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Pedrêz recipes: How to make cast hemp concrete in situ



In 2018, the Pedrêz workshop began, at Rua do Paraíso 331, in the historic center of Porto, the construction of the first building of 1200 m2 in hemp concrete with a “Portuguese character”, composed of national ingredients, going against the prevailing current , the importation of foreign formulations.

Pedrêz decided to adapt this technology to the context of Portugal, its climate, rules, regulations and resources. In formulating the recipes, national aerial lime, national natural hydraulic lime and national clay were used, with the exception of hemp shavings, which were imported, as the quantity and quality of shavings available for carrying out a construction of this kind do not yet exist. dimension.

A distinctive factor in Paraíso 331 is the experimental character of the entire work, where different construction systems were crossed – steel, concrete, wood, textile fabrics – and daring different applications of hemp. Several construction solutions were tested, from formwork hemp concrete , also to hemp concrete for filling slabs, roofing brick masonry, bricks for interior walls and facades with different formulations for each of the situations.

Exploitation of hemp shavings in construction resulted in a set of experiences with undeniable relevance at national and international level. The application of multi-layer bricks and concreting on textile screens are practices that, until now, were unexplored.

Formwork hemp concrete ,
This first article focuses on formwork hemp concrete on-site. Its mixture consists of 5 ingredients, of which three are essential and two are optional:

As key ingredients we have hemp shavings, aerial lime and water. Aerial lime makes the bond between wood and hemp fibers. It is important, however, to remember that the carbonation (hardening) of aerial lime only happens on the surface of the wall, as it lacks contact with air. The interior of the hemp wall, as it does not come into contact with air, requires a hydraulic binder. For this reason, we must add a pozzolana (ingredient that converts aerial lime into hydraulic lime, causing it to react with water) and in this way the wall gains strength, also inside. This pozzolana can be a metakaolin or simply go through the addition of a hydraulic ingredient, such as cement, to the mixture. It is also important to explain that the curing of the mortar can be prolonged, reaching 60 days in circumstances of poor ventilation and high humidity, so fungal attacks can occur particularly in spring and autumn, when the weather is humid, but the temperature is warm. . To prevent fungal attacks, antifungal ingredients such as Borax can be added.


Formwork hemp concrete , from the work Paraíso 331 was produced according to a recipe formulated with national aerial lime and clay. 

Here is the Pedrêz base recipe, for traditional Portuguese hemp in formwork, without hydraulic adjuvant.

– Hemp shavings (Association “la Chanvriére”, Kanabat product) – 20 kg;

– Air lime (“Lusical Companhia Lusitana de Cal”, product H100) – 30 kg;

– Clay (company “Mibal Minas de Barqueiros”, Metacaolino product) – 9 kg;

– Borax (without supplier definition) – 0,2 kg;

– Water (supply from the public network) – 45 to 50l;

The recipe can be supplemented with a hydraulic binder, such as white cement – ​​3kg

The equipment used for the work was just a vertical shaft mixer with a capacity of 350 liters, where water sprinklers and a timer were installed for better distribution and control of water in the mixture.

The facade walls used jute as formwork lost from the outside and OSB formwork removable from the inside. The OSB sides were removed immediately after concreting. The jute remained an integral part of the wall from the outside, protected from external agents by a ventilated façade made of galvanized sheet metal or simply plastered.

An untreated pine wood substructure was used to support the walls. Occasionally, wires were also stretched between the pieces of the wooden substructure to reinforce the connection of the hemp concrete to that substructure. The Pedrêz recipe for hemp concrete must always cover the substructure with a thickness of 5 cm or more, otherwise it will crack during drying.

The concreting of very high walls (6 m), due to the resulting natural compaction and the contraction of the material, ended up opening horizontal cracks, something common in formwork hemp concrete. ,🇧🇷 The holes were always made with great ease and closed with the same mortar mixture mentioned above. The thickness of the outer walls was 25cm and the inner walls 15cm.

For good thermal performance, hemp concrete should not be compacted inside the formwork, it should only be arranged and distributed by hand. The way of arranging and distributing the material inside the formwork varies depending on the builder, so it is advisable to have a stable team from the beginning to the end of each wall, so that the final result is homogeneous.

The plastering and finishing of the exterior walls, from the outside, was done with lime plaster from “Fassa Bartolo”, product S605. Seral.

When choosing the plaster and finish, the level of protection of the walls and their breathability were important factors, seeking to obtain greater resistance to water on the outside and greater permeability to vapour, while on the inside of the walls the opposite was sought: lower permeability to steam and less demand for water. Formwork hemp concrete , it was left visible from the inside in several situations and no surface stabilization of the material was necessary, it did not show any signs of shedding dust.

This article was originally published on issue #4 da of Cannadouro Magazine


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