(Illustration 24) Boston, Massachusetts-Commonwealth Avenue. Concrete pavement laid in 1923 by the Boston Park Commission. Carries all heavy and boulevard traffic to the central and western parts of the state

(Ilustration 25) Chicago, Ill.-North Crawford Avenue. Ten-inch concrete pavement laid in 1924 on either side of car track zone. More than 1,000,000 square yards of concrete streets have been laid in Chicago during the past

two years

(Illustration 26) Chicago, Ill.-North Cicero Avenue, a heavy traffic arterial thoroughfare; paved with a ten-inch concrete slab in 1925. More than fifty per cent of all new pavements laid in Chicago during 1925 were concrete

(Illustration 27) Seattle, Washington-Union Street, in the heart of the retail shopping district, paved with concrete in 1923. The concrete is laid on top of a concrete base, replacing a brick surface

(Illustration 28) Los Angeles, California-One of the heavy-duty concrete streets in the Los Angeles Union Wholesale Terminal. Pavement laid in 1918

THEORY OF DESIGN OF CONCRETE

By Clifford Older, Consulting Engineer With Consoer, Older & Quinlan, Chicago, Ill.

It is needless to say that there are many unsolved problems relating to paving design whatever the type of the pavement being considered. Special problems are also encountered in the design of street paving that do not necessarily apply to rural highway work. This paper deals only with some of the questions that arise in planning concrete street pavements.

A street pavement should primarily afford adequate traffic service at reasonable cost considering maintenance and renewal expense and hardly less important, should present a sightly

appearance.

TRAFFIC SERVICE COST

In order to afford adequate traffic service at a reasonable cost the pavement must be so designed as to afford a surface of low tractive resistance. Professor T. R. Agg has shown that tractive resistance increases materially with surface roughness. The surface should therefore be made as smooth as may be accomplished without a disproportionate addition to construction costs. Vehicle operators may be more sensative to the discomfort incident to ordinary surface roughness than to the added operating cost chargeable to roughness, nevertheless the roughness operating cost burden may easily assume such large proportions as alone to justify much care in construction operations. A degree of roughness less than that probably necessary to cause destructive impact effects may materially increase tractive resistance and therefore the operating cost of the road or street.

By the exercise of special care in finishing a concrete street pavement, although otherwise constructed by methods in common use, it has proved feasible to secure a surface that does not vary more than 1/4 inch as determined by a ten-foot straight edge placed parallel to the grade line. A more perfect surface would be desirable, but is not easily obtained by present methods of construction. There is little excuse for penalizing traffic by a lesser degree of smoothness.

The most conspicuous element of traffic service cost is the durability of the pavement itself for the reason that although a pavement of compartively high tractive resistance may, unknown to the road user, levy a heavy toll, yet a failure of the pavement, due to a defect of design or construction, is open to the eyes of all and leads to serious complaint. The elements entering into the making of good concrete have been discussed many times and will not be attempted here. The use of a design of sufficient strength to avoid structural failure is an obvious economy.

Investigations of the Bureau of Public Roads indicate that at least as great stresses may occur along pavement edges as at corners. The Bates Road Test and common observation, however, indicate that structural failure of such character, as to impair traffic service, materially increase maintenance cost and shorten its useful life, predominantly occurs at corners when the edge and corner thickness is the same. The stress at a right angled corner, due to any given load P may be calculated with reasonable safety by the formula f= 3P (derived from the common beam formula) and if a safe working stress is assumed for "f" the edge-corner thickness may be determined by the formula t=√3P This is the thickness formula recommended by the committee on "Proposed Specifications for Portland Cement Concrete Pavements" of this society.

Theoretical considerations and studies of test results indicate that the pavement thickness at a distance of more than two feet from an edge may be made equal to 0.7t. This is based on the assumption that two or more adjacent corners, formed in the interior portion of a pavement by intersecting cracks or joints, will be so interlocked by means provided in the design as to mutually support any load applied to either corner. This end may be accomplished by means of sufficient closely spaced steel reinforcement to prevent any cracks that may form because of subgrade movement or other cause, from widening, due to the joint becoming filled with ice or dirt as it alternately widens and closes, due to contraction and expansion of the slab. If sufficient steel is used for this purpose mutual support of the adjacent corners and edges, formed by such cracks as may be unavoidable, is assured not only by the shear value of the steel, but even more certainly by the interlocked roughness of the