represents the average condition and to it add sand and other materials as outlined in 1b in letter of October 29, previously referred to. We have attempted to use compact lumps of adobe as obtained from field samples, but here we have also been unsuccessful. All the material so far examined contains pebbles or small hard particles which strike against the tool used in dressing the specimen and either tear large gashes or break it so that it can not be used. From conferences and searches in the library, we conclude at this time that it is impossible to secure valuable data on expansion. We will try out any new ideas if obtained, but we will concentrate on the program outlined which we expect will give reliable data on shrinkage. No. 4 ADOBE SOIL Very truly yours, C. T. WISKOCIL. December 25, 1920. 1. Percentage moisture (air-dry, in laboratory about three months). 3. Expansion, air-dry to saturated condition (in porous mold). 3a Specimen cut from lump. 3b Specimen compacted from dust. 4. Force of expansion, produced by absorption of water by confined mass of adobe soil. Adobe passing 100-mesh sieve. Figure 8. Apparatus and various molds specially made, but found to be unsatisfactory; also a few specimens, some of which are defective. December 23, 1920. Mr. J. B. Lippincott, Consulting Engineer, Automobile Club of Southern California. Mr. H. J. Brunnier, Chairman, Highway Committee, Gentlemen: I submit our report on 20 experimental concrete slabs, eight plain, five reinforced with mesh and seven with bars. The object of these tests was to determine the relative bending efficiency of reinforcement versus increased concrete thickness. The table on page 8 gives a summary of the slabs tested and indicates their carrying capacity at three stages during test to rupture, a-at the proportional limit. b-at the first crack. c-at the maximum load sustained. Our report not only records the loads carried, but has made careful inquiry into the behavior of the slabs under deflection. The particulars are given in the deflection load curves, pp. 16-23. CONCLUSIONS 1. The large scale load deflection curves show that the proportional limit is the most reliable basis for the comparison of slab strengths. 2. At their respective proportional limits, the reinforced slabs are not as strong as plain slabs one inch thicker. 3 At the proportional limit the reinforced slabs 4 inches and 5 inches thick had less than 50% of the strength of plain slabs 6 inches and 7 inches thick, respectively; that is, the 2 inches of extra concrete thickness practically doubled the strength of the plain slabs. 4. The reinforcement was placed 12 inches from the bottom of each slab in its tested position. Had the reinforcement been placed at the center of the slab's depth, the comparative results would have been still less favorable to the reinforced slabs. 5. If the plain and reinforced concrete slabs are compared for load-carrying capacity, either at the first crack or at the maximum load sustained, the plain slabs still have an ample margin of increased strength over reinforced slabs 2 inches less in thickness. C. DERLETH, Jr. |