Non-ductile concrete was used in the majority of concrete buildings constructed before to 1977. Buildings built before to the establishment of the 1976 building code may not be able to withstand earthquake forces safely due to a lack of reinforcing steel. Concrete's inability to stretch or bend is what causes it to be non-ductile.
Ductility is the ability of a material to deform under stress without rupture. Concrete loses strength when exposed to moisture, so it cannot be stretched or bent without breaking. However, if you cut it with a knife or drill into it, it will usually heal up fine once again.
The key word here is usually. Concrete has a tendency to crack over time, especially if it isn't properly reinforced with steel. This is because water gets into the concrete through any gap or hole and slowly weakens it until an internal pressure is created within the slab. At this point, it becomes susceptible to further cracking from even very small changes in temperature or humidity.
If left unattended, this problem would eventually cause the slab to fail. But since pre-1977 buildings do not have reinforcement in their concrete, it would have to be replaced instead. Replacement costs are high enough that most owners choose to repair their properties instead.
Concrete is prone to flaws, which might lead to structural concerns later on. Modern concrete or prefabricated structures appear immaculate on the exterior, but upon closer inspection, they are far more difficult to mortgage and insure. Concrete surfaces are also less efficient at withstanding heat compared to other materials.
Concrete destroys house footprints, making them unsuitable for modern homes that are being built today. The material needs to be poured into a mold of some sort to become solid, so it cannot be shaped after it has set. This means that any hollow spaces inside the footings or basement floor will remain even after the foundation is complete. These holes can be filled with soil, but this increases the weight on the house structure. As the title insurance company says in its disclaimer: "If the basement has not been properly reinforced against earthquake forces, then it could be said that the house does not have good concrete foundations."
Concrete also requires special care. It is sensitive to temperature changes, stains, and moisture. It needs to be painted periodically to keep it looking new. Should any part of the house become damaged or destroyed due to concrete problems, the entire structure may need to be replaced rather than repaired.
Basement flooding caused by improper drainage is one of the most common problems with concrete foundations.
Concrete has several restrictions.
Concrete is only strong against compression forces and has limited tensile and ductility. Reinforcement materials are required for the concrete to withstand shear and tensile stresses. The most common reinforcement methods for concrete are rebars, which are metal rods used in concrete construction that provide support for larger structures or buildings. Concrete bars are shorter than rebars but just as effective at reinforcing concrete.
Rebars and concrete bars can be used together in a structure to create more stability. For example, two 12-foot lengths of rebar can be tied together with a piece of wire to make one 20-foot rebar. This helps prevent the structure from being pulled over when loading it with a vehicle, for example. Rebars are also used in conjunction with cables to provide extra strength to concrete structures such as bridges and skyscrapers.
The reinforcement of concrete with steel provides tensile strength, which means that it can resist tension forces. Concrete alone lacks this ability so adding steel bars improves its overall durability. Concrete can also suffer damage if it is exposed to extreme temperatures or conditions of humidity, so steel reinforcements are used to protect it from these factors.
Finally, concrete is used in many large structures because of its cost effectiveness. However, this means that they are prone to suffering damage due to heavy loads on them.