The Bridgeport evening farmer., 1915-08-20

been made to overcome these objec- tions to a certain degree by covering the concrete pavement with a lbitum- inous wearing surface. At the pres ent time, the specialists in the Depart ment of Agriculture hold that this can not be economically justified al- though it is possible that future inves- tigation may change the situation in this respect In the present state of road science, however, it seems that Where traffic is too heavy for macad- a biturninous kurface on a, concrete road is practicable a bituminous-sur- face macadam road would be equally practicable and certainly cheaper Wheretraffic is too heavy for macad- am road, the bituminous surface is likely to give way and the uneven manner in which it fails tends to produce excessive wear on portions of the concrete.
 For a successful concrete road. hardness, doughness and uniformity are the most essential qualities. These can be secured to a great ex- tent by care~in the selection of the constituent materials and the pro protions in which they are mixed. Sample specifications are included in the bulletin, No. 249, Portland Ce-z ment Concrete Pavemerits for Coun- try Roads, These specifications are believed to typify the best engineer ing practice as it has been developed up to this time. They cover such points as materials, grading, sub grade, and construction.
 The cement, it is said, should aI-. ways conform to some standard spec ifcations for Portland cement, such as those issued by the United States Bureau of Standards or the Ameri- can Society of Testing Materials. The and the proportion of sand to coarse - sand with more coarse than fine grains is to be preferred. The coarse aggregate may consist of either crushed stone or gravel. In either case it is very desirable that the coarse aggregate be well graded in size between proper limits.
 The proportion of cement to the sand and coarse aggregate combined should not be less than about 1 to 5, and thep roportton of sand to coarse taggregate not less than F to B. nor l greater than 2 to B.. A useful formula
 when gravel is used, as coarse aggre- gate is l part of cement to F parts
 of sand to 3 parts of gravel. When crushed stone is used, sz parts of sand may be substituted in place, of t11z parts.
 In addition to discussing the en- gtneering details of construction, the bulletin already mentioned calls at | IonC1On to the 1acC that OFQ1naT1ly 1 from one-third to oue-haIf of the to-
 tai cost of constructing a concrete pavement is for labor after the ma
 terials are delivered. This empha- sizes the importance of efficient or
 ganization and proper equipment l Failure to take these factors into con
 sideration frequently results, it is 1 said, in adding from 10 to 20 per
 cent. to the cost of a concrete pave
 ment.
 The most economical method is to
 have the work of mixing and plac- ing the concrete as nearly continuous as practicable. The work should be planned with a primary view of keep ling the mixer going full time. The
 drainage structures, the grading, and the sub-grade should, therefore, be completed well in advance of the mix
 CT and provision made for obtaining
 all of the necessary materials without delay. A common error is the fail
 ure to make adequate provision for | delivering water on the work, and the
 amount which 8. given stream is ca
 pablo of supplying is frequently over estimated. fsN