The chapter delves into the optimization of tire rubber-concrete core materials for application in new sandwich-structured cementitious composites, addressing the challenge of low mechanical strength performance of rubberized concrete. It explores the potential of recycled tire rubber as an aggregate in concrete to improve shock wave absorption, crack resistance, and thermal insulation, while investigating solutions to enhance the strength behavior of rubberized mixtures. The study employs a multimethodological experimental analysis, including static and dynamic mechanical testing, porosity and water absorption evaluation, acoustic and thermal insulation analysis. The optimal mixture was determined using MINITAB software's design of experiment (DOE) approach, aiming to maximize insulation properties while maintaining suitable mechanical properties. The optimized formulation was then scaled up to the fabrication and first characterization of the rubberized SSC, demonstrating significant improvements in flexural strength over non-optimized rubberized mixtures. The chapter concludes with a discussion of future research directions, including the strengthening of the skins and evaluating the effect of layering time on the physical-mechanical characteristics of the composites.
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Abstract
Implementing tire rubber-concrete mixtures to produce sandwich-structured cementitious composites can represent an attractive route in the perspective of lightweight design, energy efficiency, and sustainability for the building and construction industry. This work deals with a DOE multi-response optimization study on rubber-concrete mixes designed with different proportions of fine and coarse rubber aggregates to achieve the best formulation to be applied in the manufacturing of cementitious sandwich composites. The “sand-free” concrete mixture made up of 70% of rubber powder and 30% of rubber granules was optimal in terms of mechanical properties, physical characteristics, and thermo-acoustic insulation behavior. Sandwich-structured composite incorporating the optimum mix as a core layer showed significant improvement in terms of flexural performance over the monolithic rubberized materials and strength value in the range of RILEM “class II” lightweight construction materials.
1 Introduction
Along with rapid urbanization, the century is observing the biggest increase in the built environment through the construction of buildings, road networks, dams, pavements, etc., leading to an increase in the consumption and demand of natural raw materials. Hence, alternative sources of construction materials are required to reduce the demand for virgin resources and to preserve the environment. The use of recycled rubber from end-of-life tires (ELTs) as aggregate in concrete has great potential to positively affect the engineering and environmental performance of cement-based materials for a wide spectrum of civil and architectural applications where the use of ordinary mineral aggregates is not needed (lightweight concrete, non-bearing concrete brick walls, noise barriers, pavement, improved thermal insulation for flooring in buildings, railway track beds). It was claimed that rubberized concrete has abilities to absorb a large amount of plastic energy under compressive and tensile loads, improve shock wave absorption, provide resistance to cracking, lower heat conductivity, and improve the acoustical environment which is advantageous in the applications mentioned above [1]. However, the use of tire rubber in concrete faces a problem of low mechanical strength performance which is the main barrier to full scalability in the construction industry. On this side, the challenge lies in investigating solutions that aim to enhance the strength behavior of rubberized mixtures, investigating both material optimization and advanced design solutions. Sandwich-structured composites (SSCs), consisting of a thick lightweight core and thin stiff skins, are well-established in lightweight design in many applications including automotive, aerospace, and building. SSCs offer high strength and stiffness while maintaining reduced weight, low heat and acoustic signature, and enhanced impact energy absorption characteristics. Core material properties govern many of the functionality of SSC including thermo-acoustic insulation, toughness, and lightweight [2]. Within a context of “green” design of building applications, energy efficiency, mechanical performance, and material optimization the tire rubber-concrete mixtures can represent an attractive solution for manufacturing cement-based SSCs in construction. A preliminary investigation on rubberized SSCs was conducted by the authors in previous research [3], demonstrating that the synergistic effect between a lightweight rubber-concrete core and stiff cementitious skins resulted in better mechanical properties (both static and impact response) over the monolithic rubberized materials and satisfactory acoustic insulation performance for paving unit applications. Comprehensive know-how of this novel approach requires addressing many aspects including the influence of production parameters (e.g., layering time) on SSC performance, the sandwich design (e.g., skin-core-skin thickness ratio), and the optimization of the constituent materials (core and skins). The present work faced one of these research gaps. Specifically, it presents the results of an optimization investigation of rubber-concrete mixtures that could be scaled as effective core layers for cementitious SSCs. The rubberized concrete samples were produced with two types of ground tire rubber particles, 0–1 mm rubber powder (RP) 1–3 mm rubber granules (RG) in different proportion ratios, as a total aggregate fraction of the mix design. A multimethodological experimental analysis, including static and dynamic mechanical testing, porosity and water absorption evaluation, acoustic and thermal insulation analysis of all the samples was performed, demonstrating how the size of rubber fractions influences the properties of the final concrete. The optimal mixture was achieved by MINITAB software using the design of experiment (DOE) “mixture design” approach that predicted the best parameters by investigating the combined effect of different factors simultaneously. The goal was to reach the “best” combination of fine and coarse rubber aggregates that maximizes the insulation properties, and, at the same time, maintains the physical mechanical properties at a suitable value. The optimized formulation was then scaled up to the fabrication and first characterization of the rubberized SSC aiming at verifying the truthfulness of the results.
2 Materials and Methods
2.1 Materials
The materials involved in the experimental work were cement, fine river sand, ground tire rubber (GTR), and tap water. CEM IV/A-type pozzolanic cement (strength grade of 42.5 R) was the cementitious binder used in this research. Fine-grained river sand (maximum nominal size of 1 mm) was procured from a local supplier. The GTR was supplied by the European Tyre Recycling Association (ETRA, Brussels, Belgium). Two rubber aggregate fractions, obtained by ambient grinding processing of ELTs, were used in the experimental work: 0–1 mm RP and 1–3 mm RG. The specific gravity of GTR, measured by the ethyl alcohol pycnometer method, was 1.40.
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2.2 Mix Proportion and Preparation of Test Specimens
The main purpose of the study was to determine the optimal rubber-concrete formulation to implement like a core layer in the manufacturing of SSC. The control (CTR) mix design, containing no rubber, was produced with the selected water-to-cement (W/C) ratio of 0.42 and slump index below 0.1. It consisted of 720 g of cement, 300 g of water, and 1200 g of fine sand, all per liter (L) of mix. Four rubberized mix designs (Table 1) were studied by total volumetric replacement of sand with the two rubber aggregates, following different combinations of RP and RG. The amount of cement was held constant while the water content was adjusted during the operation to meet the slump properties and workability of CTR mix. As reported in Table 1, the fine GTR was always preserved in all designed mixtures. In the best authors’ experience, fine rubber fraction is crucial to enhance the compactness and static stability of the compound.
Table 1.
Mix proportions of rubberized concrete formulations.
Sample ID
Cement [g/L]
Water [g/L]
W/C ratio
RP [g/L]
RG [g/L]
RP100
720
338
0.47
550
-
RP75RG25
720
327
0.45
412
138
RP50RG50
720
300
0.42
275
275
RP25RG75
720
248
0.34
138
412
All required materials to produce the samples were firstly weighted, then the cement and the aggregates (sand and GTR) were drill-mixed until homogenization. Then the water was added, and the wet mixture was further mixed to form the fresh compound. Fresh mixes were then molded in prism molds with the standard dimensions of 40 × 40 × 160 mm and vibrated to reduce the amount of entrained air. After 24 h, the concrete prisms were demolded and placed in small containers for water curing until the time of testing (28 days). Figure 1 displayed the GTR distribution within the concrete composites investigated.
Fig. 1.
GTR distribution in rubberized concrete samples.
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2.3 Materials Testing
An experimental program related to mechanical (flexural strength, flexural modulus, compression strength, and impact energy absorption) physical (porosity and water absorption), acoustic (sound reduction index), and thermal (thermal conductivity) characteristics of the materials was planned in a multi-response optimization scenario. Flexural test was conducted on three beams for each mix. The test was performed on a Zwick-Roell Z10 universal testing machine using a three-point configuration with 100 mm span length (ASTM C 348-02, 2008). Compression test was conducted on 40 mm-side cube specimens (six samples for each formulation) extracted from broken beams after the bending test. According to ASTM C 109/109M standard method, the test was performed by using a Zwick-Roell Z150 testing system, with preload equal to 1 MPa and a test speed to 2 mm/min. Puncture impact test was performed on an Instron 9400 drop weight impact testing machine on 40 \(\times \) 40 \(\times \) 20 mm samples (three tested specimens for each mix). The impact energy was set at 10 J. Permeable porosity and water absorption were determined using the vacuum saturation method, conforming to ASTM C1202 standard procedure. Four specimens for each mix were investigated. Thermal conductivity of the samples was studied by a C-Therm Thermal Conductivity Analyzer. The test was performed following the standard method ASTM D7984 on three specimens for each formulation. Sound reduction index was considered as an indicator to assess the acoustic insulation behavior of the materials. Acoustic insulation tests were made on two 50 \(\times \) 60 \(\times \) 80 mm test samples for mix by means of the impedance tube method that is explained in detail in Ref. [3].
2.4 DOE Optimization
The mixture design method (MDM) in Minitab software was used to design the optimum rubberized concrete mixture to be scale as a core layer in SSC manufacturing. The proposed methodology consisted of several phases. First, the upper and lower limits of the component variables (RP and RG) and the process variable (W/C ratio) had been decided. The w/c ratio was divided into two categories: w/c ratio over 0.42 (named 1) and w/c ratio lower than 0.42 (named 0). Then, mechanical, physical, and thermo-acoustic properties of the investigated samples (responses), from the experimental characterization described above, were inserted into the analysis. Finally, the tool allows getting the optimum performance frame by maximizing or minimizing the desirable and undesirable properties respectively. Mechanical and acoustic insulation response parameters selected for maximization were flexural strength (Fmax fle), flexural modulus (Emod), compression strength (Fmax com), puncture energy (Puncture), and sound reduction index (Acoustic). Thermal conductivity (Thermal), permeable porosity (Porosity), and water absorption (WA) were instead minimized. Minitab Response Optimizer tool identifies the combination of variable settings that jointly optimize a set of responses implemented in the model.
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2.5 SSC Manufacturing and First Characterization
The rubberized SSC was based on three different layers: two skin layers (10 mm-thick), made of CTR mix, separated by a rubberized concrete core (20 mm-thick). The mixture used for the core was the one resulting from DOE optimization. The sandwich-based sample was produced by a ‘three-steps’ casting method, using 40 \(\times \) 40 \(\times \) 160 mm prismatic plastic molds (Fig. 2). A layering time of 1 h was selected for manufacturing. After the same curing procedure described in Sect. 2.2, the optimized rubber-concrete core and the related sandwich samples were mechanically tested in flexural. The mechanical characterization aimed to provide the improvement effect of sandwich configuration with respect to the monolithic rubberized core material. In addition, flexural strength results were compared with the performance of “no-optimized” SSC investigated by the authors in their previous research [3] to verify the reliability of DOE optimization.
Fig. 2.
SSC preparation procedure and specimen configuration.
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3 Results
3.1 Materials Testing Results
Table 2 reports the mean value results of the experimental characterization conducted on CTR mix and the potential rubberized concrete “candidates” for the SSC design. The best performance detected in the GTR-based mixtures for each property investigated in the characterization is highlighted in bold.
Concrete mixtures including GTR of any type and combination proved to reduce the static mechanical strength (flexural and compression) and stiffness compared to CTR mixture. Moreover, the addition of rubber slightly increased the water permeability of the material. The advantage of using GTR as an aggregate fraction involves improvements in terms of impact energy absorption, noise reduction, and thermal resistivity. This finding matches those obtained by other researchers [4‐6]. By detailing the influence of the rubber particle size on the performance of the rubberized composites, favoring the fine fraction (RP) implied better static mechanical performance. Small-sized rubber particles tend to increase the concrete compactness of concrete, reduce the stress singularity at internal voids and hence reduce the likelihood of fracture [4]. Because of the smaller specific surface area, coarse rubber aggregates (RG) are more likely to develop interface defects with the cement matrix, worsening the material’s strength. Furthermore, the higher the RP content, the lower the thermal conductivity of the material and therefore better heat insulation performance.
Table 2.
Overview of results related to the materials characterization.
CTR
RP100
RP75RG25
RP50RG50
RP25RG75
Mechanical properties
Flexural strength [MPa]
5.28
1.15
1.29
1.13
1.19
Flexural modulus [GPa]
2.51
0.40
0.51
0.31
0.33
Compression strength [MPa]
37.60
3.29
2.34
3.06
1.81
Puncture energy [J]
1.67
5.85
7.01
7.66
8.00
Physical properties
Permeable porosity [%]
21.39
27.37
24.68
23.91
22.57
Water absorption [%]
16.55
22.53
19.44
18.36
16.88
Thermo-acoustic properties
Sound reduction index [dB]
14.39
14.82
15.94
15.99
13.66
Thermal conductivity [W/m\(\times \)K]
2.29
0.42
0.43
0.64
0.58
Indeed, the best heat insulation behavior was found in the RP100 mix with a drop in thermal conductivity of about 82% over the CTR sample. GRT had non-polar rough surfaces, permitting the entrainment of air (porosity) in concrete. Consequently, the smaller the particle size the greater the surface area for the same mass of rubber and thus, the greater the opportunity to entrain air [5]. This would verify the lowest thermal conductivity and highest permeability detected in the rubberized composites loaded with the high volume of RP. The coarse rubber fraction induced positive contributions in terms of mechanical-dynamic properties and acoustic dissipation. Compared to CTR mix, the maximum improvements occurred in RP25RG75 mix (about 380% increase in puncture energy absorption) and RP50RG50 mix (11% increase in sound reduction index), respectively. During impact load application, the failing rubber-concrete specimen will be capable of absorbing significant plastic energy and withstanding large deformations without full disintegration. The increased ability of the material to absorb and dissipate energy is consequently related to its sound insulation capabilities [7]. The dissipative behavior of rubberized concrete is more effective when coarse rubber particles are used in replacement of mineral aggregates than fine rubber [6]. However, by focusing on the noise abatement behavior, higher RG content (RP25RG75 sample) aggravated the insulating characteristics of the material due to the adverse effect of rubber-matrix interface gaps on its sound dissipation ability.
3.2 DOE Optimization
Targeting the maximum performance in mechanical behavior (static and dynamic) and acoustic insulation and the minimum value of thermal conductivity, porosity, and water absorption the optimal percentage of RP and RG in the concrete mixture was achieved by the Minitab Response Optimizer tool (Fig. 3). The optimal solution, possessing a higher composite desirability value (D), was 70% of RP and 30% of RG (RP70RG30 mix). D-value, expressed as a combination of the maximum or minimum of individual response desirability, is found 0.7005 which is close to 1, pointing out that settings are favorable for all responses. The optimal rubber-concrete formulation was therefore implemented as a core layer in the manufacturing of SSC samples (named SSC-RP70RG30).
Fig. 3.
Response optimizer for mixture design using Minitab software.
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3.3 Mechanical Characterization of the Optimized SSC
Three-point flexural testing was conducted on SSC-RP70RG30 samples and the respective core mixture to preliminarily assess the effect induced by the sandwich design on the mechanical behavior. As can be seen from Fig. 4, SSCs exhibit a flexure failure pattern typical of monolithic materials and no detachments emerged between the skins and the rubberized core. This demonstrates adequate compatibility and synergy between the layers resulting from a proper selection of the mixtures’ rheology and the layering time. Figure 5 displayed the results of mechanical testing in terms of flexural strength properties. The average flexural strength of the “optimum” RP70RG30 mix (1.32 MPa) is close to that predicted by Minitab (1.29 MPa), indicating a good match between the statistical analysis and the experiment. SSC-RP70RG30 samples showed a clear improvement in flexural properties over the core material of about 200%. Moreover, the optimized sandwich design performed better mechanically than the SSC samples investigated by the authors in a previous research work [3] where two not-optimized rubberized cores with different proportions of RP and RG were studied. Compared to CTR mix, while the rubberized core material suffered a strong mechanical decrease (75% drop in strength), the SSC sample experienced a milder loss of strength (25% drop in strength), providing satisfactory mechanical characteristics for use as lightweight construction material “class II”, as stipulated in the RILEM classification.
Fig. 4.
Flexural testing on SSC-RP70RG30 sample and failure pattern.
Fig. 5.
Flexural strength test results.
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4 Conclusions
In this study, an optimization analysis of GTR-cement mixtures to be implemented in the production of cementitious SSCs was conducted. Following a multi-methodological experimental characterization, the formulation with the optimal proportion of RP and RG was determined through DOE, targeting the maximization of the mechanical, durability and thermo-acoustic insulation characteristics. Then the optimum mix was employed in the development of SSC specimens and a preliminary mechanical characterization was performed. The main results are listed below:
In the investigation of rubberized concrete mixtures, it was ascertained that the size of the rubber aggregate had a significant influence on the characteristics of the material. A high content of RP preserved better static mechanical performance and heat insulation. The addition of coarse aggregate improved the dynamic mechanical and acoustic behaviors of the material.
The optimum mix from DOE analysis, by using Minitab Response Optimizer tool, was RP70RG30 (70% of RP and 30% of RG in total substitution of sand). The accuracy of the statistical analysis was demonstrated by the good match between the real strength properties (experimental) of RP70RG30 and those predicted by the software.
SSC produced with the optimum core mix (SSC-RP70RG30) showed significant improvements in flexural strength (+ 200%) over the monolithic rubberized mixtures (non-sandwich configuration) and over not-optimized rubberized sandwich composites investigated by the authors in past research.
SSC-RP70RG30 met the technical requirement of lightweight construction materials “class II” (RILEM classification).
In addition to providing a more complete overview of the technological peculiarities of the sandwich systems developed in this work, future research will also address further aspects related to the optimization of these innovative cementitious composites, including the strengthening of the skins, or evaluating the effect of the layering time on their physical-mechanical characteristics.
Acknowledgements
The authors would like to express their sincere gratitude to Dr. Ettore Musacchi (ETRA) for the supply of the rubber aggregates used in the research. Thanks are due to the Circular and Sustainable Made in Italy Extended Partnership (MICS) funded by the European Union Next-Generation EU (Piano Nazionale di Ripresa e Resilienza (PNRR) - Missione 4, Componente 2, Investimento 1.3 - D.D. 1551.11-10-2022, PE00000004) for financial support.
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