6.7 ASBESTOS CEMENT PIPES

6.7.1 General

Asbestos cement pipes are made of a mixture of asbestos paste and cement compressed by steel rollers to form a laminated material of great strength and desnsity. its carrying capacity remains substantially constant as when first laid, irrespective of the quality of water. It can be drilled and tapped for connecting but does not have the same strength or suitability for threding as iron and any leakage a the thread will become worse as time passes. However this difficulty can be over come by screwing the ferrules through malleable iron saddles fixed at the point of service conncetions as is the general practice. The pipes are not suitable for use in sulphate soils.

The available safety against bursting under pressure and against failure in longitudinal bending, though less than that for spun iron pipes, is nevertheless adequate and increases as the pipe ages. In most cases good bedding of the pipes and the use of flexible joints are of greater importance in preventing failure by bending, than the strength of pipe itself. Flexible joints are used at regular intervals to provide for repairing of pipes if necessary.

AC pipes are manufactured from classes 5 to 25 and nominal diameters of 80 mm to 600 mm with the test pressure 5 to 25 Kg/cm2.

AC pipe can meet the general requirements of water supply undertaking for rising main as well as distribution main. It is classified as class 5,10,15,20 and 25, which have test pressures 5,10,15,20 and 25 Kgs/cm2. respectively. Working pressure shall not be greater than 50 % of test pressure for pumping mains and 67% for gravity mains.

For futher details refer to IS 1592-1980.

6.7.2 Laying and Jointing

The width of the trech should be uniform throughout the length and greater than the out side diameter of the pipe by 300mm on either side of the pipe. The depth of the trench is usually kept 1 meter on the top of the pipe. For heavy traffic a cover of atleast 1.25 meter is provided on the top of the pipe.

The AC pipes to be laid are stacked along the trenches on the side or opposite to the spoils.

Each pipe should be examined for any defects such as cracks. chipped ends, crusting of the sides etc. The defective pipes should be removed forthwith from the site as other wise they are likely to be mixed up with the good pipes. Before use the inside of the pipes will have to be cleaned. The lighter pipe weighing less than 80 Kg can be lowered in the trench by hand. If the sides of the trench slope too much ropes must be used. The pipes of medium weight upto 200 Kg are lowered by means of ropes looped around both the ends. One ends of the rope is fastened to a wooden or steel stack driven into the ground and the other end of the rope is held by men and is slowly released to lower the pipe into the trench. After their being lowered into the trench they are aligned for jointing. The bed of the trench should be uniform.