Experience Related to Rubber Concrete Construction

Rubber concrete placed in all the test sites previously mentioned was mixed by the process described next, excluding the rubber concrete tennis court project. Fresh concrete was first mixed without crumb rubber in a batch plant, and then hauled to the job site by a concrete truck. Crumb rubber was then added to the truck on the job site, and a remix was performed. To make the crumb rubber disperse uniformly in a concrete truck, a per-metre based empirical formula of the re-mixing time needed for a regular concrete truck (about 7.6 m capacity) is proposed here that states  

Remixing time = 180 seconds for the first 5.89 kg per cubic metre 

The time computed by the above formula may be an overstatement. What has been observed is that 5-6 minutes remixing time is adequate for most rubber concrete to have a uniform distribution of rubber particles in concrete. When rubber content reaches 177 kg per cubic metre and over, the remixing time may be increased to 8-10 minutes. Also, when adding crumb rubber into a loaded mixing truck, the truck needs to be set spinning at 2 rpm to 4 rpm. When in re-mixing mode, 16 rpm is required. 

This on-site remixing method means that crumb rubber was added at least 30 minutes after water was added and setting was beginning. On the other hand, it is speculated that most rubber concrete specimens made in a laboratory environment as reported in various studies referred to previously would have rubber and other materials mixed almost at the same time. Though whether the two ways to make rubber concrete will make a difference remains unanswered, it was noticed in one case from the rubber shot-concrete project that, the compressive strength for the specimens made in the laboratory was much lower than that measured on samples made from the job site. 

It appears that addition of crumb rubber helps prevent the phenomenon of separation. When fresh, controlled concrete looks watery with coarse aggregates being wrapped by thin and fluid cement paste. With the presence of crumb rubber, the fresh mix appears more viscous or sticky , and less watery as compared to controlled concrete. This is particularly true when the rubber content is high, say above the level of 58.9 kg per cubic metre. 

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