centre. In such cases the girders are deepest at the abutment, and have somewhat the appearance of an arched rib. The position of the reinforcement is, however, radically different. If some of the very flat arches had been designed thus instead of as arches, the unsightly cracks over the haunches so common to them would probably have been avoided.

Reinforced-Concrete Trusses.-A true truss of reinforced concrete was constructed by Considère in France. The compression members were of concrete reinforced by spiral hooping as in a hooped column. The tension members were of steel surrounded by concrete. This bridge was built only as a test of the strength of the hooped member and when finished was loaded until failure took place. It demonstrated, however, that bridges of this type may be built in reinforced concrete. The economy of such a bridge is doubtful, as the cost of form work must have been excessive.

Girder bridges are occasionally constructed with open webs. The girder is thus given the appearance of a truss. Beyond the saving of a little weight, this type of bridge has no advantage over a bridge with a solid girder, and as anything approaching an exact determination of the stress acting in the open girder is impossible, they should be avoided.

Concrete Floors for Steel Bridges.-In long-span highway bridges, when steel trusses are necessary, the wood planking, which until recently was the standard flooring, is now being largely replaced by reinforced-concrete slabs, supported on the steel beams. On these slabs the wearing surface of the roadway is placed. A floor of this type is more expensive in first cost than a plank floor, but it will outlast the bridge itself, while a wood floor requires renewing in from one to five years.

In steel railroad bridges, reinforced-concrete floors are now being extensively used to replace trough and open floors. These reinforced-concrete floors are practically noiseless, and may be ballasted in the same way as the rest of the roadway, thus making a uniform roadbed throughout the line.

Reinforced concrete may also be used to strengthen existing steel bridges when same have become insufficient for the present need, or so badly corroded as to be considered dangerous. In the bridge at Perigueux, France, the lattice bars of the main girders

and the webs of the cross beams were so badly corroded by the gases from locomotives stopping under them that the safety of the bridge was threatened. The bridge was protected and strengthened by incasing all the old steel members in reinforced concrete, and a new reinforced-concrete floor was then built. This resulted in a new bridge, stronger and stiffer than the old one, that would not be acted upon by the gases from the locomotives. If it had been necessary, the strength of the bridge could have been still further increased by the addition of reinforcing rods parallel to and alongside of the beams and girders.

Abutments. The abutments of a concrete bridge may be constructed in either plain or reinforced concrete. They should be designed to resist overturning due to the pressure of the earth backing, and at the same time to so distribute the load on the foundation caused by this pressure, and the load of the bridge, that in no place will the load on the soil exceed its safe bearing value. In some cases the bridge is rigidly attached to the abutments while in others it simply rests in the seat. In the first case all tendency of the bridge to expand or contract, due to temperature stresses, must be resisted by the abutments or internally by the bridge itself. In the second case the bridge slides on its seat as this expansion or shortening takes place. For long bridges the second method is preferable while for short bridges either method will give satisfactory results.

Centring. The form work for flat reinforced-concrete bridges is essentially the same as for floor construction. Troughs are formed in the centring to receive the beams and girders when they extend below the slab, and when the girders or beams are above, the slab formwork is built up to receive them. The formwork should be as firm and unyielding as possible, so that there will be no deflection or distortion when the concrete is placed. It should also be sufficiently tight to prevent the cement and water from leaking out, thereby causing a poor porous concrete.

Depositing Concrete. In general the concrete should be deposited as quickly as possible so as to insure a monolithic structure. Beams, girders, and slabs should, if possible, be deposited at the same time, especially where the beams have been designed as Tbeams. Where the beams or girders are deep, it is sometimes in

advisable to do this, as the contraction of the beam or girder in setting may cause it to crack away from the slab. In such cases it would be well to concrete the beam or girder first, and the slab after a sufficient interval had elapsed. In this case, however, if T-action is desired, special reinforcement will be necessary to bond the beam and the slab properly together.

Finish. In some structures, where appearance is of little importance, the concrete can be left just as it comes from the moulds, and if sufficient care has been taken in building the form work and placing the concrete, a very satisfactory finish will result. A better finish may be obtained by placing against the forms a one-inch coat

TABLE XXV.-PRINCIPAL DIMENSIONS AND QUANTITIES OF MATERIALS FOR SLAB BRIDGES.

(From "Concrete in Highway Construction," published by Atlas Portland Cement Co.)

ing of cement mortar and then placing the concrete behind it. This mortar may be applied with a trowel or behind a steel plate which separates it from the concrete backing.

In the removal of forms this facing may be treated in various ways as described in Chapter XII. If the mortar is not set too hard, it may simply be brushed with a stiff wire brush and water. This will remove the outer film of cement and bring the grains of sand into prominence. If the mortar is set too hard to be acted upon by the wire brush, sand or a cement block may be used and the same effect attained. By a proper selection of the sand, various color effects may be obtained in this way.

* Distance in feet from top of footing course to bottom of slab.

If a mortar facing is not desired, the concrete itself may be rubbed, sanded, or tooled until the outer film of cement is removed and the aggregate exposed. Where proper thought has been given to the selection of the aggregate, very pleasing effects may be obtained in this manner.

If further treatment is thought advisable, the surface of the concrete may be washed with a weak solution of acid. After the acid wash it is well to again wash it with an alkaline solution to neutralize any acid that may remain in the concrete.

SECTION V

THE USES OF CONCRETE FOR
SPECIAL PURPOSES

CHAPTER XXV

CONCRETE IN SEWERAGE AND DRAINAGE
WORKS

Advantages of Concrete for Sewers.-Forms of Sewers.-Combined and Separate Systems. Dimensions of Sewers.-Construction of Sewers and Conduits.-Quantity of Flow.-Culverts and Drains.-Types of Culverts.-Imperviousness of Sewers and Conduits.-Tables of Dimensions for Culverts.

Advantages of Concrete for Sewers. In no situation perhaps are constructive materials subjected to greater destructive forces than in subsurface work, particularly where the ground is charged with corrosive chemical and electrical influences. Under such conditions, many sewers and water-carrying conduits built of iron and steel have been destroyed in the course of comparatively few years. It is therefore with reason that municipal engineers throughout the country rejoice that in concrete, both plain and reinforced, a material has been found that will not only be cheaper than brick or masonry but more enduring than steel and iron and more susceptible to use under any condition from the largest conduit to the smallest pipe.

While in Europe, factory-made cement pipe has been largely used up to 7 feet in diameter, American engineers have found it more economical up to the present time to mould all pipes and sewers exceeding 3 feet in diameter right in place. For pipes smaller than 3 feet, difficulty in securing and using adequate forms have made it advisable to manufacture them in factories specially equipped for turning them out in large quantities and standard sizes.