Developing a Sustainable Concrete using Waste Glass and Rubber for Application in Precast Pedestrian Slabs

In this piece of research, n attempt was made to produce a sustainable concrete with the partial replacement of both fine and coarse natural aggregates with two different non-biodegradable wastes. The selected wastes were fine glass and shredded rubber tires. Fine glass passing through 4.75 mm BS sieve was utilised for the partial replacement of fine natural aggregates. Coarse natural aggregates were partially replaced with shredded rubber passing through 20 mm sieve and retained on 6.30 mm sieve. Several mixes with varying % of fine glass but with a fixed 10 % of shredded rubber were tested. Optimum fine glass content was determined to be in the order of 20 %. The resulting concrete exhibited lower plastic and hardened densities (2040 and 2117 kg/m 3 respectively) in comparison to normal plain concrete. The static modulus of elasticity was found to be 18.3 GPa (mean value), while the splitting tensile strength was 2.37 MPa. The flexural strength showed a significant increase of 20.3% compared to the control mix. The results concluded that the concrete thus produced is a viable means of disposing of such non-biodegradable wastes (rubber and glass), thus reducing the loads at landfills. This new genre of concrete was produced at a lower cost than normal concrete because of the very low pre-treatment costs of the recycled wastes used. Furthermore, the properties tend to indicate that the concrete could be applied where lower strength and high durability properties are warranted. Hence precast slabs were made from the new design concrete and were tested along a stretch of a highly trafficable pedestrian walkway on the University campus. The slabs were continuously monitored for defects such as cracks, broken corners and slabs for a period of 24 consecutive weeks. After the test period it was observed that only 4 out of the 80 precast slabs had hairline cracks. Hence concluding the enhanced durability properties of the new design concrete.


Introduction
Worldwide, with increasing population and continuous development in the society, the amount of waste generated is on the rise.Resulting waste types are becoming more intricate, problematic to treat, and to dispose [1,2].Proper dumping of the non-biodegradable wastes is time bound; it may take years or even decades [3].Recent research works have focused on the partial substitution of natural aggregates in one of the most important building materials, concrete, with a reasonable percentage of waste.Recycled wastes used include demolished concrete, glass and plastic among others [4,5].Past investigations report on many advantages, which include; (i) lesser emission of heat and carbon dioxide from the chemical reaction of cement; (ii) lower cost of the recycled concrete thus produced; (iii) abundance of recycled wastes; and (iv) reduction of air, water and soil pollution in landfill sites [6][7][8].
By heating silicon dioxide, calcium carbonate among others at extremely high temperatures, a liquid is formed.After cooling a crystalline transparent material, glass, is obtained.Worldwide a high amount of waste glass is produced yearly.Waste glass, with properties matching that of fine natural aggregates, has been used in concrete [3,9,13].Resulting recycled glass concrete has numerous benefits, which include; (i) lower consumption of embodied energy; (ii) lower pretreatment cost; (iii) depending on usage, a larger amount of glass can be recycled in the concrete; and (iv) noxious glass elements are confined in the concrete matrix [9,10].The study by Etris et al. [14], with partial replacement of coarse aggregates by waste glass, reported a decrease in both the compressive and flexural strength of the concrete.Investigations carried out by Zainab and AL-Hashmi [15] showed a reduction in the fresh density of concrete, which is attributed to the low specific gravity of waste glass.Furthermore, with 20% partial replacement of the fine aggregates, a notable increase in both the flexural and compressive concrete strengths in the order of 10.99% and 4.23% respectively were observed after 28 days [15].Supplementary replacement resulted in a decrease in the compressive strength due to incomplete adhesion between the glass and cement particles [16].Lower % replacement of fine aggregates resulted in an increase in both, compressive and flexural strengths with densification of the mortar structure [17].The densification is a result of the clustering of the finer fraction of the glass, which in turn induces an improved distribution of the cement paste thus increasing the mechanical strength [18].Other studies showed increased durability of the recycled glass concrete, making them more resistant to electricity and sulphate attacks [19].Furthermore, the drying shrinkage of the concrete decreases with increasing amount of recycled fine glass since fine glass does not absorb water in the matrix [20,21].
Another non-biodegradable waste is rubber, which is obtained when tires have reached their end life.Mostly obtained from the automobile industry, its production rate, worldwide, is around 1 billion tonnes worldwide per annum [22,23].The global stock of such waste is about 4 billion [23].In 2017, in the U.S. only, approximately 40 million waste rubber tires were disposed in landfills [24].Disposal of such waste is done in landfills, some are exported or dumped illegally, and/or buried in mine sites [25].Waste rubber is an environmental threat for the following reasons; (i) on catching fire harmful gases are released [26]; and (ii) provides breeding habitats for rats, mosquitos and vermin that can trigger health issues [27].Many research works have been carried out on the performance of concrete whereby, both fine and coarse aggregates, were partially replaced by waste rubber [27] in numerous forms (powder [28], crumb rubber [29][30][31] and tire chips [32]).Resulting concrete's performance was poor [33][34][35].Around 85% and 50% reduction in both compressive and split tensile strengths were observed [27].Both the elastic modulus and the flexural strength of the resulting concrete were inferior to normal concrete [36,37].However, improvements were noted in; (i) post-peak behaviour [38]; (ii) ductility [39]: (iii) dynamic properties [40]; (iv) resistance to crack [39][40][41]; and (v) freeze -thaw attack [42].
Concrete remains a sustainable means of disposing of inert non-biodegradable waste such as glass and rubber [1][2][3].Resulting recycled concrete have higher cost of production because of the cost incurred for the pre-treatment of the waste before usage.Inclusion of such materials produces concrete with inferior performances [4,5,7].Most research works have so far focused on the partial substitution of either fine or coarse aggregates with one waste material [29][30][31].A few investigations have looked into the partial replacement of both aggregates with one waste material [32].The intent of this current piece of research is to investigate the properties of the resulting concrete, fresh and hardened, after partially substituting both the fine and coarse natural aggregates with recycled glass and shredded rubber tires respectively.

Research Methodology
The methodology adopted in this piece of research work was as per Figure 1.

Concrete Control Mix
The concrete mix used as control mix comprised of ordinary Portland cement, fine and coarse natural basaltic aggregates and fine glass as well as shredded rubber as fine and coarse aggregates substitutes.The mix was designed according to the Department of Environment (DOE) method, which is based on charts developed at the Building Research Establishment and British Cement Association.The physical properties of natural basaltic aggregates 0/4 mm, 6/10 mm and 14/20 mm were determined, Table 1.The grading curve of the basaltic aggregates is illustrated in Figure 2. The aggregates and cement used in the tests conformed to British Standards.The control mix design (MC) was batched for a target mean strength of 30 N/mm 2 at 28 days, Table 2.

Fine Aggregates Substitute
Untreated Crushed glass was used for the partial replacement of fine natural aggregates of size 0/4 mm.The fine glass used was obtained after sieving through sieve of aperture size 4.75 mm, Figure 3.The apparent density of glass was determined to be 2.53 while the water absorption was found to be 0%.The grading curve for fine glass, Figure 4, shows that it can be used to partially replace the fine natural aggregates in this experimentation.

Coarse Aggregates Substitute
From previous research works it was found that the optimum amount of rubber waste that can be partially substituted with coarse natural aggregates is 10% [43].This was kept fixed in the present investigation.Untreated shredded waste rubber passing through 20 mm aperture sieve and retained on 6.30 mm sieve was utilised, Figures 5 and 6.The apparent density of rubber was found to be 0.89.Similar to fine glass, the water absorption was found to be 0% mainly since shredded rubber is a non-porous material, whereby the water is adsorbed on the surface of the material only.

Test Mix Preparation
As discussed in the previous section, the optimum amount of shredded rubber that can be substituted is 10% by mass, which was maintained in all the other mixes.On the other hand, the percentage of replacement by mass of fine glass was 5, 10, 15, 20, 25, 30 and 35% as per Table 3.The free water content for mix MC was kept constant while for the different batches the free water content was adjusted targeting a slump ranging between 140 to 160 mm.The mix composition for the production of 1 cubic meter of concrete is as tabulated below (Table 4), however, for this experimentation only 0.03085 cubic meters of concrete was required and consequently Table 5 provides all information pertaining to the exact amount of the different ingredients required for each mix.

Batching Sequence
The sequence of batching and mixing of concrete will be carried out as outlined hereunder;  Coarse aggregates (including coarse rubber) will be added to the concrete mixer with half the amount of water required and mixed for half a minute;  The cement will then be loaded into the mixer with the free water and allowed to mix for half a minute;  Fine aggregates (including fine glass) will be added to the mix as well as the remaining absorbed water;  The mixing process will be stopped when the concrete will appear uniform after 1 and a half minutes of mixing;  Slump will be measured for the fresh concrete;  The plastic density of the resulting concrete will be determined;  The concrete will be poured in the moulds in 3 layers while compacting the concrete with the help of a vibrating table ;  The concrete will be demoulded after 1 day and will be placed in a curing tank maintained at a uniform temperature.
For each mix the following tests, both on fresh and hardened concrete, will be performed in accordance with relevant testing standards (British Standards), namely; The tests required a precise time frame as prescribed in the standard codes.A planning schedule for each batch was made as described in Table 6.

Sample Slab Preparation
The resulting concrete, once optimised, will be tested as walkway pre-cast slabs.The concrete slabs will be cast offsite against a hard concrete surface using timber as formwork.The finished unreinforced slab dimensions will be 450 mm by 450 mm by 50 mm deep.The resulting slabs will be cured using chemical compounds.Once in place the deterioration following frequent usage will be assessed weekly over a period of 24 consecutive weeks.

Fresh Properties of Tested Mixes
For each mix investigated, the water requirement had to be reduced, to maintain a target slump within the range of 140-160 mm.It was found that the hydrophobic nature of fine glass (0/4 mm) reduces the water requirement of the concrete.Furthermore, it was observed that the smooth surface of the recycled material increases the fluidity of the mixes.The elongated surface of the shredded rubber is hypothesized to adsorb water at its surface, providing a thin film of water between the cement paste and the rubber.With increased fluidity of the mix, the workability also augmented.
Table 7 shows the plastic density of the different batches.The control batch has a higher density compared to the substituted aggregate concrete batches.Figure 7 illustrates the decrease in plastic density with increasing replacement of fine glass.This can be attributed to the fact that both aggregates have lower densities [44] compared to basaltic aggregates.Shredded rubber apparent density is 69.1% less compared to coarser aggregates while fine glass is 11.9% less than fine basaltic aggregate.Increasing the percentage of fine glass results in a concrete mix with lower mass [15].Also, the shredded rubber strips will entrap air and consequently result in more air voids within the resulting concrete matrix.Hence, a lower plastic density compared to the control mix.Air bubbles can be seen from Figures 8 and 9.

Hardened Properties of Tested Mixes
The hardened densities of the different mixes under study were determined at both 7 and 28 days respectively, Table 8.All cubes were cured in water maintained at ambient temperature.The hardened density was the lowest, 2044.41 kg/m 3 , for the mix with the highest % of fine glass recycled material (35%) substituted.Hence, the new concrete thus produced was lighter than the normal concrete mix by approximately 22%.A decrease in hardened density was observed at 7 and 28 days, Table 8.The decrease in density is attributed to the increased amount of fine glass, which has a lower density [15] than basaltic fine aggregate.Also, the presence of entrapped air due to the presence of the shredded rubber [44] contributed to an increased amount of air voids thus causing a decrease in the overall mass of the specimen under investigation.

Compressive Strength of Tested Mixes
Likewise, the compressive strengths of the mixes were also determined at 7 and 28 days after casting, Table 9. Figure 10 illustrates the variation of the compressive strength with increasing fine glass while keeping the % of rubber fixed to 10.In Figure 10 a maximum compressive strength was achieved with 20% replacement of fine glass (Mix M4).Further replacement showed a decline in the strength properties.From Figure 11, Mix M4 showed a decrease in compressive strength of 64% at 7 days and 59% at 28 days when compared to the control mix (MC).The non-polar nature of rubber particles and their tendency to entrap air resulted in a concrete of higher air content, thus subsequently reducing the strength [44].Moreover, the weak adhesive bond developed between the surface of waste glass aggregates particles and the cement paste reduced the compressive strength [16].Further decrease in strength was not observed owing to the Pozzolanic properties of the fine glass particles, which appeared to offset the compressive strength at 28 days [15].
From Figure 10, the optimum amount of fine glass that can be replaced was found to be 20%.The failure mechanism of all the cubes tested were studied.Brittle failure was noted for the control cube and non-explosive failure for the different batches containing recycled aggregates.No detachment of the concrete was observed; however, presence of cracks was noted along the plane of the application of the force [44].It was observed that the shredded rubber used were holding altogether the cube parts from opening.As a consequence, additional external force was required to separate parts of the failed cubes.Ductile failure occurred for the cubes [45].Figure 12 and 13 illustrate the state of the cube after failure.Figure 14 shows the inside of the cube after the application of additional forces to split open the cubes.Both substituted aggregates, namely shredded rubber and fine glass, were visible.The failure mechanism of the concrete indicates that this new genre of sustainable concrete material can sustain impacts/loads without breaking apart.The applied loads/stresses are absorbed by the rubber present in the matrix.

Static Modulus of Tested Mixes
The static modulus of elasticity for the different dosage of fine aggregates was determined, Table 10.Compared to the control specimen, a decrease of 25.4% was noted in the modulus of elasticity, Table 10.Both the properties and the volumetric ratio of the aggregates impact on the modulus of elasticity of the resulting concrete.Replacing the coarse aggregates with shredded rubber at 10% by mass resulted in a decrease in the observed values [46], due to rubber having a lower modulus of elasticity value in comparison to natural basaltic aggregates.Glass possesses higher modulus of elasticity compared to concrete and consequently, an increase in fine glass provides a better distribution of the particles in the cement paste [18], which in turn results in an increase in the static modulus values.As shown in Figure 15.However, there is a drop in the modulus of elasticity of the concrete, which is attributed to an increase in weak bond formation between the glass particles and the cement paste [16].Maximum static modulus was witnessed with 20% replacement of fine glass in the concrete mix.
The rubber addition reduces the static modulus of elasticity of concrete, because rubber particles are effective in increasing deformability of concrete.Using rubber waste in concrete reduces the modulus of elasticity [47].The modulus of elasticity is related to concrete compressive strength and the elastic properties of aggregates.The larger amount of rubber additives is added to concrete, the lesser modulus of elasticity is obtained.By using rubber waste in concrete, the dynamic modulus of elasticity also decreases comparing without rubber additives.The reduced both static and dynamic modulus of elasticity in concrete with waste rubber aggregates may be explained by the low modulus of elasticity of small rubber particles, which is much lower than the aggregate modulus of elasticity [48].

Split Tensile Strength
The table below shows the variation of the split tensile strength for the different batches.Table 11 gives the splitting tensile strength of the different mixes, while Figure 16 illustrates the variation across the different mixes with increasing % of fine glass.Compared to the control mix, a drop of 47.4% was observed.The decrease in splitting tensile strength was attributed to the weak bonding of the glass particles in the concrete matrix [16] and to the presence of air voids entrapped on the surface of the shredded rubber [21].Figure 4.13 shows an increase before reaching a maximum splitting tensile strength, thereafter dropping in strength.Fine glass added increased the packing of the concrete, that is, it augmented the wall effect [49,50] where the fine glass particles acted as mortar filling in the space between the coarse aggregates [18].However, addition of more fine glass resulted in an increase in weak bond between the fine glass particles and the cement paste [16].Addition of shredded rubber caused bridging of the aggregates in the concrete matrix.The shearing will occur along the shredded rubber and not through the shredded rubber.After failure occurs, the cylinder remains completely intact with presence of micro-cracks only.Figure 17 and 18 show the control specimen after splitting tensile test.The specimen would detach after application of a maximum load.From Figure 16, the optimum mix was found to be 20% replacement of fine glass.

Drying Shrinkage
A decrease in drying shrinkage was observed from Figure 19.At 20% replacement of fine glass aggregates (Mix M4), the percentage drying shrinkage was 0.0177%.The control mix exhibited drying shrinkage of 0.0411%.Thus, an overall decrease of 56.9% in drying shrinkage was witnessed compared to the control mix [21].Shredded rubber [46] and fine glass are both hydrophobic materials, having a tendency to adsorb water.Both aggregates decreased the water requirement of the mix thus resulting in lower drying shrinkage [20].At 20% replacement of fine glass, an increase in drying shrinkage was observed.Finer glass particles (less than 75µm) have a tendency to act as Pozzolanic material [47]; thus, increasing the cementitious quantity of the mix resulting in a decrease in the required quantity of fine aggregates.This resulted in a decrease in the aggregate: cement ratio of the mix.According to Neville [47], shrinkage is larger since the drying shrinkage is inversely proportional to the aggregate to cement ratio.From Figure 19, the optimum mix was found to be 20% replacement of fine glass.13 it was observed that the concrete using substitution aggregates showed an increased in flexural strength of 20.3% compared to the control specimen.

Figure 20. Variation of flexural strength
The increase is attributed to the presence of the shredded rubber.Concrete is weak in tension.The shredded rubber is ductile in nature; failure occurs at the shredded rubber-concrete interface is reduced (in the tension zone).From Figure 20, addition of fine glass increases the flexural strength to a maximum.Further addition of fine glass resulted in a drop in flexural strength.The quantity of fine glass particles acting as Pozzolanic material increases (1.2% of the fine glass content exhibits Pozzolanic effect).The cement content of the mix increases, consequently causing an increase in flexural strength [15].However, the presence of fine glass particles increases the weak bonding at the concrete-glass interface.From Figure 20, the optimum mix was found to be 20.5% replacement of fine glass.

Application of New Concrete to Pedestrian Walkway Slabs
A specific section of a highly used pedestrian walkway was selected on the campus of the University for testing the new concrete, Figure 21.The existing pre-cast concrete slabs were 450 mm by 450 mm by 50 mm deep and were laid on rock-sand.A constant gap of 50 mm was maintained among the precast slabs in both directions.A condition assessment survey was carried out on each slab within the test zone.Observed defects included cracks, both horizontal and vertical, broken corners, as well as broken slabs, Table 14.On the whole, each of the slab within the test area had defects.All the slabs were replaced with new precast slabs made from the newly designed recycled concrete and monitored over a period of 24 consecutive weeks from Monday to Saturday.The pedestrian traffic flow within the test area was documented, Figure 22.A condition survey was carried out every week over the test period and defects observed were documented, Table 14.After the 24 weeks, hairline cracks were observed in the middle region of slabs F6, F7, G6, and G7.Hence, it can be concluded that the new genre of concrete exhibits high durability properties which is appropriate for application in precast walkway slabs.

Conclusion
In this piece of research work, a new type of concrete was produced by partially replacing both fine and coarse natural aggregates with two different non-biodegradable waste materials.Several mixes were tested and the concrete that exhibited the best properties had its fine and coarse natural aggregates substituted in the order of 20 and 10% respectively.Coarse aggregate substitutes comprised shredded rubber tires that passed through 20 mm BS sieves and were retained on 6.3 mm sieve.Fine aggregates were replaced by fine glass passing through sieve of aperture size 4.75 mm.The resulting concrete exhibited lower plastic and hardened densities (2040 and 2117 kg/m 3 respectively) in comparison to normal plain concrete.The static modulus of elasticity was found to be 18.3 GPa (mean value), while the splitting tensile strength was 2.37 MPa.The flexural strength showed a significant increase of 20.3% compared to the control mix.The results concluded that the concrete thus produced is a viable means of disposing of such nonbiodegradable wastes (rubber and glass).Furthermore, the properties tend to indicate that the concrete could be applied where lower strength and high durability properties are warranted.Hence, an attempt is made to manufacture and test the newly designed sustainable concrete in walkway slabs, which is described in section 4.

Figure 11 .Figure 12 .Figure 14 .
Figure 11.Compressive strength with different percentage of fine glass and the control batch

Figure 15 .
Figure 15.Variation of E-value for the different batches

Figure 16 .
Figure 16.Variation of Split tensile strength

Figure 17 .
Figure 17.Failure along plane of application of load Figure 18.Cylinder splitting into halves