Cement Treated Bast (CTB) is a compacted mixture of graded aggregate, cement, and water, which is used as a base course for the construction of highways, airport runways and taxiways. The primary difference between CTB and soil cement is that CTB usually utilizes a graded aggregate instead of the native soils. It is primarily used to provide a structurally sound base material over such things as an expansive clay.
It has been laboratory proven to equal or exceed the modulus of sub grade reaction (k) of a thicker coarse of crushed stone sub-base and the flexural stress of a thicker coarse of lime stabilized clay. Other benefits of CTB over crushed stone sub-base include the ability to distribute loads over a greater area and its resistance to sub-grade failures associated with freeze/thaw cycles.
Close control and thorough mixing of the material is required in order to meet specifications while staying within budget, both of which will be obtained through the use of continuous mixing plant. This close control and thorough mixing will not always be accomplished when mixing in place.
Gears, Inc. and PEI utilize an Aran portable continuous mixing plant whose production is rated between 100 and 1,000 tons per hour contingent upon the engineering properties of the material being used, speed of excavation, and speed of placement.
The production of the mixing plant is monitored by the use of instruments that display the speeds of various feeds to the mixer. Feed rates for soil, cement, and water are variable and are metered with a computerized system based on the weight and density of the native soil.
Since all of the ingredient feeds are volumetric and the mix density is generally specified in terms of weight, it is necessary to calibrate each feed. The purpose of this calibration is to establish an accurate relationship between speed/gate setting and the resultant throughput in tons per hour.
The aggregate is loaded in the feed hopper by means of a front-end loader or conveyor. A fixed or variable speed belt feeder draws that material from the hopper at a uniform rate and transfers it into the Aran twin shaft mixer.
The cement is metered from the overhead storage silo by means of a cleated belt feeder and enters the same zone as the native material.
Water is drawn from the lower storage tank and metered by a variable speed, positive displacement pump system. It is delivered to the mixing chamber that minimizes the potential for escape of dust during the mixing process.