the use of waste products as substitutes in sustainable construction
THE USE OF WASTE PRODUCTS AS SUBSTITUTES IN SUSTAINABLE CONSTRUCTION MATERIALS AS PART OF A CIRCULAR ECONOMY F. H. SMITH*, V. TRAMONTIN**, C. TROIS* * School of Engineering, Civil Engineering Discipline, University of KwaZulu-Natal, Durban, South Africa ** School of Engineering, Construction Studies Discipline, University of KwaZulu-Natal, Durban, South Africa
SUMMARY: The use of recycled materials as substitutes and additives for construction products has the potential to utilise large volumes of waste in a circular economy, particularly in the context of developing countries where the implementation of effective waste management strategies has not reached enough momentum. This paper presents the results of two experimental research studies conducted in the KwaZulu-Natal province (South Africa), each evaluating the incorporation of largely locally available waste products (namely polyamide (PA) fibre (trade name nylon) from recycled tyres and recycled crushed glass -cullet) into high volume construction materials, following an integrated waste management approach. The studies are oriented to provide more environmentally sustainable solutions for concrete applications, falling under the “green concrete” category. South Africa has an estimated 60 to 100 million stockpiled used tyres, to which about 11 million waste tyres are added every year. During the recycling of tyres, the PA fibre fraction is separated from the other usable materials but has not found a useful application and continues to be sent to landfill. This paper provides a preliminary evaluation of PA fibre waste from recycled tyres, used as an additive or fine aggregate replacement in a concrete mix design. The strength of the fibre reinforced concrete was evaluated and compared with conventional concrete, using 5% (weight) fibre replacement of fine aggregate. Preliminary results are promising showing an improvement in crack resistance and flexural strength. Thus, the incorporation of the waste material into a concrete mix design has a triple positive effect: the first being the reduction of the material itself from the environment, the second being the reduced use of virgin aggregate and thirdly the materials performance modification relating to the improved crack resistance. The second study focused on recycled crushed glass (cullet), which was used as a partial replacement of fine aggregate at varying addition levels in a standard concrete mix. The chemical and physical properties of cullet are similar to fine aggregate and can be manipulated by the construction industry to produce concrete, thus reducing the amount of waste glass sent to landfill, furthermore, coloured glass, often not usable for recycling can be utilised. Experimental testing was conducted using volumetric substitutions of 10%, 25% and 50% cullet
Proceedings Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium/ 2 - 6 October 2017 S. Margherita di Pula, Cagliari, Italy / © 2017 by CISA Publisher, Italy
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
and a control sample containing only natural aggregates used for comparison. The effect of the addition of cullet was tested in terms of workability (slump) and strength (compression, tension and flexural). Results showed similar physical performance to the control concrete for the 50% cullet addition, however, the lower levels of substitution of 10% and 25% had an inconsistent but generally negative effect on all strength tests. The research therefore shows possible sustainable solutions in construction following an integrated waste management approach including the treatment of the waste materials through recovery facilities allowing their reuse for green concrete applications.
1. INTRODUCTION The requirements for a sustainable built environment demands the maximum reuse of resources through a circular economy approach. Many waste products are investigated by exploring the opportunity of recovering and reusing these as additives or substitutes in traditional construction materials or as new materials. Concrete is one of the most widely used materials in construction and offers a high degree of adaptability in terms of the possible addition or substitution of individual constituents to form composite materials. “Green concrete” (Jin and Chen, 2013) is generally the term used to identify concrete materials incorporating alternative or recycled waste materials aimed at reducing the natural resource consumption and the environmental impact of construction. The production of green concrete, targeting sectors of largely locally available waste products, can form part of a more holistic and integrated waste management approach, oriented to offer new opportunities to municipalities and the private sector in terms of circular economy and job opportunities. This is even more significant in developing countries, such as South Africa, where the implementation of effective waste management strategies has not reached enough momentum yet. This paper focuses on two sectors of waste products which are critical in the South African context and explores possible ways of reusing these as substitutes or additives in construction materials, by the collection and treatment through recovery facilities. The Recycling and Economic Development Initiative of South Africa estimated that between 60 and 100 million scrap tyres are stockpiled in South Africa (SAPA, 2012), and around 11 million waste tyres are added each year. Waste tyres pose serious environmental and health risks, clogging up landfills and create breeding grounds for mosquito larvae and rats which spread disease. When burnt for their small scrap metal content, waste tyres create air pollution both from the emitted black smoke and toxic fumes. Furthermore, the inhalation of smoke from burning tyres can cause respiratory infections degenerating to asthma. During the recycling of waste tyres, the PA fibre fraction is separated, but has not found a useful application and continues to be sent to landfill. The second waste product which was investigated in this study is recycled crushed glass (cullet). The chemical and physical properties of cullet are similar to fine aggregate used for concrete therefore it is suitable for aggregate substitution in concrete mix design (Umapathy et al., 2014). This can reduce the amount of waste glass sent to landfill and is significant for coloured glass, which is often not usable for recycling. In order to investigate opportunities for reusing the two mentioned waste products, which are critical in the South African context, the research conducted experimental testing to evaluate the performance of two green concrete materials. Partial replacement of fine aggregate was evaluated following the incorporation of 5% PA fibre waste from recycled tyres and secondly by
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
using recycled crushed glass (cullet) at 10, 25 and 50% replacement in the concrete mix designs. The results of the experimental studies are presented in this paper and critically evaluated in the light of the performance of the proposed green concrete solutions and in consideration of their contribution toward sustainability. The following section provides a review of the literature on the topic, with specific regard to waste management issues related to the two above-mentioned waste products. Then the methodology is explained in detail, followed by the presentation and discussion on the main results of the analyses. Recommendations for further research and development work are finally proposed and discussed.
2. LITERATURE REVIEW The objective of achieving zero waste to landfill and the drive towards a workable circular economy has seen more focus on waste streams that were not previously considered. Two potential waste routes are explored in this paper. The literature review focuses on the objectives of the two research studies being reviewed in this paper, namely the effect of the incorporation of waste PA fibre from car tyres and glass cullet from waste glass bottles on the workability and performance of a conventional concrete compound. 2.1 Concrete Technology General concrete is a mixture of cementitious material (binding agent), aggregate (course and fine) and water. Aggregates are inert substances (free from reactive substances) that are added to the concrete mix to give it dimensional stability and economic feasibility and make up roughly 70% of the mix design. Their shape, size and texture influence the concrete workability and final strength requiring a compromise between workability and bond strength. Workability and strength are fundamental properties of concrete. The workability is greatly influenced by the proportions of course and fine aggregates used as well the properties of the aggregates. The workability of concrete is an important factor which describes the ease of production, handling and placing and finishing. A common test for workability of concrete used in South Africa is the ‘slump test’ which is a widely adapted method due to its simplicity and consistent results. A fundamental property of concrete used in design is compressive strength which is typically 10 times more than its tensile strength and normally a measure of the value after 28 days’ cure. A knowledge of tensile strength is important to determine the loads at which tensile cracks will form; this is an important serviceability limit state, especially for water retaining structures. Since a direct tensile test is difficult to perform on concrete, two indirect methods are typically used: Split Cylinder Test and Flexural Test. This paper specifically focuses on the partial substitution of fine aggregate through the use of recycled glass cullet intended as a direct replacement and the use of recycled PA fibre which draws on data from the historical use of fibre in concrete increasing flexural strength and / or reducing crack propagation. 2.2 PA fibre from used tyres The concept of using fibres for concrete reinforcement is not a new one. Previous studies investigated the effects of incorporating fibres in concrete, for example using fibre derived from
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
recycled PET drink bottles. Generally, addition of fibres to concrete would act as crack inhibitors and substantially increase the tensile strength, cracking resistance, impact strength, wear and tear, fatigue resistance and ductility of concrete (Bon-Min Koo et. al., 2014). Kandasamy and Murugesan (2011) determined that the flexural strength of specimens with replacement of the fine aggregate with PET bottle fibres increases gradually with increase in the replacement percentage. Al-Hadithi (2015) concluded that although the workability of the concrete mix decreased, the compressive strength increased with an addition of up to 1.5% and flexural strength increased with addition up to 1.75%. Shi Yin et al. (2015) describe this a sewing effect, increasing the ductility. Furthermore, Shi Yin et al. (2015) noted when testing beams that plain concrete beams failed almost instantaneously with the occurrence of the first crack, but the fibre beams (polypropylene fibre) failed over a period of additional bending as the bending force was transmitted to the fibres once the first cracks occurred, this was a result of the matrix action that is attributed due to the concrete mixed with the fibres. The ductility of the beam was improved (Figure 1). Pešic et al. (2016) experimented with HDPE varying fibre length and concentrations concluding that the tensile strengths increased for all the substitution proportions.
Figure 1 (Shi Yin et al., 2015) 2.3 Glass cullet from used bottles The second research study considers the feasibility of utilizing waste glass as a partial replacement of natural aggregates in concrete. As the chemical and physical properties of cullet are similar to fine aggregate, cullet used in concrete can reduce the amount of waste glass that ends up in landfills (Umapathy & Mala, 2014), including coloured glass which is typically not recycled. However, currently there are very few applications of waste glass in concrete due to lack of knowledge and long term data on the behaviour of concrete containing glass aggregate (Kumar & Baskar, 2015). The Glass Recycling Company (TGRC) was established in South Africa in 2006 as a glass industry association body with the overall objective of reducing the total volume of glass being sent to landfills. There are however numerous challenges faced by TGRC. The logistics involved in transportation of waste glass poses a great challenge due to its weight and shape. Although recycling of waste is a top priority in the waste hierarchy, the cost of producing glass has decreased substantially over the past few years, hence recycling is becoming as expensive or even more expensive than producing new glass from virgin resources. This has led to a decrease in price paid for glass and an increased focus on potential alternative applications for this waste stream. Since the main component of glass is sand, the properties and specifications are very similar and concrete containing glass aggregate meets the requirements of conventional concrete (The Clean Washington Centre, 1996). A potential problem recognised by Stanton (1940) is effect of the alkali-silica reaction (ASR) which may occur between the cement which is alkaline and reactive silica, theoretically creating ‘silica gel’ which is prone to microscopic cracking in the presence of moisture. However, the presence of ASR gel may not necessarily result in
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
destruction of the concrete; it is however considered as a possible risk and should be mitigated effectively (Swamy, 1992). Positive effects of glass introduction were reported, whereby particles smaller than 1.18 mm showed lower expansion than natural fine aggregates and particles smaller than 75 µm in size, showed an increase strength development, which could possibly attribute to the cementitious properties of glass powder (Zhu, et al., 2009). In South Africa, this unconventional way of producing concrete may receive scepticism because it is perceived that waste materials are inferior to virgin materials (Caldeira, 2011) and for this reason more extensive testing is important to demonstrate the benefits and provide technical confidence.
3. METHODOLOGY The methodology followed a quantitative approach based primarily on experimental testing of green concrete materials incorporating various percentages of aggregate substitution using the two waste materials under investigation. For both waste materials, the option of evaluating their potential and feasibility as possible replacement of natural fine aggregate in concrete derives from more overarching considerations extended to the barriers and challenges for the waste management process of the two products in South Africa. For crushed glass, in particular, this argument is specifically related to the barriers to glass recycling in the Province of KwaZulu-Natal, which is the context of this study. Waste PA fibres extracted from waste tyres were tested as partial aggregate substitution in concrete. The PA fibre comes from a process called de-beading that a local waste reclamation facility uses to remove tyre wire and tyre rubber and extracting the PA fibres. The large availability of stockpiled tyres in South Africa and the associated environmental and health risks led to the investigation of alternative ways to reuse the waste sub-products, also in consideration of the gap in the literature of the behaviour of waste PA fibres from tyres in concrete. Previous research on clean PA fibres as substitute in concrete were carried out on volumetric substitutions under 2%, therefore with the objective of utilising and evaluating large quantities of fibre the present research used a weight substitution percentage of 5% of fine aggregate, which equates to 13% in volume. The modified sample was evaluated in terms of workability and strength properties of the resulting concrete against conventional concrete (0% substitution), keeping the water:cement ratio constant. The concrete properties were tested through the slump test and compressive, tensile and flexural strength testing procedures. In the province of KwaZulu-Natal the glass recycling facilitator is the Recovery Action Group, here the collected glass is crushed into cullet to maximise transport efficiency, then the mixedcoloured glass is transported to cullet processing plants in other provinces (Gauteng, Western Cape), where contaminants (metals, ceramics, labels) are removed and the glass is separated into different colours, and then melted to be reused for various applications. The increased transportation costs and the decrease in the price paid for the material have reduced the quantity of waste glass transported to the processing plants by 50%. The option of reusing crushed glass in concrete material can avoid these logistical issues since the collected glass can be crushed and graded, without colour sorting, and used for this type of application. With regard to crushed glass, the feasibility of its utilisation in concrete as partial replacement of fine aggregate was evaluated through samples containing respectively 10%, 25% and 50% fine aggregate replacement by volume, using 2mm and 5mm cullet (glass type: soda-lime glass). These were tested at 7, 14, 21 and 28 days. All mixes were proportioned in order to obtain a target compressive strength of 32.5 MPa at 28 days, as with the PA fibre assessment the water:cement ratio and cement type was unchanged and the same performance indicators
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
were assessed. For both waste materials, an economic analysis was finally conducted to compare the costs between conventional concrete and the modified concrete solutions.
4. RESULTS AND DISCUSSION 4.1 Glass Cullet The percentage of cullet substitution had a small but noticeable influence on the workability. The average slump for the control (0% substitution) was 95mm and as the percentage of glass increased the workability decreased to 75mm for the 50% cullet mix, all within the target range of 50-100mm. Strength results from the 50% cullet replacement are reviewed and displayed in Figure 2. Considering that each result represents one test sample the strength can be considered as approximately equivalent to the control. The strength of the 10% and 25% additions gave irregular results at all cure times and were typically lower than the control. Compressive strength was equivalent to the control sample after 28days (All cure times provided compression strength higher or equivalent to the control). Flexural strength was equivalent to the control sample at all cure times (with the exception of the 14-day cure sample). Tensile strength was equivalent to the control sample after 7, 14 and 21 days and higher with the 28-day cure sample. A cost analysis revealed that the utilization of cullet is not viable at the current pricing level. However, there may be a possible reduction in costs after considering the logistics and by noting is that successful recycling initiatives take time to develop and mature into successful operations and should not be primarily driven on a profit basis.
Figure 2. Compressive, flexural and tensile strength of the control sample and the sample with 50% cullet addition at 7, 14, 21 and 28 days 4.2 PA fibre The 5% substitution of fine aggregate with PA fibre gave a significant reduction in workability. The average slump for the control (0% substitution) was 120mm, whereas the fibre containing mix had a slump of only 10mm which, although well below the experimental slump for this mix of 75-150mm, actually meets the typical requirement for the category of ‘Kerbs and bedding for pipework of 10-40mm. The fibre introduction resulted in a significant and consistent reduction in compression strength after each cure period (-17% average) but a converse increase in flexural strength (+10% average) and a substantial increase in tensile strength (+65% average). It is therefore
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
clearly evident that the addition of PA fibre significantly changed the concrete from a less brittle to a more ductile material. A cost analysis determined that although the PA fibre was supplied free of charge there was virtually no change in the cost per Kg of the concrete however the lower density of the PA will result in a volume saving of approximately 8% The comparative strengths at 7, 14 and 21 days of cure are displayed in Figure 3.
Figure 3. Compressive, flexural and tensile strength of the control sample and the sample with 5% polyamide fibre substitution at 7, 14 and 21 days
5. CONCLUSIONS The substitution of 50% fine aggregate (sand) with an equal proportion of 3mm and 5mm cullet in the concrete mix provided similar physical performance to the control concrete. This is consistent with research conducted by The Clean Washington Centre (1996) which showed that concrete containing glass aggregate meets the requirements of conventional concrete. The lower levels of substitution of 10% and 25% had inconsistent but generally negative effect on all strength tests. The reason for this phenomenon is unclear and provides grounds for further study. One possible area to investigate is the control of the fineness modulus of the cullet as the as the cullet had specific 3mm and 5mm particle sizes and lacked the smaller finess particles that play an important part in the workability and final performance. Furthermore, the cementitious properties of glass powder (Zhu, et al., 2009) may further amplify this effect. The substitution of 5% fine aggregate with PA fibre provided some very significant changes in the concretes property, changing it from a less brittle to a more ductile or ‘plastic’ material whilst maintaining a reasonable 83% of the compression strength of the control mix. Testing was limited to 21-day cure due to the available time frame but the consistent changes in the concrete properties indicate that the extrapolated 28day cure are anticipated to continue with the same trend. These results lead the way for more extensive test work to determine fully the effect of different mix ratios on the concrete performance with the potential of providing and excellent material for applications where a less brittle and more ductile concrete is beneficial such as man-hole covers, paving, water retention etc. with the bonus of reduced cost and less landfill, contributing to a circular economy.
ACKNOWLEDGEMENTS The authors would like to thank Mr Jesse Mark Naicker and Mr Sachrin Naidoo that conducted the experimental studies for their BSc Eng (Civil) final year dissertations at the University of KwaZulu-Natal.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
REFERENCES Al-Hadithi A. (2015). The effects of adding waste plastic fibers on some mechanical properties of gap-graded concrete curing by drainage water and sewage water. Proceedings of the 10th Asia Pacific Conference on Sustainable Energy & Environmental Technologies (APCSEET 2015), Korea. Bon-Min K., Jang-Ho J. K., Sung-Bae K. and Sungho M. (2014), Material and structural performance evaluations of hwangtoh admixtures and recycled PET Fiber-added eco-friendly concrete for CO2 emission reduction,. Materials 2014, vol. 7, 5959-5981. Caldeira R. (2011). Glass to Sand? The Civil Engineering Contractor, 45, 46-49. Fulton F.S. (2009) Fulton’s Concrete Technology, ninth edition, Cement and Concrete Institute, South Africa. Jin R. and Chen Q. (2013). An Investigation of Current Status of “Green” Concrete in the Construction Industry. 49th ASC Annual International Conference Proceedings, California Polytechnic State University, United Stated. Nemati K. M., Monteiro P. J. and Scrivener K. L. (1998). Analysis of compressive stressinduced cracks in concrete. ACI Materials Journal, vol. 95, 617-630. Pesic N., Zivanovic S, Garcia R & Papastergiou P (2016). Mechanical properties of concrete reinforced with recycled HDPE plastic fibres. Construction and Building Materials, vol. 115, 362370. SAPA (South African Press Agency) (2012). New tyre recycling law to tackle waste problem. [Online] Available at: http://www.engineeringnews.co.za/print-version/new-tyre-recycling-law-totackle-waste-problem-2012-01-09 (Access date: 2017/04/12). Senthil Kumar K. and Baskar K. (2015). Recycling of E-plastic waste as a construction material in developing countries. Journal of Material Cycles and Waste Management. Official Journal of the Japan Society of Material Cycles and Waste Management (JSMCWM) and the Korea Society of Waste Management (KSWM), vol. 17, 718-724. Swamy R. N. (2002). The alkali-silica reaction in concrete, CRC Press. Stanton T. E. (1940). Expansion of Concrete Through Reaction Between Cement and Aggregate. American Society of Civil Engineers Proceedings, vol. 66, 1781-1811. The Clean Washington Centre (1996). Best Practices in Glass Recycling, The Behaviour of Glass Aggregate under Structural Loads. The Glass Recycling Company (2013). Annual Review 2012/2013. Umapathy U., Mala C. and Siva K. (2014). Assessment of concrete strength using partial replacement of coarse aggregate for waste tiles and cement for rice husk ash in concrete. International Journal of Engineering Research and Applications, vol. 4, n. 5, 72-76. Yin S., Tuladhar R., Collister T., Combe M., Sivakugan N. and Deng Z. (2015). Post-cracking performance of recycled polypropylene fibre in concrete. Construction and Building Materials, vol. 101 (Part 1), 1069-1077. Zhu H., Chen W., Zhou W. and Byars E. A. (2009). Expansion behaviour of glass aggregates in different testing for alkali-silica reactivity. Materials and structures, vol. 42, 485-494.
SUMMARY: The use of recycled materials as substitutes and additives for construction products has the potential to utilise large volumes of waste in a circular economy, particularly in the context of developing countries where the implementation of effective waste management strategies has not reached enough momentum. This paper presents the results of two experimental research studies conducted in the KwaZulu-Natal province (South Africa), each evaluating the incorporation of largely locally available waste products (namely polyamide (PA) fibre (trade name nylon) from recycled tyres and recycled crushed glass -cullet) into high volume construction materials, following an integrated waste management approach. The studies are oriented to provide more environmentally sustainable solutions for concrete applications, falling under the “green concrete” category. South Africa has an estimated 60 to 100 million stockpiled used tyres, to which about 11 million waste tyres are added every year. During the recycling of tyres, the PA fibre fraction is separated from the other usable materials but has not found a useful application and continues to be sent to landfill. This paper provides a preliminary evaluation of PA fibre waste from recycled tyres, used as an additive or fine aggregate replacement in a concrete mix design. The strength of the fibre reinforced concrete was evaluated and compared with conventional concrete, using 5% (weight) fibre replacement of fine aggregate. Preliminary results are promising showing an improvement in crack resistance and flexural strength. Thus, the incorporation of the waste material into a concrete mix design has a triple positive effect: the first being the reduction of the material itself from the environment, the second being the reduced use of virgin aggregate and thirdly the materials performance modification relating to the improved crack resistance. The second study focused on recycled crushed glass (cullet), which was used as a partial replacement of fine aggregate at varying addition levels in a standard concrete mix. The chemical and physical properties of cullet are similar to fine aggregate and can be manipulated by the construction industry to produce concrete, thus reducing the amount of waste glass sent to landfill, furthermore, coloured glass, often not usable for recycling can be utilised. Experimental testing was conducted using volumetric substitutions of 10%, 25% and 50% cullet
Proceedings Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium/ 2 - 6 October 2017 S. Margherita di Pula, Cagliari, Italy / © 2017 by CISA Publisher, Italy
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
and a control sample containing only natural aggregates used for comparison. The effect of the addition of cullet was tested in terms of workability (slump) and strength (compression, tension and flexural). Results showed similar physical performance to the control concrete for the 50% cullet addition, however, the lower levels of substitution of 10% and 25% had an inconsistent but generally negative effect on all strength tests. The research therefore shows possible sustainable solutions in construction following an integrated waste management approach including the treatment of the waste materials through recovery facilities allowing their reuse for green concrete applications.
1. INTRODUCTION The requirements for a sustainable built environment demands the maximum reuse of resources through a circular economy approach. Many waste products are investigated by exploring the opportunity of recovering and reusing these as additives or substitutes in traditional construction materials or as new materials. Concrete is one of the most widely used materials in construction and offers a high degree of adaptability in terms of the possible addition or substitution of individual constituents to form composite materials. “Green concrete” (Jin and Chen, 2013) is generally the term used to identify concrete materials incorporating alternative or recycled waste materials aimed at reducing the natural resource consumption and the environmental impact of construction. The production of green concrete, targeting sectors of largely locally available waste products, can form part of a more holistic and integrated waste management approach, oriented to offer new opportunities to municipalities and the private sector in terms of circular economy and job opportunities. This is even more significant in developing countries, such as South Africa, where the implementation of effective waste management strategies has not reached enough momentum yet. This paper focuses on two sectors of waste products which are critical in the South African context and explores possible ways of reusing these as substitutes or additives in construction materials, by the collection and treatment through recovery facilities. The Recycling and Economic Development Initiative of South Africa estimated that between 60 and 100 million scrap tyres are stockpiled in South Africa (SAPA, 2012), and around 11 million waste tyres are added each year. Waste tyres pose serious environmental and health risks, clogging up landfills and create breeding grounds for mosquito larvae and rats which spread disease. When burnt for their small scrap metal content, waste tyres create air pollution both from the emitted black smoke and toxic fumes. Furthermore, the inhalation of smoke from burning tyres can cause respiratory infections degenerating to asthma. During the recycling of waste tyres, the PA fibre fraction is separated, but has not found a useful application and continues to be sent to landfill. The second waste product which was investigated in this study is recycled crushed glass (cullet). The chemical and physical properties of cullet are similar to fine aggregate used for concrete therefore it is suitable for aggregate substitution in concrete mix design (Umapathy et al., 2014). This can reduce the amount of waste glass sent to landfill and is significant for coloured glass, which is often not usable for recycling. In order to investigate opportunities for reusing the two mentioned waste products, which are critical in the South African context, the research conducted experimental testing to evaluate the performance of two green concrete materials. Partial replacement of fine aggregate was evaluated following the incorporation of 5% PA fibre waste from recycled tyres and secondly by
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
using recycled crushed glass (cullet) at 10, 25 and 50% replacement in the concrete mix designs. The results of the experimental studies are presented in this paper and critically evaluated in the light of the performance of the proposed green concrete solutions and in consideration of their contribution toward sustainability. The following section provides a review of the literature on the topic, with specific regard to waste management issues related to the two above-mentioned waste products. Then the methodology is explained in detail, followed by the presentation and discussion on the main results of the analyses. Recommendations for further research and development work are finally proposed and discussed.
2. LITERATURE REVIEW The objective of achieving zero waste to landfill and the drive towards a workable circular economy has seen more focus on waste streams that were not previously considered. Two potential waste routes are explored in this paper. The literature review focuses on the objectives of the two research studies being reviewed in this paper, namely the effect of the incorporation of waste PA fibre from car tyres and glass cullet from waste glass bottles on the workability and performance of a conventional concrete compound. 2.1 Concrete Technology General concrete is a mixture of cementitious material (binding agent), aggregate (course and fine) and water. Aggregates are inert substances (free from reactive substances) that are added to the concrete mix to give it dimensional stability and economic feasibility and make up roughly 70% of the mix design. Their shape, size and texture influence the concrete workability and final strength requiring a compromise between workability and bond strength. Workability and strength are fundamental properties of concrete. The workability is greatly influenced by the proportions of course and fine aggregates used as well the properties of the aggregates. The workability of concrete is an important factor which describes the ease of production, handling and placing and finishing. A common test for workability of concrete used in South Africa is the ‘slump test’ which is a widely adapted method due to its simplicity and consistent results. A fundamental property of concrete used in design is compressive strength which is typically 10 times more than its tensile strength and normally a measure of the value after 28 days’ cure. A knowledge of tensile strength is important to determine the loads at which tensile cracks will form; this is an important serviceability limit state, especially for water retaining structures. Since a direct tensile test is difficult to perform on concrete, two indirect methods are typically used: Split Cylinder Test and Flexural Test. This paper specifically focuses on the partial substitution of fine aggregate through the use of recycled glass cullet intended as a direct replacement and the use of recycled PA fibre which draws on data from the historical use of fibre in concrete increasing flexural strength and / or reducing crack propagation. 2.2 PA fibre from used tyres The concept of using fibres for concrete reinforcement is not a new one. Previous studies investigated the effects of incorporating fibres in concrete, for example using fibre derived from
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
recycled PET drink bottles. Generally, addition of fibres to concrete would act as crack inhibitors and substantially increase the tensile strength, cracking resistance, impact strength, wear and tear, fatigue resistance and ductility of concrete (Bon-Min Koo et. al., 2014). Kandasamy and Murugesan (2011) determined that the flexural strength of specimens with replacement of the fine aggregate with PET bottle fibres increases gradually with increase in the replacement percentage. Al-Hadithi (2015) concluded that although the workability of the concrete mix decreased, the compressive strength increased with an addition of up to 1.5% and flexural strength increased with addition up to 1.75%. Shi Yin et al. (2015) describe this a sewing effect, increasing the ductility. Furthermore, Shi Yin et al. (2015) noted when testing beams that plain concrete beams failed almost instantaneously with the occurrence of the first crack, but the fibre beams (polypropylene fibre) failed over a period of additional bending as the bending force was transmitted to the fibres once the first cracks occurred, this was a result of the matrix action that is attributed due to the concrete mixed with the fibres. The ductility of the beam was improved (Figure 1). Pešic et al. (2016) experimented with HDPE varying fibre length and concentrations concluding that the tensile strengths increased for all the substitution proportions.
Figure 1 (Shi Yin et al., 2015) 2.3 Glass cullet from used bottles The second research study considers the feasibility of utilizing waste glass as a partial replacement of natural aggregates in concrete. As the chemical and physical properties of cullet are similar to fine aggregate, cullet used in concrete can reduce the amount of waste glass that ends up in landfills (Umapathy & Mala, 2014), including coloured glass which is typically not recycled. However, currently there are very few applications of waste glass in concrete due to lack of knowledge and long term data on the behaviour of concrete containing glass aggregate (Kumar & Baskar, 2015). The Glass Recycling Company (TGRC) was established in South Africa in 2006 as a glass industry association body with the overall objective of reducing the total volume of glass being sent to landfills. There are however numerous challenges faced by TGRC. The logistics involved in transportation of waste glass poses a great challenge due to its weight and shape. Although recycling of waste is a top priority in the waste hierarchy, the cost of producing glass has decreased substantially over the past few years, hence recycling is becoming as expensive or even more expensive than producing new glass from virgin resources. This has led to a decrease in price paid for glass and an increased focus on potential alternative applications for this waste stream. Since the main component of glass is sand, the properties and specifications are very similar and concrete containing glass aggregate meets the requirements of conventional concrete (The Clean Washington Centre, 1996). A potential problem recognised by Stanton (1940) is effect of the alkali-silica reaction (ASR) which may occur between the cement which is alkaline and reactive silica, theoretically creating ‘silica gel’ which is prone to microscopic cracking in the presence of moisture. However, the presence of ASR gel may not necessarily result in
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
destruction of the concrete; it is however considered as a possible risk and should be mitigated effectively (Swamy, 1992). Positive effects of glass introduction were reported, whereby particles smaller than 1.18 mm showed lower expansion than natural fine aggregates and particles smaller than 75 µm in size, showed an increase strength development, which could possibly attribute to the cementitious properties of glass powder (Zhu, et al., 2009). In South Africa, this unconventional way of producing concrete may receive scepticism because it is perceived that waste materials are inferior to virgin materials (Caldeira, 2011) and for this reason more extensive testing is important to demonstrate the benefits and provide technical confidence.
3. METHODOLOGY The methodology followed a quantitative approach based primarily on experimental testing of green concrete materials incorporating various percentages of aggregate substitution using the two waste materials under investigation. For both waste materials, the option of evaluating their potential and feasibility as possible replacement of natural fine aggregate in concrete derives from more overarching considerations extended to the barriers and challenges for the waste management process of the two products in South Africa. For crushed glass, in particular, this argument is specifically related to the barriers to glass recycling in the Province of KwaZulu-Natal, which is the context of this study. Waste PA fibres extracted from waste tyres were tested as partial aggregate substitution in concrete. The PA fibre comes from a process called de-beading that a local waste reclamation facility uses to remove tyre wire and tyre rubber and extracting the PA fibres. The large availability of stockpiled tyres in South Africa and the associated environmental and health risks led to the investigation of alternative ways to reuse the waste sub-products, also in consideration of the gap in the literature of the behaviour of waste PA fibres from tyres in concrete. Previous research on clean PA fibres as substitute in concrete were carried out on volumetric substitutions under 2%, therefore with the objective of utilising and evaluating large quantities of fibre the present research used a weight substitution percentage of 5% of fine aggregate, which equates to 13% in volume. The modified sample was evaluated in terms of workability and strength properties of the resulting concrete against conventional concrete (0% substitution), keeping the water:cement ratio constant. The concrete properties were tested through the slump test and compressive, tensile and flexural strength testing procedures. In the province of KwaZulu-Natal the glass recycling facilitator is the Recovery Action Group, here the collected glass is crushed into cullet to maximise transport efficiency, then the mixedcoloured glass is transported to cullet processing plants in other provinces (Gauteng, Western Cape), where contaminants (metals, ceramics, labels) are removed and the glass is separated into different colours, and then melted to be reused for various applications. The increased transportation costs and the decrease in the price paid for the material have reduced the quantity of waste glass transported to the processing plants by 50%. The option of reusing crushed glass in concrete material can avoid these logistical issues since the collected glass can be crushed and graded, without colour sorting, and used for this type of application. With regard to crushed glass, the feasibility of its utilisation in concrete as partial replacement of fine aggregate was evaluated through samples containing respectively 10%, 25% and 50% fine aggregate replacement by volume, using 2mm and 5mm cullet (glass type: soda-lime glass). These were tested at 7, 14, 21 and 28 days. All mixes were proportioned in order to obtain a target compressive strength of 32.5 MPa at 28 days, as with the PA fibre assessment the water:cement ratio and cement type was unchanged and the same performance indicators
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
were assessed. For both waste materials, an economic analysis was finally conducted to compare the costs between conventional concrete and the modified concrete solutions.
4. RESULTS AND DISCUSSION 4.1 Glass Cullet The percentage of cullet substitution had a small but noticeable influence on the workability. The average slump for the control (0% substitution) was 95mm and as the percentage of glass increased the workability decreased to 75mm for the 50% cullet mix, all within the target range of 50-100mm. Strength results from the 50% cullet replacement are reviewed and displayed in Figure 2. Considering that each result represents one test sample the strength can be considered as approximately equivalent to the control. The strength of the 10% and 25% additions gave irregular results at all cure times and were typically lower than the control. Compressive strength was equivalent to the control sample after 28days (All cure times provided compression strength higher or equivalent to the control). Flexural strength was equivalent to the control sample at all cure times (with the exception of the 14-day cure sample). Tensile strength was equivalent to the control sample after 7, 14 and 21 days and higher with the 28-day cure sample. A cost analysis revealed that the utilization of cullet is not viable at the current pricing level. However, there may be a possible reduction in costs after considering the logistics and by noting is that successful recycling initiatives take time to develop and mature into successful operations and should not be primarily driven on a profit basis.
Figure 2. Compressive, flexural and tensile strength of the control sample and the sample with 50% cullet addition at 7, 14, 21 and 28 days 4.2 PA fibre The 5% substitution of fine aggregate with PA fibre gave a significant reduction in workability. The average slump for the control (0% substitution) was 120mm, whereas the fibre containing mix had a slump of only 10mm which, although well below the experimental slump for this mix of 75-150mm, actually meets the typical requirement for the category of ‘Kerbs and bedding for pipework of 10-40mm. The fibre introduction resulted in a significant and consistent reduction in compression strength after each cure period (-17% average) but a converse increase in flexural strength (+10% average) and a substantial increase in tensile strength (+65% average). It is therefore
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
clearly evident that the addition of PA fibre significantly changed the concrete from a less brittle to a more ductile material. A cost analysis determined that although the PA fibre was supplied free of charge there was virtually no change in the cost per Kg of the concrete however the lower density of the PA will result in a volume saving of approximately 8% The comparative strengths at 7, 14 and 21 days of cure are displayed in Figure 3.
Figure 3. Compressive, flexural and tensile strength of the control sample and the sample with 5% polyamide fibre substitution at 7, 14 and 21 days
5. CONCLUSIONS The substitution of 50% fine aggregate (sand) with an equal proportion of 3mm and 5mm cullet in the concrete mix provided similar physical performance to the control concrete. This is consistent with research conducted by The Clean Washington Centre (1996) which showed that concrete containing glass aggregate meets the requirements of conventional concrete. The lower levels of substitution of 10% and 25% had inconsistent but generally negative effect on all strength tests. The reason for this phenomenon is unclear and provides grounds for further study. One possible area to investigate is the control of the fineness modulus of the cullet as the as the cullet had specific 3mm and 5mm particle sizes and lacked the smaller finess particles that play an important part in the workability and final performance. Furthermore, the cementitious properties of glass powder (Zhu, et al., 2009) may further amplify this effect. The substitution of 5% fine aggregate with PA fibre provided some very significant changes in the concretes property, changing it from a less brittle to a more ductile or ‘plastic’ material whilst maintaining a reasonable 83% of the compression strength of the control mix. Testing was limited to 21-day cure due to the available time frame but the consistent changes in the concrete properties indicate that the extrapolated 28day cure are anticipated to continue with the same trend. These results lead the way for more extensive test work to determine fully the effect of different mix ratios on the concrete performance with the potential of providing and excellent material for applications where a less brittle and more ductile concrete is beneficial such as man-hole covers, paving, water retention etc. with the bonus of reduced cost and less landfill, contributing to a circular economy.
ACKNOWLEDGEMENTS The authors would like to thank Mr Jesse Mark Naicker and Mr Sachrin Naidoo that conducted the experimental studies for their BSc Eng (Civil) final year dissertations at the University of KwaZulu-Natal.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
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