Influence of Styrene-Butadiene Rubber Doping on the Dynamic and Mechanical Properties of Polymer Concrete

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In ﬂ uence of doping with styrene-butadiene rubber on dynamic and mechanical properties of polymer concrete

Kinga Deredas

^a

, Norbert K ę pczak

^b,^⇑

, Mariusz Urbaniak

^c

aFaculty of Mechanical Engineering, Lodz University of Technology, Stefanowskiego 1/15, 90-537 Lodz, Poland

bInstitute of Machine Tools and Production Engineering, Faculty of Mechanical Engineering, Lodz University of Technology, Stefanowskiego 1/15, 90-537 Lodz, Poland

cDepartment of Strength of Materials, Faculty of Mechanical Engineering, Lodz University of Technology, Stefanowskiego 1/15, 90-537 Lodz, Poland

A R T I C L E I N F O

Keywords:

Polymer concrete

Rubberized polymer concrete Dynamic properties Modal analysis Mechanical properties SBR

A B S T R A C T

The paper studies the inﬂuence of doping polymer concrete with styrene‐butadiene rubber (SBR) on its dynamic and mechanical properties. The research was divided into two stages. First, experimental modal analysis was carried out in the time and frequency domain from 0 to 2000 Hz to elaborate on the dynamic properties of the polymer. The results indicated that the sample with 30% of SBR had the lowest vibration amplitude value, while samples with 20 and 30% SBR had the highest values for the damping ratio. In the second stage, the mechanical properties of the samples were investigated–compression andﬂexural tests were performed. In both cases, the highest decrease of material strength was noticed for the 30% SBR samples.

Although, increasing the amount of rubber granulate (SBR) in the polymer concrete improved the damping ratio and reduced the vibration amplitude. Based on these investigations it is not recommend to dope polymer concrete with more than 20% of SBR by volume.

1. Introduction

Polymer concretes (PC, mineral casts) have been known and used in the construction of machine tools for several decades. A mineral cast is a cast made of a composite material combine ofﬁne particles of inor- ganic aggregates–silica materials[1–4], bounded with a resin binder –usually epoxy, polyester or vinyl resin[5–7]. Due to this material’s structural analogy to concrete (the cement is replaced with a two‐ component resin), the colloquial name of the composite as polymer concrete has been adopted. Mineral cast turns out to be a particularly interesting material in theﬁeld of dynamic properties, especially for vibration damping. It characterized by a several times higher damping ratio in comparison to cast iron[8]. Furthermore, polymer concrete has high heat capacity and low thermal conductivity, relatively low density, corrosion resistance and hydrophobicity[7,8]. These advan- tages determined the use of polymer concrete as a material for machine tool beds and machine bodies, especially in constructions where the basic assumption is to stabilize the system by damping vibrations[9–11].

The mineral cast structure and its production method favor the ease of enclosing various types of materials in its structure, increasing speci- ﬁc properties of polymer concrete. Many cases of doping mineral cast

have been found in the literature. Such materials asfly ash[12], clay powder, lime powder, marble powder[13], steelfiber, glassfiber, steel slag, silica dust, basaltfiber and nanoglycine[14]have been used. All these additions were introduced into the material structure asfillers in exchange for fine aggregates. Bedi, Chandara and Sinht [12], after reviewing the literature, recommended the addition of fly ash as a microfiller to improve mechanical properties. The addition of 15%

ofﬂay ash resulted in a 30% increase in compressive strength.

Barbuta, Rujanu and Nicuta[13]studied the impact of additions such asfly ash, clay powder, lime powder and marble powder, used asfillers, on the microstructure and mechanical properties of polymer concrete. They carried outflexural, compressive and tensile strength tests on the modified composites. The samples were made with 12.4% epoxy resin content and 12.8% of the chosen type of powder.

The results indicated that most of the additives used do not change or reduce the mechanical strength of the mineral cast. Only lime powder signiﬁcantly increased each of the tested strengths, by approximately 20% compared to a sample of pure polymer concrete.

Attempts were also made to dope polymer concrete with post‐

production waste such as electronic plastic waste[14], textileﬁber [15], and crushed rubber tires[16,17]. These treatments were aimed at partially utilizing garbage which has barely decomposed. Further-

https://doi.org/10.1016/j.compstruct.2021.113998

Received 31 July 2020; Revised 6 April 2021; Accepted 13 April 2021 Available online 17 April 2021

This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

⇑Corresponding author.

E-mail address:[email protected](N. Kępczak).

Contents lists available atScienceDirect

Composite Structures

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more, waste rubber tires were also successfully added to the structure of concrete[18,19].

Bulut and Sahin investigated the impact of adding shredded electronic plastic waste as part of thefiller materials (up to 25% of the sample volume) on the mechanical properties of polymer concrete [14]. Additionally, samples with different unsaturated polyester resin content–10, 15 and 20%–were tested. As a result of their experiments, they found that an increase in the resin content increases the compressive strength, without having a significant impact on the bending and tensile strength. They also stated that while a higher electronic plastic waste content decreases the composite’s mechanical strength, it improved the plasticity. They determined the optimal composition of the doped mineral cast as 15% resin and 5% electronic waste, which allows the recycling of plastics, without a significant loss of the mechanical properties.

Wang et al.[16]attempted to add rubber crumbs to polymer concrete in a target application as a surface layer or for concrete repairs.

They dopped mineral cast with rubber crumbs of an average size of 0.279 mm, in two different proportions: 5% and 10% by weight of the resin. The results of the tests showed there was an increase in tensile and compressive strength for the samples with 5% rubber content, and a slight decrease for the samples with 10% rubber. Furthermore, they noticed a decline in thermal conductivity with increased rubber content. The study of material topography using SEM allowed the identiﬁcation of strong bonds between the rubber particles and the epoxy resin. Their research conﬁrmed the strengthening of polymer concrete when admixed with rubber crumbs, also emphasizing the possibility of utilizing rubber tires in the material.

In his studies, Laredo dos Reis conducted bending and compression strength tests of polymer concrete samples with the addition of 0, 1 and 2% crushed textilefibers[15]. He also considered different resin mass ratios in the material–10 and 12%. The crushed textilefibers, used as a replacement for some of thefiller, came from the waste of a Brazilian underwear manufacturer, and consisted mainly of cotton, polyester, natural and artificial silk. The research has established a significant decrease in the mechanical properties of mineral cast doped by textile fibers. Barely 1% of textiles in the material results in a 20% decrease in the bending and compressive strength. It was also observed that a higher textile content caused a smoother failure than in unreinforced polymer concrete. Despite contributing to the reduction of environmental pollution, by utilizing waste textiles in the material, this idea does not seem to be very successful due to its negative impact on the composite’s properties.

Similar results have been observed by Hu and others in their paper [20]. They incorporated ramie and sisalfibers in the structure of epoxy polymer concrete. Research carried out established that higherfiber content significantly decreases material strength properties due to insufficient wetting of thefibers with the resin, thus reducing interfa- cial bonds. Merely a slight addition of these naturalfibers had a posi- tive impact–0.36%fiber content improved bending strength by up to 25%.

The combination of rubber granulate and polymer concrete allows a material with even better dynamic properties to be obtained. During preliminary tests, Kępczak examined the effect of the addition of SBR rubber granulate (which is a product from the recycling of rubber tires) on the dynamic properties of mineral cast[21]. Polymer concrete (PC) samples doped in 10% volume with rubber granules of various fractions: 0.6÷ 2 mm, 1 ÷4 mm and 2.5÷6 mm have been evaluated by modal analysis. The results indicate that the addition of SBR to mineral cast enhances its damping properties by three times.

The highest increase in damping coefﬁcients and the fastest relaxation time was recorded for the sample with a 0.6÷2 mm rubber granulate fraction.

So far, relatively few studies on the dynamic properties of rubberized PC have been done. The introduction of SBR rubber granulate into a mineral cast seems to be a promising idea for obtaining a material

with very good damping properties. The aim of this paper is to explore that issue by investigating the optimum volume content of styrene‐ butadiene rubber in mineral cast. In this context, both dynamic and strength tests were carried out.

2. Specimen preparation

To conduct dynamic properties tests, four cuboidal samples (dimensions 40x40x500 mm) with 0%, 10%, 20%, and 30% SBR rubber granulate volume content were prepared. The basic material for the samples was Epument 140/5 A1–a polymer concrete material produced by RAMPF. The manufacturer supplies all the components for a self‐making mineral cast. The kit includes three components: epoxy resin, hardener and a mixture of aggregates. The manufacturer also encloses instructions for preparing the composition and the dedicated mixture ratio: 2.2 (epoxy resin): 0.6 (hardener): 27.2 (aggregates), respectively[22]. Based on the results of previous studies[21], SBR rubber granulate with a fraction of 0.6 ÷2 mm was used to create the mixture. The rubber granulate was used as replacement for part of theﬁller, with appropriate proportions to achieve a volume of 10, 20 and 30% SBR in the material. The mass of the individual components in the mixture was determined by assuming the density of mineral cast equals ρmc = 2.4 g/cm³ and the density of rubber ρSBR= 1.2 g/cm³.Table 1summarizes the masses of the ingredients used to prepare the mixtures for the individual samples.

Sample preparation started with mixing the right amount of epoxy resin and hardener. After that the rubber granulate was added and mixed precisely. The mineralfiller was then added and thoroughly mixed again to distribute the ingredients evenly throughout the mixture. The specimensflooded in the mold solidified for 24 h, allowing 80–90% of the target hardness to be obtained. After this, they were removed from the mold and aged for another 14 days to achieve full hardness. The created samples retained constant mass proportions in the composite, i.e., 9.3% epoxy resin and hardener and 90.7%filler (aggregate + rubber granulate), in accordance with the recommenda- tions of the mineral cast manufacturer.

The samples for the strength tests were created in the same way: for compression tests with cross‐section dimensions of 40 × 40 mm and height of 60 mm, and for ﬂexural tests with dimensions of 30 × 30 × 200 mm.Fig. 1presents views of the prepared specimens for the compressive strength tests.

3. Experimental analysis

3.1. Methodology

To determine the dynamic properties of the prepared samples, it was decided to perform an experimental modal analysis using a modal hammer in the time and frequency domain from 0 to 2000 Hz. During experimental modal analysis, the vibrations were excited in the object, with simultaneous measurement of the extorting force and the response of the system. On this basis, it is possible to create the object's frequency response function (FRF). The transfer function describes the relationship between the response spectrum and force in the frequency domain. FRF is used to determine the inherent dynamic characteristics of the object in the forms of natural frequencies, damping ratios and mode shapes[23–27].

It was decided to carry out the Single Input Single Output (SISO) method, performing a single excitation and a single measurement of the system response.Fig. 2presents the scheme of the test stand.

For measuring and data acquisition, the PULSE Lite system from Brüel & Kjær was used, which includes:

•Piezoelectric accelerometer 4514 53,958[28];

•Modal hammer 8206–003 54,990[29];

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•Data acquisition system 3560‐L.

In addition, the manufacturer provides PULSE LabShop software enabling registration and processing of the collected data. The software allows a Fast Fourier Transform (FFT) of the collected data to be processed.

Experimental modal analysis of the samples has been performed in the time domain and frequency domain in the range from 0 to 2000 Hz. Ten measuring points were determined on each specimen.

Each sample was tested three times and each point was excited three times, in succession, by vibrations.

As a result of the impact tests carried out using a modal hammer, the system response to single impulse excitation was found, as well as the frequency response function. On this basis, the dynamic characteristics of the samples were obtained, such as: natural frequencies, mode shapes, amplitudes and damping ratios.

The classical method of determining the damping at a resonance, using a frequency analyzer, is to identify the half power (–3 dB) points of the magnitude of the frequency response function. For a particular mode, the damping ratio dr can be found from the following Eq.(1):

dr¼Δf 2fr

ð1Þ where: Δf – is the frequency bandwidth between the two half power points;

fr–is the resonance frequency.

Data acquisition system 3560‐L contains a built‐in standard cursor reading which calculates the modal damping.

3.2. Results of modal analysis and discussion 3.2.1. Time domain graph (functions)

Fig. 3presents the obtained time courses of the system response to a single excitation.

The waveforms of all the samples, just after excitation, behave chaotically atﬁrst. Then the waveforms pass into a state of clearly pul- sating waves. For the tested samples, the relaxation time decreased with the increase of the rubber granulate content in the material.

The sample with 10% SBR was marked by a relaxation time of tr= 0.17 s, which gives almost a double decrease in its value in comparison to raw polymer concrete. The measured relaxation time for the Table 1

Composite ingredient proportions.

Sample Epoxy resin Hardener Filler Mass of sample

Aggregates SBR

[g] [g] [g] [g] [g]

0% SBR 140.8 38.4 1740.8 0.0 1920.0

10% SBR 133.8 36.5 1557.8 96.0 1824.0

20% SBR 126.7 34.6 1374.7 192.0 1728.0

30% SBR 119.7 32.6 1191.7 288.0 1632.0

Fig. 1.Specimens for compressive strength tests: a) 0% SBR, b) 30% SBR, c) 20% SBR, d) 10% SBR.

Fig. 2.Test stand: 1–computer, 2–data acquisition system, 3–accelerometer, 4–sample, 5–modal hammer, 6–clamping device.

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a)

b)

c)

d)

Fig. 3.Time courses of a single excitation: a) 0% SBR, b) 10% SBR, c) 20% SBR, d) 30% SBR.

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20% SBR sample was further reduced to tr= 0.15 s. The shortest relaxation time was noted for the 30% SBR sample, which was tr= 0.13 s.

At this stage, it can already be seen that an increased rubber granulate content in the polymer concrete caused an increase in the damping properties.

3.2.2. Frequency domain

Derived from research carried out in frequency domain from 0 to 2000 Hz, three natural frequencies with sample mode shapes have been identiﬁed.Fig. 4presents one of the frequency domain graphs –this example is for the 0% SBR specimen. The visible peaks are the points where resonant frequencies appeared.

The identiﬁed mode shapes present the same form (shape) for each tested sample, independent of the rubber granulate content. However, mode shapes have appeared at different frequencies.Fig. 5presents the mode shapes obtained for the free vibrations (resonant frequencies). In Table 2, the results of the experimental modal analysis of the samples with different SBR content are shown.

The results are now discussed along with the relevant charts.Fig. 6 shows the frequency of natural vibration forms for individual samples.

The bar graph presents a clear relationship between the increase of SBR rubber granulate content in the sample and the earlier occurrence of the resonance frequency. Every doping sample has modal frequencies lower than the raw mineral cast. For the specimens containing 10% SBR, the natural frequencies for the next modal shapes decreased by 34.8%, 29.1% and 19.4%, respectively, relative to the undoped polymer concrete. For the samples with 20% rubber granulate, these values went down by 50.0%, 41.1% and 33.9%. For the samples with 30% rubber granulate, these values went down by 40.7%, 41.8% and 42.8%. It can be said that decreasing resonance frequencies were observed along with increasing amounts of rubber granulate in the composite.

Fig. 7presents the resonance vibration amplitudes for the samples containing different volumes of SBR.

InFig. 7, a similar dependence for amplitudes as for modal frequencies can be observed. The highest amplitudes for resonance vibrations were noticed for the raw mineral cast. The tests indicate that as the amount of SBR in the material increases, the amplitude decreases. In comparison to the raw specimen for the 10% SBR sample, amplitudes dropped for the next modal shapes by 19.8%, 18.2% and 15.4%

respectively. For the 20% SBR sample, the amplitudes have gone down by 41.7%, 29.3% and 17.2%. The lowest amplitudes have been identi- ﬁed for the 30% SBR specimen, where they have decreased by 60.9%, 58.6% and 38.9% respectively. It is worth emphasizing that the per-

centage differences for the amplitude decreases were the most consid- erable for theﬁrst modal shape.

Fig. 8shows the values of the damping ratios for samples containing different volumes of SBR.

All the doped samples showed a signiﬁcant increase in damping ratios for every modal frequency. The sample containing 10% SBR achieved a minimum double value of the damping ratio relative to the undoped mineral cast. For the subsequent modal forms of free vibrations, this increase was 261.1%, 202.8% and 235.8%, respectively. For the ﬁrst modal frequency, the highest damping ratio– 5.65%–was observed for the sample with containing 20% SBR (a rise in the damping ratio of 392.6% in comparison to the 0% SBR specimen). For the second and third modal forms, the 30% SBR sample assumed the highest values of the vibration damping ratio–7.64%

and 2.91%. This was a rise of 266.5% and 285.9%, respectively, compared to the undoped mineral cast. The results indicate that rubber granulate improves the damping ratios of polymer concrete. The most valuable growth was noted for theﬁrst and second modal frequencies.

Fig. 9presents the correlation between the modal frequency and the damping ratio for samples with different volumes of rubber granules in the mineral cast.

For all samples with rubber granulate, an increase in the value of damping ratio has been noted for all modal frequencies relative to the undoped casting. Between thefirst and second modal forms, a significant increase in the damping coefficient has been observed for each sample. Followed by a decrease in the damping ratio appearing for the third resonance frequency. Each sample has a similar nature of variation in the damping ratio with frequency range. The graph presented above (Fig. 9) clearly shows the shift of subsequent resonance frequencies toward lower values with the growth of SBR volume content in the polymer concrete. According toFig. 9, the samples with a content of 20 and 30% rubber granulate in their volume show very similar damping parameters. Admixing the composition above 20% SBR caused an earlier occurrence of the next resonance frequencies (modal forms) without a profitable growth in the damping ratio value.

The research carried out so far has established that an increased amount of rubber granulate in polymer concrete caused a decrease in the amplitude and a rise in the damping ratio, however, it also led to the earlier appearance of modal forms with lower modal frequencies.

Based on the experimental modal analysis for samples with different volumes of SBR content, it is concluded that too high doping of polymer concrete (above 20% rubber granulate) is unprofitable. It does not bring significant dynamic benefits, and presumably deterio- Fig. 4.Frequency domain graph (waveform) for the 0% SBR sample.

a) b) c)

Fig. 5.Mode shapes of samples: a)ﬁrst mode, b) second mode, c) third mode.

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rates the values of the mechanical properties of the composites which will be investigated in a further part of the study.

4. Mechanical properties investigation

4.1. Methodology

The experimental study of the mechanical properties includedﬂex- ural strength and compressive strength tests. Compressive strength tests were carried out on aﬁrst‐class Mannheim testing machine. This allows conducting tests in the load range from 30 to 350 kN. It is worth mentioning that the machine has a hydraulic mechanism. Fig. 10 shows a sample of mineral cast just before the compression test on a Mannheim testing machine.

The compressive strength was determined as the quotient of the maximum compressive force and the cross‐sectional area of the sample according to the equation(2):

Rc¼Fcmax

A ð2Þ

where:Rc–compressive strength [MPa], Fcmax–maximal compressive force [N], A–the sample’s cross‐sectional area [mm²].

Flexural strength was tested on a Shimadzu universal testing machine performing three‐point bending test. The machine allows testing in the load range up to 50 kN, with a load speed from 0.05 to 1000 mm/min. The machine records data with a frequency of 800 Hz and operates in a temperature range from 5 to 40 °C.Fig. 11 presents the sample just before the bending test on a Shimadzu machine.

The tests were conducted based on the PN‐EN 206 + A1:2016–12 standard[30], which speciﬁes, among others, the methods for determining the strength of concrete. Determination of concreteﬂexural strength can be carried out according to two schemes: a free‐

supported beam loaded with two forces or one force. The tests were carried out according to the second scheme, loading the polymer con- Table 2

The results of experimental modal analysis for samples with different SBR content.

Parameter Unit 0% SBR 10% SBR 20% SBR 30% SBR

Mode I

Frequency [Hz] 86 56 43 51

Decrease in frequency value [%] – 34.8 50.0 40.7

Amplitude [(m/s²)/N] 1.51 1.21 0.88 0.59

Decrease in amplitude value [%] – 19.8 41.7 60.9

Damping ratio [%] 1.44 3.76 5.65 4.65

Increase in damping ratio [%] – 261.1 392.6 322.0

Mode II

Frequency [Hz] 550 390 324 320

Amplitude [(m/s²)/N] 3.96 3.24 2.80 1.64

Damping ratio [%] 2.87 5.82 7.33 7.64

Increase in damping ratio [%] – 202.8 255.7 266.5

Mode III

Frequency [Hz] 1760 1418 1163 1007

Amplitude [(m/s²)/N] 8.68 7.34 7.19 5.3

Damping ratio [%] 1.02 2.4 2.06 2.91

Increase of damping ratio [%] – 235.8 201.9 285.9

Fig. 6. Modal frequencies for samples with different rubber granulate volume content.

Fig. 7.Resonance vibration amplitudes for samples containing different volumes of SBR volume.

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crete beam with one force (Fig. 11). The beam was loaded by force with a displacement of 5 mm/min. Theﬂexural strength value is the quotient of the maximum bending moment and the beam’s cross‐

sectional strength index. The ﬂexural strength for a one force load was determined according to the Eq.(3):

Rz¼ 3Fl 2d1d2

2 ð3Þ

where:Rz–ﬂexural strength [MPa], F–maximal bending force [N], l–beam support spacing [mm],

d1, d2–cross‐sectional dimensions of the specimen [mm].

4.2. Results of mechanical properties and discussion 4.2.1. Compression strength

As a result of the tests carried out on the Mannheim testing machine, the curves of the force values in the displacement domain were obtained for each sample. From these curves, the maximum compressive force causing destruction of the sample has been obtained.

The values of the maximum compressive forces from each test were

used to calculate the value of the compressive strength by dividing the maximum compressive force by the cross‐sectional area of the sample. The measurements were carried out for 12 samples: three for each composition–raw mineral cast and mineral cast doped with SBR containing 10%, 20% and 30% rubber granulate.Table 3 presents the results of tests performed with some of the statistical parameters.

The highest variances and standard deviations were obtained for the raw mineral cast and for the polymer concrete doped with the 20% by volume rubber granulate. In both cases, the result range is wide: the variance for the raw polymer concrete sample was 17.5 MPa, and for the 20% SBR sample 10.3 MPa. The highest coefﬁ- cient of variation–6%–was acquired for the specimen doped with the Fig. 8.Damping ratios for samples containing different volumes of SBR.

Fig. 9.Correlation between damping ratios and modal frequencies for samples containing different volumes of SBR.

Fig. 10.Specimen prepared for a compressive strength test.

Fig. 11. Specimen prepared for a ﬂexural strength test on a Shimadzu machine.

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20% rubber granulate. The highest standard deviation–4.2 MPa–was obtained for the raw mineral cast. Samples with a 10 and 30% rubber content have a slight difference in results, where the standard deviation does not exceed 2.0 MPa, and the coefﬁcient of variation amounted to 4%.

Fig. 12presents the compression strength results as a bar chart.

The obtained compressive strength for the raw mineral cast equal to 136.7 MPa slightly differs from the range declared by the manufacturer of the mineral cast components which is 140 ÷ 160 MPa.

For a 10% SBR sample, the compressive strength was 65.2 MPa. A strength of 54.0 MPa was gained for the 20% SBR doped sample.

The composition with the highest rubber content–30% by volume– has a compressive strength of 38.1 MPa, so compared to the raw mineral cast, a 72.1% decrease in strength was noticed.

The research has shown that with increasing content of rubber granulate in the mineral cast, the compressive strength of the material decreases drastically. Merely adding 10% SBR to the sample results in a 52.3% decrease in strength. Admixing 20% by volume of rubber granules results in a decrease in the compressive strength relative to the raw polymer concrete by 60.5%.

On the basis of the compressive strength test results, the exponential function has been matched to values, reflecting the above variability fairly well. The equation is presented on the graph inFig. 12. The determination coefficient is R² = 0.9267. Based on the calculated coefficient of determination, it is concluded that the results of the compressive strength tests are reliable. However, in order to confirm the equation presented above it is recommended conducting studies on a larger number of samples with smaller intervals in SBR volume content.

4.2.2. Flexural strength

Theﬂexural strength test of the samples on a Shimadzu machine allowed typical load–displacement curves for the four types of rubberized polymer concrete to be obtained. For each specimen, the maximum force causing its breakage was noted. Based on this parameter, theﬂexural strength was determined in accordance with formula(2).

During the tests, one of the samples with 30% SBR content was destroyed after loading with a force of 340 N, so the result has been considered as unreliable. The sample may have been prepared incor- rectly (too little thickening in the material when added to the mold, too many pores in the sample), as a result of which it burst too early.

This measurement has been rejected from the results, so the bending strength for material with 30% rubber granulate content was determined as the average of only two measurements.Table 4 presents the results of theﬂexural strength tests.

The highest variance and standard deviation of measurements were recorded for the raw mineral cast. At an averageﬂexural strength of 35.2 MPa, the variance was 8.4 MPa, and the standard deviation was 2.9 MPa. The coefﬁcient of variation also reached the highest value of 8.2% for this sample. As little statistical values testify to the reliability of the tests,Fig. 13presents the obtained results as a bar chart.

The value forflexural strength obtained for the raw mineral cast equals 35.2 MPa and is in the range declared by the manufacturer of 35÷45 MPa. Increases in the content of rubber granulate in the sample results in a decrease offlexural strength. Admixing the mineral cast with 10% rubber granulate by volume results in a 15.3% fall in strength. Increasing the SBR content to 20% causes a drop in bending strength to 21.8 MPa, i.e., a decrease of 38.1% relative to the raw mineral cast. The polymer concrete doped with 30% rubber granules was characterized by aflexural strength of 16.9 MPa, which is only 48.0%

of the strength of the undoped polymer concrete.

It was noted that the relationship between bending strength and the content of rubber granulate in the sample is exponential. The exponential function with its equation has been determined (Fig. 13). This allows predicting theflexural strength for any SBR content in the sample. The determination coefficient is R²= 0.9876, which means that the model found explains almost 99% of the variability. This coefficient close to 1.0 indicates the high veracity of the observed nature of the variability.

Signiﬁcant decreases in the mechanical properties of the mineral cast doped with rubber granules may be a result of the lower bond strength between the resin and rubber granules than with the aggregates in polymer concrete. An additional component in the composite –rubber granules–may increase the porosity of the material, which also negatively affects its mechanical strength.

5. Summary and conclusion

The research carried out in this paper allowed the dynamical as well as the mechanical characteristics of a polymer concrete doped with SBR rubber granulate to be obtained. An increase in the rubber content of the polymer concrete results in lower resonance frequen- Table 3

Results of the compression strength tests [MPa].

0% SBR 10% SBR 20% SBR 30% SBR

Test 1 [MPa] 135.6 63.8 53.1 36.3

Test 2 [MPa] 133.1 67.4 51.3 38.8

Test 3 [MPa] 141.3 64.4 57.5 39.4

Average value [MPa] 136.7 65.2 54.0 38.1

Variance [MPa] 17.5 3.8 10.3 2.7

Standard deviation [MPa] 4.2 2.0 3.2 1.7

Coefﬁcient of variation [%] 3 3 6 4

Decrease in compression strength [%] – 52.3 60.5 72.1

Fig. 12.Compression strength of polymer concrete samples with different SBR content.

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cies. Material containing 10% rubber has been distinguished by a minimum double increase in the damping coefﬁcient for each modal form, relative to the undoped casting. For the 10% SBR sample and the second mode shape, the damping coefﬁcient was 5.82%, and for a 20%

SBR doped cast, this coefficient achieved 7.33%. The increase in rubber content in the sample results in a significant increase of the damping coefficient and decreased amplitude. Based on this study, it is not recommended doping the mineral cast with more than 20% of rubber granulate by volume. In excess of this value, the rise in damping properties is not so pronounced, while the mechanical properties drastically decrease.

The addition of styrene‐butadiene rubber (SBR) to polymer concrete results in a decrease in its mechanical properties. The drop in compressive strength is drastic. Admixing mineral cast with rubber in a 10% volume ratio decreases the compressive strength by 52.3%.

Weaknesses inflexural strength are not so drastic and can be characterized by an exponential function. The addition of 10% rubber granulate to the volume of the polymer concrete causes an approximately 15.3% reduction inflexural strength. The significant decrease in the mechanical properties of the mineral cast doped with rubber granules may be the result of the lower adhesion of resin to rubber than for the resin to the aggregates in polymer concrete and a higher level of material porosity.

Based on the conducted research, it is recommended doping polymer concrete with styrene‐butadiene rubber (SBR) with grains of 0.6

÷2.0 mm gradation in a volume range not exceeding 20%. This results in a signiﬁcant increase in the damping parameters, with a limited impact on the material’s mechanical properties.

A summary of the range of parameters obtained for such doped material is presented below.

The range of properties for a mineral cast doped with SBR granules in volumes of 10 to 20%:

resonance frequencies for theﬁrst three modes:

•ﬁrst mode: 43÷56 Hz

•second mode: 324÷390 Hz

•third mode: 1160÷1418 Hz

•amplitudes for theﬁrst three modes:

•ﬁrst mode: 0.88÷1.21 (m/s2)/N

•second mode: 2.80÷3.24 (m/s2)/N

•third mode: 7.19÷7.34 (m/s2)/N

•damping factor for theﬁrst three modes:

•ﬁrst mode: 3.76÷5.65%

•second mode: 5.82÷7.33%

•third mode: 2.06÷2.40%

•compressive strength: 54÷70 MPa

•ﬂexural strength: 21÷29 MPa.

Declaration of Competing Interest

The authors declare that they have no known competingﬁnancial interests or personal relationships that could have appeared to inﬂu- ence the work reported in this paper.

References

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Table 4

Flexural strength test results.

0% SBR 10% SBR 20% SBR 30% SBR

Test 1 [MPa] 31.9 29.6 22.3 16.6

Test 2 [MPa] 36.4 28.6 22.9 –

Test 3 [MPa] 37.3 31.2 20.3 17.2

Average value [MPa] 35.2 29.8 21.8 16.9

Variance [MPa] 8.4 1.8 1.8 0.2

Standard deviation [MPa] 2.9 1.3 1.3 0.4

Coefﬁcient of variation [%] 8.2 4.5 6.1 2.5

Decrease inﬂexural strength [%] – 15.3 38.1 52.0

Fig. 13. Flexural strength of polymer concrete with different SBR volume content.

(10)

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