POETLAISTD CEMENT CONCRETE POADS. 41 



because the excess of water reduces the strength of the concrete, and 

 there is a tendency for the mortar to separate from the coarse aggre- 

 gate. 



The concrete should be mixed thoroughly to a uniform con- 

 sistency. The time of mixing bears an important relation to the 

 quality of the output. Generally speaking, the longer the time of 

 mixing within practical limits, the greater will be the strength and 

 the resistance to wear. On the other hand, longer-mixing means re- 

 duced production and more expensive concrete. The time of mixing 

 should be long enough to secure the maximum of strength at a 

 minimum of cost. One minute of mixing appears to meet this con- 

 dition. Certainly, the time allowed should not be less, and it is 

 questionable whether the increased strength obtained with a longer 

 mix justifies the increased expense. To insure the mixing of every 

 batch for the proper length of time the mixer should be equipped 

 with an automatic timing device, or a combination timing and lock- 

 ing device that will prevent its discharge until all the materials have 

 been mixed together for the minimum time required. 



The consistency of the concrete also affects its strength and wear- 

 ing qualities. For maximum strength and wear, only sufficient water 

 should be used in mixing to secure a good workable consistency. The 

 water-measuring tank on the mixer should be used as a means of ob- 

 taining the proper amount of water for each batch. A test loiown 

 as the slump test is employed as a check on the consistency. The 

 slump test is made by filling a metal form with the concrete to be 

 tested, tamping it down until all the voids are filled and a slight film 

 of mortar appears on the surface. The form is then removed and 

 the vertical settlement or slump is noted as a measure of its con- 

 sistency. The form may be either a cylinder or a frustum of a cone. 

 If a cylinder is used, it is 6 inches in diameter and 12 inches in height. 

 The settlement or slump in this case should not exceed 2 inches for 

 proper consistency. If the frustum of a cone is used, the top diame- 

 ter should be 4 inches, the base diameter 8 inches and the height 12 

 inches. The slump in this case should be not gTeater than 1 inch, nor 

 less than ^ inch for proper consistency. (See Fig. 1, PL IX.) 



After the concrete has been mixed the required length of time it 

 should be placed between the side forms to the full thickness of the 

 pavement and in successive batches for the entire width. If the sub- 

 grade is dusty, it should be sjjrinkled lightly before the concrete is 

 placed. Each batch of concrete should be dumped as nearly in place 

 as practicable and leveled off with shovels. If for any cause a wet 

 batch or a batch in which portions of the aggregate are separated is 

 deposited in the road, it should be thinly distributed over the sub- 



