The use of shotcrete in the Washington, D.C. Metro System is reviewed. Shotcrete in this instance is defined as coarse aggregate as opposed to the ACI definition which is broader and includes fine aggregate and non-structural uses. Six-inch shotcrete linings in fair to good rock with moderate overbreak used in conjunction with rock bolts was considered to be an economical solution. Such linings could also be used as temporary supports eliminating steel ribs, and serve as the first step in building up a permanent lining with further layers of shotcrete. This paper which outlines basic steps utilized in developing shotcrete, also outlines the experience gained in implementing the specifications. The early set of shotcrete was selected at 500 psi at 8 hours and the 28 day strength was specified to be 5000 psi. The high early and final strength values must be selected for optimum effect and a trade-off has to be made between high early strength and high final strength for durability. The two gradations are presented which were specified for the combined coarse and fine aggregates. In the accelerating admixture needed to reduce rebound, two to 3 percent by weight of cement is used. A maximum of 3 minutes for initial set and 12 minutes for final set for the cement-accelerator paste, and a paste strength of 800 psi in 8 hours are specified. Control aggregate gradation was achieved by selecting fine-coarse aggregate gradation bands which could be met with available local aggregates. In the testing of shotcrete mix in laboratory for conformity with specifications, two sizes of aggregate with maximum sizes of 0.75 and 0.5 inch were selected. Field trials by nozzlemen, quality control during production and construction testing are also discussed. Of the accelerators tested, Sigunite and Triacasol T-1 conformed to requirements. Tests were made to determine the compatibility of cements with the selected acceleration and the ability of the combinations to produce the desired early strength.

Media Info

  • Features: Figures;
  • Pagination: p. 33-49

Subject/Index Terms

Filing Info

  • Accession Number: 00263680
  • Record Type: Publication
  • Report/Paper Numbers: SP-45 Conf. Proc
  • Contract Numbers: H0111881
  • Files: TRIS
  • Created Date: Dec 19 1981 12:00AM