SAMPLING AND TESTING HIGHWAY MATERIALS. 15 



high). Add about 5 c. c. of dilute ammonia water and about 200 c. c. of water. Close 

 with a cork or glass stopper and shake thoroughly for 20 minutes. Allow the sample 

 to settle eight minutes and decant carefully or siphon off the supernatant liquid to 

 a depth of 8 cm. below the surface of the liquid. (The depth of the liquid in the 

 bottle should be sufficient to leave about 4 cm. below the point of siphoning.) Fill 

 the bottle again with water, shake for three minutes, allow settlement, and siphon 

 off as before. Repeat the process until the supernatant liquid is clear. Be careful 

 to wash the stopper and neck of the bottle free from coarse material before decanting. 



(13) Dry the bottle and washed material to constant weight at between 100° C. 

 (212° F.) and 110° C. (230° F.), weigh and determine the net weight of washed material. 



Original weight-washed weight cent d and ^ 



Original weight 



(14) As a check the washings drawn off shall be collected and evaporated to dryness 

 for direct recovery of the fine sediment classed as clay and silt. 



Weight of residue 

 Original weight r J 



The determinations on the two samples shall check within 1 per cent to be accept- 

 able. 



13. TESTS FOR SEMIGR^VEL, TOP SOIL, AND SAND-CLAY. 



The amount of clay and silt is first determined in accordance with test No. 12, then 

 proceed as follows : 



(1) Wash the contents of the bottle cleanly into a porcelain evaporating dish and 

 carry to dryness on a water bath. The dried residue shall be carefully scraped from 

 the dish and passed through a nest of 20. 60, 100, and 200 mesh sieA^es. The residue 

 retained on each sieve shall be weighed and recorded as sand of the respective sizes. 

 Their sum constitutes the total "sand." The residue passing the 200-mesh sieve and 

 caught in the pan shall be weighed and recorded as ' ' silt. ' ' Duplicate samples should 

 check within 1 per cent. 



(2) The coarse material shall be examined for hardness and with the magnifying 

 glass to identify its character as quartz, hard iron compounds, feldspar, schistose 

 material, or indurated clay. Hard quartz or iron gravels are valuable in themselves 

 and as indicating the quality of the finer aggregate. Feldspar, mica, and clay nodules 

 are worthless and indicate that the accompanying soil is poor for road building. 



(3) The sands shall be examined with the magnifying glass for identification as 

 quartz and for the presence of mica scales or feldspar needles. If mica or feldspar 

 is present in appreciable amounts the sample should be rejected. 



(4) When the clay is recovered by evaporation it can be examined for tenacity by 

 cementing together two glass plates, each 1 inch wide, set at right angles, with a layer 

 of clay whose thickness is fixed by a fine bent wire laid between the plates. The 

 moist clay shall cover the wire on one plate, and the other plate shall be squeezed 

 down tightly on the wire. After drying, the one plate shall be held firmly against 

 cleats, wire slings shall be run symmetrically from the ends of the upper plate to one 

 arm of a beam balance, and the tension necessary to separate the plates shall be given 

 by shot or weights in the other pan of the balance. This test is tedious, and is of 

 service chiefly on low-grade samples which are of doubtful efficiency, but which 

 represent the only available material for local construction. 



(5) Approximate tests for tenacity of mixture can be made as follows: 



Make cylinders 25 mm. in height and 25 mm. in diameter from the material passing 

 the 10-mesh sieve. Work the material into a stiff mud and mold under 132 kg. per 

 square centimeter pressure. Dry thoroughly at 100° C. (212° F.) and break by the 

 small Page impact machine for testing cementing value, using a 1-kg. hammer and. 

 1-cm. drop. Record the number of strokes as the relative measure of tenacity. 



