La posa

The working possibilities of a natural stone like porphyry are innumerable.
Endowed with many good qualities which let us classify it in the category of the valuable stones, porphyry is now assuming a primary role within many projects of recovery and exploitation of the urban areas with particular reference to the historical centers and the pedestrian zones.

It is the load-bearing structure of any floor, whose solidity depends on the inalterability of the floor. As a solid road foundation, ballast was once used: it was made of big boulders placed next to one another with their widest base laying on the foundation. The upper interstices among the elements were filled with smaller stones and with gravel. Today such operation would be extremely expensive and slow, that is why it has been abandoned. As an under-foundation, the Macadam (a term deriving from the name of the Scottish engineer Mac Adam) is still used nowadays: it consists of a more or less compact layer of shattered stones, compressed with smooth or vibrating rolls.

The currently most used technique for the constitution of under-foundations (employed also because it is faster), is founded on the mixing, in a dry mixture, of several gravel, rubble and clay types with mortar, cement and bitumen, to compose the so-called "stabilized earths". Their nature and composition are different because they tend to employ the materials deriving from the zones close to the place where they will be used (as in the case of the highways).

However, concrete (a mixture of sand, rubble, cement and water) is the under-foundation of largest use, owing to its rapid execution and remarkable resistance.

For a good mixed-traffic road, it is enough to employ only 10 cm of concrete mixed with 200 kg of cement per square meter and it would be even better if a light electric-soldered network were drowned in it. The expansion joints are the only drawback of concrete, because they are necessary in the case of wide surfaces. They determine the crack and the breakup point of the floor above.

The real road is constituted by the last layer, i.e. the top layer. It can consist of a layer made of simple rubble and glue: in this case we have the "white road " or beaten-earth road. These generally lay directly on the foundation, not on an under-foundation. It is the case of a lot of forest roads or of limited-traffic road. In order to give more solidity to this mantle it can be sprinkled with a layer of cold bituminous emulsion attenuating its permeability and giving it a better-finishing aspect.

In another case the rubble is previously mixed to warm-spread bitumen, in rolled layers of various thickness: these are the bituminous floorings that represent the most used coverings. Their strong employment is due to their rapid spreading out, to the perfection that can be obtained on the surface thanks to the use of sophisticated finishing machines, to the possibility of immediate use and at last to the limited costs, especially in the case of thin thickness.

The STONE COVERING is the covering type that concerns us much more than the others, and particularly all the floors that can be made with porphyry manufactured articles.

Therefore, the at least general knowledge of the above-cited concepts is very important. The checking of the quality and resistance of foundations and under-foundations is not the layer's task, but it goes without saying that a good layer has to be able to recommend the execution of a suitable under-foundation and this in relation to the foundation and the traffic that will affect the floor (pedestrian, car, truck traffic).

In fact, when the layer thinks that the under-foundation already predisposed does not present the necessary resistance qualities, he had better dissuade the floor laying or he should at least ask for a declaration of no-responsibility to avoid a bad result which might be attributed to the material or to the laying work. For example, the surface of the under-foundation can happen to present pronounced irregularities compared to the levels of the finished flooring.

In this case, two hypotheses can verify, i.e. there can be either too high or too low an under-foundation. In the first case a certain difference can be tolerated, but (for floors made of small cubes) it must not reduce the sand bed after the compression to less than 3 cm. In the second case the difference can be corrected with sand, provided that after the compression the layer is not thicker than five or six cm (for the floor made of 4/6 or 6/8 type). For bigger differences, in the first case the road will have to be unloaded and in the second case it will have to be reloaded, to avoid probable irregularities or any subsidence in the finished flooring.

Then if the road must have a certain level when finished, this meaning it must respect some fixed elevations, the under- foundation will have to be as many cm as the specific cube type requires lower than the final elevation.

THE FUNDAMENTAL RULE IS THAT THERE MUST ALWAYS BE TWO HALF ARCHS ON THE ROAD SIDES.

The working and laying possibilities of porphyry are innumerable. In particular with small cubes, that can be used to create a mosaic, with great results both from a geometric and from a chromatic point of view.
With small cubes one can draw initials, trade-marks, symbols, boundaries, trees and so on, as well as drawings and decorations made of other materials.

Here we give some information, even though very synthesized, concerning the laying of the small porphyry cubes.

Indeed, for tiles, irregular slabs, cordons and others, the laying techniques show what is used for all the similar products, either for internal or for external laying.

Here we want to describe - in a general way - the essential data regarding a standard laying work with cube type 6/8.

Presupposed - as necessary and essential - a resistant under-foundation with the necessary longitudinal or cross inclinations pointing to the drains or towards the free spaces, a 5-cm-thick bed of granulated (or dry-premixed with cement) sand is spread out, and on it the laying of the cubes is carried out.

The chord of the arch will have a development of 120/145 cm with an arrow of about 25/30 cm. It is essential, and not only for aesthetical reasons, that - in the case of a road - there is half an arch on its sides pressing on the edges rather than a whole arch.

The longitudinal or cross inclinations of the finished floor will have to be of 2% (at least 1.1/2%) in case of cubes and 1.1/2% in case of regular tiles.

The same inclination will be needed if this carries the rain water to the center of the road rather than to the sides.
The cubes can be laid down in the form of:

- concentric circles
- a contrasting arch
- a "peacock tail"
- "straight rows"

In the case of the "peacock tail" and the concentric circles laying it is possible and often advisable to employ cubes of different colours or of different materials to better highlight the drawing.

It is important that these complementary materials have a remarkable atmospheric-agents and sliding-friction usury resistance and this to equate - as far as it is possible - the endowments of porphyry.

The hammering of the laid surface will be made with a vibrating tool and in the presence of water until the refusal.

The sealing can be made of sand but also - and better - of liquid cement.

The cleaning, that will have to be very accurate, will be done with jet of water or better with three sawdust passings.

The section of a porphyry cubes floor, with sidewalk made of tiles, porphyry boundary cordon and porphyry tiles.

FOUNDATIONS, UNDER-FOUNDATIONS AND COVERINGS MADE OF PORPHYRY
First of all, we have to explain the general meaning of the three main building elements regarding the construction of a road: foundations, under-foundations and coverings.
THE FOUNDATION
It is the natural plane on which any construction (road or building) is founded and it necessarily must have characteristics of solidity and strength. Foundations not suitable to sustain a road are the sandy ones (except for the case when sand is contained within the sides thus determining incompressibility), and the country lands, above all in the presence of clay.
THE UNDER-FOUNDATION

		It is the load-bearing structure of any floor, whose solidity depends on the inalterability of the floor. As a solid road foundation, ballast was once used: it was made of big boulders placed next to one another with their widest base laying on the foundation. The upper interstices among the elements were filled with smaller stones and with gravel. Today such operation would be extremely expensive and slow, that is why it has been abandoned. As an under-foundation, the Macadam (a term deriving from the name of the Scottish engineer Mac Adam) is still used nowadays: it consists of a more or less compact layer of shattered stones, compressed with smooth or vibrating rolls.
The currently most used technique for the constitution of under-foundations (employed also because it is faster), is founded on the mixing, in a dry mixture, of several gravel, rubble and clay types with mortar, cement and bitumen, to compose the so-called "stabilized earths". Their nature and composition are different because they tend to employ the materials deriving from the zones close to the place where they will be used (as in the case of the highways). However, concrete (a mixture of sand, rubble, cement and water) is the under-foundation of largest use, owing to its rapid execution and remarkable resistance. For a good mixed-traffic road, it is enough to employ only 10 cm of concrete mixed with 200 kg of cement per square meter and it would be even better if a light electric-soldered network were drowned in it. The expansion joints are the only drawback of concrete, because they are necessary in the case of wide surfaces. They determine the crack and the breakup point of the floor above.
COVERING
The real road is constituted by the last layer, i.e. the top layer. It can consist of a layer made of simple rubble and glue: in this case we have the "white road " or beaten-earth road. These generally lay directly on the foundation, not on an under-foundation. It is the case of a lot of forest roads or of limited-traffic road. In order to give more solidity to this mantle it can be sprinkled with a layer of cold bituminous emulsion attenuating its permeability and giving it a better-finishing aspect. In another case the rubble is previously mixed to warm-spread bitumen, in rolled layers of various thickness: these are the bituminous floorings that represent the most used coverings. Their strong employment is due to their rapid spreading out, to the perfection that can be obtained on the surface thanks to the use of sophisticated finishing machines, to the possibility of immediate use and at last to the limited costs, especially in the case of thin thickness. The STONE COVERING is the covering type that concerns us much more than the others, and particularly all the floors that can be made with porphyry manufactured articles. Therefore, the at least general knowledge of the above-cited concepts is very important. The checking of the quality and resistance of foundations and under-foundations is not the layer's task, but it goes without saying that a good layer has to be able to recommend the execution of a suitable under-foundation and this in relation to the foundation and the traffic that will affect the floor (pedestrian, car, truck traffic). In fact, when the layer thinks that the under-foundation already predisposed does not present the necessary resistance qualities, he had better dissuade the floor laying or he should at least ask for a declaration of no-responsibility to avoid a bad result which might be attributed to the material or to the laying work. For example, the surface of the under-foundation can happen to present pronounced irregularities compared to the levels of the finished flooring. In this case, two hypotheses can verify, i.e. there can be either too high or too low an under-foundation. In the first case a certain difference can be tolerated, but (for floors made of small cubes) it must not reduce the sand bed after the compression to less than 3 cm. In the second case the difference can be corrected with sand, provided that after the compression the layer is not thicker than five or six cm (for the floor made of 4/6 or 6/8 type). For bigger differences, in the first case the road will have to be unloaded and in the second case it will have to be reloaded, to avoid probable irregularities or any subsidence in the finished flooring.
	In all the three above-listed cases (ballast, macadam and concrete under-foundation), the under-foundation must be moulded in the sense of the length and of the width, so that it presents the same inclinations as the finished flooring; this to allow the draining of the meteoric waters. One should always remember that the inclinations in the under-foundation have a fundamental importance, because in many cases it is the layer who is asked to create them "with sand". This entails the employment of an irregular sand layer in the different points of the floor: these points determine - if the difference is remarkable - a smaller resistance to the traffic and along with it the imaginable negative effects. Therefore, it is better for the layer to know how to safeguard oneself and above all it is important for him to be able to give good advices.