Compaction with pneumatic tired and vibratory equipment.-Base compaction is an all-important construction function, and many surface course failures take the blame for weaknesses which actually originate in the base. Where efficient and effective compaction is being performed, the type of equipment used will vary with the character of the materials. If the base contains fine-grained materials or is stabilized with binding agents, the self-propelled pneumatic roller with tire contact pressures of 80 pounds per square inch and higher will generally obtain the desired density in the fewer number of passes. If the base materials are granular in nature or of the macadam type, dynamic compactors will be most effective in reducing ai voids which cause settlement under the vibrating effects of heavy traffic.

Aggregate gradation standards

One of the problems involving the use of coarse aggregates, particularly those used in highway structures and high-type pavements, is the wide variation of sizes and gradations of stone called for in the construction specification of the several States.

In June 1948, after years of effort, the Department of Commerce obtained agreement on and issued "Simplified Practice Recommendation R 163-48" covering sizes and grading of coarse aggregates, including crushed stone, gravel, and slag. These were adopted by the American Association of State Highway Officials.

A review of the coarse aggregate sizes specified by the 50 State highway departments, the District of Columbia, and Puerto Rico for portland cement concrete for pavements and structures reveals wide variations from established standards. They are now using some 215 dissimilar coarse aggregate sizes for that item alone. Only seven aggregate sizes are recommended for use in concrete pavements and structures in the approved standard. Wide variation also prevails for the sizes of coarse aggregates specified for bituminous surfaces.

One manufacturer of asphalt plants maintains an inventory of 21 different screen opening sizes for the hot bins in increments as low as one thirty-second and one-sixteenth of an inch in order to meet the various aggregate size requirements of the State, city, and county highway departments and other customers. Normally, not over four aggregate sizes are used in an individual mix design. Lack of practical standardization of aggregate sizes in some areas tends to discourage the movement of crushed stone or other coarse aggregates from one State to another, even though it may be within more economical hauling range than the nearest in-State source.

A comprehensive study on the aggregate gradation problem will be the subject of a forthcoming report by the Bureau of Public Roads. This report stresses the need for simplification, standardization, and uniform application. A new graphical evaluation chart is provided to facilitate the selection of aggregate size for bituminous mixes. Lightweight aggregates

Lightweight aggregates produced from expanded shale and minerals are being used to an increasing extent in highway bridge decks to lessen the amount of dead load. Twelve States have provisions for the use of lightweight aggregates. Expanded shale concrete has generally a good service record on over 100 major bridges in various sections of the country. Precast deck sections have also been installed and are giving good service on over 800 smaller bridges. There have, however, been serious failures in isolated instances.

Analysis of one failure indicated excessive shrinkage due to inadequate wetting of the aggregate prior to mixing and the presence of underburned aggregates. In order to minimize such failures in the future, the Physical Research Division of the Bureau of Public Roads has recently prepared a "proposed specification for lightweight aggregates for structural concrete" which provides for more rigid controls over quality than existing specifications. This proposed specification has been forwarded to all Bureau field offices.

Plastic foams.-Research is also being conducted by other industries. on plastic foam expanded by air or other gases to develop a light lowthermal conductive material.

Plastic foams have already been used experimentally on one highway bridge in New York State and another in Illinois. A 1-inch thick coating was sprayed on the underside of these bridge decks to insulate them for the purpose of reducing the premature formation of ice on the bridge surface. Performance data are not available on the installations because of the short period these materials have been in place.

Steel uses

STEEL

Steel is used extensively in modern structures. The Bridge Division of the Bureau of Public Roads reported that 1,933 new steel-type bridges were constructed during the calendar year 1960 for Federalaid primary, secondary, urban, and interstate road systems. Of this number, more than 1,000 were constructed on the Interstate System. The total cost of construction for these steel bridges amounted to more than $493 million.

The major forms in which steel is used include structural steel shapes, structural plates, reinforcing bars, and welded wire fabric to improve the physical strength of concrete. Steel is also used in corrugated metal pipe for culverts and drainage structures. It is an important material in fencing, guardrail, lighting standards, bridge railing, and other subsidiary items of highway construction.

New steel alloys

New construction steel, together with new, lighter, more efficient beam sections now available, has influenced advanced concepts in bridge design. The new steels, together with the advanced design concepts, have resulted in improvements or benefits not only for designers of bridges, but for fabricators, contractors, and State highway departments as well.

The new steels include American Society for Testing Materials' designations A-36, A-440, A-441, and heat-treated alloy steels such as T-1, type A, which is a trade name steel. The A-36 steel is being used quite extensively in bridge construction. It was developed in answer to the demand for a more weldable structural carbon steel. This steel is intended for riveted, bolted, and welded fabrication. It has been accepted for use in highway bridges at basic allowable stresses 2,000 pounds per square inch higher than those permitted for A-7 steel, the former type of steel most commonly used in bridge construction.

The A-440 and A-441 steels are high strength and high-strength low alloy steels developed to improve the design application of these types of steels.

New heat-treated alloy steels have been developed and used for bridges when high strength is required in thinner sections. In addition to high mechanical strength properties, the new heat-treated alloy steels have atmospheric corrosion resistance two times that of type A-7 carbon steel. The corrosion resistance of these new steel alloys will add life to the structural members where these steels are used and also lower the maintenance costs for the highway departments.

Bridge construction is also making use of most of the 11 new wide flange beam sections that are now available from the various steel rolling mills. Some economies are being achieved through the use of these new beam sections.

Combining steel types in bridge design

Bridge engineers are continually looking for better means of creating the most efficient structures at the least cost. Some new design concepts in structural steel have been developed recently and used by engineers to make full use of steel as a design material. One new concept and type of bridge construction is combination steel construction. This type of construction employs different grades of steels for the individual members of a structure to obtain greater strength without increased weight, reduction in weight and dimensions without loss of strength, greater durability, and reduced erection costs.

Examples of this design concept are incorporated in the Carquinez Straits Bridge in California and in the bridge spanning the Ohio River between Louisville, Ky., and New Albany, Ind. Each of these trusstype bridges was shop welded and erected in the field with highstrength bolts.

This concept of combination steel construction for a structure has been extended in bridge design. Within an individual member of a structure, different grades of steel have been used to obtain weight and cost savings. The Whiskey Creek Bridge in California is an example of the application of this new concept. In this deck plate girder bridge, portions of the plate girders subjected to the highest bending moments are made of 100,000 pounds per square inch minimum yield steel alloy. Portions of girders subjected to moderate bending moments are fabricated of A-441 steel, and those portions with low bending moments are of A-373 structural carbon steel.

Hybrid designs. The combination steel construction idea in bridges is being extended still further where different grades of steel having different strengths are being used within the same normal section of a beam. The use of steel in this new manner is referred to as "hybrid" beams. One example might be the use of beams having heat-treated alloy steel in the flanges and structural carbon steel, such as A-373 steel, in the web. The different steels are shop welded to form the hybrid beam. A pilot or test bridge using hybrid structural steel beams is being built in Iowa to explore the practicality of this new concept.

Composite designs.-Modern bridge design also makes use of composite construction with dissimilar materials. By employing dissimilar materials, such as steel and concrete, to make up structural members, it is possible to reduce deadweight and at the same time increase strength and stiffness. In this arrangement, a structural steel section and concrete are combined to form a single member.

High strength reinforcing steel.-High strength reinforcing steel bars or wires are being used in prestressed concrete in nearly all States. Wherever and whenever prestressed concrete in used for bridge construction, the latest material developments and design methods have been used in the building of these bridges. Also, the best and newest types of reinforcing steels have been used to make the most efficient structural member. Another example illustrating that the highway industry is making use of recent material developments wherever possible is in a bridge in the State of New York, using high strength reinforcing steel in a reinforced concrete bridge deck. In this example, the engineers are reducing the total weight of reinforcing steel required with no reduction in the strength and stiffness of the concrete. Some material and cost savings should result from this

application of a new steel. Also, field erection problems or work involving the placement of the reinforcement and the concrete for the bridge deck should be eased or lessened.

Orthotropic plate bridges

Another idea or design using new developments in steel material is the orthotropic plate as a bridge floor. This system uses steelplate welded to reinforcing ribs to form an integral floor. A number of such bridges have been built in Europe, especially in Germany, since World War II. Some studies are being made concerning this new type of bridge in this country, particularly on design principles, fabrication of structural components, and erection methods. It has been concluded by some engineers that orthotropic plate construction can compete favorably in cost in the various States with other types of construction. A bridge built according to the orthotropic plate design would have a much lighter superstructure than conventional construction. Much weight and material is saved in orthotropic plate design. In addition, there can be savings in substructure costs because of the lighter orthotropic bridge floor system.

Two orthotropic plate bridges were designed as part of the study mentioned. One of the highway departments is considering the construction of such a bridge for a pilot study. The availability of steel in a wide range of strength, and improvements in welding techniques are making orthotropic plate construction for bridges available to the highway departments.

Simplified bridge designs with standardized components

The need for simplification and standardization of bridges and component parts has been apparent for some time. At the county level, particularly in our less populous areas, the shortage of engineers has focused attention on this matter. It is felt that a great potential for standardized bridges and components exists with those less urbanized counties, as well as in comparable cases at State and municipal level. Some progress has been made by both industry and the engineering profession in fulfilling needs and providing materials and services in that area.

Standard bridge plans by Bureau of Public Boads.-The Bureau of Public Roads has prepared and published standard plans for highway bridges. The first edition of these plans appeared in 1953. A revised edition was published in 1956, and a second revision is scheduled to be published in 1962. These plans are intended to serve as a useful guide to State, county, and city highway departments in the development of suitable and economical bridge designs for primary, secondary, and urban highways.

In the development of the Bureau of Public Roads standard plans, a particular effort was made to give sufficiently complete information on all plans so that they would approach contract drawings as nearly as practicable. For any given bridge location, however, it must be expected that the requirements imposed by local conditions will necessitate a certain amount of modification. It was thought that the Bureau of Public Roads standard plans for bridges would be particularly valuable to smaller highway departments with limited engineering staffs.

The Bridge Division of the Bureau of Public Roads is constantly searching for ideas and means to build better and more economical