Mechanical properties of concrete using different types of recycled plastic as an aggregate replacement

Abstract

This study examined the quality of concrete using polyethylene terephthalate (PET) and polypropylene pipe (PEP) as aggregate replacements. In concrete mixtures, PET and PEP replaced up to 15% of the aggregates. Several laboratory tests were conducted to assess the effects of these replacements on the quality of concrete mixtures. Workability, unit weight, modulus of elasticity, compressive strength, indirect tensile strength (splitting), and flexural strength were tested. This study reveals that the two types of plastic waste materials can be successfully used as partial replacements for sand or coarse aggregates in concrete mixtures. PEP reduces workability, whereas PET enhances this material property. A lighter concrete density, up to 10% less than that of the control mixture, was obtained. The use of recycled plastic reduced the compressive strength, tensile strength, and flexural strength by up to 31%, 22%, and 60%, compared with normal concrete, respectively. A finite element model was used to simulate the flexural beam test using the modulus of elasticity obtained from the experimental tests to examine the behavior of normal concrete, PET, and PEP concrete. Dedicated results were obtained using FEs for the failure mode and flexural capacity.

Keywords:

• shredded plastic
• green concrete
• recycled PET
• recycled polypropylene
• recycled aggregate concrete

1. Introduction

The annual generation of billions of tons of plastic waste affects the environment worldwide. Therefore, reviews and experimental studies have focused on the use of these materials in building construction (M. Batayneh et al., 2007; Rabar, Hunar, et al., 2019). Recycling rubber and plastic waste significantly affects pollution, CO₂ emission mitigation, and the reduction of raw materials (J. J. Li et al., 2016; Torgal et al., 2012). The accumulated non-putrefaction materials, such as rubber and plastic, can significantly perturb the environment (Hsie et al., 2008). The utilization of recycled materials such as rubber, plastics, and even bagasse ash in the production of concrete is one of the newly suggested strategies for reducing the effect of waste materials on the environment (Chen et al., 2013; Iftikhar et al., 2023; Javed et al., 2021). Several types of waste, such as glass, rubber, recycled building materials, and plastics, have been used as filling materials for the production of green concrete (Asghar et al., 2023; Babatunde et al., 2022; Colangelo et al., 2016; Lucolano et al., 2013). The major components of solid waste are polymeric plastics and rubber, which are mainly produced from polystyrene (PS), polyethylene terephthalate (PET), polypropylene (PP), high-density polyethylene (HDPE), polyethylene (PE), and polyolefin (PO) (Adibi et al., 2020; X. Li et al., 2020; Rabar, Aryan, & Ako, 2019; Rabar, Aryan, Lamyaa, et al., 2021; Radhi et al., 2022).

In this study, the effects of recycled shredded polypropylene pipe (PEP) and shredded polyethylene terephthalate (PET) materials on the mechanical properties of concrete were investigated. The concrete workability, compressive strength, tensile strength (splitting), modulus of elasticity, and flexural strength (modulus of rupture) were investigated using standard concrete, PEP, and PET. This work was extended to examine the behavior of flexural strength using the finite element method.

1.1. PET as an aggregate replacement

The adoption of polyethylene terephthalate (PET) bottles is increasing in modern lifestyles. Therefore, the quantity of polymeric waste produced from these bottles has increased rapidly. Consequently, studies have been conducted to investigate the advantages of using these materials as a partial replacement for aggregates in concrete, as an alternative to disposal in landfills.

Frigione (2010) studied the effect of partially substituting fine aggregates in a concrete mixture with 5 wt% unwashed PET particles. This study examines the effects of this substitution on the workability, split tensile strength, and compressive strength of concrete. The sand and PET particles used in this study ranged in size from 300 μm to 2.36 mm. The findings revealed a slight decrease in the compressive strength and split tensile strength (between 1.6% and 2.4%), whereas the workability of concrete samples containing waste PET remained largely unaffected.

Mohammed (2017) examined the behavior of PET waste on reinforced concrete beams. The beams were made to fail in flexure and have minimal steel-rebar reinforcement. According to the test data, employing shredded PET waste particles instead of sand in concrete led to a 12–21% decrease in the compressive strength. However, a slight reduction in maximum load capacity was observed. This form of recycled reinforced concrete beam for structural purposes can be produced safely with up to 15% well-graded PET waste (Mohammed, 2017).

Almeshal et al. (2020) conducted a study to assess the effects of incorporating shredded PET waste (0.075–4 mm) as a partial replacement (10–50% by volume) for sand in concrete. The investigation focused on the unit weight, workability, and compressive strength of the concrete. The results indicated that increasing the proportion of PET significantly affected the workability of the mixture, as demonstrated by the slump values. Concrete specimens containing 50% PET exhibited a slump of 10 mm, whereas the control specimens exhibited a slump of 90 mm. The decrease in workability was attributed to PET’s lower density of PET compared to that of regular sand. Furthermore, the compressive strength of the concrete decreased by 31% and 60% compared to that of the control samples as the percentage of PET in the mixture increased to 40% and 50%, respectively (Almeshal et al., 2020).

Dawood et al. (2021) conducted a study to investigate the physical and mechanical properties of concrete that incorporated shredded PET waste as a partial replacement for fine aggregates, ranging from 5% to 20% by volume. Although the PET fibers and fine aggregates measured less than 4.75 mm in size, they exhibited a wide range of gradations (ranging from 0–4.47 mm), with most PET fibers falling between 2.36 mm and 1.18 mm. In contrast, sand had a uniform distribution. They observed that as the amount of shredded PET waste in the concrete specimens increased, the workability of the concrete decreased. Moreover, the compressive strength increased when the percentage of PET replacement ranged from 0% to 15%, with the highest gain of 7.5% appearing in concrete containing 12% PET compared to the control. However, beyond this point, the compressive strength decreased (Dawood et al., 2021).

The performance of concrete incorporating RPET waste in granular form as a replacement for fine aggregates was studied by Kangavar et al. (2022). Various concrete specimens were prepared using PET granules as a partial replacement for fine aggregates in grade 32 concrete mixtures with replacement percentages of 0%, 10%, 30%, and 50% by volume. Key parameters such as compressive strength, density, elastic modulus, flexural strength, and tensile strength were assessed. The experimental results indicated that replacing fine particles with 10% RPET granules improved the concrete properties. Furthermore, the findings demonstrated that concrete containing RPET granules exhibits increased ductility (Kangavar et al., 2022).

1.2. PP as an aggregate replacement

The number of studies using PP as an aggregate replacement is relatively scarce compared to those using PE. Most of these studies used additives such as silica fume and fly ash with PP, whereas others focused on self-compacting concrete, lightweight concrete, and polypropylene fibers. To the best of our knowledge, no research has been conducted using waste polypropylene pipes (PEP) as an aggregate replacement.

Li et al. (2016) studied high-performance polypropylene (HPP) fiber lightweight aggregate concrete (LWC). This study focused on various mechanical properties, including the compressive strength, splitting tensile strength, flexural strength, flexural toughness, and impact resistance. The experimental results indicate that the mechanical characteristics of LWC can be significantly enhanced by incorporating HPP fibers. While the compressive strength remained unaffected, the presence of HPP fibers led to improvements in the flexural strength, splitting tensile strength, flexural toughness, and impact resistance. Additionally, the inclusion of an appropriate quantity of HPP fibers significantly improves the post-cracking behavior of LWC (Li et al., 2019).

The mechanical, fracture, and durability properties of self-compacting high-strength concrete (SCHSC) incorporating recycled polypropylene plastic particles (RPPP) with and without silica fume (SF) were examined by Rabar et al. 2019. The test results revealed that while RPPP significantly decreased the other measured parameters of the SCHSCs, it significantly enhanced the fracture and ductility attributes. However, the use of SF significantly improves the mechanical properties and durability. The investigations showed that with RPPP content up to a 40% replacement level by total medium aggregate volume and 10% SF, it is possible to generate SCHSC with a compressive strength greater than 70 MPa at 90 d. The work extended by Rabar et al. (2021) examined the workability and rheological properties of (SCHSC) produced using fly ash, silica fume, and recycled (RPPP). Mixes containing FA, SF, and RPPP may have superior fresh properties compared with those without SF. Additionally, all the developed SCHSC mixes satisfied the restrictions required for SCC’s fresh properties (Rabar, Hunar, et al., 2019 and Rabar et al., 2021).

Islam (2022) studied plastic aggregates, specifically polypropylene (PP) and polyethylene terephthalate (PET), as substitutes for coarse aggregates in concrete, and replacement levels of 10%, 20%, and 30% by volume were tested. The preparation processes for both types of plastic aggregates were similar. Waste plastics were initially subjected to a prewashing step, followed by shredding to reduce them to smaller particles. These smaller plastic particles were melted in an oven and poured into molds for cooling. After cooling, the samples were crushed in a crushing machine. The study examined three different water-cement ratios (0.42, 0.48, and 0.57) and compared various properties, such as workability, density, compressive strength, and tensile strength. When using a polypropylene (PP) aggregate in concrete, it was found that the compressive strength increased by up to 39% compared with that of the reference concrete. The study also showed that the density of the PP-aggregate concrete was 9% lower than that of the reference concrete. Moreover, the inclusion of polyethylene terephthalate (PET) aggregates led to a significant reduction of up to 53% in the compressive strength compared to the reference concrete. However, the PET aggregate concrete exhibited improved workability and lower density. Considering the significant increase in compressive strength, up to 10% of PP aggregate can be incorporated into the concrete mixture (Islam, 2022).

1.2.1. Significance of the present work

• Most studies used powder plastic materials rather than irregular particles, such as those used in the present work. The advantages of using shredded plastics collected from local industrial areas of Mosul City with a small machine is that it requires less energy than that required to produce small granular plastics.

• Most previous works used SCC with additives (such as fly ash and silica fume) and high-strength or lightweight concrete. However, the present study aimed to investigate regular and traditional concrete mixtures with a normal concrete density without additives.

• The applicability of shredded plastic materials with local aggregates available in Mosul City is one of the main research targets. Introducing polypropylene pipe waste-shredded particles is another new approach that has been investigated.

• The applicability of finite element simulation with the aid of data obtained from the experiment (modulus of elasticity and modulus of rupture) to simulate concrete with recycled plastic is also one of the main targets of the present work.

2. Experimental work

This study examined the influence of using two types of shredded plastic particles in standard-strength concrete on its mechanical and fracture properties, as shown in Chart 1. The study was limited to local materials available in Mosul City, Iraq, which has moderate weather conditions. Eight plastic concrete mixtures were used in the experimental study to evaluate the impact of shredding plastics on the mechanical properties of concrete. Plastic particles made from shredded polyethylene terephthalate (PET) and polypropylene pipes (PEP) have been substituted for normal fine and coarse aggregates (Bach et al., 2012). The fine and coarse aggregates were replaced with weights of 2.5%, 5%, 10%, and 15%. Therefore, 63 concrete cylinders with a diameter of 150 mm and height of 300 mm, in addition to 18 concrete beams with dimensions of 100 × 100 × 600 mm, were used to conduct the study. All material preparations, concrete casting, curing, and tests were conducted at the Materials Testing Laboratory, Department of Civil Engineering, College of Engineering, University of Mosul.

Chart 1 Working processes flowchart.

2.1. Materials

2.1.1. Shredded Plastics

As shown in Figure 1 and Table 1, PET was shredded into thin sheets, whereas PEP was chopped into pieces ranging from approximately 3 mm to 10 mm. These materials were collected from an industrial area in Mosul City.

Figure 1. Shredded plastic, PET, and PEP materials.

Table 1. Physical Properties of RPET and RPEP plastics

Type	Color	Shape	Density (gm/cm3)	Aspect ratio (L/d)	Width (mm)	Thickness (mm)
PET	Transparent	Rectangular	1.35	2 to 3	3–5	0.3
PEP	Black	Irregular	0.96	1 to 3	3 to 10	1 to 2

2.1.2. Normal Aggregate

Rounded river natural aggregate that complies with the Iraqi standard Iraqi Specification IQS. 45/(1984, 2016), as shown in Figure 2, was used as the fine and coarse aggregate for the mixes, as shown in Table 2. Tables 3 and 4 display the results of the sieve analyses of the coarse and fine aggregates (sand). The sieve analysis results matched Iraqi Specifications, IQS. 5/(1984) (2016), as shown in Figure 3, with a nominal maximum aggregate size of 14 mm.

Figure 2. Fine and coarse aggregates. (a) Fine aggregate (Sand). (b) Coarse aggregate (Gravel).

Figure 3. Sieve analysis of coarse and fine aggregates.

Table 2. Concrete mixing proportion, compression, and splitting strength

No.	Specimen Type	Cement	Sand	Gravel	Water	% Replacement of Sand	% Replacement Coarse Aggregate
1	Control	1	2.35	3.40	0.40	0	0
2	2.5 % PET	1	2.29	3.40	0.40	2.5	-
3	5% PET	1	2.23	3.40	0.40	5.0	`-
4	10% PET	1	2.12	3.40	0.40	1.0	-
5	15%PET	1	2.00	3.40	0.40	15.0	-
6	2.5 % PEP	1	2.35	3.32	0.40	-	2.5
7	5% PEP	1	2.35	3.23	0.40	-	5
8	10% PEP	1	2.35	3.06	0.40	-	1.0
9	15%PEP	1	2.35	2.89	0.40	-	15

Table 3. Classification of coarse aggregate (gravel)

Sieve Size (mm)	Passing %	Passing% Limits (Iraqi Specification IQS. 45/1984, 2016)
20	100	100
14	94	90–100
10	61	50–85
5	4	0–10

Table 4. Classification of fine aggregate (sand)

Sieve Size (mm)	Passing %	Passing% Zone 2 Limits (IQS:45/2016)
4.75	100	90–100
2.36	81	75–100
1.18	69	55–90
0.6	43	35–59
0.3	16	8–30
0.15	5	0–10
Specific Gravity	2.64
Fineness Modulus	2.86
Specific Gravity	2.68

2.1.3. Cement

Ordinary local cement produced by the Badosh Factory in Mosul was used. The cement sample was tested and matched with Iraqi Specification IQS. 45/(1984) (2016), as shown in Table 5.

Table 5. Physical and chemical properties of the cement

Physical Characteristics	Units	Value	Limitations
Standard Consistency w/c	-	0.285	—-
Initial setting	Minutes	105	≥45
Final setting	Minutes	240	≤600
Compressive strength (3 d)	MPa	21.2	≥15
Compressive strength (7 d)	MPa	29.8	≥23
Fineness (sieve no. 170)	%	5	≤10
Chemical Components	Units	Value	Limitations
SiO2	%	20.26	-
AL2O3	%	4.8	-
Fe2O3	%	3.5	-
CaO	%	60.41	-
MgO	%	2.95	≤5
SO3	%	2.32	≤2.5
Free Lime	%	1.23	-
Loss on ignition	%	2.94	≤4
Insoluble residue	%	0.7	≤1.5
Total	%	99.72	-
C3S	%	39.65	-
C2S	%	29.61	-
C3A	%	6.79	-
C4AF	%	10.68	-
L.S.F.	%	88.99	-
Solid Solution	%	16.06	-

2.1.4. Mixing and Curing Water

Drinking water was used to prepare the concrete mixtures (ASTM C1602/C1602M–06, 2006) and cure the concrete cylinders.

2.2. Preparing of samples and curing

The concrete mix was prepared by mixing the aggregate in the SSD condition with cement and then gradually adding water. The plastics were added later by spreading them by hand to the mixer to prevent coagulation of plastics. The concrete was poured into molds, as shown in Figure 4(a,b, and c). After 24 hours, the molds were removed, and all samples were cured in a basin submerged in water for up to 28 d at ambient temperature (25 °), as shown in Figure 4d (ASTM C192/C192M°02, 2002). The final shape of the concrete indicated that the PET and PEP plastics were well distributed in the concrete, as shown in Figure 4 (e) and (f).

Figure 4. Beams and cylinders molds and fresh concrete casting and curing.

2.3. Fresh concrete properties: slump test

2.3.1. Slump Test

An average value of two slump test samples was recorded for each group (ASTM C143/C143M–03 and Neville, 2003). The results of the slump tests are listed in Table 6 and shown in Figure 5.

Figure 5. Slump samples for concrete with and without plastics.

Table 6. Concrete slump and density results

No.	Group	Slump 1 mm	Slump 2 mm	Average Slump mm	Diff. %	Sample 1	Sample 2	Av, Density kg/m3	Density diff. %
1	Control	52	68	60	0.0	2340	2356	2348	0.0
2	2.5 % PET	64	62	63	5.0	2259	2286	2273	−3.2
3	5% PET	66	68	67	12.0	2198	2234	2216	−5.6
4	10% PET	71	72	71	19.0	2152	2168	2160	−8.0
5	15%PET	74	82	78	30.0	2180	2064	2122	−9.6
6	2.5 % PEP	55	56	56	−7.0	2313	2270	2292	−2.4
7	5% PEP	55	54	55	−9.0	2186	2359	2273	−3.2
8	10% PEP	53	52	53	−12.0	2297	2173	2235	−4.8
9	15%PEP	48	49	49	−19.0	2106	2252	2179	−7.2

2.3.2. Concrete Density

The concrete density of each group was measured according to the ASTM C29/C29M–97 specifications (ASTM C29/C29M–97, 1997). The results for the concrete density are listed in Table 6.

2.4. Hardened concrete mechanical properties

2.4.1. Concrete Compressive Strength

Three cylinders were used to test the compressive strength of the concrete for each group according to (ASTM C39/C39M–03, 2003). A compression machine with a capacity of 10 tons was used to test each cylinder, as shown in Figure 6a. The failure load (Pc) was recorded for each cylinder, and the compressive strength was estimated according to Equation (1)(1) $\mathit{\sigma }=\mathit{Pc}/\mathit{Ac}$(1) . The results are listed in Table 7 and shown in Figure 7a (Neville, 2004).

Figure 6. Cylinders’ compressive and splitting strength testing.

Figure 7. Cylinders’ compressive and splitting strength testing results.

Table 7. Compressive and splitting strength of the testing groups

No.	Group	C1	C2	C3	Av. compressive strength MPa	% diff.	T1	T2	Av. tensile splitting MPa	% diff.
1	Control	30.1	30.6	31.2	3.6	0.0	2.80	2.79	2.80	0.0
2	2.5 % PET	28.9	28.7	27.9	28.5	−7.0	2.68	2.70	2.69	−3.6
3	5% PET	26.4	26.4	24.9	25.9	−15.5	2.52	2.52	2.52	−9.8
4	10% PET	21.2	21.0	19.7	2.6	−32.7	2.33	2.34	2.34	−16.4
5	15%PET	21.1	21.1	21.7	21.3	−30.4	2.26	2.24	2.25	−19.5
6	2.5 % PEP	29.1	28.4	30.2	29.2	−4.6	2.55	2.60	2.57	−7.9
7	5% PEP	23.6	23.0	23.0	23.2	−24.3	2.38	2.38	2.38	−14.9
8	10% PEP	24.3	24.2	21.9	23.5	−23.4	2.23	2.25	2.24	−20.0
9	15%PEP	21.2	20.9	20.7	2.9	−31.7	2.17	2.15	2.16	−22.7

(1) $\mathit{\sigma }=\mathit{Pc}/\mathit{Ac}$(1)

where Pc is the applied load (N), Ac is the cylindrical area (mm²), and (σ) is the compressive strength (MPa).

2.4.2. Concrete Cylinder Splitting Test

The splitting strength of concrete was tested using two cylinders for each group, according to ASTM C496/C496M–04 (2004). The cylinder was placed horizontally in the testing device once its diameter was specified. Figure 6b shows the maximum applied load recorded for the test device (Pt). Equation 2 was used to determine the splitting tensile strength (T) of the cylinder; the results are listed in Table 7 and shown in Figure 7b.

(2) $\mathit{T}=2\mathit{Pt}/(\mathit{\pi }.\mathit{L}.\mathit{D}.),$(2)

where L is the length (mm), D is the diameter (mm), Pt is the maximum applied load at failure (N), and T is the tensile splitting strength (MPa) (ASTM C496/C496M–04, 2004; Neville, 2004).

2.4.3. Modulus of Elasticity of Concrete

The modulus of elasticity of concrete with different percentages of plastics was calculated using two cylinders for each group, according to (ASTM C469–02, 2002). The test was conducted using a dial gauge connected to a computer in a universal testing machine, as shown in Figure 8, and the results are listed in Table 8.

Figure 8. Modulus of elasticity (Ec) test setting.

Table 8. Modulus of elasticity (Ec) and modulus of rupture (fr) of the testing groups

No.	Group	Ec1	Ec2	Av. Modulus of Elasticity (Ec) MPa	% diff.	fr1 MPa	fr2 MPa	Av. Modulus of Rupture fr MPa	Diff. %
1	Control	27416.5	27233	27600	0.0	5.1	4.8	5.0	0.0
2	2.5 % PET	24347.83	24000	24695.65	−10.5	3.0	3.4	3.2	−35.5
3	5% PET	23797.67	23762	23833.33	−13.6	2.7	3.0	2.8	−43.7
4	10% PET	22841.67	22350	23333.33	−15.5	1.9	2.1	2.0	−60.5
5	15%PET	27220.59	27500	26941.18	−2.4	1.8	2.0	1.9	−61.8
6	2.5 % PEP	23563.33	22460	24666.67	−10.6	2.7	3.0	2.8	−43.7
7	5% PEP	29327.5	28780	29875	8.2	2.2	2.4	2.3	−53.4
8	10% PEP	26366.05	25890	26842.11	−2.7	2.2	2.5	2.3	−53.0
9	15%PEP	26213.18	25790	26636.36	−3.5	2.1	2.3	2.2	−55.6

2.4.4. Modulus of Rupture (Flexural Beam Test)

The flexural strengths of normal concrete and concrete with different percentages of plastics were tested for two beams measuring 100 mm x 100 mm x 600 mm for each group, as shown in Figure 9, according to (ASTM C78–02, 2002). The load was continuously and steadily applied to the specimen until it cracked and fragmented, as shown in Figure 10. After the crack is initiated on the tension surface within the middle third of the span length, the modulus of rupture can be computed using Equation (3)(3) $\mathit{fr}\mathit{\hspace{0.17em}}=\mathit{\hspace{0.17em}}\mathit{PL}/\mathit{b}{d}^2,$(3) .

(3) $\mathit{fr}\mathit{\hspace{0.17em}}=\mathit{\hspace{0.17em}}\mathit{PL}/\mathit{b}{d}^2,$(3)

Figure 9. The flexural beam test setting.

Figure 10. The flexural beam test mode.

where fr is the modulus of rupture (MPa), P is the maximum applied load indicated by the testing machine (N), L is the span length (mm), b is the average width of the specimen (mm) at fracture, and d is the average depth of the specimen (mm) at fracture.

3. Finite element simulation

An ANSYS finite element program was used to simulate the experimentally tested flexural beam model, as shown in Figure 11. An 8-node Solid65 brick element with three degrees of freedom at each node was used to simulate the concrete (Madenci & Guven, 2006; Moaveni, 1999). Three hundred thirty-six nodes were used to simulate 320 identical Solid65 brick elements. The x, y, and z dimensions were set to 25, 25, and 30 mm, respectively. Constraints in the x- and y-directions were set at 30 mm from the beam ends, and a uniform line load was applied along the beam at 210 mm from each side. The load was applied in 10 N increments until failure occurred. Numerical simulations and analyses were performed using the inelastic concrete model and full Newton—Raphson iterative method (Moaveni, 1999). The finite element model adopts the Drucker—Prager DP yield criterion and linear elastic model to model concrete cracking and crushing (Al-Darzi, 2007; Hognested, 1951). The experimentally generated cracks depicted in Figure 10 are compatible with the crack patterns obtained from the finite element model shown in Figure 12. The experimentally determined compressive strength, tensile strength, and modulus of elasticity were used to create 9-finite element models and estimate the flexural resistance of the beams for each group. The finite element analysis results are presented in Table 9.

Figure 11. Finite element discretization of beam.

Figure 12. Finite element model cracks pattern.

Table 9. Modulus of rupture (fr) from experimental and finite element

No.	Group	Finite Element Modulus of Rupture fr MPa	Experimental Av. Modulus of Rupture fr MPa	Diff. %
1	Control	4.02	5.0	−19.27
2	2.5 % PET	3.91	3.2	21.75
3	5% PET	2.70	2.8	−3.74
4	10% PET	2.27	2.0	15.50
5	15%PET	1.70	1.9	−10.61
6	2.5 % PEP	2.36	2.8	−15.86
7	5% PEP	1.95	2.3	−16.01
8	10% PEP	1.96	2.3	−16.24
9	15%PEP	1.83	2.2	−17.26

4. Experimental test results

The slump test results for all nine groups reported in Table 6 show that workability improves with increasing PET content and deteriorates with increasing PEP content. The replacement of 2.5%, 5%, 10%, and 15% PET resulted in 5%, 12%, 19%, and 30% increments in the slump, respectively; these results coincide with those reported by Khalid et al. (2018) (Foti, 2011; Khalid et al., 2018). Conversely, the PEP replacement of 2.5%, 5%, 10%, and 15% resulted in decrements of 7%, 9%, 12%, and 19%, respectively, because the plastic particle size affects the concrete’s workability. However, the PET strips may decrease the friction between the concrete mix particles. Rabar, Hunar, et al. (2019) reported that plastic particles with a smooth texture can improve the fresh properties of concrete. This may be the underlying factor behind the slump increase compared to that of the control mix. Irregular PEP shapes may increase the interaction between the mixed contents, causing a decrease in slump.

The density results for the experimental groups are listed in Table 6. Notably, adding 3.2%–9.6% PET to natural fine aggregates resulted in lighter concrete. This reduction slowed as the PEP content increased from 2.4% to 7.2%, which agreed with the results obtained by the previous studies Yin et al. (2015) and Kangavar et al. (2022).

The compressive strength results illustrated in Table 7 and Figure 7 indicate that the compressive strength was reduced by 7%, 15.5%, 32.7%, and 30.4% when sand was replaced with PET plastic by 2.5%, 5%, 10%, and 15%, respectively, compared to the control specimens. The compressive strength decreased by approximately 4.6%, 24.3%, 23.4%, and 31.7% when PEP plastic was used to replace the coarse aggregate at 2.5%, 5%, 10%, and 15%, respectively, compared with the control specimen. The reduction in the compressive strength was normal when plastic with less stiffness compared to the normal aggregate was used. These results are consistent with those of Yin et al. (2015) and Hsie et al. (2008). Replacing the fine aggregate with PET significantly affected the compressive strength, as reported in previous studies. However, the PEP materials exhibited better compressive strength for the same replacement percentage.

Table 7 and Figure 7 present the experimentally obtained cylindrical splitting tensile strengths. The results demonstrated that the use of 2.5%, 5%, 10%, and 15% PET reduced the splitting tensile strength by approximately 3.6%, 9.8%, 16.4%, and 19.5%, respectively, similar to the behavior obtained by Juki et al. (2013). In addition, the addition of 2.5%, 5%, 10%, or 15% PEP reduced the splitting strength by approximately 7.9%, 14.9%, 20.0%, and 22.7%, respectively. Therefore, the worst condition was observed when PEP was used for splitting strength rather than PET as a replacement material.

The results of the elastic modulus tests are listed in Table 8. Using 5 %, 10 %, and 15% PET replacement, the modulus of elasticity of the concrete decreased by approximately 10.5%, 13.6%, 15.5%, and 2.4%, respectively, compared with that of the control group. The modulus of elasticity is correlated with the interaction between the concrete mix components. The modulus of elasticity results showed that the replacement of 5% PEP enhanced the modulus of elasticity by approximately 8.2% compared to the control group. The modulus of elasticity decreased by 10.6%, 2.7%, and 3.5% when PEP replaced the coarse aggregates by 2.5%, 10%, and 15%, respectively, compared with the control group.

The results obtained from experimental results for the modulus of rupture are listed in Table 8. This reveals that when using plastic aggregates made from PET as sand replacements at 2.5%, 5%, 10%, and 15%, the modulus of rupture of the concrete was reduced by approximately 35.5%, 43.7%, 60.5%, and 61.8%, respectively, compared with that of the control beam. The modulus of rupture decreased by approximately 43.7%, 53.4%, 53.0%, and 55.6% when the aggregate was substituted with PEP by approximately 2.5%, 5%, 10%, and 15%, respectively, compared with the control samples. These results show that the inclusion of PEP reduces the modulus of rupture to a lesser extent than that achieved through the incorporation of PET.

5. Finite element analysis results

After conducting the finite element analysis, the results were compared with those obtained experimentally, revealing that the crack patterns obtained by the finite element shown in Figure 12 were the same as those in the experimental beam test shown in Figure 10. Using the modulus of elasticity obtained experimentally in simulating finite element models serves as a good indication of the ability of a finite element to simulate concrete with plastic aggregates. The finite element results are more consistent with the experimental data when PET and PEP are used, as shown in Table 9 and Figure 13. This may be attributed to the ductile behavior of the concrete owing to the presence of plastics in the concrete mix. The finite element results for the rupture modulus deviated by 19% from the experimental data for the control group. Based on these findings, the modulus of rupture calculated using the finite element model was found to be higher than the experimental results by approximately 21.75% versus 2.5% replacement and 15.5% against 10% replacement of the aggregate by PET plastic, and lower by 3.74% and 10.61% for 5% and 15% replacement of PET plastics, respectively. Moreover, the modulus of rupture was reduced by 15.86%, 16.01%, 16.24%, and 17.26% according to the finite element results for the 2.5%, 5%, 10%, and 15% PEP plastic replacements, respectively.

Figure 13. Variation of modulus of rupture obtained by the finite element method and experimental test.

However, a general examination reveals a variation in the results obtained by the finite element method compared with the experimental results. Figure 14 shows the dissimilarity between the finite element findings and the PET and PEP results. Based on these results, we recommend that the behavior of such concrete be investigated in more detail by adopting each type of plastic and conducting further analysis by finite elements using different nonlinear models.

Figure 14. Differences between the modulus of rupture obtained using the finite element model and experimental test results.

6. Conclusions

Several conclusions can be drawn based on the preceding results depending on the experiments conducted under laboratory temperature conditions. The conclusions are as follows:

1. The experiments conducted in the present study led to the conclusion that the addition of PET plastic pieces improved the workability of concrete, which differs from previous studies that primarily utilized powder plastic replacements. However, the addition of PEP particles notably deteriorated the workability.

2. The experimental works also showed that replacing the fine aggregate with PET significantly reduced the compressive strength compared to PEP for the same replacement percentage. The reduction in compressive strength using 10% PEP plastic was more pronounced than that using 5% PET.

3. Within the limits of the experimental studies, the decrease in splitting strength was more pronounced when PEP was employed compared to when PET was used.

4. Except for the mixtures containing 5% PEP, with increasing plastic content, the modulus of elasticity was reduced owing to the nature and smooth surface of PET and PEP plastics, which required less time to flow compared to natural rough aggregates. However, a smaller decrease was observed with PEP compared to PET. However, this trend requires further investigation.

5. The results of the flexural beam test showed no systematic relationship between the percentage of plastic content and the modulus of rupture. For both the concrete groups, a reduction in the modulus of rupture was observed. The mixtures containing (PET) showed the lowest modulus of rupture, whereas the mixtures with PEP exhibited higher values.

6. Finite element analysis with the available nonlinear concrete model allowed for reliable estimates of the failure mode and location of the first cracks in both normal and plastic concrete with acceptable differences.

7. The finite element model with different moduli of elasticity produced different flexural resistances that agreed well with the experimental results, which seemed to be an effective approach for simulating the behavior of plastic concrete.

The results indicate that the limits of the tested concrete with PET and PEP could be fulfilled. This product can be practically useful in terms of its appropriateness and sustainability. Consequently, the problem of plastic trash disposal can be partially addressed. It is advisable to adopt each type of plastic and perform additional finite element analyses utilizing different nonlinear models to explore the behavior of each type of plastic concrete in more detail.

Acknowledgments

We are grateful to the Department of Civil Engineering, College of Engineering, University of Mosul, for their support of this research.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

The authors received no specific funding for this study.