How long does concrete dry in formwork, earth, screed.

How concrete dries properly determines its strength and durability. When organizing the construction process, SNiP standards are taken into account, which regulate the time allotted for hardening. On average, it takes 28 days for concrete to dry at 20℃. Several factors influence this. How much concrete to dry, and why it depends – in this article.

The hardening period is influenced by the brand of cement, proportions in the solution, humidity, temperature, mixing technology, pouring density and compliance with operating standards. More gravel – the mixture gets rid of excess water faster, more slag – less evaporation.

Concrete Strength Calculator

You can calculate the increase in strength of concrete grade up to B12 (M200) on Portland cement, at a temperature of 15 ℃ and in a humid environment, using a calculator.

Given the popularity of M200 concrete, the calculator will be of great help. The formula with empirical dependence is used for calculation: Rb(n)=R28* log⁡(n)/log⁡(28). You can read it below in the text. Note that the result is quite accurate starting from the 7th day.

•   4 days – 42%. After 3 days, you can walk on concrete.
•  Day 7 – 58%. The strength is sufficient to remove the formwork.
• 14 – 79%. In some cases, it is considered possible to start construction.

It’s important. The use of a calculator in calculations for high strength concrete will result in discrepancies.

The graph clearly shows the strength gain by day depending on temperature.

Drying stages

After the concrete has been placed, there are two stages:

1. setting (thickening);
2. hardening (hardening).

Setting time

The setting of concrete is a short-term process and lasts about one hour. The initial set of hardness occurs due to a chemical reaction – hydration.

Hydration of cement – This is a chemical reaction of cement with water, in which crystalline hydrates are formed. Concrete loses its plasticity and withstands minor loads. During the hydration process, the liquid or plastic cement adhesive turns into a cement stone.

Cement is the main ingredient in concrete. Its brand and the amount of water added to the mixture directly affect the cure rate and strength.

The composition of cement is complex, it is added:

• aluminate;
• calcium silicates;
• aluminoferrites.

When in contact with water, the ongoing chemical reactions proceed with the release of heat. During the setting period of the concrete, this increases the rate of hydration.

Important! The ratio of the amount of water to the amount of cement for the normal course of the process should be 0,3–0,5.

For grades M200 and M300, popular in construction, the setting time is 1–1,5 hours. With a decrease in temperature, the time increases to 5 hours, and if it is “zero” or “minus” outside, the concrete will harden after 10 hours. In hot weather, the coupling occurs in half an hour, and the mixture passes into the hardening stage.

Curing stage

The process lasts on average for concrete up to 28 days and after its completion a monolith with the specified characteristics is formed. Still soft concrete becomes motionless, undergoing crystallization, acquiring stiffness parameters corresponding to the brand. The strength of concrete also depends on the curing time. To calculate the dynamics of strength, the formula is used:

Rb(n)=R28(lg(n)/lg(28))

where Rb(n) is the strength of concrete for day n, n (at least three) is the number of days after filling, R28 – brand strength.

Strengthening of concrete (at a temperature of 20 °C):

• 3 days – about 40%
• 7–14 days — from 60-80%
• 28 days – 100%

There are normative-safe strength, it determines after how many days you can walk on concrete or remove formwork, and full strength, which comes to 28 days.

On a note. When modifiers are added to the composition of the mixture, the hardening time can be reduced and the concrete is able to gain grade strength within two weeks.

Dependence of drying on external factors

The key factors affecting the setting time of the mixture are climatic – temperature and humidity, but the brand of concrete must also be taken into account. These parameters affect how many days the concrete will gain strength sufficient to start further work.

Temperature

The optimum temperature for concrete drying is 20–22 °C. If the temperature is higher, the concrete sets and hardens faster. But a rapid loss of moisture leads to the formation of voids in the monolith and, as a result, a decrease in the quality of the product.

In cold weather, hydration decreases, and with sub-zero temperatures it almost completely stops. To maintain the process, artificial (chemical or mechanical) stimulation measures are used.

Chemical measures include adding additives to the solution. These are:

• potassium carbonate (potash);
• calcium chloride;
• sodium chloride and others.

Additives affect the solubility of cement, and crystallization is faster.

Mechanical methods to speed up drying include:

• the use of a heat gun;
• use of electricity;
• insulated formwork device;
• cover with polyethylene to retain heat.
  

The table shows the dependence of the curing of the concrete mixture M200-M300 on the external temperature.

Humidity

Humidity directly affects the hydration capacity. At low – freezing will slow down, high – will accelerate. Normal conditions for drying poured concrete are defined as 80–100%. The lack of moisture is compensated by watering or covering the entire surface with moistened sawdust or sand.

Concrete grade

The curing of concrete is affected by the proportions and composition of the mixture. The more porous materials in it (expanded clay, slag), the slower the evaporation and the longer the drying time.

The higher the grade, the faster the process of strengthening the concrete structure. But even in these cases, it is recommended to withstand at least 28 days before starting work.

Poured Concrete Care

The laid concrete will need proper care. After pouring, normal conditions should be created for the concrete to dry and harden, in particular, protection from rain, sun (ultraviolet).

Complete drying time

The time required for complete drying depends on a number of factors. An important role in this process is played by temperature, the mass of poured concrete and its location. The data in the table above shows that the higher the temperature, the faster the strength gains. At 30°C, the drying time is reduced to two weeks, but this accelerated drying will affect the quality.

Here it is necessary to make a clarification – it is desirable that the drying time of the concrete coincides with the time of curing, and this is 28 days. Subject to the rules for the care of the laid concrete, the structure will be durable and without defects.

In hot weather, in order to slow down drying, the concrete is shed with water, and in winter, it speeds up drying, insulates and warms. That is, the time for the complete transformation of the cement slurry into calcium hydrosilicate is artificially regulated.

With proper care, the M150 sand concrete indoors dries for the same 28 days as the concrete screed outdoors. The same period is released for the strip foundation in the formwork. However, everything must be approached from a rational point of view. For example, the screed can continue to gain strength, and finishing work can begin from 4-7 days.

In industrial construction, massive concrete structures dry for 90 days or longer.

There are no such restrictions in private construction. If the concrete hardens in the ground and cool weather, then the drying time will increase, but you do not need to wait longer than the set time.

What will disrupt technology lead to?

If you do not fully withstand the time and prematurely load the concrete structure, then already at the very beginning of operation, problems are possible:

• Cracking. The biggest problem is with the foundation. It leads to the appearance of cracks in floors and walls – up to through. Even if M400 concrete has a shorter hardening time, you should not rush to remove the formwork and start construction work.
• Deformation. Minor damage. It is connected with insufficient drying time of concrete. It can affect the finishing work: plaster falls off, tiles come off. Such errors are usually associated with foundations that are in the formwork or the ground. It takes a little more time to dry these structures.

You should be guided by the requirements of SNiP when pouring and holding the concrete mixture. The current set of standards SP 70.13330.2012, which replaced SNiP 3.03.01-87.

How to speed up drying

There are several ways to speed up the drying of concrete. They are used if they do not violate the construction technology. They have already been mentioned, there are two of them:

1. Heat by 10 °C accelerates the reaction by 2-4 times, for example, curing at 60 °C and the curing rate will increase by 16 or more times. For 12 hours of smooth heating, you can gain 90-100% of the “brand” strength.
2. Adding additives to the mixture: potassium carbonate, calcium chloride and sodium.

It is safe to concrete even in frosty winter, but after pouring it is necessary to maintain the temperature of the mixture from +10 ° C for three days. This can be achieved by covering the structure and installing an air gun or, before pouring the solution, lay an electric heating element throughout the volume.

How to test the strength of concrete yourself

Accurate parameters on the strength of concrete can be obtained in the laboratory. The samples taken are subjected to tests using measuring instruments. There are portable devices that measure the strength of concrete using ultrasound, but they are very expensive and it is not advisable to purchase them for home use.

But you can evaluate how durable the poured concrete turned out to be on your own.

For a more reliable result, the measurement must be carried out in several places. To do this, take a hammer weighing 400-600 grams and hit with a blunt part and medium strength in the selected place.

The depth of the recess can be judged on the strength:

• dent 2 mm and deeper – concrete M50;
• 1 mm – concrete M75;
• small dent – concrete M150;
• no visible damage – concrete M200 or higher.

In the second part of the test, you will need a sharp object such as a chisel. The experiment is repeated with a blow to the chisel and the conclusion is drawn from the residual traces:

• the chisel is stuck in concrete – the grade is lower than M50;
• 10 mm chip – M75 concrete;
• 5 mm – brand M100;
• small fragments – brand M200;
• no damage – concrete M300 or higher.

You can judge the strength of a concrete structure by the sound it makes when it hits. A dull sound indicates a low strength of concrete, a sonorous sound indicates a high one.

Does concrete need to cure?

A common mistake of some builders is the opinion that a concrete structure needs to stand before construction begins. Following this statement, the foundations need to be poured in the fall to allow it to overwinter. It is believed that during this time the concrete structure additionally gains strength and “shrinks”.

However, this is not confirmed by regulatory documents and there is no need to wait more than 28 days. In addition, leaving an unloaded foundation in the winter is harmful and even dangerous. On some types of soil, sandy loam or loam, a light foundation will easily be pushed out by frozen soil. This leads to its deformation and the appearance of cracks.

Conclusion

Each case of pouring a concrete structure is individual. When building or repairing, it is important to plan all the stages correctly. This will minimize time losses. When pouring a foundation or screed, correctly estimate the right time for concrete to harden. In the future, this will help to avoid trouble with him. And at the end of the topic, watch the video: “How much concrete hardens.”