A pile cap is a thick

concrete Concrete is a composite material composed of fine and coarse aggregate bonded together with a fluid cement (cement paste) that hardens (cures) over time. Concrete is the second-most-used substance in the world after water, and is the most wi ...

mat that rests on

timber Lumber is wood that has been processed into dimensional lumber, including beams and planks or boards, a stage in the process of wood production. Lumber is mainly used for construction framing, as well as finishing (floors, wall panels, wi ...

piles that have been driven into soft or unstable ground to provide a suitable stable foundation. It usually forms part of the

deep foundation A deep foundation is a type of foundation that transfers building loads to the earth farther down from the surface than a shallow foundation does to a subsurface layer or a range of depths. A pile or piling is a vertical structural element ...

of a building, typically a multi-story building, structure or support base for heavy equipment, or of a bridge. The cast

pile cap distributes the load of the building into the piles. A similar structure to a pile cap is a "raft", which is a concrete foundation floor resting directly onto soft soil which may be liable to

subsidence Subsidence is a general term for downward vertical movement of the Earth's surface, which can be caused by both natural processes and human activities. Subsidence involves little or no horizontal movement, which distinguishes it from slope move ...

Design

Construction

The mat is made of concrete which is an aggregate of small rocks and

cement A cement is a binder, a chemical substance used for construction that sets, hardens, and adheres to other materials to bind them together. Cement is seldom used on its own, but rather to bind sand and gravel ( aggregate) together. Cement mix ...

. This mixture has to be supported by a framework to avoid sagging and fracture while setting. This process is known as shuttering and

reinforcing In behavioral psychology, reinforcement is a consequence applied that will strengthen an organism's future behavior whenever that behavior is preceded by a specific antecedent stimulus. This strengthening effect may be measured as a higher freq ...

. The materials used are long steel bars with longitudinal protrusions between the piles held in shape by thinner tie wires. Once this steel mat is laid, timber is attached around the perimeter to contain the wet concrete mixture. Once poured, (usually as a series of small loads), the concrete is stirred to remove any air pockets that might weaken the structure when set. The concrete undergoes a chemical change as it hardens and this produces a lot of heat. Sometimes, if the mass of concrete is very large, pipes carrying

refrigerant A refrigerant is a working fluid used in the heat pump and refrigeration cycle, refrigeration cycle of air conditioning systems and heat pumps where in most cases they undergo a repeated phase transition from a liquid to a gas and back again. Ref ...

coolant A coolant is a substance, typically liquid, that is used to reduce or regulate the temperature of a system. An ideal coolant has high thermal capacity, low viscosity, is low-cost, non-toxic, chemically inert and neither causes nor promotes corrosio ...

are used in the mass to assist the setting process to prevent the concrete from cracking.

References

Structural engineering Building engineering Foundations (buildings and structures) {{engineering-stub

Design

Construction

See also

References