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During construction, repair, reconstruction, you often have to save money, since the budget is not rubber. Many processes and solutions can be done independently, and without compromising quality. But this is not always possible. Most often they try to save on the preparation of solutions. The price for concrete from the factory seems too high. If you calculate how much the “correct” components cost, and not any (cheapest), then the savings, if any, are very small. In general, if it is worth mixing concrete or mortar with your own hands, then the grades are no higher than M250. An ordinary concrete mixer will cope with this task. Heavier ones – M400 concrete, for example – are already difficult to do normally. Achieving design strength will not be easy.
Characteristics of concrete M400
According to the new classification, the concrete mortar of the M400 brand is designated as B30, but we use the old name M400 more. It is described in the standard as heavy concrete. Among the people, it is referred to as the “middle” class, although this is not in the classification. It’s definitely not “light”, but it’s also not heavy enough. This is probably why the weight of a cube of M400 concrete depends on the exact characteristics and averages 2310-2350 kg / m³. This brand has high strength, frost resistance, low surface wear. Here are the characteristics of concrete grade M400 according to GOST 7473-94:
- mobility – from P 1 to P 4;
- water resistance – W6-W12;
- frost resistance – from F150 to F300;
- strength B30.
Most of all, any concrete stone characterizes strength. This type can withstand a load of 400 kgf / cm² (according to the standard 293 kgf / cm²). All other parameters may vary depending on the requirements for the implementation of a particular project. The mobility of concrete affects the ease of its installation. Somewhere a fluid mixture is required (so that it can get under the reinforcement), somewhere a rigid one is needed. Fluid formulations (containing more water) have more cement added to maintain design strength. To increase frost resistance and water resistance, special additives are used, which are strictly standardized.
Application area
M400 brand concrete is used where high strength and frost resistance are extremely important:
- In the manufacture of beams, piles, lintels and other elements of the supporting structures of buildings.
- For filling sites for heavy equipment.
- During the construction of basins, dams, collectors, etc.
- In the construction of bank vaults, tunnels and other facilities where high strength and water resistance are needed.
- Construction of monolithic walls for high-rise buildings.
- During the construction of bridges, viaducts, interchanges.
That is, M400 concrete has mainly industrial applications. In private construction, even when building pools or foundations for cottages, it is excessive in strength and therefore is used very rarely by private traders.
In addition to too high strength, a high hardening rate also interferes. On the one hand, this is a plus: concrete quickly gains strength. So fast that you can walk in 8-10 hours after pouring. But it is possible to lay it without cold seams only with continuous supply and the presence of a qualified team. In all other cases, there is a high probability of getting delamination and cracks. The introduction of additives to increase the time until the onset of setting increases the already considerable price. And the price is high, because to ensure the required strength, high-quality cement is needed, and even in large quantities. So private traders use concrete M400 extremely rarely.
Composition and ratio of components
It is possible to speak accurately about the composition of M400 concrete only in relation to specific materials: cement, sand and gravel. Replacing one of the components leads to a change in the content of the other two. The amount of water and plasticizing additives depends on how fluid or rigid the solution is required. And the introduction of “softening” additives allows you to reduce the amount of water and cement. So the recipe (any) is given only approximately. The exact composition is determined in the laboratory, since M400 concrete is used at serious facilities, both the composition and the mixing process must be strictly controlled.
As for the choice of cement, we can say that only PC 400 and higher grades can be used. Optimal – PC 500. Lower ones are needed in very large quantities to obtain the required strength, which is economically unprofitable. Cement must be “fresh” – no older than a week from the date of production. This is critical. And the packaging should be factory, not warehouse.
Requirements for aggregates for self-production
In the manufacture of high-quality concrete, the choice of aggregates is very important. Crushed stone must be of all fractions. It cannot be crushed gravel, much less crushed cobblestones. The gravel must be clean, free of dust, dirt and other impurities. If it is dirty, it should be washed and dried.
For high-quality concrete, which includes M400 concrete, the composition and quality of sand is important. Sand should be river, washed, without clay, dust and other foreign inclusions. In order to be able to obtain the required strength, the sand must contain fine, medium and coarse fractions. You can check the purity of the sand if you take it in your hand and rub it, as if you are salting a dish. If dirt remains on the fingers, it will be necessary to either rinse and dry the sand, or slightly increase the amount of cement. The second option is risky – you can get concrete of a lower density.
How to cook yourself
If you want to get M400 concrete at home, you will need an auger or rotary concrete mixer. Ordinary (such as a pear) will not cope with the task. After laying the mortar, additional compaction will also be required. To do this, you need a submersible or surface vibrator – it depends on how large the layer thickness will be. Post-lay compaction is a must if you want close to design strength.
And the kneading process is no different from the standard. First, crushed stone is thrown into the concrete mixer, sand and cement are added in parts, achieving their uniform distribution. If dry additives are used, they are introduced with sand. If liquid – together with water.
Water is added to the concrete solution in parts. It is measured based on the normalized water-cement ratio. For M400 concrete, this is 0,47-0,54 parts of the amount of cement. First, about half of the liquid is poured into the mixer. The mixture is stirred until even, then water is added in small portions. As soon as the fluidity of the composition satisfies the conditions, the water is stopped.
It only seems that excess water will not interfere with concrete. An extra bucket for one batch can reduce the strength of concrete by 10-40%. So be very careful with water. Don’t be afraid to underfill, be afraid to overfill. Even in semi-dry solutions, there is enough liquid to set the design hardness. But only under the condition of proper care – maintaining a sufficient level of moisture in the ripening concrete. To do this, the concrete is covered with a film and periodically moistened. This is really important for the process of crystallization to take place, and not for the evaporation of moisture.