mix design such as double coating method [41] and three equations method [42] were also used to approximate the proportions of concrete components to match ACI 211.1. Mixes were developed using Cherat cement (local brand name) complying to ASTM C 150 [43]. A total of 162 concrete cylinder samples were cast; these were 144 samples for normally cured compressive strength test and 18 samples for accelerated cured compressive strength test. Furthermore, each batch of normally cured samples (i.e., 3, 7, 28, 56, 90, 120, 210, and 365 d) composed of 18 samples, which means that 18 samples were tested at a specified test day. EVA was mixed by weight of cement up to 4%, 8%, 12%, 16%, and 20% at a constant water to binder ratio of 0.40. Fine aggregates were collected from Lawrence-pur quarry in KP Pakistan. Due to the coarseness and high strength of Lawrence-pur sand, it has commonly been used in KP province. Coarse aggregates were collected from Margalla in the form of natural crushed stones. EVA was obtained in the form of fine-grained powder from crushed waste of shoe soles as shown in Fig.1, and was tested for its properties in the PCSIR laboratory, Peshawar, Pakistan. Physical properties and Vicat softening temperature of EVA are tabulated in Tab.1 and Tab.2, respectively. Tap water was used in the preparation of mixes; before employing it in the concrete it was tested for salts and other impurities. Fine aggregate and coarse aggregate were tested for specific gravity, gradation and unit weight and voids in compliance with ASTM C 127-15 specification [44], ASTM C 136 specification [45] and ASTM C 29 specification [46], respectively. Three samples of concrete cylinders were cast for each dosage of EVA (0%, 4%, 8%, 10%, 12%, 16%, and 20%, respectively) and an average of the three samples was taken for analysis. The detail of the mix design is given in the Tab.3. After casting concrete cylinders, the samples which were to be normally cured, were de-molded after 24 hours, however, the samples which were to be rapidly cured were de-molded after 27.5 h ± 15 min. The mix design and preparation of specimens is shown in the Fig.2.

3.2 Curing

The strength of concrete is the result of a hydration reaction between the constituents of cement. Hydration of cementitious particles is not transitory but creates long-lasting bonding. The beginning of the hydration reaction indicates formation of the gel pattern of calcium silicate hydrate (C-S-H) which depends on the degree of the hydration process. Therefore, curing becomes vital in the initial days after concreting, helping the early age hydration process. In this study, two different ways of curing (i.e., normal and accelerated) were used in order to predict the later age compressive strength using regression models.

3.2.1 Normal curing

In this study, normal curing refers to the curing of concrete specimens carried out under normal weather conditions by maintaining a moist environment around concrete specimens at a temperature of (23 ± 2) °C. Also, the term ‘normal curing’ is used in order to differentiate it from accelerated curing. The compressive strength of normally cured concrete cylinder specimens was determined at the age of 3, 7, 28, 56, 90, 120, 210, and 365 d in accordance with the specifications of ASTM C192 [44].

3.2.2 Accelerated curing

The accelerated curing of concrete cylinder specimens was performed in accordance with ASTM C 684-99 using a rapid curing tank at a temperature up to 98 °C for 3.5 h [45]. The specimens were then removed from the tank, dried on their surfaces, and tested in a compression testing machine at a loading rate of 2.5 kN/s.

3.2.2.1 Design of curing tank

A rapid curing tank was designed and manufactured locally to fulfill the heating operation of the tank for the specified time period of (30 ± 5) min. The curing tank had the maximum capacity of 12-cylinder specimens at a time. Fig.3 describes the detailed design dimensions of the tank. The required electric power for heating operation of the tank was represented in terms of wattage and can be calculated as follows.

Total volume of the rapid curing tank = 472.5 liters.

Volume displaced by the concrete cylinders = 63.585 liters.

Volume of the free board (150 mm) = 118.125 liters.

Volume of heated water = 472.5 − 118.1 − 63.6 = 290.8 liters.

To calculate the electric power in terms of watt, the following basic equation was used:

(1)\mathrm{P}\mathrm{o}\mathrm{w}\mathrm{e}\mathrm{r}(\mathrm{w}\mathrm{a}\mathrm{t}\mathrm{t})=\frac{\mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}(\mathrm{l}\mathrm{b})\times \mathrm{S}\mathrm{p}\mathrm{e}\mathrm{c}\mathrm{i}\mathrm{f}\mathrm{i}\mathrm{c}\mathrm{h}\mathrm{e}\mathrm{a}\mathrm{t}(\mathrm{b}\mathrm{t}\mathrm{u}/{\mathrm{l}\mathrm{b}}^{\circ }\mathrm{F})\times \mathrm{c}\mathrm{h}\mathrm{a}\mathrm{n}\mathrm{g}\mathrm{e}\mathrm{i}\mathrm{n}\mathrm{t}\mathrm{e}\mathrm{m}\mathrm{p}\mathrm{e}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{u}\mathrm{r}\mathrm{e}({}^{\circ }\mathrm{F})}{3412\left({\displaystyle \frac{\mathrm{b}\mathrm{t}\mathrm{u}}{\mathrm{k}\mathrm{W}\mathrm{h}}}\right)\times \mathrm{t}\mathrm{i}\mathrm{m}\mathrm{e}\mathrm{o}\mathrm{f}\mathrm{h}\mathrm{e}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{n}\mathrm{g}\left(\mathrm{h}\right)}.

According to the characteristics of liquid, water boils at 212 °F (100 °C) while the initial temperature of water was 86 °F (30 °C), hence the difference between the two temperatures becomes 126 °F (52.2 °C) and this temperature can be achieved in (30 ± 5) min as specified in ASTM C684.

(2)\mathrm{P}\mathrm{o}\mathrm{w}\mathrm{e}\mathrm{r}=\frac{640\times 1\times 126}{3412\times 0.5}=47.27 \mathrm{k}\mathrm{W}.

From Eq. (2), 47.27 kW of electric power is required to boil water of the tank in (30 ± 5) min and it would require 16 screw plug immersion heaters having capacity of 3000 W with a supply of 240 ampere current for 4 h. However, such arrangement is not practically possible in any advanced laboratory. The feasible design of the curing tank was satisfactorily performed by providing the tank with sealed top lid for maintaining the pressure. Four screws plug immersion heaters (3000 W) were provided at each side, requiring a current supply of 55 ampere as shown in the Fig.4. The required current was 4.5 times smaller than the previous calculations which would boil the water of the tank in an hour.

Regulation of curing temperature for concrete specimens as stated in ASTM C 684 was established by installing the digitally immersed thermostat and analogue heating sensors in the rapid curing tank. The surplus pressure was released with the safety valve and steam was carried away by an outlet valve fitted in the rapid curing tank. The rapid curing tank was also provided with a digital control panel to sustain the tank temperature and to manage the screw plug immersion heaters, as depicted in Fig.5. Steel bars were also provided in the tank to support the concrete specimens.

After successful installation of the curing tank, concrete cylinders were placed for curing purpose. Curing of concrete specimens was carried out for 3.5 h ± 5 min at the boiling temperature of water. However, due to sealed top lid the temperature decreased to 205 °F (96 °C). Finally, at a specified curing period, cylinders were removed from the tank and placed in open environment for at least 1 hour to regain normal room temperature prior to testing as shown in Fig.6.

4 Results and discussion

The results are discussed in the following subsections.

4.1 Compressive strength results

The compressive strength tests were performed in accordance with ASTM C39 [46]. The compressive strength results are tabulated in Tab.4 and graphically shown in Fig.7. The specimens were tested in universal testing machine as shown in Fig.8. The compressive strength test was carried out for all specimens at the curing age of 3, 7, 28, 56, 90, 120, 210, and 365 d. The performance of strength gain by the EVA-modified concrete against time can be seen in Fig.7. It was observed that while increasing the dosage of EVA, the compressive strength of EVA-modified concrete was enhanced. However, the maximum strength was achieved at 16% EVA content and the strength development became almost constant beyond this percentage.

4.1.1 Discussion

The influence of EVA on the behavior of compressive strength is depicted in Fig.7. Compressive strength of the EVA-modified concrete increases with the addition of EVA up to 16%, however, the increase in compressive strength reduced at further addition of EVA content. One previous study [6] concluded that the compressive strength of concrete increases by incorporating up to 19% EVA. The enhancement in compressive strength was due to the water reduction capability of the EVA as it restricted the amount of water needed for concrete, so that cement hydrates form strong bonds with coarse aggregate. The bond formation between the cement hydrates after hydration of cement and the filling up effect of the polymer within the concrete are the two important factors in enhancing the compressive strength of PMC [24]. Additionally, during polymer modification in concrete, the particles of polymer accumulate at the surfaces of cement particles and form layers around the cement particles, inducing a water retention capability in the concrete. Such a type of concrete can withstand a longer dry curing phase, contributing to the higher strength of PMC relative to conventional concrete. In addition to film formation around the cement particles, polarity of the copolymers is increased by the development of numerous random copolymers and thus strong hydrogen bonds are formed due to the polar radical present at the surface of aggregates [23]. In polymer concrete, the cement hydration phase is followed by polymer film formation around cement particles that leads to the formation of fibrous and threaded structure of C-S-H. Such configuration of the hydration products affects density of the resultant concrete as a result of which the tensile strength of the polymer concrete is reduced. Therefore, to compensate for the low tensile strength, polymer concrete can be integrated with steel and FRP bars for construction [11]. In EVA modified concrete, cementitious products and coarse aggregate are more packed together which imparts four times superior compressive strength to the PMC in comparison to conventional concrete [10]. In the current study, compressive strength of the normally cured concrete incorporating 16% EVA content is increased almost 1.2 times as compared to conventional concrete. However, direct comparison of the former results cannot be concluded due to variation in the nature of experimentation.

4.2 Development of model for compressive strength prediction

Investigation of prediction of later age compressive strength of concrete blended with EVA was the prime objective of this research, introducing early age compressive strength as an input for the strength relationship models. Regression analysis was employed for the purpose of assigning together two variables of a linear equation based on the actual experimental data. The most commonly used method of fitting a regression line is least square method. This method was used to determine the best-fitting line of the collected data by minimizing the summation of the squares of the vertical deviations from each data point to the line.

Before trying to fit a linear model to the collected data, correlation between the variables of concern should first be evaluated. For this purpose, a scatter plot is appropriate to determine the level of correlation between the two variables of concern. If the scatter plot fails to show any rising or falling trend, there will be no correlation between the two variables. In such case, the experimental results would be not likely to give a very helpful model when a fit is applied to the regression model. The correlation between the two variables in a model can easily be understood from the value of coefficient of determination (R²). The value of R² lies in a range of 0 to 1, where 0 shows that there is no relation between the variables while 1 shows an indication of 100% relation between the variables, in which case the dependent and independent variables are correlated to each other in a simple linear equation. In the current study, 3.5 h accelerated curing strength was considered as an independent variable, whereas the targeted strength at a specified age was considered as dependent variable.

Compressive strength was predicted using the equations based on the trend line, derived from plotting the compressive strength data involving EVA dosage. The compressive strength results at the age of 28, 56, 90, 120, 210, and 365 d corresponding to 3.5 h early age strength is shown in Fig.9(a) to 9(h) and tabulated in Tab.5 and Tab.6.

Furthermore, a Kolmogorov-Smirnov (KS) goodness-of-fit test was performed for the collected data. It is a non-parametric test and was used to check how the data differs significantly from a reference value. From the results it was observed that the K-S test statistics value (D), in each case was lower than the critical value 0.521 for significance level of 0.05, which showed that the data was normally distributed.

4.3 Performance and validation

For predicting compressive strength at specified ages, it is essential to select the acceptable set of equations and to check their conformity. For each mix, the average compressive strength value at 3.5 h accelerated curing was taken as an input value for the equations, hence the required compressive strength values were concluded along with their particular percentage error, as tabulated in Tab.6. The predicted values were found to be reasonably close to the experimental values, signifying the compatibility of the regression equations for predicting the compressive strength.

Tab.6 and Fig.9 can be presented as follows.

1) The percentage difference in the actual strength values and predicted strength values in case of 3-d compressive strength test was about ±11% while in case of 7, 28, 56, 90, 120, 210, and 365 d, it was ±6%. Similarly, the coefficient of determination (R²) at the age of 3 d was found to be 0.89 while for all other ages, it was more than 0.93, which reflects good correlations. Hence, in the prediction of compressive strength from short term to long term period, the value of R² was found to be more than 0.94 in most of the cases, indicating the accuracy of predictions. In other words, it can be stated that with the prolongation of the curing period more accurate results are achieved. The values of R² confirmed the fact that the accelerated compressive strength at 3.5 h provided a good correlation with the compressive strength of normally cured specimens at each dosage of EVA for 3, 7, 28, 56, 120, 210, and 365 d.

2) Percentage error of the EVA-modified concrete specimens having 12% EVA content at 28 d and 20% EVA content at 56 d was quite noticeable as compared to normally cured specimens, which showed that the percentage error reduced with the curing age.

5 Conclusions

The main focus of this study was to investigate the development of regression models for compressive strength prediction at specific curing ages. The conclusions of this study are as follows.

1) The Kolmogorov-Smirnov (KS) goodness of fit test demonstrated that K-S test statistics value (D) in each case was lower than the critical value 0.521 for significance level of 0.05, which showed that the data was normally distributed.

2) The compressive strength of concrete blended with EVA increased at all the ages in comparison with conventional concrete. For instance, compressive strength of the blended mixes, that is E₄, E₈, E₁₂, E₁₆, and E₂₀, cured for 3 d increased by 8%, 13%, 19%, 25%, and 19%, respectively. Similarly, for the same blended mixes cured for 7 d, the compressive strengths increased by 5%, 7%, 12%, 15%, and 14%. Likewise, for the mixes cured for 28 d, the compressive strength increased by 9%, 13%, 16%, 23%, and 19% in comparison with conventional concrete.

3) The optimum dosage of EVA was obtained as 16% and further addition of EVA beyond this percentage showed very slow strength development.

4) The value of R² (co-efficient of determination) showed good relation between the compressive strengths of 3.5 h accelerated cured concrete and normally cured concrete for each percentage of EVA incorporated in the regression models. The regression models helped to relate the early age and later age strength at specified curing age. For compressive strength of EVA modified concrete, such models are practicable in prediction of future compressive strength at 28, 56, 90, 120, 210, and 365 d without waiting for later age test results.

5) The predicted compressive strength values were within a 6% difference for all the mixes, which indicates the practical application of the equations.

6) The prediction models suggest that the effect of high curing temperature is favorable in obtaining concrete strength of cementitious and polymeric materials. However, the authors further suggest applying additional models (such as artificial neural network, multiple regression and support vector mechanics) for predicting the strength of EVA-modified concrete.

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