382 EXPERIMENT STATION RECORD. [Vol. 41 



directly under the heavy wheel loads is never very great as long as the slab 

 rests on the subgrade, and it is unlikely that the slab design would ever be 

 controlled by this stress." 



The commercial sizes of crushed stone aggregates, F. H. Jackson and C. W. 

 MiTMAN (U. S. Dept. Agr., Public Roads, 2 {1919), No. IJf, pj). 35-40). — A survey 

 of the present practice in the States of Ohio, Kentucky, Tennessee, North 

 Carolina, and Georgia, for the purpose of securing data to be used in the 

 development of a system of standard sizes and uniform nomenclature of crushed 

 stone products, is reported. The results confirm the conclusions of a report pre- 

 viously noted (E. S. R., 39, p. 494), and it is concluded that the necessity for 

 standardizing stone sizes has been demonstrated. 



Bonding new cement-mortar and concrete to old in tests, W. E. Rosen- 

 GARTEN {U. S. Dcpt. Agr., Public Roads, 2 (1919), No. 14, j)p. 26-34, figs. 5).— 

 The results of an extensive series of tests conducted in the Bureau of Public 

 Roads to obtain definite data on the relative value of various methods of in- 

 creasing the strength of bond between new and old cement mortar and con- 

 crete are reported. These tests included " (1) tests on the bond strength in 

 tension of a 1:2 mortar in the form of the standard tension briquette, (2) 

 tests on the bond strength in cross bending of a 1 : § : IJ concrete in 4 by 4 by 

 14 in. prisms bonded to concrete slabs, (3) tests on the bond strength in shear 

 of a 1 : f : IJ concrete in specimens in the form of S-in. diameter cylindrical 

 disks, 2 in. high, [and] (4) tests on the permeability or water-tightness of 

 the joints formed by various bonding methods. The same specimens were 

 used in this series as in the shear tests." The following results were 

 reached regarding the tensile strength of bonds: 



It was found that the ability of fresh mortar to adhere to older material 

 decreased rapidly as the old set up. Where the older material had set for 

 24 hours before adding the fresh, the bonding strength was but 44 per cent 

 of that of monolithic construction. Further aging of the old mortar caused 

 a slight decrease in the bonding ability, which at 7 days was 39 per cent of 

 the original and at 28 days was reduced to 35.5 per cent. 



It was possible, however, to increase the bond by treating the old surface, 

 either by roughing with a steel tool, or by applying dilute hydrochloric acid 

 of 1 : 10 strength. Roughing the surface increased the strength of bond 20 

 per cent and the acid treatment increased it 13 per cent, except where the old 

 concrete had aged seven days, in which case there appeared an unexplainable 

 detriment to the bond, due to the acid treatment. Combining both roughening 

 and acid treatment gave but a slightly greater bonding ability than either by 

 itself. 



The application of a thin layer of cement butter as a bonding medium had a 

 decided effect in increasing the adhesive strength. Twenty-five per cent in- 

 crease in strength was developed by this method, and 28 per cent if the butter 

 layer was allowed to stand one hour prior to adding the fresh mortar. Hard 

 tamping of the new mortar against the old increased the strength of bond about 

 8 per cent. 



The broken surface showed a greater bond strength than either the plain 

 molded or natural surface, but roughing of the two latter placed them on 

 a par with the broken surface. The ability to bond to the molded and natural 

 surface appeared about equal, although surfaces molded against forms showed 

 a decrease in strength as the age of the old concrete increased. 



With reference to cross-bending strength, it was found that by careful 

 treatment new concrete can be made to adhere to old with a strength equal 

 to 60 per cent of monolithic concrete. If no special treatment other than 

 cleaning is given the old concrete, a bond strength of but 20 per cent of the 



