More than 10 million new vehicles of all types are added each year to the Indian roads leading to the increase of about 30 million discarded tyres each year pose that a serious threat to the environment. Tyres are recyclable yet significant number are added to existing tyre dumps or landfills. Interestingly, this accumulated waste materials can be very well used in construction, says Kaushal Kishore, Consulting Materials Engineer, Roorkee
Tyre rubber crumb is produced by an ambient process (mechanical sizing) or by cryogenic process (freezing). In the mechanical process, tyres are reduced to chips or shreds and then put through granulators which separate and remove loose steel and fiber and further reduce rubber particle size. Finally, the small rubber chunks are ground to produce rubber crumb of 30 to 80 mesh size.
In the cryogenic process, tyre chips are frozen in liquid nitrogen as they pass through a cryogenic tunnel then broken down by impact. They then pass through a series of screen meshes where they are shattered into their three component parts: rubber, steel and fabric. Although the cryogenic process is the more expensive of the two, it produces smoother and smaller crumb.
Properties of tyre rubber
Density: Tyres are slightly heavier than water and will sink in water unless entrapped air provides enough buoyancy to allow them to float. This generally occurs only with whole tyres or fine crumb rubber.
Moisture absorption: Tyres and shreds can trap water on the surface and in irregular contours, but they are relatively impervious to actual absorption. Various studies cited in ASTM D 6270-98 indicate maximum moisture absorption of 2-4%.
Thermal Insulation: Rubber is a poor thermal conductor, conversely providing a better thermal insulation than soil or aggregate.
Acoustic Insulation: Tyre rubber is a poor acoustic conductor and, therefore a good insulator.
Temperature Tolerance: Tyre rubber is capable of withstanding a full range of ambient temperature extremes without undergoing permanent property change. Some properties like flexibility change as a function of temperature, but the change is reversible and repeatable.
Leaching Characteristic: Tyre shred leaching characteristic have been examined under a wide range of pH conditions. Extensive practical experience has conformed the absence of any deleterious impact.
Flammability: Tyres shreds have a reported flash point of 3060C, higher than some other building materials used for Architectural purposes such as wood, paper, foam, and fabric. When crumb rubber is combined with cement and sand, this may control the flammability of the resulting product.
Early studies on the use of worn out tyres in asphalt mixes were very promising. Yet, not much attention has been given to the use of rubber from scrap tyres in Portland cement concrete. So far no research has been done in the use of rubber crumb for RCC and RB roof slab water proofing.
Cement, aggregates, rubber and water mixture has a drawback of lower strength due to its poor bonding with concrete matrix. In the early study, it was found that coarse grading of rubber granules lowered the compressive strength of rubber mixtures more than fine grading. There was about 85% reduction in compressive strength and 50% reduction in tensile strength when the coarse aggregate was fully replaced by coarse rubber chips. However, when the fine aggregate was fully replaced by fine crumb rubber, specimen lost up to 65% of their compressive strength and up to 50% of their tensile strength.
The rougher the rubber particles used in concrete mixture the better the bonding they developed with surrounding matrix and higher the compressive strength achieved. If the bond between rubber particles and the surrounding cement paste was improved, then significantly higher compressive strength of rubcrete mixtures could be obtained.
The following are the possible causes in strength reduction of rubcrete mixtures:
- The rubber is much softer than the surrounding cement paste, cracks are initiated quickly around the rubber particles due to this elastic mismatch which propagate to bring about failure of the rubber-cement matrix.
- Weak bonding between the rubber particles and the cement paste increases void, resulting reduction in strength.
- Strength of concrete depends mainly on the density, size, and hardness of coarse aggregate. When aggregates are replaced with light, flexible, and smooth surface rubber a reduction in strength is anticipated.
While in structural concrete, strength is one of the important factors. For nonstructural concrete such as rooftop rending for insulation and water proofing, rubber mixture may be a suitable material. As, it has the property of insulation as well as flexibility required due to the movement of slab.
In due course of time, cement-sand rending cracks even when mixed with polymers, and the leakage of water starts again. In the present study it is to be seen that rubber mixture when applied as rending on cracked roof surface in the natural roof movement cycle, does it cracks and cost wise is it almost equal to conventional roof treatment construction.
Materials used were Cement OPC 43 Grade conforming to IS: 8112- 1989, and River Sand & Crumb Rubber as mentioned in the Table.:
After many trials and testing, with the given set of materials, three mixes were finalized out of which Mix No. 1 is well known and it is not used in the rending. Mix No. 2 showed lower strength while, Mix No. 3 was found satisfactory in all respects.
Note: Proportions by weight can be converted to proportion by volulme, by dividing with the bulk density of the materials available for use at site. The bulk density of OPC may be taken 1.44 kg/lit.
A leaking and cracked RCC roof was applied with 25mm thick layer of mortar of Mix No. 3, After completion of curing cycle, the roof was left to dry. It was then filled with water for three days and left to dry. After three cycles, the roof was left to be dry for considerable time. It was then filled with water for three days. On the third day the roof was inspected for any leakage. No leakage was observed. The roof was again left to be dry for considerable time and then inspected for any cracks. No crack were observed.
Waterproofing of rooftop with discarded tyres rubber crumb
The treatment with rubber crumb for water proofing and insulation gives long lasting performance beyond one year at an approximate cost of Rs. 29/- per sqft. However, no fixed mix design can be considered appropriate for all applications. Mix design has to be worked as per the site materials and roof & site conditions. The advantages of the treatment include utilization of waste material, lower density resulting in reduced load on roof, flexible materials which do not crack and cost effectiveness.