Abstracts of Technical Articles from Bell System Sources (01 October 1932)

Some Recent Developments in Underground Conduit Construction in the Bell System.2 A. L. Fox. In the past the type of joint made by trowelling cement mortar around the abutting ends of multiple clay conduit has not been entirely satisfactory because in some cases it permitted infiltration of sand and silt which obstructed the ducts. Various types of joints have been investigated, including a modified 1 2 Mecli. Engg., June, 1932. Belt Telephone Quarterly, July, 1932. 622 ABSTRACTS OF TECHNICAL ARTICLES 623 form of the old trowelled joint, cement mortar collars molded in place, and joints made with different plastic materials such as asphalt compounds spread on a fabric backing to facilitate application and retention. 

None of these provided the tight seal desired. A satisfactory joint has been developed by encasing the junction in a mortar bandage consisting of a wide band of cement mortar enclosed in cheese-cloth and held tightly against the conduit by tapes passing completely around the joint and secured on top. A separate strip of cheese-cloth is imbedded in the center of the mortar to prevent slumping. W i t h cement mortar of proper consistency and an admixture to insure the desired plasticity sufficient cement paste comes through on the inside when the bandage is applied to provide a tight bond. A strip of paper placed in the bandage under the cheese-cloth in the outer side helps to distribute the pressure of the tying tapes and assists in retaining water in the mortar thereby aiding the hardening and increasing the strength of the joint.