Workability of fresh concrete using HDPE, LDPE, PP, and PET fibers was represented by subsidence values, and the results were shown in Figure 3
. The subsidence values were found to have decreased with increasing volume fraction of plastic fibers [13
]. Movement of fresh concrete/mortar, which is primarily the cause of workability or slump, is restricted by the presence of recycled plastic fibers. The occurrence of complex network structure during the concrete processing is presented in Figure 4
a. In addition, increasing the surface area of plastic fibers at high volume fractions can improve the cement paste absorption and the viscosity of fiber reinforced concrete [14
]. The low subsidence value refers to the low workability of fresh concrete, which can cause segregation in building structures. Fresh concrete dissociation brings about fracture on hardened concrete, and consequently losing concrete strength. To prevent segregation of plastic fibers (see Figure 4
b) in the reinforced concrete composites, two recommendations were proposed [13
]: (1) Increasing the amount of water used in the mix and (2) adding water reducing agents. Increasing water to cement (w/c) ratio was appropriate for this study in order not to cause chemical interaction with the non-ionic surfactant. That w/c ratio was gradually increased from 15% volume fraction so as to help reduce honeycomb concrete [50
]. From Figure 3
, the HDPE and PP fibers at various volume fractions provided lowered subsidence values in comparison with that of the mortar (Figure 4
c). In the meantime, the LDPE and PET fibers offered lower subsidence values than that of the mortar, when the volume fraction was 20% and 30%. Relatively low subsidence values of 1–6 cm were obtained when the HDPE fiber was utilized in the concrete. According to these results, PET was observed to be more rigid than other plastics, so addition of PET fibers to the concrete was likely to agglomerate and form fiber balls [51
]. PET fiber balls (even though they are hydrophobic) can adsorb water within voids during concrete processing. When produced concrete was put into the slump cone and tamped, water retained in PET balls could be released. Finally, increasing the amount of water in the mixture would increase subsidence value.
3.2.3. Thermal Conductivity
Thermal conductivity is inversely proportional to thermal resistance. Thermal conductivity depends on the material property, surface area, thickness, and temperature gradient in steady state heat transfer condition [18
]. According to Fourier’s law [54
] Equation (1), thermal conductivity is the term for material characteristic which can transfer heat from higher to lower temperature sides. According to Klein [29
], thermal conductivities of HDPE, LDPE, and PP were found to be about 0.43, 0.35, and 0.23 W/(m·K) at 25 °C, while thermal conductivities of ordinary concretes with various aggregates were normally about 1.34–2.92 W/(m·K), three to thirteen times higher [30
]. Similarly, Tae Sup Yun et al [31
] found that the thermal conductivities of mortar and concrete were about 2.0 W/(m·K). Plastic fibers are expected to help reducing thermal conductivities of concrete due to its property.
The thermal conductivity (k) of the produced concrete was calculated from the experiments, and the results were shown in Figure 6
. The thermal conductivities of concrete mixed with recycled HDPE, LDPE, PP, and PET were about 0.74–0.96, 0.72–0.86, 0.84–0.94, 0.95–1.02 W/(m·K) on average at 25 °C, respectively, whereas the thermal resistances of fiber reinforced concrete in Equation (2) were found to be about 0.08–0.11, 0.09–0.11, 0.08–0.09, and 0.07–0.08 m2
·K/W, respectively. Apparently, the thermal conductivity values of produced concrete from all plastic fibers decreased with increasing volume fractions of plastic fibers, and their thermal conductivities were lower than that of the mortar by about 2–31%. This is concordant with the work of Fraternali et al [7
] who studied recycled PET fiber and virgin PP at 1% in concrete and the thermal conductivities were found to have reduced when comparing to the thermal conductivities in plain concrete. Not only is synthetic fiber property affecting the thermal conductivity of fiber-reinforced concrete but small permeable void [32
] can also restrain heat transfer as well. According to Figure 5
, the more volume fractions of synthetic fibers are mixed in concrete, the more permeable voids of fiber-reinforced concrete were found [16
]. The results demonstrated that the plastic fibers of HDPE, LDPE, PP, and PET can decrease the thermal inducing property or heat transfer of produced concrete. The lowest thermal conductivity of 0.7–0.9 W/(m·K) was found in the produced concrete from LDPE fiber, which had higher porosity than the control. The thermal property improvement of produced concrete was in the following order; LDPE > HDPE > PP > PET. Furthermore, the thermal resistance values (R) of produced concrete were calculated, the maximal value of 0.1 m2
·K/W was recorded in the produced concrete at 30% volume fraction of LDPE fiber, which was verified to be a good thermal insulator. Therefore, the produced concrete from LDPE fiber has an energy-saving potential when it is applied as a green building material. In fact, one of the green building requirements promoted in Thailand is low energy consumption, therefore if this composite material is used in concrete precast wall or non-load-bearing structure, it can insulate heat transfer from side to side better than the ordinary mortar and help to reduce energy consumption from room temperature adjustment.
3.2.4. Tensile and Compressive Strengths
(1) Splitting Tensile Strength
Normally, tensile strength of concrete is low when compared with its compressive strength, as a result, high strength materials, such as, steel, plastic, etc., are introduced into the mixture to improve the brittle property of the cementitious composites. According to Hasan et al. [13
], adding 0.33–0.51% by volume of macro PP synthetic fibers in concrete could enhance about 10–15% of the splitting tensile strength. Other investigators had also found similar results [11
]. Choi and Yuan [11
] reported that adding 1–1.5% by volume of PP fibers in concrete increased the splitting tensile strength of concrete by 50%, and Hsie, Tu, and Song [14
] increased the splitting tensile strength (9–13%) of concrete by mixing 3.6–9.6 kg/m3
of PP hybrid fibers with concrete.
Ductile fibers, when added into cementitious composites, can enhance its tensile strength depending on several factors such as fiber toughness, fiber volume fraction, alignment, and bonding between the fibers and the cementitious matrix. In this study, the optimum volume fractions of recycled HDPE, LDPE, and PET plastic to achieve the highest splitting tensile strength of the fiber-reinforced concrete composites were found to be 10%, 10%, and 30%, respectively. Lower and higher volume fraction than these optimum amounts resulted a lower splitting tensile strength. For the case of PP recycled plastic, optimum splitting tensile strength of the composites was found to be at 5% volume fraction, thereafter, at higher volume fractions the tensile strengths of the composites reduced (see Figure 7
). In general, all the mixings in this study with different fiber volume fractions showed lower splitting tensile strengths than that of plain mortar (about 3 MPa). The strength reduction varied from 4 to 80% depending on type of fiber and volume fraction. Similar findings were reported by several investigators [11
] that increasing fiber volume fraction could cause weak bonding between plastic fiber and cementitious matrix, thus reducing its effectiveness in strengthening the concrete composites. Besides, the splitting tensile strength reduction (when mixed with 25–75% fiber volume fraction) was reported to be lower by 41% when compared with the tensile strength of normal concrete [11
]. As mentioned earlier, the lower strength might be attributed to the high volume fractions of plastic fibers used in this study. Larger volume fraction often leads to clumping or balling of fibers [51
], making them less effective in strengthening the concrete composites. It should be noted that one of the objectives of this study is to explore the potential utilization of recycled plastics in concrete as alternative to typical disposal in landfill. The use of large quantity of recycled plastic fibers is therefore a goal of this investigation, if the final fiber-reinforced cementitious composites can be used as construction materials, such as wall panel, etc.
(2) Compressive Strength
Concrete is commonly known for its high compressive strength with a rather low tensile strength, typically about one-tenth of its compressive strength. The addition of low fiber volume content (typically less than 1% for plastic fibers such as PP) into concrete often does not affect the compressive strength of concrete [7
]. In some circumstances, the presence of fibers can enhance some property of concrete composites [7
], for instance, tensile strength, micro crack reduction, crack opening reduction, and so on. However, the large volume fraction of fibers often lowers the compressive strength of concrete as the hydrophobic property of plastics is likely to increase air void [16
] in the cementitious composites. Figure 8
shows the compressive strength of concrete produced with the addition of HDPE, LDPE, PP, and PET recycled plastic fibers. The volume fraction of fibers used in this study varies from 5 to 50%. It was found that the compressive strength values of all the mixings, regardless of the fiber type and fiber volume fraction, were likely to be declined with increase in plastic contents. With a 5% fiber volume fraction, mixing with HDPE, LDPE, and PP recycled fibers showed a strength reduction of −36%, −8%, and −23%, respectively, when compared to mortar. The strength reduction escalated to −79%, −66%, and −60% when fiber volume fraction increased to 20%. However, concrete with PET recycled plastics showed a better performance with lesser strength reduction, ranging from only −38% to −17% when the fiber volume fraction increased from 10% to 50%. Similar results of strength reduction were also reported by other investigators [20
] when percentage of fiber volume fractions used in the concrete was increased.
Typical compressive strength of concrete used in the USA construction industry varies from 17 MPa for residential buildings to 28 MPa for commercial buildings [59
]. As stated, one of the objectives of this study was to investigate the potential utilization of recycled plastics of materials for concrete wall panel, which must adhere to the TIS 2226-2548 Standard for precast concrete wall panel. It is required that all concrete used for precast wall panels (non-structural load carrying) must have a minimum compressive strength of 16 MPa [60
]. In this study, only concrete mixing with HDPE and LDPE recycled plastics with 5% fiber volume fraction and PP with 10% fiber volume fraction produced sufficient compressive strength to be used for precast wall panel application. As for PET recycled plastic, all mixings (with 10% to 50% fiber volume fraction) tested in this study met the minimum compressive strength requirement for precast wall panel application. Clearly, this study shows that recycled plastics, when used in fiber-reinforced concrete, have a good potential to be used as construction materials for many non-structural load carrying members in the building system, thus paving the way for a new type of environmental friendly renewable construction materials [25
From all experiments and characterization, HDPE, LDPE, and PP have the potential to be used as additional ingredients to hinder concrete’s thermal conductivity and can be used for manufacturing of high insulation material. However, the property was found to be more effective in the produced concrete from LDPE fiber, and to a lesser degree from HDPE and PP fibers. Though PET fiber was found unsuitable for thermal resistance property in concrete because of its inability to show clear improvement over regular mortar, PET fiber compressive strength and splitting tensile strength were higher than other fibers, which was the advantage for utilizing PET waste in green building.