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Polymers 2021, 13, 1885 13 of 23 Flexural strength or bending strength reduces with the increase of HDPE when incor- porated as coarse aggregate in concrete Figure 4c. Bakri et al. [17] noted that the flexural strength decreased from 17.56 MPa to 15.47 MPa with the increase of HDPE content up to 60%. The flexural strength has dropped by 6.1 MPa when the HDPE is added from 60% to 100% [17]. It has also been observed that the 28-days’ flexural strength of HDPE coarse aggregate concrete is dropped by 1.9 MPa (from 6 MPa to 4.1 MPa) when HDPE increases from 0% to 20% in 5% intervals [21]. A 2 MPa flexural strength drop is observed with the increase of HDPE from 0% to 8% [16,18,20]. Shanmugapriya and Santhi [12] observed the opposite trend as a 2.5 MPa flexural strength gain obtained at 28 days for HDPE-15 concrete. The same experiment has achieved the optimum 28-days’ flexural strength of 6.4 MPa at HDPE-15, followed by a drop of 3.1 MPa, when HDPE is increased by another 5% [12]. It is noted that the optimum 28-days’ flexural strength is attained in different HDPE percentages: for instance, 6.4 MPa at HDPE-15 (Shanmugapriya and Santhi) [12], 4.28 MPa at HDPE-8 (Philomina and D’Mello) [18] and 1.075 MPa at HDPE-10 (Lopez et al.) [20]. The density of HDPE coarse aggregate concrete is decreased with the increase of HDPE content, Table 3 [18,20,21]. It is observed that there was a 5.2% reduction in density with the increase of HDPE from 0% to 32% and a 6.1% reduction during 0% to 20% of HDPE addition [12,21]. The density of 1 ′ ′ HDPE coarse aggregate concrete matrix has reduced by 2 776.1 kg/m3 and by 675.72 kg/m3 in 3 ′ ′ HDPE coarse aggregate concrete matrix only with 4 the increase of HDPE from 0% to 30% [20]. This reveals that a greater density drop could be achieved when the aggregate size is reduced from 3′′ to 1′′ regardless of the HDPE 42 inclusion percentage. Another notable observation is that the rate of reduction of dry density was 0.2% per volume percent of waste polymer added in HDPE coarse aggregate concrete [23]. 3.4. Durability Characteristics of HDPE Coarse Aggregate Concrete Porosity and permeability of HDPE coarse aggregate concrete have increased with the increase of HDPE content [20]. The HDPE-0 standard concrete displayed a porosity of 22.67 and increased it up to 36.21 when HDPE was added to 30% [20]. The typical value of permeability of a pervious concrete ranges from 0.135 to 1.219 cm/s and all the HDPE coarse aggregate concretes display a higher permeability than the range specified [20]. Permeability of 1′′ HDPE coarse aggregate concrete has increased by 5.28 cm/s, when 2 the HDPE increases from 0% to 30% in 10% intervals [20]. Similarly, the permeability was increased by 4.03 cm/s, when 3 ′ ′ HDPE coarse aggregates are increased in the same 4 range [20]. It can be noted that the permeability of HDPE coarse aggregate concrete of this research study has exceeded the industry standards and varies between 0.135467 to 1.2192 cm/s [20]. Moreover, the porosity of HDPE coarse aggregate concrete has increased by 65% and 59.7%, with the increase of HDPE from 0% to 30% for 1 ′ ′ and 3 ′ ′ aggregate 24 sizes, respectively. An extensive study carried out on sorptivity of HDPE coarse aggregate concrete results in a drop of 45.4% in sorptivity with the increase of HDPE from 0% to 32% [18]. Water absorption rate of HDPE coarse aggregate concrete is increased by 34.7%, when the HDPE increase from 0% to 20% [22]. In another experiment, a similar gain, 35.5% in water absorption was observed with the increase of HDPE content to 8% [16]. However, Philomina and D’Mello [18] have observed a 6.13% drop (from 4.4% to 4.13%) in water absorption rate of HDPE coarse aggregate concrete when HDPE is added from 0% to 32% [18]. At direct UPV test, the pulse velocity of HDPE coarse aggregate concrete has increased from 4200 m/s to 4650 m/s, when HDPE is added from 0% to 16% [18]. After reaching the maximum velocity, 4650 m/s at HDPE-16, a drop of 560 m/s was recorded with the increase of HDPE from 16% to 32% [18]. The same experiment has performed an indirect UPV test for HDPE coarse aggregate concrete. Initially, the velocity is increased by 560 m/s, when HDPE increases from 0% to 8% [18]. Then, an 830 m/s velocity drop during 8% toPDF Image | Engineering Performance of Concrete Incorporated with Recycled HDPE
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