concrete requiring 70 seconds of mixing was priced at approximately $18 per cubic yard. It follows that reduction in total mixing time, and recognition of transfer time as mixing time, results in substantial savings in material processing with better assurance of high quality. It also tends to make portland cement concrete more competitive with other paving materials.

Central mixing plants-Mixing time.-Another relatively new equipment development which will provide for greater economy and quality in portland cement concrete production is the highly portable central mix plants with stationary mixers of 4 to 8 cubic yards capacity. Many of these plants have automatic batching features, including moisture meters and compensators for controlling the amounts of aggregates in equivalent dry weights with accuracy and the correct amount of total mixing water. This assures the production of concrete of highly uniform consistency for each batch. Highway departments cannot realize the full potentials of improved central plant mixing if they continue the old practice of requiring an additional 15 seconds of mixing for every additional cubic yard capacity above 1 cubic yard. Studies conducted by the Ohio State Department of Highways and other organizations tend to confirm that a mixing time of about 60 seconds for any available size of mixer appears to be entirely feasible. Ohio reduced its mixing time from 120 to 90 seconds for stationary mixers up to 10 cubic-yard capacity pending further studies. Substantial savings are possible if the central plant operation can be phased to the lower mixing times.

Other new equipment developments. Actually the acceptance of centrally mixed concrete for paving is a step toward mass production, at least partially due to two other recent equipment developments. Segregation of the concrete mix had been a problem both in hauling and placing. Several new hauling truck models, including a specially designed body for nonagitated haul have now been developed. Hopper type spreaders have also been developed which can receive a full truckload of concrete and deposit it uniformly between the side forms. without segregation.

One of the newest developments in processing the concrete placed between side forms is a spreader-finisher-float which performs all three of these functions in a single pass. One man can now control all operations from a single control panel. Quality work is accomplished with a reduction in personnel and power requirements.

Concrete slip-form pavers

The slip-form paver is not a new development, but several models have been continuously improved so that this method of placing concrete is an approved alternate to the side-form method in a number of States. It has been employed in 14 States with the largest volumes of concrete having been poured by this method in California and Colorado. The machines are designed to pour two or three lanes (24 feet and 36 feet) of nonreinforced concrete in a single pass. Wire mesh reinforcement, however, has been successfully placed in slip-form operations.

The advantages of the slip-form method are the substantial reduction in the paving equipment train and in operating personnel. machine performs spreading, tamping, vibration, and finishing in a single pass without the aid of preset side forms. The concrete which

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is extruded under high pressure forms a slab with the aid of vibration to a relatively dry mix.

The effective use of slip-form pavers does require experience and operating know-how. Concrete of uniform consistency must be supplied and the speed of the paver must be kept constant and continuous if a smooth pavement is to be obtained.

Experience gained in California in the last several years has enabled them to initiate measures which will assure conformance to their rigid pavement smoothness tolerance of 7 inches of total deflection per mile. They were able to produce concrete pavements by the slipform method last year with a total deflection of less than 3 inches per mile. In earlier operations in some States this high degree of smooth

FIGURE 15.-Extruding a nonreinforced concrete slab with a slip-form paver which is fed by two dual compartment paver mixers.

ness had not been achieved and it was necessary to correct this by expensive surface grinding.

Slip-form pavers have not reached their ultimate stage of development, but with further improvements, it is thought they will contribute to greater efficiency in concrete paving operations, particularly when teamed with the highly productive central mixing plants.

Continuously reinforced concrete pavement

The first continuously reinforced concrete pavement was constructed in Arlington, Va., in 1921 by the Bureau of Public Roads. Interest in this design type has increased recently. In the last 3 years some 230 lane-miles have been constructed in seven States. Some of these sections were built as Bureau of Public Roads experimental projects,

and their inservice performance will be closely observed in the future by our field engineers.

General characteristics.-Continuously reinforced concrete pavement makes possible the construction of a slab of any length without the installation of intermediate planned expansion and contraction joints. The many fine transverse cracks which characteristically form in such pavement are usually not noticeable by sight, sound, or feel to the motorist. These cracks ravel only superficially, do not fault, and apparently do not open enough to permit the passage of damaging amounts of water to the subbase and subgrade. The periodic crack and joint sealing common to conventional pavements is not required, accompanying traffic inconveniences are thus avoided, and the cost of maintenance is reduced.

The reduced number of planned expansion and contraction joints should permit the initial riding smoothness of the pavement to be maintained over a longer period of years. Structural weaknesses and difficulties such as faulting, spalling, and concentrated pressure failures due to infiltration of inert material and pumping often associated with the use of transverse joints in conventional concrete pavements are minimized and longer useful life of the pavement may be expected. Design requirements.-Based on these potential advantages, the Bureau issued minimum design requirements in November 1959 under which Federal-aid projects for this type of construction would be approved.

Equipment for bridges and drainage structures

In the bridge construction area, several new deck finishing machines have been developed to assist in securing improved riding qualities. Cast-in-place concrete culverts have been used in the Western States for longitudinal drainage installations outside of the pavement area. Nonreinforced culverts up to 72-inch diameters can now be extruded with a new machine which operates similar to the smaller extrusion machines used to produce curbs of portland cement or asphaltic concrete. Available cost comparisons with the prefabricated types of concrete culverts indicate savings up to 20 percent may be achieved by the cast-in-place method.

Reinforced concrete culvert pipe

Portland cement concrete culverts of the reinforced type remain one of the principal materials used for small drainage installation. It is estimated that well over 3 million tons will be installed this year for highway construction and maintenance purposes.

Design standards and installation criteria.-New design and installation criteria developed by the Bridge Division of the Bureau in cooperation with AASHO, ASTM, and the American Concrete Pipe Association were issued in April 1957. This joint effort resulted in a complete redesign of reinforced concrete culvert pipe for improved strength and for increased applications in the expanded highway construction program. Five classes of pipe each having a minimum ultimate load rating were established.

Under these new criteria, reinforced concrete pipe of the proper class can be used in fills up to 100 feet in height and more as compared to a maximum cover of 25 to 30 feet which prevailed previously.

The installation criteria provide for the proper bedding and backfilling to assure keeping within the culvert design stresses and of minimizing settlement of the backfill.

Standardization.-Reinforced concrete pipe is now fabricated in several shapes including an elipsoid for low clearance installations, arch sections which may be used as cattle passes, and complete precast manhole barrels in base, riser, and top sections. Tentative specifications for precast reinforced manhole sections were developed last year as ASTM designation C478-61T.

Standardization of the smaller sizes for concrete pipe has also been accomplished in recent years. Perforated concrete pipe was standardized by ASTM designation C444-59T and concrete drain tile as ASTM designation C412-60.

One of the newer methods of sealing reinforced concrete culvert pipe joints is with preformed rubber gaskets.

Handling equipment.-Excavations for concrete culverts placed in trenches are normally made with a trencher or backhoe. Some of the horizontal boring machines, however, are capable of forming holes through existing embankments up to 36 inches in diameter. Special equipment for loading and unloading concrete culverts from trucks without damage has been developed.

Use of latex modified portland cement mortars

Next to the repainting of structural steel, the disintegration of portland cement concrete is the most pressing problem associated with the maintenance of highway structures. The disintegration of concrete bridge decks is probably the most critical area, especially where airentrainments have not been used, and it presents a difficult repair problem because the impaired area is relatively shallow and is not readily susceptible to replacement in kind. In the last several years a number of latex modifiers for sand-cement mortar have been used with considerable success in repairing scaled and disintegrated bridge floors. They show excellent promise of being an invaluable new material for this purpose. One latex used dispersion in water of polymeric particles designed to achieve compatibility with highly alkaline portland cement mortars with resulting improved adhesion, flexibility, compressive strength, chemical resistance, and greatly improved freeze-thaw stability. The ability to resist chemicals such as the chloride salts and to withstand thaw-freeze cycles is highly important since these two factors are contributory causes of the premature disintegration of portland cement concrete. Latex mortars also have the advantage of curing to almost the natural color of conSuccessful use of latex modified mortars in repairing concrete bridge decks has been made in 14 States including recent ones in Kentucky, Michigan, Maine, and Texas.

Scope of use

CLAY PIPE AND TILE PRODUCTS

There have been several improvements in the clay drainage pipe. field including stronger and smoother sections produced by modern extrusion machines. The 2- and 3-foot lengths have been replaced with new standard lengths of 4 feet 4 inches and 5 feet. This results in a substantial saving in installation costs. It is estimated that approximately 90,000 tons of clay pipe and tile for subsurface drainage will be used for all highway purposes in 1962.

As in the case of reinforced concrete culvert pipe, there is widespread use of resilient materials for compression type joints. Tentative specifications for factory-installed gasket material were issued in 1958 as ASTM designation C425-58T. In the same year tentative specifications for installing clay sewer pipes were also introduced as ASTM designation C12-58T.

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