- Concrete classification
- The proportions of the most common brands
- How does the quality of ingredients affect the strength of concrete?
- Cooking procedure
- Formwork, reinforcement, pouring and sealing
- Conditions for normal cement hydration to achieve the grade strength of concrete
A strip or slab foundation, floors on the ground, paths – this is an incomplete list of the use of concrete in private construction. Often, due to the remoteness of the facility from the plant or the lack of access roads, it is not possible to deliver the finished solution. The only way out is to make good concrete with your own hands..
First, you need to know exactly how strong the concrete should have. The choice of not only the brand of cement depends on this parameter, but also its dosage..
“Parallel” there are two classifications of concrete – by brand and class. They practically do not differ, but they have different letter and number designations. Roughly speaking, the only difference is that the concrete grade means some “average” value of strength (pressure is measured in kilogram-force) and is denoted by the letter M, and the class is the guaranteed strength value (pressure is measured in MPa, SI system) and is denoted letter B.
The most common characteristic at the household level is the familiar and understandable compressive strength, measured in kilogram-force per square meter. cm, where the numbers indicate the pressure.
For private construction, concrete grades from M200 to M350 are usually used. Such a difference cannot but affect the cost of a cubic meter. The spread of the selling price of factories reaches 20%. And it is preserved during the manufacture of concrete on its own, therefore, the “overkill” of strength significantly affects its cost.
Even for private housing construction, it is necessary to develop documentation based on preliminary engineering surveys. At this stage, the strength characteristics of concrete are also determined. But this is for residential buildings, and outbuildings (garages, summer kitchens, baths, garden houses) are often built “by eye” and in accordance with the construction experience in this area.
For example, recommendations for choosing concrete for a strip foundation depend on the materials of the structure and usually look like this:
- wood or frame-panel technology – M200 – M250;
- blocks of aerated concrete and warm ceramics – M300;
- brick – M350.
It is customary to indicate the class of concrete in the design documentation. It is easy to convert it to a brand, there are correspondence tables where the closest concrete grade is indicated for each class: B15 – M200, B20 – M250, B22.5 – M300.
The proportions of the most common brands
When making concrete with their own hands, they use a recipe for “pure” concrete of four components: cement, sand, crushed stone (gravel) and water. The plant also mixes additives that improve the technological and operational properties of the mixture, but in private construction they are ignored.
What is better to use for dosing the mixture: a measure of volume or weight? At the household level, very often the proportions for a particular brand of concrete are indicated in terms of volume. Of course, at the construction site it is more convenient to measure the ingredients in buckets rather than weigh them, but this dosage method is inaccurate..
For example, the proportions of sand / crushed stone in relation to a 10 liter bucket of M400 cement are as follows:
- 3.2 / 4.9 for concrete M200;
- 2.4 / 3.9 – M250;
- 2.2 / 3.7 – M300.
But in this case, the size of the crushed stone fraction is not taken into account, and it affects the consumption of the remaining components. Obviously, the larger the size of the volumetric “monolithic” fraction, the lower the consumption of cement, sand and water.
In addition, when pouring bulk materials (especially cement), their bulk density decreases. Therefore, the standards operate only with weight.
There are special calculation methods that take into account many factors that affect the strength of concrete. But they are guided by laboratories and quality control services of factories. In practice, use the averaged values summarized in tables. The only thing is that they are indicated for dry sand and gravel.
Below are these values in kilograms for the most common brands of cement and concrete per cubic meter..
Concrete grade Cement / sand Cement M300, weight of crushed stone with a fraction of 20 mm – 1115 kg, water consumption – 205 liters M200 310/740 M250 350/700 M300 390/660 Cement M300, weight of 40 mm crushed stone – 1200 kg, water consumption – 190 liters M200 290/700 M250 330/660 M300 360/630 Cement M400, weight of crushed stone with a fraction of 20 mm – 1100 kg, water consumption – 205 liters M200 275/810 M250 315/750 M300 355/710 Cement M400, weight of 40 mm crushed stone – 1200 kg, water consumption – 190 liters M200 255/755 M250 290/710 M300 330/680
There are such proportions for crushed stone with a fraction of 70 mm, but in monolithic structures with a reinforcing frame, this filler size is not used.
How does the quality of ingredients affect the strength of concrete?
Cement. Grade – the higher, the less cement consumption for concrete of the same grade. There are several types, but Portland cement is used in private construction. The quality of concrete is affected by the production time of cement – during storage, it loses its astringent properties. Even when stored in a dry room, in one month this is expressed in 10% of the original quality, in six months – a third.
Sand. It has no shelf life, but its “purity” means a lot. Sand mined in a quarry can contain a large amount of clay, which impairs the hydration of the cement and reduces the strength of the concrete. Use washed or river sand. Of course, the mineral composition also has an effect, but usually they are content with the raw materials that are in this region..
Crushed stone. As with sand, composition is very important. There is a whole list of regulatory restrictions on a large list of minerals in the composition of crushed stone for the manufacture of concrete, so you can request a quality certificate when purchasing. Flakiness is no less important – the percentage of fractions with a lamellar (oblong or “broken”) structure. This means the presence of voids in the raw material for the production of crushed stone, which affects its strength. The recommended value is less than 15%. It is better to use crushed granite, not recommended – lime and secondary.
Water. If the sand must be washed, then the water must be clean – at the drinking level, without dirt and biological impurities (from a water supply system, not a river or reservoir).
The mixing sequence can be different. But, given the limitation on the time of use, at first you can prepare a large volume of dry mix, and introduce water in parts (for each portion of concrete).
It is very difficult to manually mix the mixture – you need a concrete mixer.
For a large amount of work, it is necessary to think over the concrete supply system. This is especially important for pouring monolithic structures – foundations and walls.
Formwork, reinforcement, pouring and sealing
It is very important that water does not escape from the concrete during curing.
Therefore, when pouring concrete floors or sites on the ground, it is necessary to lay a geomembrane on a cushion of rubble and sand, and limit the side walls with formwork.
No less important is the quality of the formwork for vertical structures (buried foundation with backfilling of soil, basement, walls). There should be no gaps between the boards, so the use of an unedged board is not recommended – it often does not have the same width.
Concrete has high compressive strength but rather low resistance to fracture loads. Reinforcement is used to compensate for these forces..
Pouring of concrete should take place in one step. For example, each layer of the foundation is poured at one time, and the next layer is poured before the previous one sets. There is a “cold seam” casting technology, but it is much more complicated and requires a whole range of additional work.
Sealing is a mandatory procedure that allows you to eliminate bubbles and air pockets that arise during the preparation and pouring of the mixture. The easiest way is bayonetting, more modern is the use of a vibrating screed or a manual vibrator for concrete.
Conditions for normal cement hydration to achieve the grade strength of concrete
Concrete hardening will stop if water dries out prematurely.
The first week is especially critical, when the concrete stone gains 70% of its design strength. At this time, the concrete must be moistened from time to time, and covered with film or wet burlap in between.