Cinder blocks are a cheap building material. Recently, they are finding increasing use in private construction. They are used to build houses, extensions, garages and fences. Many people are concerned about the answer to the question whether it is possible to build a foundation from cinder blocks. In some situations this is completely acceptable and justified, but sometimes it leads to unfavorable consequences.
Advantages and disadvantages of a cinder block foundation
First, you need to decide what a cinder block is. This building material is produced by vibration pressing using components such as cement and filler (sand, slag, crushed stone or screenings, broken bricks, etc.). Due to its porous composition, cinder block conducts heat well and has low frost resistance. In addition, it weighs quite a lot. It is mainly used in the construction of low-rise buildings, as it is of little use as a structural building material. The disadvantages of cinder blocks also include low sound insulation, the content of toxic substances and not very high strength. Most of the shortcomings of this material can be eliminated using long-established technologies. For example, low heat resistance can be leveled by installing insulating panels, and high water absorption rates can be reduced by reliably waterproofing the walls and the base of the foundation.
The advantages of the material include low cost, low consumption of cement for masonry, simplicity and speed of construction work, and versatility of the material.
As a result, cinder blocks are often used in construction. Large dimensions and a cost in the range of 50 rubles per piece make it possible to fit construction work within budgetary limits, especially if you carry out the work yourself.
Cinder block can be hollow or solid. A hollow block is used to construct walls, since their heat and sound insulation rates are higher. A solid cinder block with crushed stone filler is more durable and can be used for laying a foundation.
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Advantages of a cinder block foundation
- such a foundation can be erected independently without losing in terms of durability and strength;
- Construction time and quantity of concrete are significantly reduced;
- when using cinder block there is no need to erect formwork;
- cinder block does not shrink.
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Disadvantages of a cinder block foundation
- It is not recommended to build houses weighing more than 100 tons on such a foundation.
- Due to its high density, cinder block is a fairly heavy material, so lifting equipment may be required.
- It is impossible to build such a foundation on heaving soils (clayey, loam, sandy loam, etc.) due to the possibility of its displacement in the horizontal plane.
- Due to its hygroscopic characteristics, the construction of a cinder block foundation is also not recommended on wet soil and in high-lying surface waters. In addition, the groundwater level must be below the freezing line.
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Technology for constructing a cinder block foundation
When constructing such a foundation, the following stages are distinguished:
- For the strip base it is necessary to dig a trench. A pit needs to be built under the columnar foundation.
- A layer of sand and gravel is poured onto the bottom of the trench or pit, on which roofing material is spread, which serves as the basis for waterproofing the lower part of the foundation. In addition to roofing felt, you can use bitumen, bitumen mastic, waterproofing, and fiberglass.
- A ready-made reinforced concrete block (type BF or FL) is mounted on roofing felt, or a reinforced base with a 10x10 cm grid is poured with M300 concrete. This is done because wall blocks cannot bend and react to changes in the soil.
- Next, the 1st row of blocks (with a strip base) or the 1st layer of blocks (with a columnar base) is laid.
- Subsequent rows are laid staggered (from the corners to the center), connecting them with layers of fastening mortar 1 cm thick. These actions are repeated until the base reaches the grillage.
- The upper part of the strip base is strengthened with a reinforcing belt poured over the surface of the blocks of the last row. The top of a columnar foundation is made of reinforced concrete, wood or metal.
- At the end, the foundation walls are covered with a layer of mastic, onto which polystyrene foam panels or other heat-insulating material are glued.
- After this, the pit is filled in and work begins on the construction of the walls of the structure.
Important! The foundation walls must be insulated without fail (even if the structure being built will be located on dry ground). It is necessary to protect not only the vertical planes of the cinder blocks, but also to lay a horizontal layer between the underground and above-ground parts of the building. This helps prevent excessive wetting of the walls of the house with capillary moisture.
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How to prepare a solution for cinder blocks
The cinder blocks are fastened with cement mortar. A prerequisite for constructing a base of blocks is to securely fasten them together using cement mortar. The mixture should be prepared immediately before erecting the walls.
To work with a material such as cinder block, the solution must be strong and plastic.
To obtain the desired consistency and plasticity, you should add red clay to it in the proportion of 1/3 of a bucket to 4 buckets of a mixture of cement and sand.
To prepare a soft and homogeneous mixture, the solution should be mixed, maintaining the correct ratio of components.
First you need to mix the dry ingredients, then gradually add water to the resulting mixture, while mixing everything thoroughly. Typically, cement and sand should be taken in a ratio of 1:4 (i.e., 1 part of cement goes to 4 parts of sand), but since cinder blocks do not like moisture, to increase the strength of the masonry, it is better to prepare a mixture in a ratio of 1:3. It is necessary to add water gradually, carefully controlling the consistency of the solution. The finished mixture must be consumed within 1-1.5 hours.
To prepare a cement-sand mortar, you should take M400 or M500 cement. Sand can be taken from both river and quarry. Quarry sand itself contains clay, so the solution is plastic and more convenient to work with. You can work with clean river sand, adding clay in the above proportion or any detergent (washing powder or dishwashing detergent at the rate of 400-450 g per 80-100 l).
You can also add store-bought plasticizer to the solution. A well-prepared mixture is the key to high-quality masonry, but a hard mortar makes the work much more difficult, and too greasy a mortar literally sticks to the spatula.
The use of slag in foundation construction is due to the savings in material resources. But it must be remembered that such a design, due to its characteristics, may not live up to expectations. As a result, the cost of restoring the house will significantly exceed the amount required to build a high-quality foundation. Experts do not recommend using slag in the construction of the underground part of residential buildings. This method can be used as a last resort or when constructing unimportant objects, such as sheds, garages, summer kitchens and other outbuildings. Although one should not ignore the fact that many owners of houses built on slag during the period of general shortage 20-25 years ago claim that they have not encountered any problems over the years.
Specifics of the material
It is quite difficult to predict what strength a slag foundation will have, since no one accurately determines the proportions of the components found in waste from metallurgical enterprises. One delivered batch of bulk material may contain heterogeneity associated with an unequal amount of metal and cement chips or other inclusions. This fact influences the fact that under the same operating conditions, and even under the same house, the constructed foundation can behave completely differently.
In addition, the standards do not describe the algorithm for calculating the slag foundation, as well as a clear technology for its installation. Masters are guided by their own experience or tips from friends. But this does not always lead to positive results.
Slag can degrade when exposed to water, so it should not be used on a building site with a high groundwater level. For the same reason, a foundation built on dry soil requires enhanced waterproofing, but for protection from rain and melt water. In addition, care should be taken in advance about the drainage system that removes moisture from the underground structure in order to avoid the formation of stagnant processes.
The load-bearing capacity of a slag foundation is insufficient to support permanent buildings, although there are many known cases of its successful construction under country houses and relatively light residential buildings.
The use of slag when constructing foundations is strictly prohibited in seismically active regions. There is no need to even doubt that the underground structure will not withstand the jerky movements of the soil and will collapse. For territories included in the risk zone due to possible earthquakes, specific structural construction schemes are provided.
Which slag to choose
Single types of slag are suitable for foundation construction. Their properties are determined depending on several components, the main ones being:
- chemical composition;
- cooling mode;
- primacy of processing;
- grain size.
Slag is formed during the smelting of ferrous or non-ferrous metals, as a result of burning fuel in thermal power plants, during the production of fertilizers, in particular phosphorus, etc. The specifics of the production where the material was obtained determine its chemical composition and proportions of components. Foundations are laid from heavy waste slag A, formed in metallurgical or steel mills.
Industrial waste generated at thermal power plants or chemical industry enterprises is not allowed to be used.
The approximate composition of the slag can be judged by its color. For example, a gray tint indicates the presence of cement, a greenish or black tint is obtained during the melting of non-ferrous metals, and a bluish tint indicates the presence of manganese inclusions. External assessment of slag is also carried out by grain size and the number of large pieces. The structure of the material should be crumbly, and the fraction should be less than 5 mm. When watered, small particles contribute to better compaction of the slag, which reduces the porosity of the future structure of the foundation block.
Slag mixed with cement, subjected to repeated processing, loses the ability to set and harden due to the maximum loss of the binder component. For this reason, it will never turn out to be a monolith. Ideally, the slag should contain about a third of the cement dust, which is important for proper hardening of the compacted mass.
When constructing foundations, it is recommended to use hot slag, or rather warm, but still smoking. It is from this material that structures with good load-bearing capacity are obtained. “Fresh” slag has a crumbly structure without the presence of lumps, which is the best solution.
If you need to store slag outdoors, you must remember that under the influence of rain or high humidity, the material begins to harden. The top crust can be quite tough and will require considerable force to break it down to release some of the usable material. Therefore, the slag brought and dumped for the time being should be protected from moisture before the start of the work process.
Construction of a slag foundation
Based on the above, it can be noted that:
- You should not build heavy buildings on a slag foundation;
- the structure of the hardened slag along the perimeter of the belt may be heterogeneous;
- the material must not be soaked before work begins;
- It is quite difficult to select high-quality waste from metallurgical production, relying on external signs;
- the finished structure is afraid of water, so it requires good waterproofing.
All these factors should be taken into account when constructing a slag foundation. But first of all, it is necessary to carry out preparatory work, which consists of clearing the area of foreign objects, removing the fertile layer and moving it to another place, marking it, and also digging a trench. For formwork you will need to stock up on solid boards and nails, and to remove communications - pipe scraps.
The depth of laying the foundation strip is taken depending on the soil freezing index. A trench is dug taking into account the construction of a crushed stone cushion 30 cm thick. To ensure better stability of the underground structure, it is laid with mandatory compaction of the bottom of the excavation and layer-by-layer compaction of crushed stone.
To reduce the risk of foundation destruction due to soil movements, deepening it by at least 1.2 m will help. The height of the trench, in this case, will be at least 1.5 m.
The formwork box is constructed based on the fact that the width of the slag foundation should be at least 60 cm. In this case, the need to raise the shields above ground level by at least 25 cm should be taken into account. To strengthen the structure, internal and external struts are installed on the box, and pieces of pipes are mounted at the communication outlets.
An important point is the correct determination of the position of the metal casings in the wooden box relative to the sewer and water outlets. Failure to comply with this condition will subsequently lead to the need to punch through the foundation tape, which is not permissible for a slag monolith.
It is recommended to lay a knitted reinforcing mesh at the bottom of the foundation. It will allow the base to become a single whole and help distribute the load more evenly. For its manufacture, reinforcing bars with a variable cross-section are used, treated with an anti-corrosion compound to avoid the occurrence of metal rusting. A frame is not installed in the body of the slag foundation in order to improve the compaction of bulk material.
Forming the tape
The most critical stage in the construction of a foundation is the correct filling of slag into the trench and careful compaction of the laid layers.
- The first condition is that the thickness of the lower and middle layers should be 20-25 cm, and the upper ones can be increased by 5 cm.
- The second condition is to pour plenty of water or cement laitance onto each layer. The liquid is selected depending on the content of binder components in the slag.
- The third condition is conscientious compaction of all layers without exception.
- The fourth condition is the installation of an upper reinforced belt, which ensures a strong connection between the underground and above-ground parts of the structure.
Impregnation of the slag with water guarantees the best quality of compaction, which helps compact the porous structure of the material. As a result, a fairly strong adhesion of particles occurs. Some craftsmen suggest, in case of low cement content in the slag, pouring the binder directly into the trench, mixed with waste from metallurgical production. Others advise using cement laitance instead of water. Surely both methods have a right to exist.
After laying and compacting each layer, take a break of about an hour to allow the cement to set. Then they begin to fill the next layer.
Without waiting for the last layer of slag to set, a reinforcing mesh is laid on it with the lower rods buried in the backfilled foundation. After this, the structure is left for approximately 12-16 hours. At the last stage, an upper concrete belt with a thickness of at least 20-30 cm is made. To do this, the concrete mixture is poured into the formwork to the design mark with mandatory compaction.
Protection device
After the curing process of the underground structure is completed, waterproofing begins. It should be understood that the durability and reliability of the slag foundation will depend on the quality of protection. It has a porous structure and quickly becomes saturated with moisture, so insulation work must be approached responsibly.
To carefully remove the formwork and gain free access to the surface of the structure, it is necessary to clear the area around its perimeter. This is done when the slopes are too close to the foundation. If there is enough space, you can skip this preparatory stage.
- plaster – helps compensate for porosity of surfaces, seals microcracks and covers sinks;
- liquid rubber - creates an impenetrable, water-repellent layer, evenly distributed through spraying.
Without additional protection, plaster cannot guarantee complete waterproofness, as it is prone to cracking.
In addition to vertical protection, two horizontal layers of waterproofing should be provided. The first of them is carried out before filling the slag, and the second is laid between the upper concrete belt and the wall. For their construction, in both cases, roll materials are used, the reliability of which is ensured by bitumen mastic. It must be remembered that the junction of vertical and horizontal waterproofing must be airtight.
After erecting the foundation part, you should not immediately load it with the above-ground superstructure. The optimal “rest” period for the structure is at least six months.
A country house begins with a foundation
First of all, we had to dig a trench for the foundation. The soil was clay, and the dimensions of the trench turned out to be small: 50 cm wide and 50 cm deep.
I decided to make a strip foundation from brick fragments up to 7 cm in size (note that the brick crushed stone must be clean, free of dust and small crumbs). The entire country house was built on clean water.
River sand was poured into an even layer at the bottom of the trench (layer thickness 15-20 mm). I laid halves of bricks on the sand with gaps of 3-5 mm, and poured prepared brick crushed stone on top in a layer 10 cm thick (Fig. 2).
In an oblong box I prepared a dry mixture of cement and sand in a ratio of 1:5 (by volume). Before preparing the solution, cement and sand were always thoroughly mixed dry until a homogeneous mixture was formed.
Since sand and cement in the solution quickly settle, the foundation was poured from a bucket. That is, first, dry solution (1/2 bucket) was poured into a bucket, and at the edge of the trench, stirring the mixture, water was added to the bucket to the top. Having prepared the solution, he immediately poured the entire bucket onto the crushed stone. In this case, the entire solution must pass through the crushed stone. If a mound of solution forms on top, it means the solution is thick. You need to pour the solution from the bucket not into one place, but along the perimeter of the entire foundation every 30-40 cm to ensure uniform pouring. After the first layer is completely filled, it would be good to place at least one thread of thick wire in the middle of the trench along the entire perimeter. I myself used thin heating pipes as fittings.
It would be nice to pour the entire foundation flush with the ground in one day. When filling the trench with mortar, I laid it in three layers. In the top layer, 5-10 cm from the surface, I provided another reinforcement. Do not forget, of course, to check the “horizontalness” of the foundation by level, spirit level, etc.
On the outside of the house I made an overlap with two whole bricks, thereby expanding the foundation by 10 cm. That is, now its width has become 60 cm, and the height of the foundation increases by 2 brick thicknesses. Now, on the inside of the foundation, we will strengthen the formwork board and fill the gap between the overlap bricks and the formwork with slag-sand-cement concrete. Concrete components (slag, sand, cement) are taken in a ratio of 6:1:1. I recommend dosing the solution components in buckets.
First, you should mix the cement and sand well, and then mix this mixture with the slag. Water is added to the concrete depending on the moisture content of the sand and slag, but keep in mind that the solution (don’t forget to do the latter!) should not be liquid and it could be compacted.
After pouring concrete into the foundation formwork flush with the top brick of the overlap, we make an even screed of cement mortar 10-15 mm thick on top. When preparing a mortar for screed, take 4-5 parts of sand for 1 part of cement.
After 2-3 days, you can lay waterproofing on top of the screed, for example, from roofing felt (preferably with one continuous strip without breaks). If you have to join the ends, you need to do it overlapping so that the ends of the roofing material overlap each other by at least 50-60 cm. The best waterproofing is obtained from roofing felt or roofing felt laid on hot bitumen, that is, a layer of heated bitumen is applied to the concrete screed , and roofing felt is laid on the bitumen. It is safer, of course, to stick another layer onto the first layer of roofing material using the same bitumen. The sides of the roofing material must hang down from the sides of the foundation by at least 5 cm.
Base
So, the foundation is finished. Now the base part is next (Fig. 3). Like the above-ground part of the foundation, we make the outer side of the base with a brick cladding, which can be folded from uneven halves, of course, laying them with the uneven parts inward, which will improve their bonding with the concrete. The height of the base is usually 60-75 cm.
Note that the higher the base, the better the underground will be ventilated. Of course, before starting the construction of the basement, it is necessary to lay a cement screed 5-10 cm thick on the waterproofing of the foundation. Be sure to provide windows in the basement for ventilation of the underground. During concreting, mesh (wire or made from reinforcement) can be fixed in the windows. At the top of the plinth, as on the foundation, a cement screed is made, waterproofing is laid with skillful hands, and again a cement screed is carefully leveled with a trowel. The composition of cinder concrete for the base is the same as for the upper part of the foundation.
Walls
First of all, for future walls it is necessary to construct formwork from wooden panels 40-45 cm high (Fig. 4). It is better to dig the stands that secure the shields into the ground. The distance between the posts, of course, depends on the thickness of the boards in the formwork panels and the width of the walls (don’t forget 5 cm for the gap). The main requirement for the formwork is that when tamping its boards do not bend, which will become clear during the first tamping together with the backfill.
The racks on top are knocked down with slats, but shields can also be fastened together. The sides are fixed using wedges driven between the shields and racks into a gap, which is also necessary for more convenient removal and installation of the shields in another place.
As a rule, the walls of houses are usually made of monolithic concrete, but due to a shortage of cement, the author decided to experiment and built walls of slag-cement concrete not solid, but with channels.
But in general, country houses made of slag-lime-cement concrete are better. Lime prevents the walls from becoming damp, and they will be warmer. If there is not enough lime, you can limit yourself to lime concrete from the waterproofing to the windows. In principle, slag-lime-cement concrete can be composed of components taken in a ratio of 10: 1: 1 (by volume).
Slaked lime should be filtered through a fine metal mesh to avoid lumps and diluted with water to better mix the concrete composition. It is more convenient to add water little by little using a watering can.
But let's return to the walls. The author concreted a country house using only two panels, so that a lot of material for formwork was not required, and it is easy to work with such panels. After installing the formwork (panels), the concrete is first spread in a continuous layer 10-12 cm thick over a cement screed and compacted. Then a piece of plywood or thick rigid sheet 35-40 cm high and 50 cm long is placed on this layer at a distance of 10-15 cm from the shield (Fig. 5). The gap between the board and the plywood is carefully filled with concrete. The layer should be of such a height that, once carefully compacted, it will not collapse if the plywood is removed. You can fill the free space between the plywood and the board with backfill, and then, carefully removing the plywood, repeat the operation at the opposite board, but do the backfill flush with the layer of already laid concrete. While working, make sure that the backfill does not get into the concrete.
In the walls, transverse partitions 10-15 cm wide are made every 30-50 cm. It is not necessary that the partitions coincide. The resulting square voids are filled with fine slag, preferably from non-coking coals, and expanded clay.
To create voids, it is convenient to use a special plywood box with slightly beveled walls to make it easier to remove the box from the wall after pouring the concrete (Fig. 6).
The corners of the house are monolithic - 50X50 cm. In the doorway the wall thickness is 25 cm, in the window - like the walls.
To decorate windows with platbands, you will need additional formwork (Fig. 7), consisting of two vertical boards (a distance equal to the width of the brick is provided between the boards). With the help of this formwork, bricks were strengthened in the sidewalls of the platbands, which, protruding from the wall, form a kind of platband, giving the window an elegant look. So, on a brick window sill, in the space between the formwork boards, a spacer is placed with a height of 65 mm and a thickness equal to the thickness of the formwork board.
Two halves of bricks are installed on the spacer in the space between the boards (dry), then the spacer and the whole brick are placed on them again. Again, a spacer, half a brick, etc. are placed on it.
The bricks are inserted during the concreting process, with the mortar being laid pressing them against the board, which is nailed to the outer sides of the additional formwork boards.
The bottom of the window sill is made like the basement of the house.
Window and door openings are covered with monolithic lintels, for which formwork is made in the form of a box. At the bottom of the formwork, to prevent water from flowing out of it, plastic film, roofing felt, and glassine are laid. It is necessary to lay wire reinforcement in the lintels (5-6 pieces), the ends of the reinforcement should be bent and extend onto the walls by at least 25-30 cm. The height of the lintels is 12-15 cm. Concrete is prepared on sand.
The cornice is laid with brick lining, like a plinth.
Under the ceiling beams - matrices - a platform of slag-sand-cement concrete (6:1:1) with a thickness of 10-12 cm is laid (Fig. 8). A frame of logs is placed on the beams, into which the rafters are cut.
Slag concrete mixtures are deservedly popular not only in industrial, but also in private low-rise construction. This is due to the characteristics of this material. Slag added to concrete is obtained as a secondary material in the metallurgical industry.
For a long time, granular material was considered simply waste, but world practice has proven its feasibility for use in construction. Slag concrete is lighter than sand and gravel and is distinguished by its durability, resistance to temperature fluctuations, and sound insulation properties.
It is the metallurgical material mixed with cement that has the greatest strength. Lime, gypsum or even clay can also act as a binding agent. Two types of granulated slag are actively used:
- with small grains (0.5-1.5 mm) – the result is fairly heavy, high-strength concrete with good thermal conductivity;
- with large grains (20-30 mm) - concrete is lighter, provides good thermal insulation, but is slightly less durable.
It is not recommended to use varieties larger than 6 mm, since the durability of such concrete will be unsatisfactory. It is preferable to replace finer fractions with sand. for construction it must be as clean as possible, without admixtures of clay, soil, or organic elements.
Considering the properties of concrete with different fractions, to create external walls, large and small grains are often mixed in a ratio of 7:3, respectively. For internal partitions, you can take one more fine slag. If you want to get the most durable mixture after hardening, it is advisable to completely remove the 12 mm fraction and replace 20% of the finest slag with sand.
How to prepare cinder concrete
Before starting to prepare the mixture (about a few hours), it is necessary to moisten the slag with water to improve the durability of the future concrete. Next, you need to mix the components in one of the above proportions and mix well. After adding water, the solution should be kneaded until smooth. To obtain an average grade of concrete, you can use 4-5 parts of slag (fractional mixture), 2 parts of cement and 2 parts of sand.
It is advisable to use the finished mass within an hour, the maximum processing time is 1.5 hours. A cheaper solution can be made by mixing cement with lime in a 3:1 ratio. The walls of the house can be erected using the cast-in-place method (monolithic structure) or from prepared cinder blocks.
The construction of capital structures requires huge resource and energy costs. At the same time, the construction of the foundation alone accounts for about 30% of the total costs. It's no wonder that many homeowners are looking for alternative options and prefer to use more affordable materials. Unfortunately, in pursuit of cheapness, many of them give preference to low-quality products, which they later regret very much. When choosing a future foundation, you need to use common sense. A bad foundation can cause irreparable harm and even completely destroy the entire structure.
Foundation based on waste slag materials
Such a foundation can be used when constructing houses in certain regions that are characterized by low groundwater levels and low rainfall. If the proposed construction site meets these requirements, then you can safely purchase waste slag materials for the foundation. This will save you a lot of money. Despite the fact that they also require the additional use of crushed stone and sand, their cost is still significantly lower than similar materials.
It is important to consider that such a foundation is only suitable for low-rise buildings. It has a porous structure and requires additional waterproofing. This is the only way to achieve the strongest and most reliable foundation. Many experts argue that foundations created on the basis of such cheap materials are only suitable for arranging garages, sheds and other outbuildings. This is only partly true. Such statements mostly apply to the work of amateurs. Professional craftsmen are even able to build residential buildings on them that can stand there for decades.
Distinctive characteristics of the material
When choosing a material, you should give preference to fresh fine-grained slag that has not been subjected to secondary processing. It has a high content of cement particles and is best suited for this purpose.
The foundation must be laid as quickly as possible so that the material does not have time to gain moisture and self-cement. It is necessary to lay a concrete strip or brick rows on top of the base.
The maximum permissible load on the foundation is no more than 20 tons, and its laying depth is at least 1.2 m. Compliance with these conditions will prevent possible deformation of the structure under the influence of ground shifts. It is best to waterproof the foundation using plaster, liquid rubber, etc. The main thing is that the waterproofing is layer-by-layer. Using a good drainage system will help avoid stagnation of liquid. From the moment of erection of the foundation, at least 6 months must pass before the start of construction of the building. Only in this case will the base have time to gain sufficient strength. As a result, the homeowner will receive a reliable and inexpensive foundation.
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