Recycled plastics for ventilated crawl spaces: which static and dynamic effects are transmitted on recycled plastic crawl spaces using different mixtures

The ancient Romans had already understood, in the construction of buildings, the importance of creating a ventilated air space, between the ground and the floor , in order to avoid the capillary rising of humidity and allow a thermal insulation of the floor. The crawl space was built using walls connected to each other or with amphorae as a filling base.

With the evolution of buildings, the ventilated crawl space has had multiple uses, not only to isolate from humidity, but it was possible to use, in the best way, the space created between the ground and the floor. Until a few years ago, before the advent of plastic in the building industry, the construction of crawl spaces was done through small tables, for horizontal parts, and bricks or concrete prisms for the vertical wall.

However, this system did not fully guarantee isolation between one floor and another. Today, with the use of recycled plastic elements, the possibilities of using the interspace have been expanded and its technical qualities improved.

Let’s see what are the possible uses of separating elements in recycled plastic:

1) The classic function for which he was born is to create, through continuous modular plastic elements, an effective separation between the inhabited plane and the foundation soil, preventing the capillary rising of humidity. Furthermore, the space that is created, allows the passage of the systems for the house functions easily.

2) The monolithic interspace formed, allows the evacuation of the Radon gas that forms in the ground. This is a colorless, odorless radioactive gas, formed by the decay of uranium 238, which has the ability to creep into the cracks of the ground and saturate the basements or floors in contact with it. Through the laying of the plastic elements on which a continuous concrete jet will be created, natural ventilation will be created, with air inlets to the north and exit to the south, so as to avoid gas stagnation.

3) The creation of ventilated roofs, especially for horizontal ones, allows a natural adjustment of the thermal changes that help, together with a correct insulation, the livability of the underlying environments and energy saving.

4) The plastic elements of reduced heights, especially those of 5 cm, help to correct sound insulation, together with damping mats, since the air stops inside the cells, helps the damping of sound waves.

5) Another function is to be able to create hanging gardens with the characteristic of being able to isolate the waterproofing layer from the roots of the plants. It is well known that most of the hanging garden defects concern the percolation of meteoric water, as the action of the roots opens gaps in waterproof bituminous membranes, with the possible passage of water. The plastic elements are extremely resistant to the drilling action of the plants.

Surely there are many other functions that the plastic crawl space can perform but, listing the most common, I tried to give an idea of its use.

Once you have decided what use must be made of the separator elements, it is important to understand how they are produced to be able to choose the elements that are suitable for our work.

The main characteristics that are asked of a set of elements that will constitute the supporting structure for our concrete casting in the upper surface are:

• Element flexibility
• Vertical compression resistance
• Resistance to bending of domes
• Dimensional maintenance of the single pieces after molding in order to be assembled effortlessly by the operators and without leaving voids
• Lack of fragility during handling
• Correct thicknesses depending on the raw material used
• Non-deformability under the weight of fresh concrete
• Minimum walkability of the element expressed in the ability to support the floor jet worker, which must not be less than 150 kg calculated on a surface of cm.8 x cm.8.

These characteristics, without prejudice to a correct design of the mold and the element itself, can be reached with a right choice of recycled raw materials, which can increase or decrease certain characteristics.

The most commonly used material belongs to the polypropylene family, in particular a mixed compound between PP and PE which allows discrete mechanical performance and a low production cost. In some cases the HDPE element is produced, which gives the elements better technical performance in the face of higher production costs.

The PP + PE recipe used has technical limitations to keep in mind:

1) The compound in PP + PE normally comes from the differentiated collection components , which consists of rigid polypropylene waste and low-density polyethylene flexible waste. The two elements are difficult to manipulate from the thermal point of view, during the molding phase, with the risk of degradation of the material and the formation of gas inside the molded element. These micro holes can weaken the element.

2) The compound obtained has, in general, good mechanical vertical characteristics , in particular as regards compression resistance, but, on the other hand, has a limited resistance to bending and torsion . The knowledge of the technical limits of this compound normally allows the resolution of these minus with an appropriate design of the reinforcing bands through the positioning of reticular septa, in the points most subject to possible breaks.

3) The search for exaggerated cost-effectiveness could induce producers to reduce polypropylene inside the mixture to the advantage of LDPE , creating situations of structural weakness that should be compensated with the addition of HDPE and / or mineral fillers.

The study of such complex recipes is certainly not recommended in the production of elements on which one has to walk safely, in order to avoid accidents, because they require a high technical competence and the control of the incoming input through frequent laboratory analyzes.

In some cases a mixture of HDPE is used which can be composed of granules deriving from the processing of caps in the beverage sector or with mixed compounds with caps and detergent bottles.

According to the data collected we can indicate some differences:

a) The production of recycled plastic crawl spaces using granules coming from HDPE plugs means having to work a raw material that has certainly a lower fluidity than the PP + PE compound, normally 1.5-2 to 2.16 Kg ./190° against a MFI 5-6 to 2.16 Kg./230°. This means that the size of the press to be used must also be taken into account as the HDPE polymer is certainly less fluid.

The mechanical characteristics of this compound can be summarized in a good compressive strength and an excellent resistance to bending and torsion of the molded elements.

However, there is an important factor that could influence the choice of this polymer. In the presence of very large laying surfaces and in correspondence of peaks of very high temperatures, it is to be considered that the HDPE element, continually coupled with other modules, inside the lattice of the beams, could undergo a deformation important given the reaction to the heat of the sun.

The problem can be solved, in the granulation phase, by adding a percentage of mineral charge that sterilizes the expansive reactions of the HDPE.

b) There are cases in which the resistance of the module is a fundamental element and, in the presence of thin thicknesses of the walls of the product, it is possible to opt for a mix formed by the granulation of caps and HDPE bottles or just the bottles .

The reduction of the fluidity of the mixture leads to an increase in the mechanical performance of the elements with the same physical characteristics of the element, with fluidity values ranging from 0.3 to 1 to 2.16 Kg./190°.

Automatic translation. We apologize for any inaccuracies. Original article in Italian.