- Concrete grades by strength
- Load prediction
- Fullness / density of common concrete
- Water absorption and frost resistance of concrete
- Mineral filler
- Mix moisture content
The complexity of the calculation and construction of foundations is aggravated by a wide variety of concrete grades used in their production. The suitable material is determined not only by the type and purpose of the structure, but also by the hydrogeological characteristics of the construction site.
Concrete grades by strength
For all types of building concrete, there is a general classification in which the compressive strength of the material is taken as a key criterion. The unit is kilogram-force per cm2 (kgf / cm2), the number in the brand designation is the limit of the destructive effect during bench tests of concrete cores that have gained the design strength after holding for 28 days. According to the marking system of concrete classes (B), the value in MPa is indicated as a numerical value, which is guaranteed not to exceed the ultimate breaking load. The actual strength of concrete becomes 1.5–2 times higher after a year’s exposure, in real-life conditions such an increase in strength is about 50–70%.
In civil engineering, concrete strength grades from M100 to M500 or the corresponding classes from B10 to B40 are used. It is worth remembering that the compressive strength of the concrete mass does not determine the final structural strength. However, this value is used in the calculation of concentrated and distributed loads of a concrete structure, which also takes into account the shape, size and reinforcement scheme. The main methodology is to calculate the deformation capacity of the reinforcement, the force of its adhesion to the mass and the compressive strength of this mass with the application of a force along the vectors of tension, compression and twisting.
The area of application of grades M150 and below is concrete preparation: screeds, lower foundation layers that do not contain reinforcement, concreting of pillars, etc. Grades M200 – M300 are used for strip foundations of simple sections with upper and lower frame reinforcement. Grades M350 – M500 are intended for MZLF of complex sections, pile and pile-grillage structures, slab and normally buried foundations of buildings with no more than two above-ground floors. Grades higher than M500 have an even narrower area of use in private construction – multi-storey basement foundations, foundations for buildings with 3 storeys and higher, heavily loaded concrete frames. In most IZhS facilities, such strong concrete is practically not used..
Table of the ratio of grades and concrete strength
Brand Average strength, kgf / cm2 Class M100 98 B7.5 M150 131 AT 10 O’CLOCK M150 164 B12.5 M200 196 B15 M250 262 IN 20 M300 302 B22.5 M350 327 B25 M400 393 B30 M450 458 B35 M500 524 B40
The required strength of a reinforced concrete foundation is determined by the load applied to it. The final impact on the base of the building includes three components: the mass of all building structures, the operational load at the rate of 100-150 kg / m2, as well as snow load according to the climatic region.
A single structure requires strength equal to the maximum possible concentrated load. Basically, the concentration occurs due to the uneven softening of the soil from soaking, as well as the natural heterogeneous density of the soil. For foundations of complex sections, indirect loads are separately calculated: lateral soil pressure, the effect of displacement of the upper layer on slopes, frost heaving forces.
Self-construction of a physical foundation model is not required today, you can use online calculators. Most of them work according to the conformity determination scheme, the result of the calculations is the safety factor, which can be negative if the selected structure or concrete grade does not correspond to the applied loads. As the initial data, the calculator takes the basic dimensions of the foundation, the estimated concrete grade in terms of strength, the number and placement of reinforcing elements, the total distributed load (that is, the curb weight of the building), the layout and location of the supports, as well as several coefficients that give the necessary corrections for the operating conditions.
Fullness / density of common concrete
In addition to the bearing function, the concrete foundation can also perform localizing, preventing the penetration of groundwater into basements. In such cases, in addition to the structural strength, the filtration capacity of the concrete is determined. It depends on the content of pores and microcracks that can pass groundwater in the presence of a pressure difference.
The void content of concrete directly affects its density. It, in turn, increases as the strength of the concrete increases. But there are other factors that affect the filling, mainly the ratio of water and cement, the conditions of hydration, the type of compaction and, in general, the technology of pouring and compaction of the concrete mixture..
In the construction of MZLF, mainly lightweight ones are used (up to 1.8 t / m3) concrete – initially there are no requirements for localizing ability to the structure. Normally buried foundations and walls of basement floors with high requirements for waterproofing are made of heavy (2-2.5 t / m3) concrete, which fully correlates with the strength grades used from M350 to M500. Stronger grades have an even higher density, such concrete is considered especially heavy and is extremely rarely used in civil engineering..
Water absorption and frost resistance of concrete
The foundation requires protection from the influence of the conditions of the environment in which it is located. The main risk is the leakage of moisture containing dissolved oxygen to the reinforcement elements. An increase in the protective layer of concrete in this case does not add special strength to the structure, because the reinforcement is not distributed further from the center, but at the same time the own weight of the foundation increases.
By choosing the correct grade of concrete for water absorption and frost resistance, you can reduce the thickness of the protective layers and thereby reduce material consumption. To begin with, with an increase in the density of concrete, its water absorption, together with the content of pores, becomes lower and for each grade in terms of strength has maximum permissible values, which are described by monograms from GOST 12730.4–78.
The water absorption of concrete is marked with the letter W and quantitatively (W4 – W20) indicates the water pressure at which a sample of standard thickness is guaranteed to show no filtration effect. The water absorption class defines frost resistance, denoted as F with values from 75 to 500 – the number of freezing cycles in which there is no significant (more than 5%) loss of strength.
Note that the required frost resistance and water absorption class is determined by climatic conditions and hydrogeological conditions at the construction site. If there is insulation or waterproofing for the foundation, it is allowed to use concretes of lower grades. The above are the ranges of brands and classes used in individual construction, the general classification includes more varieties.
The final parameters of concrete products are determined not only by the characteristics of the binder used. The qualities of the mineral filler are sometimes of no less importance. First of all, the use of different rocks and filler fractions can lead to an increase / decrease in density, and hence, in the main performance indicators..
Also, the concrete fraction is selected depending on the distance between the reinforcement elements, which should be 2–2.5 times higher than the largest size of crushed stone in order to completely eliminate wedging and blockage with the formation of unfilled areas. The fraction should also be selected commensurate with the dimensions of the concrete product, usually it is no more than 1 / 10–1 / 15 of the smallest linear size.
Usually, for small-sized reinforced concrete structures such as strip and pile-grillage foundations, a filler made of washed rock gravel or light volcanic rocks with a fraction of up to 50 mm is used. For heavy concretes used in the construction of slab and buried foundations, preference should be given to granite and basalt fillers up to 70–80 mm in size. For lightweight foundations or insulation layers of concrete, expanded clay filler can be used.
Mix moisture content
For the construction of the foundation, it is recommended to use ready-mixed concrete, or prepared on site under the supervision of a technologist. Building Foundation – Responsible construction and accurate ingredient content is of utmost importance to meet design specifications. If the cement / water ratio is disturbed, the mixture may stratify during pouring or vice versa – undergo increased cracking during cement hydration.
It is for this reason that it is not allowed to add water to the imported concrete, as well as the loss of cement laitance during long open storage is not allowed. Depending on the type and grade of cement used, not only the initial moisture content is regulated for concrete, but also the dynamics of its decrease in the first 28 days. Maintaining the appropriate conditions during the period of gaining strength with a reinforced concrete foundation is sometimes even more important than the correct determination of the grade of concrete, because even the highest quality material can be spoiled by violations of the technology of concrete work.