Conceptual design of interlocking block utilizing lightweight expanded polystyrene concrete reinforced with kenaf fiber

Concrete is the most used material in the construction industry in modern times, the concrete technology has also been well developed over the years. Carbon emissions from the built environment and its construction has been reported amounting to 38% of the total global emissions. Furthermore, waste management is a struggling issue. This study utilises the recycled expanded polystyrene and kenaf fiber, an agricultural by-product, incorporated into the concrete matrix. Compressive, split tensile and flexural strength has been done on the composite concrete and has shown good results by surpassing the target of 5.2 MN/m2, set according to the Malaysian Standard, MS 76:1972. The interlocking block is a possible application in the construction industry for cost effectiveness.


Introduction
In recent times, it is generally aware that issues such as climate change, global warming and natural disasters are pressing on the society. It is also commonly known that the activities from the construction industry are contributors of a significant amount of carbon emissions that leads to global warming, which in turns causes climate change. According to environmental reports, it was reported that carbon emissions from construction sector is responsible for 38% of total global-related carbon dioxide emissions [1], and 9% of total greenhouse gas emissions [2]. While another report states that the construction industry also consumes large amount of raw materials which leads to resources becoming [2]. Further contemplating, waste management in current society is also a factor that leads to global warming due to its large volume and disposing methods. In this paper, waste expanded polystyrene, a common material used in packaging products, and kenaf fiber, a by-product from the agricultural sector is especially highlighted, with hopes that utilizing these materials can reduce wastes and help reduce the effects of global warming, while also providing cost effective solutions for wall construction.

Recycled expanded polystyrene in lightweight concrete
In common terms, the expanded polystyrene (EPS) is known as Styrofoam. It is a popular material used in the development of concrete technology, due to its qualities such as lightweight, durable, compressive and insulating. When added into the concrete matrix, it provides good thermal insulation and acoustic properties.
Primarily in this study, the recycled EPS can be categorised into clean post-consumer and dirty post-consumer, and only clean post-consumer such as Styrofoam boards, packaging beads, etc. are accepted [3].
The comparison of virgin and recycled EPS in lightweight mortars was reported to have a slight difference. In terms of grain size, the virgin EPS pearls is almost monogranular, while the recycled ones are rougher and has less regular surface with more distributed grain size; the usage of virgin or recycled EPS does not affect the workability of concrete, but the usage of recycled EPS does increases the mortar's density and mechanical properties [4].
Although the usage of EPS increases the density of concrete [5], but it was reported that the increased dosage of EPS in the matrix lowers the compressive strength [6]. The same phenomenon was reported in another study, where the substitution of gravel with EPS particles results in a significant reduction in compressive strength, and continues as the dosage of EPS increases [7]. In order to compensate the lost in strength by the lowered density, the addition of silica fume is added to increase the compressive strength, and the higher amount of silica fume is added, the higher the rate of compressive strength development [8].
Besides the strength aspect, the failure pattern of EPS in lightweight concrete is observed and reported to have inclined cracks and vertical cracks on the surface when the concrete is subjected to compressions, and the cracks appear along the loading direction, further extending to the central section gradually with the increase in strain [9].

Kenaf fiber in lightweight concrete
The nature of concrete is weak in tensile and flexural strength, and in a lightweight concrete, there will be a further decrease in this aspect. The kenaf fiber's strength is on the opposite of the concrete, it is known to be tensible. Besides being tensible, the fiber also has advantageous qualities such as biodegradable, recyclable and low cost.
The kenaf fiber is obtained from the Hibiscus cannabis plant, mainly from its bark. The obtained fiber is tough, nonabrasive, has high filling levels and low energy consumption, but has high level of moisture absorption [10].
The fiber itself has a density of 0.3 -1.2 g/m 3 , which ultra-lightweight; 2.7 -6.9% of elongation at break, which indicates its higher ductility than other natural fibers such as, flax, jute, sisal, etc. Individually, the fiber has a high tensile strength that ranges between 170 -1627 MPa, indicating its durability and strength [11]. Similar results were obtained, where additionally, it is reported that the kenaf fiber has a Young's modulus of 53 GPa, and states the fiber is sufficient to replace synthetic fibers [12], [13].
The strength of kenaf fiber in the concrete is further recognised that it enhances the flexural toughness and impact resistance with optimal volume dosage, where in a pull-out test, the average peak pull-out load of 10 fiber strands embedded in mortar at a depth of 15 mm was 15.33 N, and 16.00 N when the fiber is embedded at a depth of 20 mm, the bond strength is calculated to be 0.25 N/mm 2 [14]. The mortar-fiber composite failure mode was also observed to be slow and gradual as compared to the sudden failure of common mortar specimens [14]. Previously, it was mentioned that the addition of kenaf fiber can enhance the tensile and flexural strength of concrete at an optimal volume, further studies show that the optimal length of kenaf fiber also has to be considered. It was found that the kenaf fiber increases the water absorption of composite mortar, but with optimal fiber volume and optimal fiber length of about 10 mm, the composite's compressive can be enhanced [15].
As mentioned previously, the kenaf fiber in the concrete mix absorbs water, this disadvantage side of the kenaf fiber can be mitigated by pre-treatment before adding into the mix. It was found that the fibers pre-treated with 6% sodium hydroxide, NaOH, obtained by diluting sodium hydroxide solid pellet in water, can reduce water absorption of kenaf fiber effectively, while also improving the surface roughness, which leads to better adhesion between the fibers and the matrix [16].

Interlocking concrete block
Previous studies and development of the interlocking block utilises the materials such as EPS, rice husk ash (RHA) and natural fibers for desired properties, as such in the study of Sayanthan, states that the concrete block that is to be used for construction should have compressive strength of 5 N/mm 2 or cube strength of 7.5 N/mm 2 [17].
In the masonry brick market, there are patented interlocking systems available. These interlocking blocks are categorised as hollow-block systems, such as Sparlock, Meccano, Sparfil and Haener systems, and solid block systems such as, Hydraform interlocking blocks, just to name a few [18]. Examples of the interlocking blocks are shown in Figs 1 and 2.  The Azar interlocking block system [18] The interlocking design of bricks was found to be cheaper to produce compared to traditional bricks and also lower building cost, shorter construction time and has the capacity to resist effects of earthquake [18]. Moreover, the plus side of interlocking blocks masonry construction, is when dry-stacking is applied, curved walls construction can be done [18].
The shorter construction time and lower material cost was studied. It was reported that pulverized EPS interlocking block takes only 9 minutes 50 seconds to construct a 1 m x 1 m wall panel, while conventional bricks with no interlocking system takes 28 minutes and 5 seconds to finish construction, as it can be observe, there is a large reduction of construction time which is a great advantage in easing workload and early completion of projects [19]. The nearest reference utilises pulverized EPS to replace coarse aggregates by 10%, 15% and 20%, after evaluating the physical and mechanical properties, it was found that at 20% substitution by volume, the material cost is much cheaper than conventional concrete bricks, and expects more profits to be made [19].

Materials and experimental programme 2.1 Materials
The materials utilised in this study to produce the EPS-Kenaf fiber lightweight concrete, along with its proportions are shown in the Table 1 below. The water/cement ratio in this study was rationalised to be 0.4. The recycled EPS in beads forms have a density of 21 kg/m 3 and its particle size ranged between 2.75 mm to 4.5 mm. The kenaf fiber were trimmed to 50 mm in length, and has a dry density of 30 kg/m 3 , where the length was decided with reference to Mahzabin's study [20]. The alkaline treatment is an important pre-treatment for the fibers to reduce water absorption and increase tensile properties of the fibers [21]. The fibers were immersed in 6% sodium hydroxide, NaOH, solution for 3 hours at room temperature for chemical elimination of the impurities on the fiber surface. After the immersion, the fibers were rinsed with water for several times until its brownish colour fades away. Then, the fibers were dried under hot sun and kept in airtight container. Before the mixing commence, the kenaf fibers and recycled EPS beads were prepped by immersing them in water to prevent water absorption by the materials when mixing.

Specimen preparation
First step in preparing the concrete mix is to mix a part of the sand and cement in the concrete mixer machine, then water is added into the mix until a cohesive cement mortar is obtained. Then, the recycled EPS beads and kenaf fiber were added into the mixture by parts and mixed thoroughly until a uniform distribution of cohesive cement mortar is obtained. The remaining sand and water were added into the mix and mixed till a homogeneous mixture is obtained. Next, the addition of superplasticizing admixture into the mixture reduces water dependency without compromising the workability of the mixture. The rest of the recycled EPS beads and treated kenaf fiber were mixed into the mixture by wrapping it in, till the mixture becomes homogeneous.
When the homogeneous mixture was obtained, the concrete mix was poured into the desired mould and compacted by hand tamping to avoid EPS beads to float to the top surface. The concrete specimens were demoulded after 24 hours and placed in the water curing tank to be cured according to ASTM C192 [22].
In brief, there were 15 cube specimens for compression test, dimensioned 100 x 100 x 100 mm; 15-cylinder specimens for tensile test, dimensioned 100 mm diameter x 200 mm length; and 15 beam specimens for flexural strength test, dimensioned 100 x 100 x 500 mm.

Testing programme
The preliminary test was performed on the cube specimens with the dimensions of 100 x 100 x 100 mm, to study its characteristics and test its compressive strength, where the compressive strength should achieve the target of 5.2 N/mm 2 or 7.5 N/mm 2 cube strength. Since the test's purpose is to find out the optimal dosage of EPS, the cubes are only tested for its 7 th day compressive strength.
The specimens with EPS and kenaf fibers were tested for its compressive strength, split tensile strength and flexural strength. The compressive strength tests on all specimens were in accordance with BS EN 12390-3 [23], while the split tensile strength test in accordance with ASTM C496 [24], and the flexural strength test in accordance with ASTM C78 [25]. These specimens were cured for 3, 7, 14, 28 and 56 days and were tested on the cured days.

Mechanical Properties of EPS-Kenaf fiber lightweight concrete
Taking into consideration, the high absorption of kenaf fibers, the water/cement ratio is rationed to be 0.40. The average slump value and bulk density of varying percentage volume of kenaf fiber content is shown in Table 3. From Table 3, it can be observed the average slump values were 127 mm to 53 mm for recycled EPS concrete with 0% kenaf fiber and recycled EPS concrete with 5.5% kenaf fiber. The slump value decreases as the fiber content increases from 0% to 5.5%. This occurrence is because of the concrete mix becoming stiffer and tougher, which shows lower workability after the addition of kenaf fibers, and resulted in a lower slump value. The lower slump value means the capability of kenaf fiber to absorb water in the concrete mix, resulting in a drier mix and lowers the slump value.
The average bulk density recorded in Table 3 shows that the densities of all specimens decrease with the increase of fiber volume. This can be attributed to the low density of kenaf fibers and recycled EPS beads as it is responsible for the overall weight of a concrete specimen. The overall density of recycled EPS concrete reinforced with kenaf fiber are ranged in between 1400 kg/m 3 to 1500 kg/m 3 . The percentage differences among the samples with varying kenaf fiber content were absolutely small where the densities reduce around 1% with the increasing kenaf fiber volume.  Table 4, it can be observed that the controlling cube specimens with no kenaf fibers has the highest compressive strength of 12.06 MPa, while 5.5% of kenaf fiber specimen has the lowest compressive strength of 4.96 MPa on the 56th day. The obvious decline in compressive strength with increase in kenaf fiber volume can be explained by the reduction of concrete volume which is responsible for carrying compressive load. The fiber agglomeration is higher when more fiber content is in the matrix, making the concrete become more porous, because the contents in the matrix is less well distributed [26]. The cracking pattern is different for varying kenaf fiber volume, where control specimen shows bigger cracking failure from edge to edge, while kenaf fiber specimens shows more yielding failure with slightly smaller cracking patterns. This phenomenon is because the kenaf fiber holds the matrix firmly and slightly decreases the rate of sudden failure.
In the splitting tensile strength tests the cracking behaviour reflects the potential of kenaf fiber under tensile loads. The control specimens have shown brittle failure along the vertical diameter of the cylinder samples because its weakness in tensile resistance, while a more distributed cracking pattern can be observed from the kenaf fiber reinforced samples. Samples with the inclusion of kenaf fibers exhibits ductile failure mode as kenaf fiber can be found bridging across the cracks.
In the flexural strength tests, samples with higher kenaf fiber content have higher flexural strength in beam specimens. Flexural strength grows significantly with the increase in kenaf fiber content in the matrix. The bending failure of the control specimen has shown a more brittle manner compared to the kenaf fiber reinforced samples. The characteristic ductility has been enhanced with the addition of treated kenaf fiber, where the specimens exhibited a higher ability to deflect gradually without sudden failure. This is because the treated kenaf fibers helps in holding the crack and prevents the occurrence of brittle failure.

Proposal of Conceptual Design of interlocking block
With the concrete mix studied above, and the aims of achieving those advantages mentioned previously, an attempt to design a novel interlocking block is done. The Figs. 4 and 5 show the design of interlocking block attempted.

Conclusion
This study concludes the mechanical strength of EPS-Kenaf fiber concrete can achieve the minimum target strength for masonry with the optimal EPS and kenaf fiber volume, which is nominally 8.7%, as obtained from the material proportions and 3.5%. With the concrete mix in hand, there is room to propose the conceptual design of interlocking blocks which utilizes the material investigated in this paper for future studies.