64 BULLETIN 949, U. S. DEPARTMENT OF AGRICULTURE. 



45. PROPOSED FIELD DETERMINATION OF CLAY AND SILT. 



The apparatus used in this test is the same as that described under the volumetric 

 method of making sieve analysis, test 44. Two hundred c. c. of the sand or other fine 

 aggregate are measured in the graduated cylinder and transferred to the small cylin- 

 drical container. Water is added and the sand washed by agitation. The large min- 

 eral particles are allowed to settle and the water containing the clay and silt is poured 

 into the large outside cylindrical container. The operation of washing with new 

 portions of fresh water is repeated until the wash water remains clear. The water in 

 the large container is allowed to stand over night to permit the clay and silt to settle 

 out. The clear s'upernatant water is then poured off and the sediment of clay and 

 silt remaining is transferred to the 200 c. c. graduated cylinder. Water is added to 

 bring the contents of the graduated cylinder to 200 c. c. The volume of sediment in 

 the cylinder is determined at the end of three hours. This volume divided by 2 gives 

 the percentage by volume of clay and silt on a wet basis, three hours standing. This 

 value is usually from 2£ to 4 times the value obtained when determining the clay and 

 silt by dry weight. If we assume, therefore, that the specifications require not more 

 than 5 per cent of clay and silt by dry weight, less than 1\ per cent by volume of clay 

 and silt would indicate that the fine aggregate complies with the specifications, while 

 more than 12 per cent by volume of clay and silt would indicate an excess of material 

 removed in washing. 



46. PROPOSED METHODS OF FABRICATING AND TESTING COMPRESSION 

 FIELD SPECIMENS OF CONCRETE. 



As a guide to the selection of the sample of the concrete and to the method of mak- 

 ing compression specimens in the field see the procedure outlined in Appendix 1, in 

 the report of Committee C-9 in the Proceedings of the A. S. T. M. vol. 17, part 1. 



The essential part of the report is as follows: 



Sampling the concrete. — Concrete for the test specimens should be taken immediately 

 after it has been placed in the forms. All the material for each sample should be taken 

 from one place. A sufficient number of samples — each large enough to make one 

 test specimen — should be taken at different points so that the specimens made from 

 them will give a fair average of the work. The location from which each sample 

 is taken should be clearly noted for future reference. 



In securing samples, the concrete is taken from the mass by a shovel or a similar 

 implement and placed in a large pail or in some other receptacle for transporting to 

 the place where the specimens are molded. Care should be taken to see that each 

 specimen represents the total mixture of the concrete at that place. 



Molding the specimen. — The pails containing the samples of concrete should be taken 

 to the place selected for making the test pieces as quickly as possible. To offset 

 segregation of materials during transportation, each samjale should then be dumped 

 out of the pail into a nonabsorbent water-tight receptacle and without further mixing 

 immediately placed in the mold. Different samples should not be mixed together, 

 but each sample should make one specimen. 



(The conference recommends that a £-inch rod 2 feet long should be used for 

 puddling the concrete instead of the f-inch rod recommended by Committee 

 C-9. The conference also recommends that the material be placed in the mold 

 in layers ?> inches deep and each layer puddled 20 times with the rod.) 



" Ramming should be avoided, but care should be taken to remove air pockets. 

 The freshly made specimen should be struck off and troweled level with the top of 

 the form. The specimen should preferably be capped in the field while it is in the 

 mold so as to be ready for the testing machine. After the concrete has stiffened 

 appreciably and before the molds are removed, neat cement or a rather stiff 1 : 2 mortar 

 may be used to fill the molds level full. A piece of plate glass or machined metal 

 plate should then be worked around on the top of the mortar until it rests on the form. 



