Effect of Mix Parameters on the Strength Performance of Waste Plastics Incorporated Concrete Mixes

Disposal of solidwastes has been amajor problem all over theworld.Out of all the different types of solidwastes, themajor challenge of disposal is posed by the ever increasing volumes of plastic wastes. While several methods are in practice, producing newer useful materials by recycling of such plastic wastes is, by far, the best method of their disposal. One such possible method is to use the waste plastics as an ingredient in the production of the concrete mixes in the construction industry. The present study aims to investigate the relative contributions of the various mix parameters to the mechanical properties of concrete mixes produced with waste plastics as partial replacement (10–30% by volume) to coarse aggregates. Initially, strength test results of a set of trial mixes, selected based on Taguchi’s design of experiments (DOE) method are obtained. A detailed analysis of the experimental results is carried out to study the effect of using waste plastics as a partial replacement to coarse aggregates on the strength parameters of these concrete mixes. It is found that all these trial mixes have performed satisfactorily in terms of workability in the fresh state and strength properties in their hardened state.


Introduction
In the urge of making a cleaner and sustainable environment, one important concern has been on disposing of the solid wastes. There have been many attempts to use different solid wastes as possible construction materials. The present study explores the use of waste plastics as a possible replacement to coarse aggregates in concrete mixes. The study aims at investigating the mechanical properties of concrete mixes with waste plastic aggregates incorporated in them as a partial replacement to their coarse aggregate fraction. This work is focused on developing mixes with as maximum replacement of waste plastic as possible but retaining a minimum compressive strength of grade M20.
In general, the inclusion of waste plastics into concrete has been found to decrease the strength performance characteristics of concrete. This decrease in performance is expected both in fresh and hardened properties. In fresh state, the workability of the concrete mixes produced with recycled plastic aggregates was found to be smaller as compared to the control mixes [1][2][3][4][5]. The hardened properties of concrete such as the compressive strength, tensile strength, flexural strength, and modulus of elasticity reduce with increased percentage of waste plastics [1,2,[5][6][7][8][9][10]. Though many works have been reported on the strength performance characteristics of waste plastics incorporated (as a partial replacement to both fine and coarse aggregates) concrete mixes, very few works evaluating the effect of mix parameters such as binder content and water/binder ratio along with plastic incorporation have been reported. Hence an attempt is being made to investigate the mechanical properties of such concrete mixes by considering these major control factors. Taguchi's design of experiments methodology is employed herein to analyse the test results.     Table 3. The results of sieve analyses for CGC, waste plastics, and their blends, all conforming to Grade II (graded aggregates) of IS 383-2007 [14], are presented in Table 4 and Figure 2. The various trial mixes tested herein are then designed based on the general guidelines of IS 10262-2009 [15].

Design of Experiments (DOE).
Strength properties of concrete mixes depend on variety of factors associated with quality and proportions of ingredients, production methods, curing regimes, and loading conditions. This is true whether natural coarse aggregates are used or they are replaced partially (or fully) by the waste plastics. An effort is made in the present work to explore the possibility of employing Taguchi's DOE technique to understand the strength performance of various waste plastics incorporated concrete mixes. Based on a general appreciation of issues connected with the strength   development in concrete mixes, the following parameters are initially identified as primary parameters affecting the strength characteristics of a waste plastics incorporated concrete mix: (a) binder content ( ); (b) water-binder ratio ( ); (c) percentage level of replacement of aggregates with waste plastic ( ).
Initially trials were done with different binder contents, / ratios, and plastic replacement levels. It is found that, as expected, mixes with increased plastic replacement had lesser strengths. It was also observed that, with 40% waste plastic replacement (by volume), compressive strengths lower than specified (M20 grade) were obtained. So, it was decided to replace only up to a maximum of 30% of the volume of the coarse aggregate with waste plastics. Details of specimens tested and test methods used are given in Table 5.
The following precautions were also taken during the study in order to compensate for loss in strength.
(a) Use of higher binder content with fly ash as mineral admixture is preferred. A constant fly ash percentage of 35% of total binder content was maintained in all the mixes tested herein. (b) Lower range of water to binder ratio ( / ) is selected.
In this study, / ratio is kept in the range of 0.32 to 0.36. Approximate dosages of a superplasticizer based on polycarboxylate ether (PCE) polymer were then used in the mixes, to attain required levels of workability.
The waste plastics with specific gravity around 0.94 are lighter than water and other ingredients in the concrete mix. During trial mixes, it was found that when the slump values of concrete mixes were higher than 100 mm, the plastic particles tend to come up and float on the surface when vibrated. Hence, it is decided to keep the slump range 50-75 mm for which segregation is limited and the vibration effort was kept at a minimum.
Mixes are designed so as to attain a minimum specified compressive strength of 20 MPa. With the three factors listed earlier taken at three representative levels (as given in Table 6), for the full factorial set of experiments, 3 3 = 27 different mixes are to be tested to investigate the effect of the three parameters on the strength performance of this class of mixes since number of test specimens will be large; alternatively, Taguchi DOE method is used to reduce the number of the tests to be performed. Thus, an L9orthogonal array of test matrix is taken into consideration which now consists of evaluating the strength performance by the experimental results obtained with just 9 different combinations [16]. The details of mix proportions of the nine waste plastic incorporated concrete mixes and the three control concrete mixes are given in Table 7.

Fresh Concrete Mixes.
The effect of inclusion of waste plastics on workability characteristics is investigated by keeping constant both the binder content and the / ratio but varying the percentage content of waste plastics. In general, it is found that the workability of the concrete mixes decreased on increased addition of plastics and hence increased water demand for the same level of workability (50 ± 10 mm slump) ( Figure 3). The average increase in water demand was about 1.5% for every 10% increase in waste plastic replacement to obtain similar range of workability. This decrease in workability (slump) is compensated by using appropriately increased dosage of a superplasticizer (SP). A polycarboxylate ether (PCE) based superplasticizer was used and the required dosage used in each of the mixes in the L-9 orthogonal array is given in Table 8.  The rupture surfaces of all test specimens after failure under compression testing were visually inspected for assessing the distribution of waste plastic particles. It is found that the plastic aggregates had been distributed uniformly all over the concrete matrix. The cubes failed in a normal trapezoidal type of failure pattern.

Tests on Hardened
By using Taguchi's DOE method, the response table for mean of means of 28 days of compressive strength of waste plastics incorporated concrete mixes is generated (Table 9). Figure 5 shows the main-effect plots of the three chosen mix parameters with respect to 28-day compressive strengths of the different concrete mixes. Also, relevant calculations   based on analysis of variances (ANOVA) for the series of the experiments conducted herein for 28-day compressive strengths are presented in Table 10. From both Taguchi's mean of means (highest rank) and ANOVA (highest dominant factor, ) methods, it is found that amongst the parameters considered herein, the mix parameter (waste plastics replacement %) is the most significant factor which determines the strength characteristics of these mixes followed by the parameters, binder content, and water-binder ratio, in that order.

Split Tensile and Flexural Strengths.
The split tensile and flexural strengths of waste plastics incorporated concrete mixes are also determined and are given in Figure 6. It is found that, as expected, these mixes possess lower flexural and split tensile strengths also, as compared to the corresponding mixes with CGC aggregates only. The reductions in flexural strengths are not as pronounced as that of split tensile strengths. The lower tensile strength is due to the loose bonding characteristics of waste plastic coarse aggregates with the cement paste. Weak transition zone in waste plastic coarse aggregates may also be the reason for these lowered strengths.
The overall reductions in various strengths were determined for each of the mixes. The summary of percentage reductions in compression, split, and flexural strengths of the mixes is given in Table 11. Strengths of each set of three mixes are compared with the corresponding control mix. For example, strengths of first set of 03 mixes, that is, MIX 111, MIX 122, and MIX 133, are compared with CMIX-1 as a reference. Similarly second and third sets of the mixes are compared with CMIX-2 and CMIX-3, respectively.

Modulus of Elasticity (E).
Experiments were conducted to obtain the modulus of elasticity ( ) for all the concrete mixes and it is observed that the values are lower compared to the respective control mixes (Figure 7). The reduction in values varied in a limited range of 1.7-8.7% of the values of the control mixes.

Regression Equations for Predicting Strength of the Mixes.
The results of the initial set of nine experiments specified by the L-9 array have suggested the relative importance of the An attempt was also made to study the relation between compressive and tensile strengths of the plastics incorporated mixes and an empirical formula has been obtained for the same (Figure 8).

Conclusions
An exploratory study on the strength characteristics of a class of waste plastics incorporated concrete mixes has been reported. Based on the experimental results obtained in this  study, it can be said that waste plastics can be used as partial replacement to coarse aggregates, in preparation of structural grade concrete mixes (say M20-M25).
(i) It is found that the workability of the concrete mixes decreased on increased addition of waste plastics. The average increase in water demand so as to obtain similar range of workability was about 1.5% for every 10% increase in waste plastic replacement.
(ii) It is found that the strengths of the waste plastics incorporated mixes are always lower than mixes with only natural coarse aggregates. The reduction in strengths increases with increase in the percentage of waste plastics replacement. Mixes with a maximum replacement of up to 30% by volume of the coarse aggregates are found to be satisfactory.
(iii) Of the three factors considered to be influencing the strength performances of waste plastic incorporated  mixes, replacement level of plastics is shown to be the most dominant factor, followed by the binder content and / ratio in that order.
(iv) The waste plastics incorporated concrete mixes are also found to possess lower flexural and split tensile strengths and so also lower modulus of elasticity than the corresponding mixes with natural CRC aggregates only.
(v) Concrete mixes with flexural strengths greater than 4.5 MPa and above are considered satisfactory for concrete pavements for rural roads as per IRC-58 (2002) [17]. In the present study, all the mixes qualify this flexural strength criterion. Hence, it can be concluded that the waste plastic incorporated concrete mixes could be used for concrete pavements for rural roads.
(vi) Multilinear regression equations, with satisfactory predictive capabilities, have been derived for each of the strength parameters, based on DOE approach ( 2 = 0.85-0.99).