Mechanical Properties Of Rubberized Concrete Research Articles

Experimental Investigation and Machine Learning Prediction of Mechanical Properties of Rubberized Concrete for Sustainable Construction

Concrete is widely used in civil engineering applications and the natural aggregates which used in concrete are scarce, but its demand is increasing. The disposal of rubber tyres poses a significant environmental challenge, as their decomposition releases harmful chemicals into the soil and water bodies over many years. Decomposition of tyres should be done in a smart way and hence came the emergence of mixing recycled rubber crumbs into concrete as Rubberised Concrete (RC). This paper provides an in-depth analysis of the mechanical properties of concrete such as Compressive Strength (fck), Tensile Strength (ft), Flexural Strength (fcr) of 7, 14, 28 days in replacement of fine aggregate with fine rubber (FR), and Coarse Aggregate with Coarse Rubber (CR). The results indicate that RC is more suitable for structural applications, including Reinforced Concrete columns, beams, slabs, than conventional concrete. The primary objective of the article is to explore the potential use of recycled rubber crumbs in concrete, referred to as Rubberised Concrete (RC), and to analyze its mechanical properties such as compressive strength, tensile strength, and flexural strength over different curing periods. Additionally, machine learning (ML) based prediction model has been developed for various strength characteristics of concrete mixtures at 28 days. The hyperparameter optimization using Grid Search CV with fivefold cross-validation have been performed to obtain the best hyperparameters. The model’s performance is evaluated using metrics like MAE, MSE, RMSE, and R-squared values. Results reveal varying performances among different ML algorithms for predicting flexural, tensile, and compressive strengths.

A study on mechanical properties of rubberized concrete with polypropylene fibres

A study on mechanical properties of rubberized concrete with polypropylene fibres

Mechanical properties of cement-based composites incorporating eco-friendly aggregate of waste rubber

The purpose of this study is to evaluate the waste tires that harm the environment as aggregate in concrete and to examine the effect on the mechanical properties of concrete. Three different classes (powder, crumb and chips) of waste rubber with seven different ratios (0%, 4%, 8%, 12%, 16%, 20% and 24%) and two different water to cement ratio (0.4 and 0.5) were used in concrete production. In this regard, this study conducted various experiments consisting of 28 and 90 days of the curing process to determine the properties of unit weight, compressive, flexural, and splitting tensile strength, along with ultrasonic pulse velocity (UPV) and dynamic modulus of elasticity. The waste rubber concrete with the highest compressive strength at the end of 90 days is the concrete that includes 4% waste rubber (0.4WR4) with 58.81 MPa. Concrete containing 8% waste rubber has the highest UPV of 5660 m/s after 90 days. The increase in the water/cement ratio from 0.4 to 0.5 and the waste rubber ratio cause deterioration in the mechanical properties of concrete. Although the use of waste rubbers does not bring an increase in strength, it is feasible to produce high strength concretes with 4% and 8% waste rubber substitution ratios. The water/cement ratio and curing time were highly effective on the mechanical properties of rubberized concrete.

High-Temperature Stirring Pretreatment of Waste Rubber Particles Enhances the Interfacial Bonding and Mechanical Properties of Rubberized Concrete

This paper innovatively proposes a method of 180 °C high-temperature stirring pretreatment for waste rubber particles and compares this method with untreated, NaOH-treated, and silane coupling agent KH570-treated waste rubber particles. Fourier-transform infrared spectroscopy, X-ray diffraction analysis, water contact angle measurement, scanning electron microscopy, and energy-dispersive X-ray study are used to investigate the effects and mechanisms of different pretreatment methods on waste rubber particles. The results indicate that compared to NaOH-treated and KH570-treated waste rubber particles, the 180 °C high-temperature-stirred pretreated waste rubber particles show significantly improved cleanliness and form a hard oxide film. The study also investigates the effects of different pretreatment methods on the mechanical properties and interface binding performance of rubber concrete made from pretreated waste rubber particles. The results demonstrate that rubber concrete prepared using 180 °C high-temperature-stirred pretreated waste rubber particles substituting 20% fine aggregate exhibits the best mechanical properties and interface bonding performance. The compressive strength recovery rates after 7 and 28 days are 41.6% and 37.3%, respectively; the split tensile strength recovery rates are 47.3% and 60.6%; the axial compressive strength recovery rates are 34.1% and 18.8%; and the static compression moduli of elasticity recovery rates are 46.8% and 26.3%. High-temperature stirring pretreatment of waste rubber particles is simple to operate and suitable for scaled production. Its pretreatment effect is superior to those of the KH570 and NaOH methods, providing a reference value for the scalable application of waste rubber particles as a substitute for fine aggregate in rubber concrete.

Studies on Concrete Replaced with Waste Tyre Rubber as Fine Aggregate

Concrete is one of the most extensively used construction material in the world and the mineral aggregates used to make the concrete are a finite resource, which is fast dwindling. On the other hand, Waste tyre treatment is now a serious global threat. Waste tyre dumping, disposal of these materials or burning these tyres cause serious environmental and health problems. The public, governments and industry are all greatly interested in green environment and engineering approaches towards better sustainable development. At the same time, these studies can help to make the environment clean and minimize waste. An emerging use is the production of concrete, in which rubber tyre particles Partially replace natural coarse aggregates. One of the solutions suggested is the use of tyre rubber aggregates as additive in cement based material. Waste tyres are cut into small pellets of size matching the nominal aggregate size and are used to replace the coarse aggregate used in concrete. For comparative analysis, concrete mix of M20 grade is prepared for various concrete mixes by varying percentage replacement of mineral coarse aggregates by 4, 6, and 8 rubber aggregates and the mechanical properties of rubber concrete are compared with that of conventional concrete. Different combinations of crumb rubber with traditional coarse aggregate were evaluated based on compression strength and flexural strength tests were conducted according to Indian Standard.

Effect of rubber surface treatment on damping performance of rubber-mortar ITZ in rubberized concrete

The reduction of the mechanical properties of rubberized concrete can be alleviated by surface treatment of rubber particles, which changes the interface between cement and rubber particles. The dynamic properties of the rubber-mortar interface under different bonding states were explored by performing four-point bending tests on the rubberized concrete beams. The free decay method was used to test the damping ratios of rubberized concrete beams with different interfaces and damage states. Isight was used to build an inversion analysis platform to determine the damping ratios of the rubber-mortar interfaces with different bonding states. The results show that: (1) Compared with the water treatment, the damping ratio of rubberized concrete modified by sodium hydroxide decreased by 3.01 %, while zinc stearate modification increased the damping ratio by 15.37 %; (2) The average increase in the damping ratio of zinc stearate rubberized concrete was 14.51 % for each damage level. Sodium hydroxide resulted in a 75.4 % decrease in the rubber-mortar interfacial damping ratio, while zinc stearate increased the interfacial damping ratio by 381.9 %; (3) The relationship among the normal bond strength, fracture energy and interface damping ratio was fitted by a planar relation formula.

Analyzing the behavior of graphene oxide on high-strength rubberized concrete properties using different optimization techniques

Applying rubber aggregates (RA) obtained from used rubber tires in concrete is an innovative method to substitute natural aggregate (NA) to perform sustainable construction practices. RA in concrete is not only helpful to remove the burden of used rubber tire heaps from the earth but may also give a better alternative to the fast-depleting NA. Graphene oxide (GO) was mixed in the RC to modify the physical and mechanical properties of rubberized concrete (RC). An experimental study was performed on the sixteen different concrete samples of GO-mixed RC with the help of workability, density, air content, compressive strength, tensile strength, and flexural strength tests. The research findings indicated that GO had a beneficial impact by enhancing the density, compressive strength, tensile strength, and flexural strength by counteracting the detrimental effects caused by RA on concrete. Increments up to 0.72 %, 16.23 %, 8.82 %, and 6.48 %, respectively, were reported in the density, compressive strength, tensile strength, and flexural strength of RC after the inclusion of GO. However, the incorporation of GO tended to reduce workability (up to 13.04 %) and air content (by 1.5 %) in the RC. Different optimization techniques like response surface methodology (RSM), analysis of variance (ANOVA), and heatmap correlation were applied to the experimental results. Coefficients of determination were higher than 92 % for each GO-mixed RC property, indicating a higher goodness of fit for the experimental results.

Mechanical Properties of Rubberized Concrete at Elevated Temperatures

The use of rubberized concrete has become increasingly popular as a means of disposing of waste materials, such as used and end-of-life tires, while also providing an effective solution for construction applications. The strength and durability of rubberized concrete can be negatively affected by temperature fluctuations, but little is known about the performance of this material. Hence, the work presented herein aims to evaluate the performance of rubberized concrete when it is exposed to different temperature levels. In this study, rubberized concrete specimens were prepared by replacing 5–20% of crumb rubber by volume of fine aggregate. The specimens underwent a curing process for 28 days, followed by exposure to temperatures of 200 °C, 400 °C, and 600 °C for a period of 2 h. The residual test and normal cooling method were adapted. Surface characteristics by visual inspection, the residual weight, compressive strength, splitting tensile strength, ultrasonic pulse velocity, and dynamic modulus of elasticity were assessed and compared to unheated specimens. The study’s findings revealed that, when exposed to temperatures between 200 °C and 400 °C, rubberized concrete containing a 5% to 15% rubber content experienced less reduction in compressive strength than conventional concrete, which showed a reduction of 43% to 48.5%. Also, it was observed that the splitting tensile strength was more sensitive to elevated temperatures than the compressive strength.

Mechanical Properties and Microstructure of Rubber Concrete under Coupling Action of Sulfate Attack and Dry–Wet Cycle

In order to study the mechanical properties of rubber concrete (RC) with different rubber particle sizes after dry–wet cycles in a sulfate environment, apparent morphology analysis, mass loss analysis, relative dynamic elastic modulus analysis, compressive strength loss analysis, internal microscopic characteristics and deterioration degree analysis of ordinary concrete (NC) and rubber concrete after dry–wet cycles were compared and analyzed. The results show that with the increase in the number of dry–wet cycles, the surface caves of rubber concrete increase, the internal microcracks develop and penetrate, and the macroscopic strength increases first and then decreases significantly. The high elasticity of rubber effectively improves the expansion force caused by sulfate attack and the dry–wet cycle. The deterioration degree of RC in each dry–wet cycle stage is obviously better than that of NC. When the rubber particle size is 0.85 mm, the performance of the sample is the best. After 120 days of dry–wet cycle, the compressive strength is reduced by 37.4%, and the compressive strength of concrete with a rubber particle size of 0.85 mm is reduced by 11.2%. After cyclic loading, the deterioration degree of concrete is 5.1% lower than that of ordinary concrete.

A Comprehensive Evaluation of the Mechanical Properties of Rubberized Concrete

Most metropolitan areas in the world are facing major solid-waste-disposal problems. The solid-waste problem is considered one of the major environmental problems that countries and environmental organizations are paying increasing attention to at present, not only due to its negative effects on public health and the environment, but also due to the dangers it may cause to the nearby residential communities. One of the visible solutions is to reuse solid waste as a partial replacement of concrete constituents. In this investigation, fine aggregate was replaced with crumb rubber at four different volumetric percentages, ranging from 5 to 20% with a 5% step size. A novel treatment technique based on a combination of chemical and thermal treatments of a crumb rubber surface was adopted. A superplasticizer was added to improve both the workability and the strength of the concrete mixtures. The mixtures were assessed in fresh and hardened phases and compared with a control mix. In the fresh phase, the mixtures were evaluated regarding workability and wet density; and in the hardened phase, compressive strength after 180 days, tensile and flexural strength after 90 days, dry density, and absorption were investigated. Additionally, the mixes were assessed using non-destructive tests, namely, the ultrasonic pulse velocity test, rebound hammer test, and core test. The results showed that the addition of rubber particles to concrete decreased the compressive strength, tensile strength, and flexural strength in comparison with control concrete. An empirical equation based on combined analysis with R2 = 0.95 was derived. At the age of 180 days, the compressive strength of rubberized concrete varied from 34 to 42 MPa. From a structural point of view, its strength is regarded as acceptable.

Performance of Rubberized Concrete and the Effect of Temperature and Stainless Steel Fibers

Rubberized concrete is widely used in construction by utilizing the advantages of partially replacing fine or coarse aggregate with rubber to enhance several properties of concrete and provide an environmentally friendly solution. This paper experimentally explores the influence of utilizing crumb rubber (CR) as an alternate coarse aggregate in concrete. Concrete specimens were prepared with different percentages of rubber (0%, 5%, 10%, 15%, and 20%). Additionally, other parameters, such as freezing–thawing cycles, temperature, and stainless steel fibers (SSFs), were investigated. The workability of fresh concrete and the compression properties of hardened concrete were examined. Reductions in the mechanical properties of rubberized concrete were obtained. The compressive strength reductions ranged between 13% and 50%, based on the percentage of CR in the concrete mix. However, a lesser unit weight and higher toughness were obtained relative to conventional concrete. The average unit weight decreased by 1.3%, 2.5%, 3.4%, and 5.7% of the control mixture when 5%, 10%, 15%, and 20% of the CR were incorporated into the concrete mixtures, respectively. Regression models to predict the compressive strength and unit weight of concrete with CR were developed. In addition, a life cycle cost analysis (LCCA) to identify and quantify the possible benefits of using CR in concrete mixes was carried out. Using rubberized concrete mixtures for thin whitetopping offered a slightly lower net present value compared to the ordinary concrete mix.

Experimental effect of pre-treatment of rubber fibers on mechanical properties of rubberized concrete

The current experimental research encompasses the outcome of the pre-treatment of rubber fiber (RF) derived from discarded tires when utilized in concrete as a limited substitution of sand by its volume. To overcome the causes of reduction in strengths of concrete after inclusion of rubber particles (RP) as fractional or complete replacement of fine aggregates, three different pre-treatments are adopted in this study by using sodium hydroxide (NaOH), hydrochloric acid (HCL), and water spray method, and the obtained results compared with controlled concrete and rubberized concrete (RC) with untreated RF. The fine aggregates from concrete are swapped with discarded tire RFs of size 2.36–1.18 mm and a maximum length of 20 mm from 0% to 20% with an increase of 5%. RC's mechanical properties are studied, and they are contrasted with those of controlled concrete that has no sand substitute. Fresh properties like workability, density, and hardened properties such as compressive, flexural, and splitting tensile strengths were assessed. In addition to this abrasion resistance to check wear and tear of concrete, load–displacement relationship, elastic modulus, and microstructural analysis of RC were also studied. The research study investigated that the strengths varied with the pre-treatment methods adopted. Altered concrete with RFs has shown the enhancement in strengths for a replacement of sand by RFs up to a certain limit of replacement. The maximum reduction in strengths is observed for concrete with a 20% inclusion of untreated RFs as a substitution for sand. Concrete with 10% RFs pretreated with NaOH and HCL has shown enhancements in strength and decline in rate of reduction for strengths as compared to controlled concrete and concrete with untreated RFs. Based on the major findings of this study, the better strain rate behavior of RC was observed.

Prediction of Mechanical Properties of Rubberized Concrete Incorporating Fly Ash and Nano Silica by Artificial Neural Network Technique

The use of enormous amounts of material is required for production. Due to the current emphasis on the environment and sustainability of materials, waste products and by-products, including silica fume and fly ash (FA), are incorporated into concrete as a substitute partially for cement. Additionally, concrete fine aggregate has indeed been largely replaced by waste materials like crumb rubber (CR), thus it reduces the mechanical properties but improved some other properties of the concrete. To decrease the detrimental effects of the CR, concrete is therefore enhanced with nanomaterials such nano silica (NS). The concrete mechanical properties are essential for the designing and constRuction of concrete structures. Concrete with several variables can have its mechanical characteristics predicted by an artificial neural network (ANN) technique. Using ANN approaches, this paper predict the mechanical characteristics of concrete constructed with FA as a partial substitute for cement, CR as a partial replacement for fine aggregate, and NS as an addition. Using an artificial neural network (ANN) technique, the mechanical characteristics investigated comprise splitting tensile strength (Fs), compressive strength (Fc), modulus of elasticity (Ec) and flexural strength (Ff). The ANN model was used to train and test the dataset obtained from the experimental program. Fc, Fs, Ff and Ec were predicted from added admixtures such as CR, NS, FA and curing age (P). The modelling result indicated that ANN predicted the strength with high accuracy. The proportional deviation mean (MoD) values calculated for Fc, Fs, Ff and Ec values were −0.28%, 0.14%, 0.87% and 1.17%, respectively, which are closed to zero line. The resulting ANN model’s mean square error (MSE) values and coefficient of determination (R2) are 6.45 × 10−2 and 0.99496, respectively.

Mechanical Properties of Rubberized Concrete and Its Application in Green Buildings

Global warming has become an increasingly pressing issue today due to excessive carbon emissions. The application of rubberized concrete material provides a new method for the disposal of waste rubberized tires. In terms of mechanical properties, rubberized concrete has poorer compressive strength, weaker splitting and tensile strength, but higher toughness and deformation capacity than ordinary concrete. In this paper, some of the properties of rubberized concrete are verified through finite element analysis. In terms of thermal performance, by synthesizing relevant literature, it is concluded that rubberized concrete has lower thermal conductivity and less thermal expansion than the traditional one. In terms of applications in green buildings, rubberized concrete materials are superior in thermal and acoustic insulation and have vibration damping effects, yet the peak performance is low. The research on the mechanical properties, thermal conductivity, thermal expansion, and acoustic and thermal insulation properties of rubberized concrete will provide a broader vision for rubber concrete to become a new sustainable construction material.

Comparative Effects of Chemical Pretreatments on Mechanical Properties of Sustainable Rubberized Concrete

Chemical pretreatments are known to have significant influence over mechanical properties of concrete and being able to quantify the effect of chemical pretreatments will be of great help in trying to anticipate the rubberized concrete’s possible mechanical properties. This paper presents an experimental study that was conducted on cylindrical concrete samples prepared by using different proportions of natural coarse aggregates (NCAs) replaced by pretreated; with different concentrations of chemicals, rubber coarse aggregates (RCAs). The aim was to ameliorate the mechanical properties of concrete using sustainable alternative; rubber coarse aggregate. After chemical treatment, washing and air drying, RCAs were first coated with cement paste and then dried and cured for 28 days to enhance their bonding behavior in concrete. The results confirmed the efficiency of pretreated RCAs, in improving the mechanical properties of rubberizes concrete especially the compressive strength. Key words: Compressive strength, Rubber coarse aggregates, Rubberized concrete, Pretreatment, Sustainability corresponding author: pc.ahmadali@gmail.com

A Comparative Assessment of Regularized Regression Techniques for Modeling the Mechanical Properties of Rubberized Concrete

Background: Over the last few decades, many researchers have investigated the properties and behavior of concrete mixtures incorporating rubber-based solid wastes as a partial substitution of natural aggregates. Within this context, they have conducted experimental studies and developed numerical models that simulate the nature of rubberized concrete. Some of these mathematical simulations were intended to provide a rapid mixture of proportioning approaches and property estimation methods. Currently, it is believed that regression analysis provides an effective tool to simply construct a mathematical expression that models a set of data. For that reason, multiple linear regression was extensively utilized in predicting rubberized concrete properties in the literature. However, the performances of regularized regression analysis approaches were not evaluated even though they provide better alternatives to traditional regression methods in terms of controlling the overfitting issue. Objective: This study aims to assess the performance of Ridge, Lasso, and elastic net regression models in estimating the compressive and tensile strengths, and modulus of elasticity of rubberized concrete. Additionally, it intends to benchmark their capabilities against the traditional multiple linear regression method. Methods: Multiple linear regression, Ridge regression, Lasso regression, ElasticNet regression, Bayesian ridge regression, Stochastic gradient descent, Huber regression, and Quantile regression methods were used in the study. Result: In general, the research findings illustrated the superior performance of regression assessment in modeling the mechanical properties of rubberized concrete. Conclusion: Indeed rubberized concrete mechanical properties can be better modeled using regularized regression techniques, such as ElasticNet-based SGD compared to traditional methods, such as MLR.

Mechanical properties and shear strength of rubberized fibrous reinforced concrete beams without stirrups

• Presenting the experimental shear strength results of sixteen rubberized RC beams with/without fibers. • Investigating the impact of crumb rubber on mechanical properties and shear strength of concrete beams. • Investigating the effect of fiber types and ratios on the shear strength of rubberized reinforced concrete beams. • Investigating the effect of shear span to depth ratio on the shear strength of rubberized RC beams with/without fibers. This study investigates the mechanical properties and shear strength of rubberized reinforced concrete (RC) beams with/without fibers with no web reinforcement. A total of 16 RC beams were cast and tested under four points bending to fail in shear. The study parameters are; replacement ratio by volume of fine aggregate by crumb rubber (CR) (0, 15, 30, and 40%), types of fibers (hooked long steel fiber, smooth short steel fiber, and polypropylene fiber), fiber volume fraction (0, 0.4, and 0.8%) and shear span to depth ratio a/d (2.5, 3.5, and 4.5). The results show that the mechanical properties of rubberized concrete and its weight are reduced with increasing the CR content due to the low strength, stiffness, and elastic modulus of CR, while the addition of different types of fibers, particularly hooked large steel fiber, significantly improves the mechanical properties of rubberized concrete. Using lightweight CR contributes to increasing the deformability of the tested beams and reducing their self-weight. However, increasing the CR content decreased their ultimate shear force, post-diagonal crack shear resistance, and energy absorption capacity. The inclusion of CR and fibers in concrete production can be considered as a potential technique to develop new types of eco-friendly concrete with decreased self-weight and exhibits higher shear strength than that of the control beam. Moreover, the comparison of experimental results with the predicted results of five codes and six equations from the literature showed that most of them overestimate the shear capacity of rubberized RC beams; therefore, a new predicted equation is required to predict the combined effect of rubberized fibrous RC beams.

Effectiveness of surface treatment on rubber particles towards compressive strength of rubber concrete: A numerical study on rubber-cement interface

• Surface modification methods on rubber are employed and compared. • Steered molecular dynamics simulations of rubber-SCA-cement interface are performed. • Peridynamics models of rubber mortar are developed. • Novel reinforcing method with compressive casting is proposed. Waste rubber can be added to concrete for recycling if the addition does not reduce mechanical properties of the concrete significantly. The reduction in mechanical properties of rubber concrete can be alleviated via surface treatment of the added rubber particles, which modifies the interface between cement and rubber particles. Effectiveness of the modification for compressive strength of rubber concrete is evaluated through experiments, peridynamics and molecular dynamics studies. Upper bound of the positive effect of the surface treatment for compressive strength is predicted, which can be achieved by using silane coupling agents. On top of the surface treatment, novel modification methods for rubber concrete like improvement of mechanical properties of rubber particles and of casting method are envisioned.

Influence of Basalt Fiber on Mechanical Properties and Microstructure of Rubber Concrete

The utilization of waste rubber in concrete will reduce pollution and improve the efficiency of resource utilization. The effects of rubber particles and basalt fibers on the compressive strength and splitting tensile strength of concrete were investigated. In addition, the influence of basalt fibers on the mechanical properties and micropore structure of rubber concrete (RC) were analyzed using scanning electron microscopy (SEM) and X-ray computed tomography (CT). The distribution of rubber particles in concrete was also studied. The results indicate that the effects of basalts fibers on the mechanical properties of rubber concrete were significant. The rubber particles were evenly distributed in the concrete. Compared with normal concrete (NC), rubber concrete with 10% rubber particles had lower compressive strength and splitting tensile strength. Compared with rubber concrete, basalt fiber rubber concrete (BFRC) with 2% basalt fibers had no obvious effect on the compressive strength, while significantly improving the splitting tensile strength, refining the pores of rubber concrete, and reducing the porosity of the matrix. The effects of basalt fiber on the properties and pore distribution of RC should be considered in future applications.

Influence of crumb rubber on the response of concrete beams against low velocity impact

High impact resistance and energy absorption capacity are desirable characteristics of concrete for several structural applications such as roadside barriers, bridge columns, rockfall protection barriers, and industrial buildings. Crumb rubber has been proven to enhance these characteristics of concrete. In past, numerous experimental investigations have been performed to study the mechanical properties of rubberized concrete. However, a detailed inspection of rubberized concrete under low velocity impact is still lacking. Also, the design guidelines for the employment of crumb rubber in concrete have not been explored. In the current study, the influence of partial replacement of sand with crumb rubber on the impact response of concrete has been studied. Concrete mixes with different proportions of sand replacement (5–30% by volume) have been prepared. The impact tests are conducted using drop impact test setup on concrete beams of dimension 500 mm × 100 mm × 100 mm. The specimens are subjected to an impact of 6 kg mass from an elevation of 500 mm. The results reveal that the replacement of sand with crumb rubber enhances the flexibility, energy absorption capacity, and peak impact force. Although, a curtailment in the compressive strength, elastic modulus, and split tensile strength is observed. Moreover, the microstructural properties of rubberized concrete are also investigated using SEM (Scanning Electron Microscopy), and tentative design guidelines are proposed for selecting an optimum percentage of crumb rubber in concrete.