- November 29, 2020
The basis of any building is the foundation; the reliability and durability of the entire structure depends on it.
When laying it, special knowledge and compliance with all building codes and requirements are required, and it is imperative to take into account the climatic conditions in a particular location. Not so long ago, foundation pouring was carried out only in the warm season, at subzero temperatures; the concrete froze before hardening. Subsequently, the entire mass was deformed, cracks and failures appeared. If the process took place in cold weather, the base was covered with reed mats, mineral wool, and various methods were used to prevent premature freezing of the mass.
The scientific world has also tried to find a way in which work with concrete mixture can be carried out at subzero temperatures. In the course of scientific research, the optimal temperature was established - from +5 to +15C. It is these conditions that contribute to obtaining a solid foundation that will last for many years without damage.
How concrete gains strength
Concrete is an artificial stone. It consists of crushed stone, sand, cement, water and special additives. Crushed stone and sand provide strength, while cement and water form the “glue” that forms the monolith. Special additives make it possible to give concrete the necessary characteristics: rapid strength gain, the ability to lay in winter, frost resistance, and so on.
Concrete gains strength gradually; this process consists of two stages.
Grasping
– this is a loss of mobility of the solution. Its speed depends on the air temperature. For example, if it is +5 °C outside, the concrete will set in 15–20 hours, but the reference, or design, temperature is considered to be +20 °C. With it, setting occurs 1–3 hours after mixing the concrete mixture, that is, mixing all components with water. Before setting begins, you need to have time to place the mixture in the formwork, compact and level it.
Strengthening, or hardening
is the process of achieving a given strength by a concrete mixture. The optimal conditions for its collection are a temperature of +20 °C and a humidity level of 80%. The estimated period of “hardening” to brand strength at normal temperature and without additional measures is 28 days. Then the strength gain of concrete slows down, but does not stop growing throughout its entire service life.
Brand strength is an important characteristic of concrete. It is designated by the letter “M” and numbers from 50 to 1000 - the higher the index, the stronger the concrete. Physical and mechanical characteristics are determined by the selection of the composition of the concrete mixture, its water-cement ratio, as well as the quality of the components used: fillers, cement, water and additives.
Preheating components
All components of concrete except cement are heated to a certain temperature before mixing:
- fillers - up to 55-60°C;
- water - up to 90°C.
It is forbidden to heat cement, as it may lose activity. First, hot water is poured into the concrete mixer, and after several revolutions, aggregates are added. Cement is placed into the already cooled mixture. The temperature of the finished concrete before pouring should not be lower than 35°C.
The warm mixture is placed in the formwork. While it cools, the cement hydration reaction begins in the solution. Additional heat kickstarts primary hardening, maintaining the temperature of the concrete at the desired level.
Why increase the water resistance of concrete?
The higher the water resistance of concrete, the less susceptible it is to water pressure, as well as the effects of chemically aggressive substances in the soil and groundwater: acids, alkalis, salts, and so on. The water permeability of concrete is negatively affected by excess mixing water, undercompaction of the concrete mixture and lack of maintenance of freshly laid concrete.
Water that has not entered into the hydration of cement (the so-called transformation of a plastic mass into a monolith) forms pores in the concrete after evaporation. Some of them are not closed and form end-to-end channels. Moisture penetrates through them into the foundation, which expands when freezing, which contributes to the further destruction of concrete.
To reduce the amount of mixing water while maintaining the mobility of the concrete mixture, plasticizers are used. They reduce the water-cement ratio, reduce the pore volume in concrete and increase its density. It should be taken into account that it is necessary to select the composition of a specific concrete mixture in laboratories, taking into account the features and characteristics of the materials used: cement, coarse and fine aggregates and various additives.
During installation, special attention is paid to compacting the concrete. The concrete mixture must completely fill the required space without creating voids. This is especially important with dense reinforcement of the structure. For these purposes, special devices are used - deep and surface vibrators.
Warming up the pit
The hydration reaction is possible at temperatures above +5°C. Therefore, cooling of the concrete mixture below this indicator should not be allowed.
Preparation for installation begins with heating the pit. In the conditions of individual private construction, open-flame fires, reflector stoves, thermomats, hot coolants - sand, slag - are used.
The hardest thing is to prepare the foundation pit for the slab foundation. A large area of soil heats up slowly and cools down quickly. But it is much easier to warm the trenches under the strip base. The route can be prepared not entirely, but in parts, and monolithic work can also be carried out in separate sections.
How to care for concrete after pouring
In concrete with a low water-cement ratio, it is necessary to retain water, which is needed for the hydration process. If this is neglected, the quality of the final product will noticeably decrease. The standard concrete care regimen involves moistening the freshly laid foundation every 3-4 hours for the first 3-5 days after pouring, depending on the ambient temperature. Also, the concreting area is covered with damp burlap or film, and special film-forming compounds are used.
What are penetrating additives
To increase the water resistance grade of concrete, mineral materials of penetrating or penetrating action are often used. They are added to the mixture during preparation or applied to a prepared surface, that is, cleaned of dirt and cement laitance, as well as a water-saturated surface using a plaster sprayer or brush.
Active chemical additives in the material react with the components of the concrete mixture. As a result, insoluble compounds or crystals are formed. They create a continuous barrier that prevents the flow of water. Depending on the type of concrete being processed, the degree of its waterproofness can be increased by two or three levels.
What to look for when concreting a foundation
In order to competently carry out construction work and reduce further costs of repair and operation of the foundation to a minimum, you must adhere to a few simple rules:
- The foundation design should not allow cracks to form under any load.
- The entire volume of concrete must be poured without technological and “cold” joints, that is, in one technological cycle.
- If the work involves the formation of technological seams, they must be sealed using swelling cords, sealants or waterstops.
- Working reinforcement must be installed in strict accordance with the design; the thickness of the protective layer of concrete must be at least 15 mm.
- It is necessary to compact the laid concrete with vibrators or use self-compacting concrete.
- Newly laid concrete needs to be maintained.
Are there any advantages to winter concrete work?
In general, working with concrete in harsh conditions of low temperatures entails additional difficulties, but it is impossible to stop construction for six months every time autumn comes, and besides, winter work also has significant advantages:
- Winter discounts on building materials and the decline in demand for labor make it possible to save money.
- In winter, foundations can be concreted on weak or brittle soil.
- Frozen access roads make it possible to easily deliver heavy equipment and materials to a construction site.
What time of year is best to pour the foundation?
Difficult weather conditions negatively affect the strength gain of concrete.
Abundance of precipitation
at the stage of setting the concrete mixture can lead to its saturation with excess moisture. This leads to an increase in the water-cement ratio and a decrease in the strength of concrete.
Frost
turn mixing water into ice, which disrupts the hydration process of cement. When water freezes, it expands and internal stresses arise in the freshly laid concrete, which leads to damage to the foundation.
High air temperature, low humidity and wind
increase the rate of water evaporation from freshly laid concrete. This leads to disruption of the process of cement hydration and destruction of concrete in the surface layer, the appearance of shrinkage cracks.
Based on this, and also taking into account that the process of concrete gaining strength lasts 28 days, the optimal time for pouring the foundation is the period from late spring to early summer.
In addition to the advantage of comfortable conditions for carrying out work, there will be enough time ahead to eliminate possible defects, arrange waterproofing and additional protection. As a result, a foundation built in the spring will not be afraid of autumn temperature changes and winter frosts.
Selection of composition
For winter concrete, the components are prepared more carefully than for concrete that gains strength under normal conditions. Recommended:
- use quick-hardening cement, Portland slag cement or ordinary PC grade M400 and higher;
- antifreeze additives should be combined with plasticizers;
- do not use PMD in mixtures with pozzolanic or aluminous cement;
- prevent inclusion of snow and ice in aggregates;
- prescribe the amount of antifreeze additives taking into account the air temperature;
- reduce the water-cement ratio to the minimum possible.
Sodium nitrite and nitrate in liquid or crystalline form, as well as nitrodap and lignosulfonates are used as PMD. But they cannot be used in concrete with alumina binder and at high, variable groundwater levels.
How to pour a foundation in winter
If it is still necessary to concrete the foundation in winter, several technological methods are used.
Antifreeze additives and hardening accelerators
– they allow work to be carried out at sub-zero temperatures without loss of speed and quality.
Heating concrete with an electric cable
– it is attached to the reinforcement frame and connected to the network, and then remains in the body of the concrete.
Warming up concrete with heat mats
– unlike cables, mats are installed on the surface of a heated structure and can be used repeatedly.
Arrangement of the greenhouse
− this is a structure similar to a greenhouse, which is assembled above the work site. Heat guns are installed inside to maintain positive temperatures, so in the “warmhouse” you can safely carry out foundation construction work: knitting the reinforcement frame, concreting, installing waterproofing and installing waterstops.
Bottom line
The optimal time for pouring the foundation in most regions of Russia, except the northern ones, is the second half of spring or early summer. During this period, the optimal weather for concreting work is established. Here are the main points to pay attention to when conducting them:
- Entrust the work of decoupling the reinforcement frame and concreting to a specialized organization. This way you will avoid mistakes, the correction of which can be very expensive.
- Use only ready-mixed concrete of appropriate strength and water resistance grades. Methods of preparing concrete on site in a metal bath and using shovels are strictly unacceptable.
- Carry out concrete work without forming cold joints.
- Increase the water resistance of concrete using special additives. For example, penetrating.
- Freshly laid concrete needs to be maintained. Cover it with damp burlap and water it every two or three hours, even at night, for 3 to 5 days. If the weather is hot, it is better to water within 7 days.
- Carry out foundation work in winter using special methods.
- If possible, check the thickness of the concrete cover using a rebar locator. It must be at least 15 mm.
- Additionally, protect foundation structures from the negative effects of soil and groundwater.
Filling an armored belt at sub-zero temperatures
The reinforcing belt is designed to evenly distribute the load from the upper rows of masonry to the lower ones. It seems to connect the entire structure into a single whole. which significantly increases its overall strength and durability. The armored belt is made from steel rods laid around the perimeter, which must be welded together into a single structure.
When constructing a structure from gas silicate blocks, a reinforced belt is especially important. Such blocks quickly crack at the slightest movement of the soil or during shrinkage of the base. When installing a roof on such blocks, an armored belt is also necessary, since it is impossible to attach the timber to the blocks - they can crack. To avoid deformation of the walls and the building as a whole, a welded reinforced belt is simply necessary. It is welded into a solid structure, laid on top of the masonry and filled with concrete. It is sealed with mortar on both sides so as not to disturb the thermal insulation of the walls. Concreting of the belt is allowed in the cold season only using the above methods.
Working with concrete mixture at low temperatures is allowed in cases where there is no other option, because the optimal hardening temperature of the mass is from +5 degrees to 25.
