HOW TO CREATE AN ARCHITECTURAL
FLOORING IN WASHED GRAVEL

The bed must be made of a thickness consistent with the load that the flooring, which will be built on it, must withstand. On average it must have a thickness varying from 25 to 40 cm in thickness and it will be necessary to provide for containment and drainage of wastewater and percolation.

Prepare the appropriate concrete containment formworks (wood to be removed, concrete or natural stone curbs, straight or curved plates to maintain, etc.) to the project dimensions to delimit the casting areas.
The height of the formworks must be the overall height of the base concrete (about 10 cm) plus the thickness of the “deactivated” concrete (about 5 cm).

Proceed to the packaging of the “traditional” concrete for the realization of the base support for the “washed” concrete. The recommended thickness should be around 10 cm (unless otherwise indicated by the construction supervisor). During packaging, in addition to clean aggregates and in a continuous grain size curve suitable for the thickness to be made (usually 28 mm for a thickness of 10 cm) and for gray or white cement (according to the design requirements) of class 32.5 R or 42.5 R (based on design requirements) Type I (CEM I Portland) or Type II A-LL (CEM II Portland composite with limestone) based on the performance to be achieved and the seasonal temperature.

At the end of the concrete casting operations on the project dimensions, the surface of the same must be appropriately roughened (using a rake or stiff broom) to favor the best coupling of the next concrete layer to be deactivated.
PLEASE NOTE: if the concrete to deactivate is to be cast on an existing concrete substrate, make sure it has been roughened before. Alternatively, after appropriate high-pressure washing, a polymeric primer (such as BOND HG from AZICHEM srl) can be applied as a construction joints immediately before applying the deactivated concrete.

To achieve an optimal aesthetic result, the concrete mix design must include a mixture with more gravel and less sand than normal.This operation will create a so-called “broken” curve that will deviate from the normal “continuous grain size curve” typical of a traditional commonly used concrete.
This “broken” curve, due to the low presence of fine parts, will penalize the casting, which can be spread only through the “slide/channel” of the concrete mixer truck or manually, absolutely avoiding the use of the “concrete pump”.
The concrete packaged as said, can be mixed with colored ferrous oxides to modify the pigmentation to obtain, together with the gravel colors (ranging from pearl gray, to red, up to various shades of blue) a pleasant surface appearance that will be integrated perfectly to the surrounding architecture.

INDICATIVE MIX DESIGN FOR THE PACKAGING OF 1 CUBE OF WASHED CONCRETE:

• Cement Type I (CEM I) or Type II / A-LL (CEM II) – 32.5 R or 42.5 R (gray or white): 350-450 kg
• Clean and selected aggregates (rounded or angular) diam. 6-12 mm (approximately): about 1500 kg
• Clean and selected sand diam. 0-3 mm (approximately): about 100 kg
• PRERIT COMPOUND (multifunctional additive): 25 kg
• PROTECH OXICROM (synthetic iron oxide in the chosen color): 15-25 kg
• READYMESH PM-120 (12 mm multifilament polypropylene fibers): 1 kg
• Water (as required): About 250 lt
• TOTAL MIXTURE: Approximately 2300 kg

The table shown above is purely indicative. Given the desire to conceive washed gravel flooring as “natural, low environmental impact”, we will often or almost always be in the condition of having to use local aggregates from the quarries adjacent to the construction site, and therefore we will have to adapt to what nature puts to layout.

Manually spread the “special” concrete in the framework of the formworks previously prepared with the use screeder that appears like a rake without teeth; thickness of about 5 cm.
Spread the surface with an aluminum support following the project dimensions and the appropriate incline.

Once the spreading operations are completed, the surface will be adequately smoothed with the help of a metal trowel or special smoothing tool with a long handle (magnesium board), possibly avoiding any type of vibration or stress that could lead to the sinking of the aggregates.

Spread evenly over the entire surface of the concrete involved in deactivation (without bleeding), by means of a manual or electric low pressure pump, the PRERIT SOLUTION surface deactivator for an indicative consumption variable from 100 to 250 gr/m² (depending on the type and quantity of cement used, the period of execution of the work as regards the external temperature and the millimetric depth of removal of the surface) which temporarily inhibits the setting of the concrete. PRERIT SOLUTION it is a water-based, ready-to-use, light-blue deactivator, specific for cortical removal of the concrete surface. The product essentially allows the surface of the concrete to be peeled by a few millimeters, so as to bring out the aggregates it contains.

Within a maximum of 24 hours, depending on the conditions of humidity, temperature, quantity and class of cement used (with an outside temperature between 18 and 23 °C, the ideal time for washing is about 6-8 hours), for spreading and smoothing of the “special” concrete, the cortical part of the surface will be removed to highlight the gravel. The removal must be carried out using a pressure washer, with water at room temperature, with at least 150 bar of pressure. The result of the removal is strongly conditioned by the environmental temperature, the type of cement used, the grain size curve, the consistency and quality of the concrete, additives, etc. (for this reason we recommend suitable preliminary tests to calibrate the quantity of retardant product for deactivation purposes).

A single operator will be able to deactivate a maximum of 150 m² in 8 hours of continuous work. For this reason it is not recommended to lay more than 150 m² per day of “special” concrete. Otherwise, depending on the availability of more operators and more pressure washers, the surface may be increased. It is advisable to match the end of each flow with a formwork, an insert or an eyelash. Interrupting the flow and then taking it up again with new concrete will only create a concrete casting that after washing will be shown as it is on the floor.

Depending on the size of the paved area and its geometrical conformation, suitable expansion and fractioning joints must be made with mechanical disc cutters. This operation must be carried out within a maximum of two days after completion of the floor and will be completed with an adequate seal. The correct dimensioning of the sealing section must include a width of 7-12 mm and a depth equal to 1/4-1/5 of the thickness of the flooring. In order to prevent the sealant from slipping on the bottom of the joint and to correctly dimension its depth, the compressible non-stick expanded closed cell polyethylene cord must be installed in the joint seat FILTENE FONDOGIUNTO, having a diameter 20% higher than the width of the joint. To perfectly delimit the seal, the joints will be taped part and the other with adhesive paper tape.

The joints must be sealed, by extruding with a special gun, with mono-component sealant PROTECH FLEX, moisture-curing, fast-curing polyurethane sealant, with medium/high modulus of elasticity, after careful cleaning and dust removal from the joint seat with compressed air.

Flattening to remove excess parts and immediately seal the sealant in the joint will be done with a metal spatula slightly moistened with soap and water. Once the operations are complete, the adhesive paper tape will be removed.

The surface of the flooring, once dry and clean of any dust, will be treated by spraying with a manual or electric pump at low pressure, for a consumption of about 100-200 g/m² with the water-oil repellent PRERIT SURFACE according to the atmosphere of exposure and service, however characterized by a high protection and curing effect, freeze-thaw cycles and de-icing salts, where present, as well as the overall durability of the works, without determining any criticality in terms of possible delaminations.