Foundations

It is prohibited to design foundations without proper engineering and geological justification. 

1. Outlining the site:

• When determining the depth of the foundation, you must take into account: 
  - the intended purpose and design features of the structure being designed, loads and impacts on the foundation;
  - the depth of laying the foundations of adjacent buildings and utilities; 
  - the relief of the existing and planned area of development after site preparation; 
  - the engineering and geological conditions of the construction site; 
  - the groundwater conditions and their possible changes in the course of construction and operation of the structure;
  - the seasonal frost depth.
• In the case of laying monolithic foundations, it is necessary to ensure that the natural or artificial base retains its physical and mechanical properties, the ground is sufficiently compacted and has an adequate load-bearing capacity.

2. Site preparation:

• If the site has loose soil, compact it using a plate compactor.
• Create drainage of gravel or sand, which must be laid in a small layer and compacted.

3. Formwork and preparation for concreting:

• The formwork must correspond to the design and withstand the weight of the volume of fresh RMC and the additional load created by concrete mix vibration.
• To prevent leakages of concrete, the formwork elements must be tightly connected.
• Put polyethene or roofing felt on the bottom to provide waterproofing.
• Before pouring the concrete, clean and sufficiently moisten the formwork.

Dry wooden formwork will absorb some of the water from the concrete mix, which will worsen the demoulding process and reduce the strength of the structure.

• There must be a distance between the rebars and the formwork for the protective layer of concrete. Its thickness depends on the structure and the conditions of its concreting, but not less than 20 mm. However, the thickness of the protective layer should be 35 mm in the presence of base concrete and 70 mm without such base concrete.

When laying a monolithic reinforced foundation, ensure that the formwork is free of defects and that the rebar frame is secured correctly.

• Clean and moisten the base concrete before placing the main concrete layer.

4. Concreting process:

• For the base concrete for the foundation, use concrete of strength class B7.5 (M100), and for the foundation itself, use concrete between B15 (M200) and B25 (M300), depending on the design load on the structure.
• Lay the concrete mix in horizontal layers of the same thickness without interruptions, consistently directing it in one direction in all layers. 
• The thickness of each layer depends on the tool used for compacting:
  - when using a heavy, suspended vertical vibrator, the thickness of the layer should be 5-10 cm less than the length of the working part of the vibrator; 
  - when using vibrators at an angle to the vertical (up to 35°), the thickness of the layer should be equal to the vertical projection of the length of the working part of the vibrator; 
  - when using manual immersion vibrators, the maximum thickness of the laid layer should not exceed 1.25 of the length of the working part of the vibrator; 
  - when laying the mix with surface vibrators, the layer thickness should not exceed: 250 mm in non-reinforced structures and structures with single reinforcement, 120 mm in structures with double reinforcement. 
• The step of movement of immersion vibrators should not exceed the 1.5 radius of its action, and the depth of immersion should ensure it penetrates 5-10 cm into the previous layer. 
• When moving surface vibrators, the step should ensure that the compacting plate overlaps the already vibrated area by 100 mm.

Scheme of vibration by immersion

• The duration of vibration depends on the power of the compacting device, the composition of the concrete, reinforcement and the shape of the solid structure. Outwardly, the RMC is considered to be compacted if cement laitance begins to appear.

Over-compaction of RMC is as harmful as under-compaction. 

• The formwork should be filled with a slightly more significant amount of RMC because after compaction, the concrete shrinks by the volume of the entrained air, which is released during vibration (usually 1-3%).
• To avoid the segregation of RMC, the height of its fall from the spout should be low.

It is essential to ensure the continuity of concrete feeding to the structure. The gap between the laying of individual layers should not exceed 2 hours.

5. Levelling of RMC 

to a smooth surface is the final stage of laying the foundation.

Concrete must be protected from moisture loss, especially during the first week after laying. Keep freshly laid concrete moist, protecting it from damage, shocks, and sudden changes in temperature and rapid drying. Read more about curing concrete here.