Physical and mechanical properties of synthetic beams from high density polyethylene waste

. Improper processing of plastic waste can cause several environmental problems, such as air pollution, soil damage, respiratory diseases, and others. This research utilizes high density polyethylene (HDPE) waste as a basic material for synthetic beams. The manufacture of synthetic beams goes through several stages, such as washing, drying, crushing, chopping, and melting HDPE waste, as well as moulding into a block. The physical properties examined in this study include compressive strength, flexural strength, density, and ductility which refer to SNI 03-3958-1995 and SNI 03-3959-1995 codes. From the test results, it was obtained that the average compressive strength and flexural strength of synthetic beams were 1.52 – 2.53 MPa and 0.61 – 0.95 MPa, respectively. Synthetic beams have an average modulus of elasticity of 109.4 MPa, and a density of 834.8 kg/m 3 . The test results from this study indicate that the strength of synthetic beams from HDPE is low and does not meet the requirements for use as a construction material. In further research, innovation is needed to obtain stronger synthetic beams.


Introduction
The population growth and the increasing prosperity level of the community can trigger the emergence of more waste.If waste is not managed properly, waste accumulation can occur and raises environmental pollution, which is a problem for all countries in the world [1].One type of waste that is increasing in number is synthetic materials that are easily formed into various forms of goods or called plastics.
Plastic waste in Indonesia contributes to 11% of the world's plastic waste and is ranked 2 nd in the world [2].The large amount of plastic waste in Indonesia is related to the increasing use of plastic-based equipment [3].The contribution of plastic waste to total waste production in Indonesia reaches 15% with an average growth of 14.7% per year.This is the second largest contributor after organic waste [4].
Plastic waste management is a hot issue because plastic is a material that cannot be decomposed naturally (non-biodegradable) so plastic waste management with landfills and open dumping is not appropriate.One alternative for a plastic waste solution is to carry out the recycling process to be made into new plastic or other materials that are more valuable [4].
Recycling plastic granules has been widely studied in the field of civil engineering, including being used as a mixture of rock materials in flexible pavement construction [5], an alternative binder for paving blocks production [6], as row material for reducing alternative fuel [4], and as concrete aggregate replacement in *Corresponding author: restu.faizah@umy.ac.id concrete footpaths [7].Enhancing plastic recycling have some benefit in reducing greenhouse gas emissions, reducing plastic waste accumulation, limiting the use of petroleum, and increasing the sales value of plastic waste [8].

Plastic waste
The processing of plastic waste into other goods requires further research because there are various types of plastic waste in the field that has their own characteristics.In general, plastics are grouped into 2 types, namely thermoplastic and thermosetting.Thermoplastics can be recycled and changed into other forms by heating, whereas thermosetting cannot.Thermoplastics can be easily found in the field because they are widely used as daily goods.There are 6 types of thermoplastics, namely Polyethylene Terephthalate (PET), High-Density Polyethylene (HDPE), Polyvinyl Chloride (PVC), Low-Density Polyethylene (LDPE), Polypropylene (PP), and Polystyrene (PS) [4].Each type has an unique symbol and characteristic as shown in Table 1 [9].
This research utilizes HDPE plastic waste, which is the most widely used type of plastic in households, as the basic material for making synthetic beams.The use of HDPE plastic waste in the field of civil engineering has been extensively studied, including as a mixture of bricks [3], BATAKO [10], AC-WC wear layer (Asphalt Concrete Wearing Course) [11], and LASTON-WC mixture [12].This study aims to determine the properties of synthetic beams from HDPE waste to obtain recommendations for the use of these beams, whether they can be used as structural materials or not.Beam properties studied include compressive strength, flexural strength, elastic modulus, density, and modulus of elasticity.In further research, innovation can also be proposed to improve its properties.

Synthetic beam
The synthetic beam referred to in this study is a longshaped element with a square cross-section, which is made from HDPE plastic waste.In the field, this synthetic beam can be used as a structural element such as a column or beam but must meet certain strength requirements.Structural columns and beams will bear the loads on them, so they must have a compressive strength of at least 17 MPa [13].Meanwhile, the structural elements of the building must also have the ability to withstand earthquake forces with adequate ductility [14][15].To determine the ability of a synthetic beam as a column, it is necessary to examine its strength to withstand axial forces or known as compressive strength, while as a beam, it is necessary to examine its flexural strength (Fig. 1).

Materials and methods
The primary material used in this research is waste plastic of the HDPE (High-density polyethylene) type (Fig. 2).HDPE is usually used in the manufacture of grocery bags, milk bottles, juice bottles, detergent containers and more.HDPE is widely used for household packaging because it is safe to use and can prevent chemical reactions between plastic packaging made from HDPE and other food/beverage packaging.Besides that, HDPE is also known as a material that has stronger, harder, opaque, and more resistant to high temperatures.However, HDPE is only recommended for one-time use because it contains antimony trioxide compounds which continue to increase over time [10].

Synthetic beam manufacturing process
Manufacture of synthetic beams through several stages as shown in Fig. 3. HDPE waste is washed first and dried in a dryer machine (Fig. 4a), then cut into small pieces using a crusher machine (Fig. 4b), so that HDPE plastic pieces are obtained as shown in Fig. 5. Then the HDPE plastic pieces are melted using an induction melter at 350 o C (Fig. 6).After all plastic pieces has melted, then moulding is done using a beam mould of size 50x50x800 (mm) (Fig. 7).

Specimens
In this study, a compressive strength test and a flexural strength test were carried out with 6 specimens for each test, as presented in Table 2.There are 3 variations of cooling treatment after specimen moulding.The first treatment was given to CD1, CD2, FD1, and FD2 specimens, namely direct cooling after moulding by soaking the specimen in water.In the second and third treatments, the specimens were exposed to free air after moulding, for 15 minutes to CA151, CA152, FA151, and FA152 specimens, but 30 minutes for CA301, CA302, FA301, and FA302 specimens, then soaked.Each treatment had 2 compressive strength test specimens and 2 flexural strength test specimens.The tests refer to SNI 03-3958-1995 [16] for compressive strength tests and SNI 03-3959-1995 [17] for flexural strength tests.The specimens have a crosssectional area of 50x50 mm 2 with a length of 200 mm and 350 mm respectively for compressive strength and flexural strength testing.The specimens were obtained from the original moulded beams measuring 50x50x800 mm 3 (Fig. 8) and then cut to the desired size.To obtain a flat surface, the specimen is caped using gypsum.

Laboratory testing
Testing used a Universal Testing Machine (UTM) located in the laboratory of building materials and structure of the Civil Engineering Study Program, Universitas Muhammadiyah Yogyakarta.The setting for testing specimens in UTM is shown in Fig. 9 and 10.The results of testing using UTM obtained a relationship curve between load and displacement which can be developed into a stress-strain curve.From these results, several synthetic beam properties can be calculated, including compressive strength, flexural strength, and modulus of elasticity.The compressive strength (σ, MPa) is obtained by dividing the maximum load (P, N) by the compressive area (A, mm 2 ) according to Equation 1 [16].
Flexural strength testing using a centre point loading method, with nominal flexural strength obtained using the formula in Equation 2 [17].Where fb is flexural strength (MPa), P is maximum load (N), L is distance between supports (mm), b is specimen width (mm), and d is specimen height (mm).
To determine the modulus of elasticity of the specimen, it can be analysed from the compression strength test and flexure strength test results.In the compression strength test, the modulus of elasticity is expressed as the slope of the stress-strain curve in the elastic phase.As for the flexural test, the value of the modulus of elasticity can be calculated using the formula in Equation 3. Where, E is Modulus of elasticity (MPa), P is maximum load (N), l is specimen length (mm), b is specimen width (mm), d is specimen height (mm), and Δ is maximum displacement (mm).In addition, it is also possible to obtain the density of the specimen, by dividing its weight by its volume.

Result and discussion
The laboratory test has been carried out following the test method and obtained some results as described below.

Compressive strength
Compressive strength testing was carried out on a sample of synthetic beams measuring 50x50x200 mm 3 , which had 3 variations of treatment.In this test, the UTM tool will result in a load-displacement curve as shown in Fig. 11.From this figure, the maximum P value is known, and then the compressive strength can be calculated using Equation 1.The test results in the form of an average of each type of treatment are shown in Table 3.  From Table 3, synthetic beams have a compressive strength between 1.52 -2.67 MPa.This value does not meet the beam strength requirements for use as a structural element, which is at least 17 MPa [13].From the type of load-displacement curve shown in Fig. 11, it can also be seen that the synthetic beam has a brittle behaviour.The results of the compressive strength test of synthetic beams were also influenced by the cooling treatment after moulding, where the highest strength was obtained by soaking in water after 15 minutes of exposure to free air first.
Based on these results, it is necessary to carry out further research to find solutions to increase the compressive strength of the beam.Some possibilities that can be done are to find the appropriate cooling treatment method or to combine the plastic-based materials with other materials that can increase the ductility of the beam.

Flexural strength
The load-displacement curve of the flexural strength test results also shows the brittle properties of synthetic beams (Fig. 12).From this figure, the P max and Δ max can be fixed, and the flexural strength (fb) can be calculated using Equation 2. The results of the flexural strength testing of synthetic beams with 3 variations of cooling treatment are shown in Table 4.  From Table 4, it appears that the flexural strength has the same tendency as compressive strength, where the higher the compressive strength, the higher the flexural strength.In this study, the results obtained were that the flexural strength of synthetic beams was less than the compressive strength, which was around 38 -40%.In the flexural strength test, the highest strength was obtained by the beams that take in cooling treatment by being exposed to free air for 15 minutes before being soaked.This phenomenon is the same as the result of the compressive strength test.Therefore, the recommended cooling method in this study is exposing the synthetic beams to free air for 15 minutes after moulding is finished and then soaking them in water.However, the possibility of a better cooling method can still be investigated.

Modulus of elasticity
After the compressive strength test, the modulus of elasticity can be determined from the tangential stressstrain curve in the linear (elastic) phase or the stressstrain ratio.The modulus of elasticity (E) of synthetic beams obtained from the compressive strength test is presented in Table 5, which is between 77.3 -106.4MPa.The highest E value was also found in the beam that had the highest compressive strength and flexural strength, namely the beam with the cooling treatment by soaking in water after 15 minutes of exposure to free air.Meanwhile, the flexural strength test also obtains the modulus of elasticity (E) which is calculated using Equation 3 and shown in Table 6.The E value of the synthetic beam obtained from this flexure test was slightly higher than the E value from the compressive strength test but showed the same trend, where the highest E was found in the soaked synthetic beam after 15 minutes of exposure to free air.The E value of this synthetic beam is also relatively small, lower than the E value of rubber, around 500 MPa.This low value of E is related to the physical properties of the beam which cannot withstand high strength at small deflections.Consequently, if the load is too high, the beam will experience a large displacement, which may exceed the allowable displacement and the beam will suffer damage.

Density
By weighing and measuring the specimens, the density of the synthetic beam can be obtained, which is between 650 -1000 kg/m3 as shown in Fig. 13.This synthetic beam is classified as a lightweight material because its density is less than 1850 kg/m3 so it is recommended for use as an earthquake-resistant building material [18].

Synthetic beams properties
The properties of synthetic beams obtained from this study include physical and mechanical properties.The physical properties consist of modulus of elasticity and density averaged at 109.4 MPa and 838.4 kg/m3, respectively.Mechanical properties include compressive strength and flexural strength with the results shown in Fig. 14.

Conclusion
This study has several conclusions as follows: 1. Synthetic beams have an average modulus of elasticity (E) of 109.4MPa. 2. Synthetic beams have an average density of 834.8 kg/m 3 .3. The compressive strength of synthetic beams ranges from 1.52 -2.53 MPa 4. The flexural strength of synthetic beams ranges from 0.61 -0.95 MPa 5. Synthetic beams are classified as light materials, so they are suitable for earthquakeresistant building materials 6.Further research is needed to improve mechanical strength of synthetic beams.

Fig. 1 .
Fig. 1.Flexural strength and compressive strength of beam and column.

Fig. 11 .
Fig. 11.Load-displacement curve from the UTM result of compressive strength test.

Fig. 12 .
Fig. 12. Load-displacement curve from the UTM result of flexural strength test.

Table 1 .
Types of thermoplastics and their characteristics.

Table 2 .
Distribution of the specimens.

Table 4 .
Results of flexural strength test

Table 5 .
The modulus of elasticity (E) obtained from the compressive strength test.

Table 6 .
The modulus of elasticity (E) obtained from the flexural strength test.