A monolithic concrete foundation made of “pure” concrete, unfortunately, has poor resistance to loads other than compression. When the soil shifts, the concrete can crumble. Cracks may form on it, which can ultimately lead to damage to the entire structure being built.
To prevent damage to the foundation from multi-directional loads, a metal reinforcing cage is created inside it. However, instead of a structural framework made of metal rebar, large-fraction fillers such as rubble stone or large crushed stone can also be used to increase the strength of the concrete foundation in various directions.
As filler, relatively large stones such as limestone fragments, sandstone, or more expensive granite can be used. The stone itself, when laid in concrete mortar, must have a strength rating of at least 1 ton per square centimeter.
Advantages of Rubble Concrete Foundation
The undeniable advantages of a rubble concrete foundation are:
- High strength. A quality rubble concrete strip foundation can easily support the weight of a 2-story brick building.
- Availability of building materials, which significantly reduces construction costs.
- Simplicity of construction, not requiring special technological skills from builders.
For a rubble concrete foundation to withstand heavy loads, it must be created in certain proportions. The mixing recipe is very simple – equal parts of concrete mix and rubble stone. The proportions can vary slightly, but each component cannot be less than 40 percent.
What Kind of Rubble Stone Should Be Used?
As a filler for the concrete mix, rubble stone of not too large a fraction, with sizes from 20 to 30 centimeters, should be used. To more accurately select the size of the rubble stone used, you can use a simple formula: the size of the rubble stone should be approximately one-third of the width of the concrete strip foundation.
Before being laid into the foundation, the rubble stone must be cleaned of dirt and dust. Also, before laying, the rubble stones are moistened, which increases their adhesion to the concrete mix.
The strength of the stone can be checked by tapping it with a hammer – a rubble stone should produce clear, ringing notes.
Preparing the Concrete Mix for the Foundation
For mixing the concrete mix, cement with high strength grades, in the range of 400 to 500, is used. Medium-fraction sand is also used for forming the concrete mix, and it is mixed with cement in standard “one to three” proportions.
Water is added gradually when mixing the mortar for a rubble concrete foundation. The concrete mix should be more liquid than for ordinary foundations, as it needs to penetrate narrow gaps between the rubble stones.
Rubble Concrete Foundation Design
When designing a rubble concrete foundation, its width should be laid out 8-10 centimeters wider than the thickness of the load-bearing outer wall. This is because the rubble stone should not touch the formwork walls.
The strip foundation shape is the most optimal for a rubble concrete base. This achieves savings in building materials used and at the same time creates a strong base for all load-bearing structures.
If you are creating a rubble concrete foundation for a heavy brick structure, its base should be wider to ensure even distribution of the load from the structure onto the ground.
Building a Rubble Concrete Strip Foundation
On the cleared construction site, we mark out the foundation base according to the chosen project. When creating the foundation project, the groundwater level and the freezing depth of the soil must be taken into account.
- The top fertile layer is removed from the construction site, and a trench for the foundation is dug according to the marking.
- Cushions of compacted sand and gravel are laid at its bottom. If the cushions tend to absorb moisture, a layer of roll waterproofing, such as roofing felt, is laid at the bottom of the trench. The sheets are laid overlapping to prevent moisture leakage.
- First, we pour the bottom layer of pure concrete, approximately 15 centimeters thick. This prevents the rubble stone from being squeezed into the “footing.”
- Above the “footing,” we begin to erect the wooden formwork. We carefully strengthen it so that the heavy mixture of stone and concrete does not destroy the wooden structure.
- Pour a layer of concrete approximately 25 centimeters thick. Embed a row of rubble stones into it so that they protrude about halfway above the concrete layer. A distance of approximately 5 centimeters should be maintained between the stones.
- Pour the next layer of concrete so that it covers the layer of rubble stones and rises above them by half the size of the rubble filler.
- Then, again, lay another layer of rubble stone with partial immersion and repeat these operations for the entire height of the strip foundation.
- The final layer of the rubble concrete foundation is a layer of pure concrete that completely covers the rubble stones.
The poured rubble concrete foundation must be protected from drying out, especially in hot weather. For this, the surface of the strip foundation is watered or covered with a moisture-insulating pad.
After the concrete has fully cured, another layer of leveling concrete mix can be poured onto the top surface.
Some Features of Rubble Concrete Foundation Construction
Finding rubble stone for the foundation construction will be quite difficult during the building process. We recommend planning the construction of a rubble concrete foundation only if there is a stone quarry near your construction site from which you can easily deliver building materials.
When building a rubble concrete foundation, try to rationally organize the placement of building materials on the construction site. Moving large masses of rubble stone from place to place is unlikely to add to your enjoyment of the construction process. It is advisable to lay out the rubble stone for construction along the foundation trench.
However, you should not place the building material directly on the edge of the trench. This approach can lead to potential injuries. It is sensible to arrange the stone supplies in small piles approximately 20 centimeters from the edge of the trench.
Rubble concrete foundations are also quite sensitive to frosty winters. The stone in the foundation can crack and cause damage to the foundation. To prevent such occurrences, the rubble concrete foundation must be carefully insulated and waterproofed. This approach will also allow you to create a full-fledged habitable space inside the rubble concrete strip of the foundation.