5-24, SLABS ON EARTH: Before proceeding to construct concrete slabs on earth, all pipes under concrete floors on earth shall have received the required tests, The earth fill shall be compacted as approved by the Engineer, and any drainage fill required under the slabs on earth, as in- dicated on the drawings or specified, and compacted by rolling and tamping. shall be deposited earth under the slabs or drainage fill, In compacting the any unsuitable ma- terial encountered in the subgrade shall be removed and replaced with material approved by the Engineer. The subgrade shall be brought to a true even plane, solid bearing. and compacted to a Gravel drainage fill, where indicated, shall be installed to the thickness shown and shall be properly leveled to a reasonably true and even surface, The gravel fill shall be covered with draft paper, lapped at least 4 inches at the edges and ends. The concrete shall be of comparatively dry consistency and shall be screeded level or to the required grades. After compacting, the surface shall be prepared to receive the required type of finish treatment specified. 5-25. PROTECTION AND CURING: &, Protection Against Moisture Loss: Immediately after placing of finishing, concrete surfaces not covered by forms shall be protected from the loss of surface moisture for a period of not less than 7 days where a normal Portland cement has been used, or 3 cays where a high-earlystrength Portland cement has been used by using membrane curing compounds or by covering with kraft paper, quilts or burlap, lapped 4 inches at edges and ends, used, shall be sealed, Kraft paper, if Materials for curing shall be as specified under heading 5-05 above. Surfaces from which forms are removed before the curing period has elapsed shall be protected as specified for surfaces not covered by forms. Membrane curing shall net be used on surfaces that are to receive additional concrete or concrete fill, nor on cement finish coats that are to receive dust-proofing and hardening treatments. b. Curing: Curing may be accomplished by effective protection of all exposed surfaces against moisture loss as specified above, or by either method specified below. 1. Water cur ing: ed by keeping the Water curing shall be effect- forms sufficiently wetted with clean water to reduce cracks and to prevent the joints in forms from opening and, by keeping the other protective material thoroughly wet, prevent hair cracks from occurring in surface of the concrete. During damp periods, this water curing routin: 5-16