LIFE CYCLE ASSESSMENT OF LATTICE JOIST PRECAST SLABS WITH TIRE RUBBER RESIDUE ADDITION

1 INTRODUCTION

The construction industry is extremely significant for a nation, as it directly influences environmental, social, and economic aspects (MARCELINO-SADABA et al., 2017; SILVA et al., 2018; YUSOF; IRANMANESH, 2017). Brazilian construction sector strongly impacts the economy, representing 3.3% of the Gross Domestic Product (IBGE, 2020) and employing more than 1.8 million workers (PDET, 2019) in 2018.

Some possible solutions to deal with the negative particularities of civil construction would be to encourage: (i) the use of materials or construction methods that generate less waste and are more efficient; (ii) the incorporation of waste in constructions, through the development of new composites, reducing the amount of raw material extracted (AZEVEDO, 2015; JI et al., 2018; ZHENG et al., 2019). Two residues with the potential to be incorporated into constructions are tire rubber residues (TRR) and rice husk ash (RHA).

In recent years, numerous studies have been carried out on the use of alternative materials in civil construction, generally considering only technical aspects, such as mechanical properties (HUANG et al., 2020; MESGARI; AKBARNEZHAD; XIAO, 2020; ROYCHAND et al., 2020; WANG et al., 2019) and, eventually, evaluating economic or environmental aspects (ARRIGONI et al., 2017; NAKIC, 2018; STAFFORD et al., 2016). However, it is not enough just to incorporate these residues without carrying out a detailed study, since, when modifying the composition of a conventional material, the environmental, social, and economic impact will not necessarily be mitigated to justify a wide production of the new compositions developed.

The methodology that integrates environmental, social, and economic aspects from a life cycle perspective is called Life Cycle Sustainability Assessment (LCSA). LCSA allows a more holistic understanding of products and processes among existing sustainability tools, providing better support for decision makers (FINKBEINER et al., 2010; UNEP/SETAC, 2011).

Therefore, this work intends to apply the LCSA to evaluate the environmental, social and economic performance of precast lattice slabs made with conventional concrete, compared to slabs produced from concrete with the addition of TRR and RHA.

2 METHODS

For the production of the concrete slabs, the following materials were used: cement, sand, gravel, water, TRR, RHA, superplasticizer, hollow clay bricks, and steel bars.

The objective of the study is to carry out a LCSA comparing conventional concrete with concrete mixtures with the addition of TRR and RHA, used in the lab-scale production of precast lattice slabs, to verify if the additions bring environmental, social and economic benefits to the final product. The approach that will be used in this work is the one suggested by Klöpffer (2008), in which LCSA = Life Cycle Assessment (LCA) + Life Cycle Cost (LCC) + Social Life Cycle Assessment (SLCA), without any formal weighting between them.

In an LCSA, the inventory and impact indicators must be related to the same functional unit of the product or service (UNEP/SETAC, 2011). Therefore, the functional unit adopted will be one square meter (1 m²) of slab.

The precast slab model, shown in Figure 1, have a minimum compressive strength of 25 MPa, at 28 days of age, and the volume of concrete used to make 1m² of slab is 0.046 m³.

Figure 1. Slab model cross section.

The system studied will be the production of concrete used in the manufacture of slabs (conventional and with residues), considering the production and processing of raw materials, transport and mixing the components, resulting in concrete. A "cradle-to-gate" approach will be adopted, from raw material extraction to production.

The use of waste TRR and RHA in precast lattice slabs did not significantly affect the structural behavior regarding the Ultimate and Service Limit States (FAZZAN, 2011; SOUSA, 2014). Thus, the use and disposal phases of both compositions will be disregarded in the comparison. In the same way, the materials that were used in equal amounts in the two slabs (hollow clay bricks and steel reinforcements) were not considered from the analysis.

The three scenarios analyzed are as follows:

• Scenario 1: production of conventional concrete slab;

• Scenario 2: production of concrete slab with TRR (replacing sand);

• Scenario 3: production of concrete slab with TRR (replacing sand) and RHA (replacing cement).

Secondary data was used in the project. The energy consumption used in the concrete mixing process was obtained from Van den Heede and Belie (2014). Data on raw material production and electricity generation were obtained from Ecoinvent v.3 (WERNET et al., 2016). Data on transport distances for sand, gravel, ceramic tiles and steel bars were obtained from local suppliers of construction materials and previous studies (CELIK et al., 2015; SOUZA et al., 2016). Social data can be obtained from the International Labor Organization database (ILO, 2020) and from official Brazilian government databases, such as IBGE and PDET. Economic data was collected from local suppliers and building materials manufacturers.

The method used to carry out the assessment of environmental impacts was the ReCiPe 2016 Midpoint Hierarquist v.1.01 (HUIJBREGTS et al., 2016), using the SimaPro software v.8.5.0.0 (PRÉ CONSULTANTS, 2019),

For the LCC, the costs of acquiring materials, transporting these materials to the concrete plant and the energy costs of production will be analyzed.

The product SLCA approach will be the Reference Scale Approach (RS S-LCIA), using four subcategories of the Workers category, as materials from different productive sectors are used (UNEP, 2020 ).

The quantities of materials that were used in each scenario are shown in Table 1.

Inputs:	Scenario 1	Scenario 2		Scenario 3
Portland cement (kg)	15.730		16.560	15.730
Sand (kg)	40.710		35.490	35.490
Gravel (kg)	46.180		54.880	54.880
RHA (kg)	–		–	0.830
TRR (kg)	–		1.660	1.660
Superplasticizer (kg)	–		0.132	0.126
Tap water (m³)	0.0089		0.0065	0.0065
Hollow clay brick (kg)	40.70		40.70	40.70
Steel bars (kg)	2.50		2.50	2.50
Electricity (kWh)	0.176		0.218	0.218

Table 1. Quantity of materials used.

3 RESULTS

The processes described were modeled in the SimaPro software version 8.5.0.0 (PRÉ CONSULTANTS, 2019), using the Ecoinvent database (WERNET et al., 2016). Figure 2 presents a comparison of the impacts generated by the production of 1 m² of slab for each scenario, verifying the alternative that has the greatest impacts in each category analyzed.

Figure 2. Potential impacts of the proposed scenarios.

Therefore, for the conditions of the study, in all the categories analyzed Scenario II is the most impacting from an environmental point of view, followed by Scenario III, with Scenario I presenting the lowest impacts potentials. This result can be explained by the fact that Scenario II has more cement in its composition, in addition to requiring superplasticizer and more gravel. Scenario III, in turn, has the same amount of cement as Scenario I, but it also needs superplasticizer and a greater amount of gravel.

The purpose of the LCC is to determine the cost-effectiveness of alternative investments and business decisions, from the perspective of an economic decision maker, such as a company or a consumer (NORRIS, 2001). The monetary values were obtained mainly from suppliers and manufacturers and are shown in Table 2. As the data were all collected in the year 2020, it was not necessary to adjust the values for inflation to compare them.

Inputs:	Scenario 1	Scenario 2		Scenario 3
Portland cement	R$ 10.63		R$ 11.19	R$ 10.63
Sand	R$ 1.01		R$ 0.88	R$ 0.88
Gravel	R$ 1.71		R$ 2.03	R$ 2.03
RHA	R$ -		R$ -	R$ 0.02
TRR	R$ -		R$ 0.04	R$ 0.04
Superplasticizer	R$ -		R$ 1.07	R$ 1.03
Tap water	R$ 0.00		R$ 0.00	R$ 0.00
Electricity	R$ 0.13		R$ 0.16	R$ 0.16
Total	R$ 13.47		R$ 15.37	R$ 14.78

Table 2.: Cost of materials acquisition and transportation.

Analyzing the cost, Scenario I was the cheaper alternative, with Scenario II being 18.10% more expensive, and Scenario III about 13.51% more expensive. It is possible to observe that concrete was the main contributor to material costs, representing 77.04%, 68.67% and 67.87% of the costs of Scenarios I, II and III, respectively. It is also important to highlight the impact of the superplasticizer, which, despite representing just over 0.1% of the mass of the functional unit (1 m² of slab) in Scenarios II and III, was responsible for 9.78% and 9.71%, respectively, of the total cost of materials.

SLCA aims to assess the social impacts of products and services, considering the life cycle approach. It is a systematic method that combines quantitative and qualitative data, generating information, recommendations, and conclusions regarding social and socioeconomic scopes, serving as a foundation for decision-making at operational (products) or organizational levels (UNEP, 2020). Figure 3 shows the results obtained for the analyzed materials and its productive sectors. Green represents an above-average result in the subcategory, meanwhile Red represents alarming results compared to the average worker in that productive sector. Yellow stands for the average value for that sector in each subcategory.

Figure 3.: SLCA Results.

Classifying the sectors according to the results on the reference scales, considering that all categories have equal weights, we have that the order from the worst to the best classified sector from the workers' point of view is: TRR; RHA, Sand and Gravel (tied); Cement; Superplasticizer

Connecting the Scenarios with the sectors, Scenario I have a predominance of Sand, Scenario II has a greater amount of Cement, Gravel, Rubber and has Superplasticizer, and Scenario III has a greater amount of RHA, TRR and Gravel, and also have Superplasticizer. Therefore, from a social point of view, the most beneficial scenario for workers would be Scenario II, followed by Scenario I, while Scenario III is the worst classified.

4 CONCLUSIONS

Based on the results obtained, from the combined economic, environmental, and social point of view, it is possible to conclude that Scenario I (conventional concrete), within the conditions and hypotheses adopted in this study, was the most adequate alternative, even without alternative materials in its composition, followed by Scenario III, and finally Scenario II. The addition of TRR caused a reduction in mechanical strength, which required a greater amount of cement so that all mixtures had the same strength.

According to the results, however, the most efficient way to improve the sustainable profile of the slabs studied would be to replace or reduce the amount of cement, while avoiding the use of superplasticizer as much as possible. This is noticed comparing Scenarios II and III, in which when partially replacing cement with RHA, there is an improvement in the environmental and economic footprint.

ACKNOWLEDGMENTS

This research was financed by the Fundação e Amparo a Pesquisa do Estado de São Paulo – FAPESP (Proc. nº 2018/23233-4).

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