Modulus of concrete surface: definition, examples of

What is this parameter - surface modulus? We have to get acquainted with a new concept for ourselves and explore ways of calculating its values ​​for real structures. In addition, we will touch on the fundamentals of winter concreting and the influence of the surface modulus on the methods of work used in this process.

What it is

Definition

The ideal time for concrete work outdoors is a warm season. Alas, it is not always possible to wait for the spring: in some cases, monolithic construction is carried out at negative temperatures.

In addition: in some regions of the country the warm season is simply too short. In Yakutsk, for example, the average monthly temperature above zero is only five months a year.

When concreting in frost, the main problem is to give concrete to gain strength before the crystallization of water in it begins. The main methods of its solution are reduced to the thermal insulation of the formwork or the heating of the laid mixture. In this case, the choice of a particular solution is determined primarily by how quickly the mold with concrete will cool.

The speed with which a particular structure will lose heat is determined by the ratio of the area of ​​its cooled surface to the volume.

Surface modulus бетонной конструкции - это, собственно, и есть отношение ее охлаждаемой площади к внутреннему объему. Формула модуля поверхности бетона предельно проста: Мп = S/V, где Мп - модуль поверхности; S - площадь поверхности конструкции, контактирующая с холодным воздухом, грунтом или охлажденными ниже нуля прочими элементами конструкции; V - полный объем монолита.

Since in the numerator of the formula the value is indicated in square meters (m2), and in the denominator in cubic (m3), the desired parameter will be measured in strange units, described as 1 / m, or m ^ -1.

An important point: since the process of gaining concrete strength practically stops when cooled to 0 degrees (water crystallization temperature), only those parts of the monolith surface that are in contact with colder air, the base or structural elements are considered to be cooled.

Calculation examples

Let's calculate the parameter we are interested in for a slab foundation with a size of 6x10 m and a thickness of 0.25 m, laid at negative ambient temperature on thawed soil.

1. It is obvious that all surfaces of the slab will cool except the bottom: because it is in contact with the ground, which has a temperature above zero. We fold their areas: (6 x 0.25) x 2 + (10 x 0.25) x 2 + 6 x 10 = 3 + 5 + 60 = 68 m2.
2. Calculate the volume of the plate. It is equal, as we remember from the school course of geometry, to the product of the sides of a rectangular parallelepiped: 10 x 6 x 0.25 = 15 m3.
3. Calculate the surface module: 68 m2 / 15 m3 = 4.5 (3) 1 / m.

In practice, calculations of beams, cylinders with transitions of diameters and other structures can be quite complicated and take considerable time. Like all people, builders tend to simplify their lives whenever possible; For this purpose, there are several simplified calculation formulas for the main structural elements.

What to do with it

So, we learned to calculate a certain parameter that affects the rate of cooling of the array in the cold. And how to apply it in real construction?

Heating and cooling rate

Since it is impossible to provide simultaneous heating or cooling of concrete throughout the entire volume of the array, any change in the conditions will, willy-nilly, lead to the appearance of a delta of temperatures between the core and the surface.

Warning: this delta will be the larger, the more massive the structure. That is, simply put, the smaller the ratio of its area to volume.

An increase in the temperature difference between the core and the surface will inevitably lead to an increase in internal stresses in the material; since we are talking about concrete that has not gained strength, cracks are not only possible - guaranteed.

Output? It boils down to as much as possible to slow the change in temperature of the surface of the array.

The temperature stability during cooling is provided, as a rule, by thermal insulation of the concrete monolith; when heated - adjustable power cable for concrete or heat gun.

Choosing a way to maintain temperature

This use of the obtained value of the modulus of the surface is directly related to the calculation of the rate of heating / cooling: on the basis of the performed calculation, the method of stabilizing the temperature to a set of concrete strength is chosen.

For a modulus of a surface not higher than 6, the so-called thermos method is sufficient. The form is simply qualitatively thermally insulated, which significantly reduces heat transfer.

In addition: in the process of hydration (chemical reactions of Portland cement with water) a rather significant amount of heat is released, which contributes to the self-heating of the mixture.

For MP in the range of 6 - 10 1 / m, several solutions are possible:

• The mixture is heated before laying in the form. In this case, with proper insulation, the period of its cooling to a critical temperature (0 degrees) increases; moreover, hot concrete seizes and gains strength much faster.

• Additives are added to the mixture to accelerate its hardening. As an option - quick-hardening Portland cements of high grades are used, which, besides accelerated curing, are useful because in the process of hydration they release more heat.
• An alternative approach is to reduce the crystallization temperature of water in a solidifying concrete mix. Thanks to the corresponding additives, the curing continues at low temperatures.

Useful: it is worth warning against the use of saline for this purpose. Their price is really lower than specialized synthetic additives; however, it is leveled by a high (from 5%) salt content in mixing water. At the same time, high salt content reduces the final strength of concrete and contributes to accelerated corrosion of reinforcement.

Finally, for a surface module over 10, the only sensible solution is to heat the concrete with a heating cable or heat guns up to a certain percentage of design strength. The value of the minimum strength before freezing depends on the class of concrete and the area of ​​operation of the monolith; full instructions on the selection of values ​​are contained in SNiP 3.03.01-87.

Scrapping

After a set of minimum required strength and stabilization of the temperature of the monolith, the formwork is removed and the insulation is removed. Since this occurs at negative temperatures, the delta between the surface of the concrete and the surrounding air is also important and is also tied to the surface module.

• When Mp lying in the range of 2-5, and the coefficient of reinforcement (the ratio of the total cross section of the reinforcement to the cross section of the monolith) to 1% of the maximum allowable temperature delta is 20 C.
• With a reinforcement ratio of 1 to 3 percent, the maximum temperature delta is 30 degrees.
• With a reinforcement ratio of over 3%, the air can be 40 degrees colder than concrete.
• With a surface module of more than 5 1 / m, the maximum allowable temperature drops for different reinforcement factors are 30, 40, and 50 degrees, respectively.

Processing of winter concrete

If, after a set of full strength, winter concrete and monoliths from unprepared concrete of normal humidity are processed quite conventionally, then the perforation and the device of the openings in the monolith have their own specifics before they gain strength.

Simply put, not gaining brand strength and frozen concrete should not be crushed with a jackhammer and a perforator. In this case, the appearance of cracks.

The best way to set up openings is to form the formwork for them even at the stage of pouring monolith. Among other things, in this case, it is possible to fully anchor the edges of the reinforcement at the edges of the opening. Where this is not possible and the opening will have to be cut in place, corrugated reinforcement is used: the grooved on its surface itself serves as an anchor for the rod.

It is useful: for arranging a hole (for example, blowing air or entering communications in a strip foundation), when pouring it with your own hands, it is enough to lay an asbestos cement or plastic pipe of the appropriate diameter in the formwork.

For the actual processing, where it can not be done without, diamond tools are preferable. Diamond drilling of holes in concrete does not require the use of a percussion mode; as a result, the probability of cracks and chips is less. Cutting reinforced concrete with diamond circles leaves the edges of the cut perfectly smooth and, which is very convenient, does not require changing the cutting wheel when cutting reinforcement.

Related concept

A simple associative chain will force us to touch on another concept related to concrete structures. This is the so-called Young's modulus for concrete (it is also the elastic modulus or deformation modulus).

The value of the module is determined experimentally, according to the results of testing the sample, measured in pascals (more often, taking into account high values, in megapascals) and denoted by the symbol E. To be honest, this parameter is of interest only to specialists and is not taken into account in low-rise construction.

Simply put, this parameter describes the ability of a material to deform briefly under significant loads without irreversible damage to the internal structure. Easier yet? Please: the higher the modulus of elasticity, the less likely it is that when you strike with a sledgehammer a piece of concrete will break away from the foundation.

After such a determination, it is logical to assume that the modulus of elasticity (or deformation) is associated with compressive strength and, accordingly, the brand (class) of the material.

Indeed, the dependency is almost linear.

• For heavy concrete of natural hardening class B10, the modulus of deformation is equal to 18 MPa.
• Class B15 corresponds to a value of 23 MPa.
• B20 - 27 MPa.
• The modulus of deformation of concrete B25 is 30 MPa.
• Class B40 - 36 MPa.

Conclusion

We hope that they did not tire the reader with an abundance of boring definitions and dry numbers. As usual, additional thematic information can be found in the attached video in this article. Successes!