Concrete is a mixture of cement and D’aggregateskneaded witheau. Depending on the characteristics sought, the nature and dosage of the components differ in variable proportions, which must be known and respected. This is an essential concept for obtaining concrete suitable for the intended use. There is no need, however, to carry out clever calculations to obtain a high-performance product perfectly suited to everyday work, which any handyman is faced with. The few simple recipes developed in this article make it possible to carry out, using common and safe equipment, all the concrete works of the house.
Choose concrete ingredients
1 – cement
It is the binder that ensures the cohesion and solidity of the concrete. Cements are classified according to their resistance measured after 28 days of hardening. For our current work in reinforced or unreinforced concrete, cements Portland CEM II/A or Bresistance class 32,5 fit perfectly. It is important to know that the hardening curve of concretes is exponential the first days, then gradually flattens. Thus, under normal manufacturing and drying conditions, the concrete reaches 35 % of its hardening from the second day, 70 % at 7 days and near 85% at 14 days. The concretes of type R (e.g. CEM II/BR), harden faster. This concept determines, in particular, the time taken to strip the formwork of the structures.
2 – The aggregates consist of:
- sand quarry, gravel or river 0/4 or 0/6. This classification indicates the minimum and maximum diameters of the grains that compose it (for example, between zero and four millimeters, for the 0/4). Concrete sand must be clean, non-porous, inert and non-freezing. 0/2 sand is used for thin structures. The role of sand is to fill the spaces between the grains of gravel.
Attention : never use sea sand or sand containing salt (formation of saltpetre).
- gravel and rolled gravel 4/20, Routine. For finer works, reduce the size of the gravel (from 2/8 to 4/10). The gravels give volume and contribute to strengthen resistance concretes.
Depending on the characteristics sought (thermal insulation, weight reduction, etc.), countless mineral or organic materials can enter the concrete aggregates (pozzolan, expanded clay, polystyrene beads, vegetable fibers, synthetic fibers, animal fibers, etc.) . These are “special concretes”, the resistance of which is affected by these contributions.
Use : gravel pits also produce mixed aggregates of different grain sizes. Composed of pre-measured sand and gravel, these “concrete mixes” simplify the manufacture of concrete, to save precious time on the site. For low-volume work, finally, the “ready-to-wet” industrial mortar is a safe, fast, practical and simple solution to implement. These products are available from materials dealers.
3 – Water
She must be soft and clean. It is used to transform cement powder into a malleable and fluid material. It is also water that activates the chemical reaction responsible for hardening. Water dosage is essential. The natural tendency to overdose water to increase the fluidity of fresh concrete must be strictly prohibited, given the disastrous effects on the quality of the concrete: cracking, cracks, lower resistance, porosity, freezing, corrosion of reinforcement, accumulation of gravel in formwork bottom (segregation), etc. The practice of adding water to the concrete delivered ready-to-use (BPE), to allow the deliverer to save a little time when emptying the rotors, is strictly prohibited by the DTU. Refuse it, categoricallyeven if it means asking the supplier to include an adjuvant in their formulation, to make the concrete more fluid.
The dosage of ingredients is essential and must be respected, to ensure the safety and durability of the works. There are 4 main categories of concrete, characterized by the binder load, per cubic meter of ready-mixed fresh concretethat is :
- 150 kg of cement/m3for concretes of cleanliness (leveling the bottoms of excavations, for example);
- 250 kg/m3for concrete classics sans armature metallic ;
- 300 kg/m3for concrete unwired loaded (foundations, pedestrian slabs, garden paths, etc.);
- 350 kg/m3for the reinforced concrete (running footings of foundations, stringers, lintels, posts, beams, slabs, thresholds of garage doors or gates, etc.);
- 400 kg/m3for concrete heavily armed (smoothed concrete slabs, interior slabs, drive-over exterior slabs, etc.).
Concrete dosed at 350 kg/m3 is the most used.
The multiplication of units of measurement, necessary for the dosage of ingredients (weight for cement and volume for water), presents a difficulty that is difficult to implement on a construction site. To remedy this, usage defines equivalences between aggregates, measurable litres. The standard measures become, the mason’s wheelbarrow (90 liters), the mason’s bucket (10 liters) and the shovelful (5 liters). 1 kg of cement is more or less equivalent to 1 liter of product, but it is more practical to resonate in bags or ½ bags, for large volumes.
Concretely, to obtain 1 m3 of concrete dosed at 300 kg, you must mix:
- 300 kg of cement (12 bags of 25 kg or 8.6 bags of 35 kg);
- 880kg batten = 550 l (6 wheelbarrows, or 55 buckets);
- 1,100 kg of gravel = 730 l (8 wheelbarrows, or 73 buckets);
- In between 140 and 160 liters of water (14 to 16 buckets), depending on the degree of humidity of the aggregates.
To get 1 m3 of concrete to 350 kg, it is necessary:
- 350 kg of cement (14 bags of 25 kg or 10 bags of 35 kg);
- 800kg saber = 500 l (5.5 wheelbarrows or 50 buckets);
- 1,200 kg of gravel = 800 l (9 wheelbarrows or 80 buckets);
- 170 to 180 liters of water (17 to 18 buckets).
Another method, more empirical and simpler, but sufficient for current work, consists in mixing:
- Concrete dosed at ≃300 kg/m3 : 1 bucket of cement + 2,5 buckets of sand + 3,5 buckets of gravel (or 6 buckets of concrete mix), with ½ bucket of water.
- Concrete dosed at ≃350 kg/m3 : 1 cement bucket, 2 buckets of sand and 3 buckets of gravel (or 6 buckets of concrete mix), with ½ bucket of water (method 1, 2, 3).
For different dosages, convert the data proportionally (rule of three).
How to mix concrete by hand?
Making concrete manually is a work physically testingreserved for the works of low volume. The concretes thus obtained are rarely mixed homogeneously, so unsuitable to works subject to heavy constraints. For this job, you will need a shovel, trowel, bucket, and a clean, hard, smooth surface (tub, trough, wheelbarrow, or concrete surface). The procedure is simple:
- deposit the ingredients in a heap, in order next : gravel, sand (or concrete mix), then cement ;
- mix all, dried up, shoveling the base of the heap to form a new one next to it. Each shovelful is deposited on the top of the new cone, so that the materials roll down;
- when all the materials have been transferred, repeat the operation a second, then a third time, or more, until a uniform color is obtained;
- Dig a crater of sufficient volume to receive the volume of water prepared;
- Pour them 2/3 the volume of water in this hole;
- Crumble the shores of the crater, for humidifier gradually mixing;
- Brew the mixture, adding water until a soft dough is obtained, but not liquid.
The right consistency is achieved when a trowel’s worth of fresh concrete remains in a heap, without spreading.
How to mix concrete with a concrete mixer?
The concrete mixers have a capacity of between 60 and 450 litres. These devices ensure good homogeneity of the concrete provided the tank is only loaded at 70 to 80% of its volume nominal. Thus, a 130 liter concrete mixer can mix a maximum of 100 liters, corresponding to a concrete at 350 or 400 kg/m3, to 35 kg of cement (1 bag). Always start your work, perfectly clean tank, concrete mixer wedged horizontally, ingredients and water hose at hand.
- Tilt the tank (rotating) at 45°;
- Charge the basin of a bucket of water, then, in order, gravel, sand and finally cement;
- Leave to mix for 5 to 7 minutes, tapping the outer bottom of the tank with a rubber mallet to loosen the adhesions of the mixing fins. Gradually add water until you get the desired consistency;
- The concrete is ready ! The emptying is carried out by gravity, by tilting the tank towards the transport wheelbarrow, positioned below.
After the last batch (or before residual concrete dries), load the tank with 1 bucket of water et ½ bucket of gravel. Let it run like this for about ten minutesthen empty and rinse. This cleaning avoids removing, later, the hardened residues with a chisel and a hammer, to make the concrete mixer operational again.