Nowadays, many techniques are invented to prevent structures from the damages caused by earthquakes but the most effective design is isolation technique. However, the technique still has some disadvantages which limiting its application to all the building in the world. These problems are their large size, expensive cost and complex installation. To extend its application to more structures in the world, the weight and cost, therefore, must be reduced. Thus, in the recent years, many approaches are developed for a simple and affordable seismic isolation system
This article presents an overview of the techniques using scrap tires as seismic isolation material. Experimental investigations were conducted on scrap tire pads (STPs), scrap tire rubber pads (STRPs), rubber-soil mixtures (RSM) and recycled tire isolator (RTI).
From the test results, STPs are determined to fail in compression at about 8.5Mpa axial stress. Researches on STRPs and RTI show an improvement on performance, specially STRPs. The expected average axial compressive pressure of STRPs is about 3.3-7.5 MPa with a 150% shear strain, which make it a feasible isolation system for low-rise or medium-rise buildings in earthquake prone zone. RSM method provides a viable solution for an environmental threat over decades, which is a huge amount of disposed tires all over the world. The method still has some potential problems prior to its practicable application, such as liquefaction, no-linear site response, internal heating of scrap tires, ground settlement and environmental effects. Nevertheless, all these approaches provide a promising earthquake protection method for low-rise residential buildings in developing countries.
List of Figures
Figure 1 Response Spectrum shows different earthquake protection techniques
and their effects
Figure 2 Example of current largest base isolated buildings in the world
Figure 3 The scrap tire problem in the United States
Figure 4 Tire production line
Figure 5 Components of a car tire
Figure 6 Preparation of STP specimens
Figure 7 Specimens used in shaking table test
Figure 8 A typical section of STRP
Figure 9 Preparation of STRP specimen
From the past, earthquakes often cause tremendous loss of lives and collapse of structures. In developing countries, the result is more devastated as most of buildings are made of poorly constructed concrete or masonry. Nowadays, many techniques are developed aiming to minimize the effect of earthquakes. And it continues to be a popular and growing topic in the field of structural engineering since it is invented.
There are also other available techniques to reduce the earthquake effect, such as bracing system, damping device, and increasing the earthquake resistant capacity . Compared to the isolation techniques, bracing system and increasing earthquake capacity techniques mainly rely on increasing the stiffness of structure instead of reducing seismic demand. Damping devices may be an appropriate solution but it is not widely adopted because of its expensive cost.
Figure 1: Response Spectrum shows different earthquake protection techniques and their effects (source: )
Presently, the base isolation is one of the most effective methods of earthquake resistance system. The technique is innovative compared to conventional structural systems because it takes a different approach to the earthquake resistant design problem . Conventional structural reinforced systems aim to increase the seismic capacity of the structure, while base isolation system is designed to reduce the seismic demand of the building . However, the technique still has some disadvantages which limiting its application to all the building in the world. These problems are their relatively large size, expensive cost and complex installation. In order to extend its application to more structures in the world, the weight and cost, therefore, must be reduced. Thus, in the recent years, developing a simple and cheap seismic isolation systems is demanded for the purpose to extend the use of this method in countries where resource and technology are deficient. And one of many notable approaches is the use of recycled tires to replace a huge source of rubber material required.
Base isolation History
Base isolation, in a general sense, separates the superstructure from direct contact with the ground. Elastic bearing, sliding and hybrid systems are the most used seismic isolation devices. These bearings are designed to sustain a large displacement when an intense ground shaking occur and result in a lower natural frequency of the buildings .
The base isolation system is first considered by Italian after Messimo-Reggio earthquake of 1908. in the aftermath, almost all of the unreinforced structures were collapse and resulted in about 160,000 killed . The first case of an elastomeric bearing implementation was in 1969 in Skopje. The bearings used were solid unreinforced natural rubber block. In the same year, the steel rubber bearing was developed in Japan. It takes almost three decades for base isolation to become an accepted seismic design solution in many seismically active areas of the world.
Figure 2: Example of current largest base isolated buildings in the world. The building on the left is Tokyo Skytree East Tower, Tokyo, 229,237 square meters. It was completed in 2012. The building on the right is Isparta City Hospital, Isparta, Turkey, 221,000 square meters. It was completed in December 2016.
At present, the country that has highest numbers of buildings with base isolation is Japan. A total of 4,100 base isolated commercial buildings are built as of December 2015. Other countries currently pushing base isolation include China, Turkey, New Zealand, Peru, Colombia and Chile. It is a growing trend that base isolation technique is taken into consideration on design of both high-rise apartments and low-rise residential buildings all around the world.
Use of Rubber and Scrap Tires
In the recent years, the disposal of used vehicle tires has become a serious environmental threat. Due to the increased population of automobile drivers every year, over billion of scrap tires are produced worldwide in countries like Japan and Mexico . From the Fig 3, Specifically, it is observed that a maximum value of 300 million scrap tires can be generated in a single year, and the amount of waste is predicted to rise annually. This problem has become more severe in 21st Century due to the rapid economic growth of a number of developing countries, such as China, India, and so forth . In addition, new tires production is expected to follow a non-stop increasing trend since 2000.
Figure 3: The scrap tire problem in the United States (source: )
Over decades, the used tires have found application in civil engineering projects, such as road construction, embankment, road pavements, and backfill. However, the scope of waste tires application is still narrow and limited. According to U.S. environmental protection agency, only about 20% of 300 million of scrap ties are recycled or used in civil projects. It is urgent to find other applications to consume the huge scrap tires stockpiles. Recently, using rubber component in tires as an alternative approach for the cheap earthquake isolation system in developing countries that have inadequate financial resource has been a promising solution to consume scrap tires worldwide.
Tires we use today are produced from synthetic rubber. The creation of synthetic rubber can go back to industrial age. In 1839, Charles Goodyear first invented the process of rubber vulcanization. This method is still used today but a refined version to strengthen rubber in a wide variety of applications. He first mixed rubber with various dry powder hoping to make it not only waterproof, but also weatherproof. Then he discovered adding sulfur changes its property to like leather, which has both plastic and elastic behavior. The best result is achieved when he applied a 270 degrees Fahrenheit steam heat to the sample. This is the early version of vulcanization process. Today, automobile tires are being produced by means of refined version. A production line is shown in Fig. 4 for automobile tires production.
Figure 4: Tire production line. Raw materials are placed in a special mixer and prepared for construction process. The fabric and steel cords are manufactured in another workbench in the meanwhile. Finally, all parts, including steel cord, bead, tread and sidewalls are assembled together.
Figure 5: Components of a car tire
Development of many practical base isolation devices has been done over decades. However, the implementation of such techniques is still limited to high-rise buildings or valuable structures because of its expensive cost and complex execution. In the recent years, a new approach that involved the use of scrap tires emerges in the research field and aims to develop a low-cost base isolation system in developing countries.
Scrap Tire Pads
In 2008, Ahmet Turer conducted a research on application of scrap tire pad (STP). The experimental specimens are prepared by stacking the rectangular shape layers cut from tread parts of used tires on top of another without using any adhesive chemical. The assumption is that the frictional force between tire layers would be high enough to maintain the stability of STP layer . However, the result shows that STPs begins to fail in compressive stress 8.5Mpa axial stress within an around 70% of shear strain . A different test was conducted later with using nails and bolting rod for the purpose to increase their stability . This research can be considered as first innovative approach on the low-cost isolation system.
Figure 6: Preparation of STP specimens (source: )
From literature, elastomeric bearings are determined to have a damping ratio of nearly 2% to 3% and 10% to 20% for the for the low damping and high damping rubber, respectively . It is expected that the rubber pads have similar property with the pads made of scrap tires and the vulcanizing steel mesh has the same function as the steel shim inside an elastomeric isolator. Typically, the specimen is a 20 cm long piece cut off from the tread part of a tire.
Discussion and observation
In literature, several tests have been conducted on specimens to obtain the basic behavior of STPs. Axial compression test and static shear experiments were performed to obtain the compression behavior and horizontal deformation behavior of STPs. Three dynamic free vibration tests were aimed to get damping ratios of specimens. Lastly, a shaking table test was conducted on a ¼ scale masonry model to exam the general performance under intense ground motion.
Compression tests indicates axial compression failure around 8 MPa level. The design stress is decreased to 4 MPa for safety concern. It is concluded that using additional steel plates are recommended to improve the axial load capacity of STPs since only a small amount of steel mesh is present in specimen originally. .
Dynamic tests reveal that damping ratios of STPs are to be around 7% to 10%. In inclined static shear tests, it is discovered that the layers do not slip until the shear strains go beyond 70%. STPs also show a higher shear modulus values as the rubber in tires are much harder than natural rubber because cat tires are designed to have high durability under extreme weather condition. .
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Finally, the shaking table test results showed the STP models were able to reduce the seismic demand on the masonry structure. It is noted that isolators used in this experiment are composed of steel sheets and soft rubber layers, since the rubber in car tires has large shear modulus than the required value. As Fig 7 shown, rubber pads and steel plate are stacked on top of each other and o any adhesive was used to keep these layers together.
Figure 7: Specimens used in shaking table test (source: )
In conclusion, there are several advantages of STPs, including nearly no-cost at preparation, light weight material and ease of handling. However, further implementation remains to be an issue since experiments mainly investigate general property of STPs . As a result, improvement of STP devices should be continued to in order to implement it with more confidence.
Scrap Tire Rubber Pads
Rubber and steel cords are two main components of scrap tire rubber pads (STRP). It is expected that rubber materials in the scrap tire exist similar properties of rubber pads, and steel cords in the scrap tire perform similarly to the steel plate in elastomeric rubber bearings . The previous research conducted by Turer on using the scrap tire for the seismic isolation purpose found out the insufficiency of vertical load capacity of the scrap tire pads. Different from STPs, STRPs use only the tread parts from radial tires of bus or truck and uneven surface is removed. All these modified treatments are expected to improve the performance and safety of STRPs . Moreover, these steel layers in radial tires are expected to provide extra rigidity and prevent slip when subjected to tensile force .
Figure 8: A typical section of STRP (source: )
On the contrary of the elastomeric bearings, which are well tested and developed, STRPs still have concerns on weather effects and durability. In conventional elastomeric bearings, steel layers are protected from corrosion by cover. The outer part of STRP, however, is exposed to environment as Fig.8 shown. In a long term, durability will become a problem when steel are exposed to arduous condition.
There are two types of specimens examined in the experiments. The first type of specimens is produced by stacking the STRPs on top of another without applying the adhesive chemical, called as layer-unbounded STRP bearings. The other type is simply the same composition but with application of adhesive between layers. Each layer of bearing is comprised of five layers of steel reinforcing cords as shown in Fig 9. A typical section of STRP is a rectangular shape with a dimension of 100mm and thickness of 12mm.
Figure 9: Preparation of STRP specimen (source: )
Discussion and observation
Layer-bonded STRP bearings are examined to have a better performance on shear strain test. The analysis shows that the layer-bonded STRP can resist a compressive force up to 150% shear strain when loaded with axial pressure of 8.6 MPa. The isolation bearings are typically used at axial pressure around 5-13 MPa. At this level, the STRPs can be concluded to serve as a feasible base isolation device for low-rise residential buildings. It important to mention that the STRPs isolator needs to be bonded to achieve the best performance on the shear deformation capacity .
The production of STRPs is relatively easy and cheap. Scrap tires are available everywhere in the world so availability of STRP shall not be a problem. STRP bearings need to be produced by mechanized process, which most of re-processing companies are able to execute even in the developing countries. The total cost of a 4 layer-bonded specimen is about 68 dollars in Japan. This price will be even lower in the developing countries due to domestic production. Compare to the cost of commercially seismic isolators in the thousands of dollars, the choice is obvious to install STRPs device in the hundreds of dollars with enough resistance to the effect of earthquake.
The huge stockpiles of scrap tires have been an environmental threat over decades. A long-term viable solution is essential to consume such large waste of used tires. Rubber-soil mixtures, a distinct technology from using the scrap tires as base isolation, has been demonstrated by numerical modeling under different ground motions in experiments.
The potential amount of scrap tires used in the method is relatively huge. Taking an example of a typical 10-story building, the bulk volume occupied by RSM is determined to be around 42,000
, which means over four million scrap tires equivalent need to be consumed [7,8]. Consequently, this amount of consumption is well beyond in all other civil engineering projects.
Figure 10: Schematic drawing of the isolation system using RSM (source: )
This approach has been categorized as a distributed seismic isolation system. Compared to conventional isolation system, RSM technique aims to modify dominant frequency and dissipate energy through surrounding soil before these waves reach the structure. From the experiment, the 10-story office building has shown a reduction of 60-70% in horizontal ground acceleration and a reduction of 80-90% in vertical ground acceleration when subjected to a intense ground motion.
Discussion and Issue
RSM, as a newly proposed technology, still has some potential problems to be considered in further investigation, including liquefaction, no-linear site response, internal heating of scrap tires, ground settlement and environmental effects, but it provides a promising earthquake protection of low to medium rise buildings and an alternative fast and reliable way of consuming huge stockpile of scrap tires around the world [5,7,8].
Recycle Tire Isolator
Recycled tire and steel plate are the main elements in the fabrication of recycled tire isolator (RTI). Tread parts of scrap tires are cut and divided into small pieces of tire pads and adhesive glue is added between each layer. The specimens are put under pressure for 24 hours to make sure a strong bond.
Figure 11: Preparation of RTI specimen (source: )
Comparison with STP Test Results
Several tests have been conducted to investigate the property of RTI, including compression test, finite element analysis and shake table test on a small scale of two-story building. The compressive strength of RTI and STPs is concluded to be quite similar with a difference of 6.7% . Beside the slight difference in dimension, the higher strength of STPs may be a mixed result of different types and brands of scrap tires been used. In the literature, the brand RTI used is Bridgestone. Also, STPs had a slightly higher compression modulus due to thicker sample.
Comparison with STRP Test Results
In the study, several tests include compression test, dynamic test, and finite analysis are conducted to investigate the effect of RTI on protecting three stories model. From the results, RTI shows a similar value of vertical stiffness with STRPs due to the same material in fabrication . In addition, RTI is stiffer than layer-unbonded STRPs. Hence, RTI may withstand a higher compressive load than STRPs although it is thinner than STRPs. From finite analysis, RTI shows a total deformation reduction of 83% compared the result for the building without isolation system. The damping ratio is examined to be around 9%, which is within the optimum range of damping ratio for rubber bearing.
RTI is introduced as a low-cost system to resist low to medium intensity level of ground shaking for low rise residential or commercial buildings. The low cost of manufacture makes it an affordable and suitable seismic resistant device in developing countries compare to other types of base isolators such as synthetic rubber bearing, damper, or lead-plug bearing .
Isolation system is currently one of the most effective method to reduce the tremendous damage due to earthquakes. The approach of using scrap tires as the rubber material replacement has not only provide an innovative way of consuming huge stockpile of scrap tire but also extend the application of this earthquake mitigation technology to low-rise and medium-rise residential buildings or housing in the developing countries where resource is deficient and technology is inadequate.
Some of issues remain to implement these techniques with more confidence, such as long-term durability under sustained loading, environmental conditions like temperature and moisture, and possibility of rocking motion. Therefore, improvement of isolation system using of scrap tires should be continued to develop a simple and affordable seismic isolation system for the structures all over the world.
 Ahmet Turer and Bayezid Ozden. Seismic base isolation using low-cost Scrap Tire Pads (STP). Material and Structures, Vol. 41, 2008.
Jie, S. W., Tong, S. Y., Kasa, A., & Osman, S. A. (2016). Effect of recycle tire isolator as earthquake resistance system for low rise buildings in Malaysia. Journal of Engineering Science and Technology, 11(8), 1207-1220.
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 Mishra, Huma Kanta, Igarashi, Akira (2012). Experimental and analytical study of scrap tire rubber pad for seismic isolation. Proceedings of the International Conference on Earthquake and Structural Engineering, Kuala Lumpur, Malaysia
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 Turer A. and Ozden B., “Seismic base isolation using low-cost scrap tire pads(STP)”, Materials and Structures, 2008.
 Tsang H.-H., Lam N.T.K., Yaghmaei-Sabegh S., Sheikh M.N. and IndraratnaB., “Geotechnical Seismic Isolation by Scrap Tire-Soil Mixtures”, inProceedings of 5th International Conference on Recent Advances inGeotechnical Earthquake Engineering and Soil Dynamics, San Diego,California, May 24-29, 2010.
 Xu X., “Earthquake protection of low-to-medium-rise buildings using rubber-soil mixtures”, MSc Thesis, University of Hong Kong, 2009.
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