SAMPLING AND TESTING HIGHWAY MATERIALS. 65 



This plate should be oiled or a piece of wax paper be placed between it and the con- 

 crete. If the forms are carefully made, this will give top and bottom surfaces per- 

 pendicular to the sides of the specimens. To prevent the specimen from drying out, 

 it should be covered or otherwise protected. If desired, the mold itself may be 

 buried in sand while the specimen is being molded. 



"At the end of 48 hours the specimens should be removed from the mold and buried 

 in damp sand." 



(It is the sentiment of the conference that oftentimes in concrete road con- 

 struction it would be advisable to cure the test pieces along the side of the slab, 

 under conditions similar to those of the pavement.) 



" Testing. — Ten days prior to the date of test, specimens should be well packed in 

 damp sand or wet shavings and shipped to the testing laboratory, where they should- 

 be stored either in a moist room or in damp sand until the date of the test. It is 

 assumed that ordinarily a 28-day test will be made, although tests at 7 and 14 days 

 will give some indications of the results to be expected at 28 days. In case 7-day 

 tests are made, the test pieces should remain at the job as long as possible to harden, 

 and should be shipped so as to arrive at the laboratory in time to make the test on the 

 required date." 



47. PROPOSED METHODS OF MAKING TEST SPECIMENS OF CONCRETE 



IN THE LABORATORY. 



The conference recommends that concrete specimens shall be proportioned by 

 volume. The measurements of the unit volumes of aggregate shall be made in accord- 

 ance with tbe method outlined under "Weight per cubic foot of aggregate" (p. 11). 

 Volumes required for the given quantity of concrete shall be measured by weighing 

 the requisite amounts of each material. The weight of 1 cubic foot of cement shall 

 be assumed to be 94 pounds. In weighing quantities of a coarse aggregate which varies 

 considerably in size of particles, it is suggested that the aggregate be graded into 

 different sizes and the proper proportions of each of these sizes be used. 



In making specimens sufficient quantities of materials u to fill a single specimen 

 mold with an excess of 10 per cent should be calculated. The dried materials should 

 be placed on a metallic or other nonabsorbent mixing tray and thoroughly mixed to a 

 uniform color with a square-nose trowel. The requisite amount of water for temper- 

 ing the mix to proper consistency should be weighed. The dry material should be 

 formed into a crater and about three-fourths of the estimated amount of water added. 

 The entire batch should then be turned until of uniform composition, water being 

 added until the required consistency has been obtained. The amount of water 

 actually used in the mixture should be recorded. 



In molding the test pieces, the form should be filled to about one-quarter of its 

 height and thoroughly puddled with a |-inch rod, using 20 strokes per layer. 



From two to four hours after molding, compression specimens should be capped 

 with a layer of neat cement in order that the top of the specimen may present a smooth 

 surface for loading. The cap can be readily formed by means of a glass plate which 

 may be worked down on the neat cement until it rests on top of the form. In order 

 to eliminate shrinkage, the cement for capping should be mixed to a stiff paste before 

 the concrete specimens are made. Adhesion of the concrete to the base of the mold 

 and to the glass can be eliminated by oiling the base and by inserting a sheet of paraf- 

 fined tissue paper beneath the glass. Specimens should be removed from the forms 

 on the day after they are fabricated, marked, and stored in damp sand, or in a moist 

 chamber until tested. 



It is recommended that tests be made at 7, 28, or 90 days; the 28-day period is the 

 most commonly used. 



ii In most cases it is preferable to use air-dry aggregate. 

 29465°— 21— Bull. 949—5 



