has had sufficient use to show that it is one of the satisfactory types for heavy mixed traffic,

oncrete, and these are so often disregarded that the average serviceability of the concrete road surface is sometimes much lower than it would be if built with due regard for the effect of traffic on concret

tortion of the road slab to produce rupture. The aggregates may be crushed under wheel loads if the material is too soft. Abrasion from steel tired vehicles wears away the concrete unless it is hard and durable. Changes in dimension due to the effect of chan

urfacing, and that is accomplished by using suitable aggregates, by proper

. The thickness at the edge is 1 inch less than at the middle except that 6-inch surfaces are usually of uniform thickness, the total crown being 2 inches. The thickness of the two course pavement is the same as would be used for a single course pavement in the same location.

Section for C

uccessfully constructed. Similar designs have been used in a few other places, but for general practice it is unsafe to de

oth. Supplies of broken stone are likely to contain a small percentage of soft pieces and such of these as are at the surface when the concrete is finished will crush under traffic, leaving a pit in the surface. Pebbles screened from gravel are also likely to be variable in

as the two course type of concrete road is employed. The wearing course usually is about 2 inches thick and is constructed with selected aggregates of good quality shipped in for the purpose. The lower course is constructed of aggregates which do not possess the desired

d coarse material to produce uniformity in the concrete, and the use of aggregates of that character is not permissi

t be clean, free from organic matter and sand, must contain no appreciable amount of mica, feldspar, alkali, shale or similar deleterious substances and not exceed two and one-half

for the wearing surface should have a crushing strength of at least 2500 pounds per square inch, and the mixture adopted is based on the requirements that will give the desired crushing strength. The common mixture fo

aggregates exhibit peculiarities or the traffic conditions are unusual. It is desired to emphasize that the

portioned at the mixer. If even reasonably near uniform wearing qualities are to be secured throughout the entire area of the concrete road surface, successive batches of concrete must be alike, and to insure that, the aggregates including the wa

ed of such shape that the contents will be a multiple of 1 cubic foot when level full. For the larger jobs, the aggregates are hauled in industrial ca

emptied and when adjusted for a mixture will introduce the proper amount of water for each bat

engineers advocate that in preparing the subgrade, the earth be thoroughly rolled; others prefer not to roll the subgrade. If fills of considerable depth are c

side forms of a height equal to the thickness of the concrete r

cut to the prescribed cross section and the concrete is shaped by drawing the template along the side forms. Sometimes the template is used as a tamper, being moved

rosswise with a slight hand roller about 8 inches in diameter and 30 inches long. The fin

is carried on wheels that run on the side forms, and the machine moves slowly along as the tamping progresses. The concrete is tamped, struck off to shape

he one-course road. The concrete for the lower course is placed and struck off by means of hand tools, and after that

on is progressive for a long time, but is more rapid during the first few hours than during the later periods, and the concrete reaches about three-fourths of its

sun. This is accomplished by first covering with canvas as soon as the concrete has hardened sufficiently and by later covering

sections by earthen dikes and the space inside the dikes filled with water to a de

one month, and in cold weather traffic may

o temperature changes, expansion joints ? inch wide are placed about every thirty feet. These con

or less and many engineers think it advisable to omit expansion joints in constructing the pave

truction is to omit the expansion joints, but

it is poured so as to form a belt around each section about 15 feet square. If the mesh type is employed, a part of the layer of concrete is placed an

cient durability. The bituminous material is applied hot to the surface and is then covered with stone chips or gravel pebbles, ranging in size from ? inch down to ? inch, the resulting coating being about ? inch thick. Many failures of this type of surface have been recorded due to the diff

is an excellent automobile road and its low tractive resistance makes it a desirable surface for horse drawn vehicles. It possesses a high d

a year, tar or asphalt being employed. The dust and detritus is cleaned out of the cr