2024 Author: Beatrice Philips | [email protected]. Last modified: 2023-12-16 05:33
Poured into the space bounded by the formwork and equipped with a steel frame of steel reinforcement, the concrete sets in the next few hours. Its complete drying and hardening occurs in a much longer time.
Influencing factors
Before starting construction, craftsmen pay attention to the reasons that directly or indirectly affect the hardening of concrete. We are talking about the speed, the duration of the complete hardening of the concrete composition, into which the supporting metal frame is immersed, preventing cracking and creeping in different directions of the parts of the poured structure.
First of all, the speed of hardening is influenced by the climate, the weather of the day of the laying and the days of the subsequent set with the building material filled with the declared hardness and strength . In summer, in 40-degree heat, it will completely dry out in 2 days. But its strength will never reach the declared parameters. In the cold season, when the temperature is above zero (several degrees Celsius), due to a 10 or more times slowdown in the rate of moisture evaporation, the period of complete drying of concrete stretches for two weeks or more.
In the instructions for the preparation of a concrete composition of any brand, it is said that in only a month it is gaining its real strength. Hardening at a relatively normal air temperature can and should occur in a month.
If it's hot outside and the water evaporates quickly, then the concrete base, poured 6 hours ago, is watered abundantly every hour.
The density of the concrete foundation directly affects the final strength of the structure poured and soon hardened . The greater the density of the concrete material, the slower it will release moisture and the better it will set. Industrial casting of reinforced concrete is not complete without vibrocompression. At home, concrete can be compacted using the same shovel with which it was poured.
If a concrete mixer has gone into business, bayonetting (shaking with a bayonet shovel) is also necessary - the concrete mixer only increases the pouring speed, but does not eliminate the compaction of the concrete mixture. If the concrete or concrete screed is thoroughly compacted, then such material will be more difficult to drill, for example, to install beams under a wooden flooring.
The composition of the concrete also plays an important role in the speed of hardening of the concrete mixture . For example, expanded clay (expanded clay concrete) or slag (slag concrete) takes up some of the moisture and not quite willingly and quickly returns it back when the concrete sets.
If gravel is used, then the water will leave the hardening concrete composition much faster.
To slow down the loss of water, the newly poured structure is covered with a thin layer of waterproofing - in this case, it may be polyethylene from the foam blocks, with which they were closed during transportation. To reduce the rate of evaporation of water, a weak soap solution can be mixed into concrete, however, the soap stretches the setting process of concrete by 1.5-2 times, which will noticeably affect the strength of the entire structure.
Curing time
A freshly prepared concrete solution is a semi-liquid or liquid mixture, except for the presence of gravel in it, which is a solid material. Concrete consists of crushed stone, cement, sand (seeded quarry) and water. Cement is a mineral that includes a hardening reagent - calcium silicate. Cement is known to react with water to form a rocky mass. In fact, cement sand and concrete are artificial stone.
Concrete hardening in two stages. In the first couple of hours, the concrete dries and partially sets, which gives an incentive, after preparing the concrete, to pour it into the prepared formwork compartment as soon as possible. Reacting with water, cement turns into calcium hydroxide. The final hardness of the concrete composition depends on its amount. The formation of calcium-containing crystals leads to an increase in the temperature of the hardening concrete.
The setting time also differs for different grades of concrete. So, concrete of the M200 brand has a setting time equal to 3.5 hours from the moment the main ingredients are mixed. After initial hardening, it dries up within a week. The final hardening ends only on the 29th day. The solution will turn into a final monolith at a temperature of + 15 … 20 degrees Celsius . For the south of Russia, this is the off-season temperature - the best conditions for the construction of concrete structures. Humidity (relative) should not exceed 75%. The best months to lay concrete are May and September.
Pouring the foundation in the summer, the master has a high risk of running into premature drying of concrete and it must be irrigated regularly - at least once an hour . Seizing in an hour is unacceptable - the structure with a high degree of probability may not gain the declared strength. The foundation becomes extremely fragile, cracks, significant pieces of it can fall off.
If there is not enough water for timely and repeated moistening of concrete, then the composition, half or completely set, without waiting for all the water to evaporate, is tightly covered with a film.
However, the more cement there is in the concrete, the sooner it will set. So, composition M300 can grab in 2.5-3 hours, M400 - in 2-2.5 hours, M500 - in 1.5-2 hours . Sawdust concrete sets in about the same time as any concrete similar to it, in which the ratio of sand to cement is similar to any of the above grades. It should be remembered that sawdust has a negative effect on the parameters of strength and reliability and increase the setting time up to 4 hours or more. Composition М200 will fully gain strength in two weeks, М400 - in one.
The setting speed depends not only on the grade of concrete, but also on the structure and the depth of the bottom edge of the foundation . The wider the strip foundation and the further it is buried, the longer it dries. This is unacceptable in conditions where land plots are often flooded in bad weather, as they are located in a lowland.
How to accelerate hardening?
The fastest way to make concrete dry as soon as possible is to call a driver on a concrete mixer, in the concrete of which special ingredients are mixed. The supplying companies in their own testing bureaus mix ready-mixed concrete samples with different performance values in different batches. The concrete mixer will deliver the required amount of concrete to the address indicated by the client - while the concrete will not have time to harden. Pouring work is carried out over the next hour - to speed things up, a concrete pump is used that is suitable for the foundation.
To speed up the hardening of concrete in cold weather, so-called thermomats are attached to the walls of the formwork . They generate heat, the concrete heats up to room temperature and hardens faster. This requires an electrical connection. The method is irreplaceable in the Far North, where there is no warm summer, but it is necessary to build.
When the concrete composition hardens, industrial additives and additives in the form of powders are used . They are added strictly at the stage of mixing the dry composition with water, during the filling of gravel. This acceleration helps to save on cement costs. Accelerated hardening is obtained using superplasticizers. Plasticizing additives increase the elasticity and fluidity of the mortar, the uniformity of the pouring (without settling the cement slurry on the bottom).
When choosing an accelerator, pay attention to the activity of the substance . It should increase the water resistance of concrete and frost resistance. Incorrectly selected improvers (setting accelerators) lead to the fact that the reinforcement can significantly rust - right in the concrete. To prevent this from happening and the structure not to fall on you and your guests, use only branded, highly effective additives and additives that do not violate either the composition or the technology of filling and hardening of the composition.
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