=Wire-cut-lug Brick.=--This is a type of non-repressed brick which has s.p.a.cing lugs provided by cutting one face in a special manner which provided lugs for s.p.a.cing. In this type the wire cut face is the one between the brick as they are laid in the pavement.
=Tests for Quality.=--The standard test for quality of paving brick is the rattler test. The brick rattler consists of a barrel of 14 sides 24 inches long, mounted so as to rotate at a speed between 29.5 and 30.5 revolutions per minute. The duration of a test is 1800 revolutions. Ten brick const.i.tute a charge and these are placed in the rattler along with 300 lbs. of cast iron spheres. The spheres are of two sizes, the smaller being 1-7/8 inch in diameter when new, and the larger 3-3/4 inches in diameter when new. Ten of the larger spheres are used and the balance of the charge is made up of the small size.
When tested in the standard manner the loss allowable for the several cla.s.ses of service are as follows:
------------+---------------+---------------- | | Maximum Loss Traffic | Average Loss | for any Brick ------------+---------------+---------------- Heavy | 20 per cent | 24 per cent Medium | 22 per cent | 26 per cent Light | 25 per cent | 28 per cent ------------+---------------+----------------
=Other Tests.=--Sometimes the absorption test is specified for paving brick, but it is rarely a vitrified brick that will pa.s.s the rattler tests which fails to pa.s.s a reasonable absorption test. Absorption of water in an amount exceeding 4 per cent indicates incomplete vitrification and failure of such brick is almost certain during the rattler tests.
The cross breaking test is also sometimes employed, but generally only to check the general quality of the brick. Failure in service more frequently occurs from excessive wear than from any other cause and the cross breaking test has little significance, except for brick less than 3 inches thick, which are to be laid on a sand bedding course.
=Foundation.=--The foundation for brick roads is usually of Portland cement concrete, the thickness varying with the nature of the traffic and the kind of soil upon which the pavement is built. For well drained soils and normal highway traffic, 5 inches is the ordinary thickness of foundation. Under favorable conditions such as locations with sandy soils or in semi-arid or arid regions where the soil is always stable, the foundation may be four inches thick, and a considerable mileage of brick road has been built with concrete foundations less than four inches thick.
In other locations the soil and traffic conditions require a base six inches or more in thickness, and the proper thickness can be determined only after all of the factors involved are known and have been a.n.a.lyzed. It is impractical to adopt a standard thickness of foundation that will be equally economical for all locations and all kinds of traffic. As the brick pavement is essentially a heavy traffic type of surface, the design cannot be varied greatly with similar foundation conditions because the weight of individual loads is the significant factor and this does not vary so much as the volume of traffic. A variation in volume of traffic may be compensated for by a variation in the quality of the brick as already set forth.
The mixtures for the concrete foundation vary widely because of the variation in the aggregates employed. If the fine and coa.r.s.e aggregate for the concrete are of good quality a mixture of one part cement, two and one-half parts sand and five parts of coa.r.s.e aggregate would insure concrete of adequate strength. A somewhat leaner mixture is sometimes employed and would be satisfactory if the aggregates were of exceptional concrete making quality. Mixtures of sand and pebbles (unscreened gravel) may also be used if care is exercised to secure a mixture of adequate strength. The proportion will of necessity vary with each particular material and the discussion of the various considerations involved may be obtained from various standard works on concrete and concrete materials.
Broken stone macadam is sometimes utilized for the foundation course of the brick pavement and such foundations are constructed as water-bound, which is described in a previous chapter. The thickness, like that of the concrete foundation, varies with the soil conditions and the weight of the loads that are expected to use the road. The macadam is placed in a single layer and is rolled and bonded with screenings as described in the chapter dealing with water-bound macadam. Six inches is a common thickness for the macadam base. This type of foundation should be employed only where the soil is quite stable and where material costs are such as to insure that the macadam base is materially cheaper than one of concrete. This would usually be in locations where the cost of cement is high because of long hauls and where suitable macadam materials may be obtained close at hand.
Old macadam roads are sometimes utilized for the foundation for the brick surface, but the instances where this is permissible are comparatively few in number. When an old macadam is to be used it is reshaped to the proper cross section and re-rolled and bonded so as to afford a stable foundation of the proper cross slope.
BEDDING COURSE FOR BRICK SURFACES
In order to equalize the variations in size and shape of the brick, they are laid on a bedding course composed of material into which the brick may be forced by rolling. In this way the upper surfaces of all brick can be brought to the proper elevation to insure smoothness and easy riding qualities. Several kinds of bedding course are now employed.
=Sand Bedding Course.=--The sand bedding course has been referred to as a sand cushion, but as a matter of experience the cushion effect is slight, although sometimes pavements have become uneven because the brick have pushed down into the sand after the pavement was used for a time. The sand for the bedding course should preferably be fine grained, all particles pa.s.sing the eight mesh sieve, but ordinary concrete sand is satisfactory. The sand need not be clean, as a comparatively large percentage of silt or clay does not impair the usefulness of the material.
[Ill.u.s.tration: Fig. 19.--Cross Sections for Brick Highways]
=Sand Mortar Bedding Course.=--In order to eliminate the tendency for the straight sand bedding course to shift because of the impact of traffic on the brick, a lean cement mortar is sometimes employed rather than the straight sand. Sand and cement in the ratio of one part cement to four or five parts of sand are mixed dry, and after the brick have been rolled, is moistened to furnish water to hydrate the cement. The sand employed is ordinary clean concrete sand.
=Green Concrete Bedding Course.=--In the monolithic type of brick road construction, the brick are laid directly on the green concrete base before the concrete has taken a set and the irregularities of the brick are taken up by rolling them until bedded in concrete.
FILLERS FOR BRICK SURFACES
The s.p.a.ces between the brick are filled with some material that will prevent the brick from being displaced and prevent water getting to the bedding course. A suitable filler must adhere to the brick and fill completely the s.p.a.ces between them. It must withstand traffic so as to remain intact in the joints and when in place it must be rigid enough to prevent displacement of the brick.
=Cement Grout Filler.=--One of the most commonly used fillers for brick pavements consists of a grout composed of Portland cement and fine sand. When properly mixed and applied the grout filler meets all requirements for a filler except that it is non-elastic and some means must be adopted for caring for pavement expansion.
=Bituminous Fillers.=--Asphaltic materials and tars are widely used as fillers for brick pavements. Such fillers are of high melting point and consequently solid at ordinary temperature. They are poured into the joints hot and when they cool are firm enough to comply with the requirements for a filler. In addition, they have enough ductility to accommodate the expansion of the pavement due to temperature changes.
=Mastic Fillers.=--Mastic consists of a mixture of about equal volumes of fine sand and a solid bituminous material. The mixture is prepared at high temperature and is worked into the joints between the brick while hot. When cool it resembles the straight bituminous filler except that the mastic is somewhat more resistant to wear than the straight bituminous filler.
EXPANSION JOINTS
It is recognized that brick will expand and contract with changes in temperature. When a bituminous or mastic filler is employed there is sufficient yield to the filler to accommodate the change in dimension in the brick, but when the grout filler is used either the expansion joint must be provided or the pavement must be designed to withstand the compression due to expansion of the brick. Expansion joints may consist of a sheet of bituminous mastic prepared for the purpose and set in place in the pavement. The sheet of joint material is simply inserted between courses of brick at the proper place.
Another method of forming an expansion joint consists in placing a strip of wood between courses of brick at the place where a joint is required. After the pavement has been grouted, the wooden strip is pulled out and the joint is filled with a suitable bituminous filler.
=Marginal Curb.=--If the sand bedding course is employed, it is necessary to provide curbing along the sides of the brick to hold the bedding course in place. The curb is usually constructed integral with the base and of concrete of the same mixture as the base. The width of the curb is usually six inches and the top of the curb is at the same elevation as the edge of brick surface.
CONSTRUCTION OF THE SURFACE
Before the construction of a brick surface should be undertaken on a road, the drainage should be provided for even more completely than for a less costly type of surface since it does not pay to jeopardize the stability of the pavement by failure to provide adequately for the stability of the supporting soil. Grades should also be reduced to the economical limit.
The earth subgrade is brought to the proper elevation and cross section and is thoroughly rolled. If there are places where the soil will not compact properly under rolling, these places are corrected by taking out the material and back filling with new material that will properly compact under the roller.
The aggregates for the concrete may be distributed along on the prepared subgrade or may be stored in stock piles or bins at convenient points. If stored on the subgrade, a traction mixer is employed which is drawn along the road as the work progresses, the materials being placed directly in the mixer. If stored at a central point, they may be transported to the mixer on the road and dumped directly into the mixer, or the mixer may be set up at the storage piles and the concrete hauled in trucks to the road where it is deposited and shaped.
The concrete is spread to the proper thickness and tamped either by hand or by machinery. If the marginal curb is to be employed, it is constructed immediately after the concrete for the base has been finished but before the cement begins to set.
After the foundation concrete has set, the bedding course is spread and struck off to the proper thickness. When the bedding course consists of sand-cement mortar, the sand and cement are mixed dry and spread to prescribed thickness. It is considered to be desirable to roll the sand bedding course with a light hand roller before the brick are placed, but the sand-cement bedding course is not rolled. The bedding course must be carefully shaped by means of a templet or strike board before the brick are placed.
The brick are laid in straight courses across the pavement, with the s.p.a.cing lugs all in the same direction if brick with s.p.a.cing lugs are employed, and with the lugs in contact with the brick of adjoining courses. If brick without s.p.a.cing lugs are used they are laid loosely so that there will be room for the filler between the brick of adjoining courses.
After the brick have been laid they are rolled to bed them in the sand or sand-mortar bedding course and thus secure a smooth surface. For this purpose a light, power driven, tandem roller is used and the rolling is continued until the brick are thoroughly bedded. Any defective brick that are noted are removed and replaced with good brick and after this culling has been completed the surface is once more thoroughly rolled. If a cement-sand bedding course is employed, the surface is sprinkled just after the final rolling so that water will flow down between the brick and moisten the bedding course sufficiently to cause the cement to set. In some cases, the sand-cement bedding course is sprinkled just before the brick are laid but in warm weather the setting would take place before the brick could be rolled if that were done. In cool weather the setting is sufficiently slow to permit rolling before the bedding course hardens.
The filler is applied to the surface after the rolling. If the bituminous type of filler is employed, the hot filler is poured onto the surface and worked into the joints by means of squeegees, with comparatively little material left on the surface. In some instances cone-shaped pouring pots are employed and the material is poured directly into the joints.
The cement grout filler is applied in the same general manner as the bituminous filler. The grout, consisting of equal parts of sand and cement, is mixed to a thin consistency and poured onto the surface and is then worked into the joints with squeegees. Two or more applications are usually required to effect a complete filling of the joints. The surface should be covered with sand and be kept moist until the cement grout has set.
CHAPTER X
BITUMINOUS ROAD MATERIALS AND THEIR USE
Tars and asphaltic materials of various kinds are widely used for road construction and maintenance, especially for road surfaces subjected to motor traffic. Materials of this character that are employed in highway work possess varying degrees of adhesiveness, and while they may be semi-solid or viscous liquids at air temperature, they melt on the application of heat and can be made sufficiently fluid to mix with the mineral aggregates that may be used in the road surface. Upon cooling, the bituminous materials return to the previous state and impart a certain amount of plasticity to the mixture, at the same time serving as a binding or cementing agent, which is sufficiently stable for many cla.s.ses of road construction.
=Cla.s.ses of Bituminous Materials.=--Bituminous materials may be cla.s.sified, according to the source from which they are obtained, as coal tars, water gas tars, native or natural asphalts and oil or petroleum asphalts.
=Coal Tar.=--Coal tar is obtained as a by-product in the manufacture of illuminating gas from coal. It is also obtained in the manufacture of c.o.ke from coal. The tar thus obtained is manufactured into products that are used for dust layers on gravel or macadam roads, binders for macadam and gravel surfaces, fillers for brick, wood block and stone block pavements and for expansion joints. These various materials differ mainly in their consistency at air temperature. (They may differ widely in chemical composition, but that need not be considered herein.)
=Water Gas Tar.=--Water gas tar is obtained as a by-product in the manufacture of illuminating gas from crude petroleum. It is used for the same kinds of construction as coal tar, and the products utilized for the several purposes, like the coal tars, differ mainly in consistency.
=Natural Asphalt.=--Natural asphalt is found in deposits at many places in the world, existing in beds or pools where it has exuded from the earth or as veins in cavities in the rocks. It is of varying composition and consistency, but those kinds in most general use are solid or very viscous liquids at air temperature. Of the deposits that have been developed on a commercial scale, the Trinidad lake in the British West Indies and Bermudez deposit in Venezuela are best known.
Both of these materials are too hard in the natural state to be used for road construction, and are softened, or fluxed as it is called, with fluid petroleum oil before being used.
=Petroleum Asphalt.=--Petroleum asphalt is a residue remaining after the fluid products have been distilled from petroleum. Residues of this sort are not always suitable for road construction, but a number of brands of road material are obtained from this source. Oil asphalts are used for dust layers, for binders for macadam roads, for asphalt cements for sheet pavement surfaces, and for fillers for block pavements and expansion joints.
=Mixtures.=--Water gas tars and asphalts are sometimes mixed to produce road materials, and likewise native asphalts and residues obtained from petroleum are sometimes mixed to produce asphalt cements for paving mixtures.
=Cla.s.sification according to Consistency.=--The various bituminous materials may be cla.s.sified according to consistency in discussing the various uses to which they may be put.