Why Concrete Fails

Why Concrete Fails

High-rise condominiums and commercial buildings are constructed of reinforced concrete. Formed around a skeleton of rebar, rods and steel mesh, reinforced concrete can resist the compressive and tensile stresses that are typical in high-rise buildings, parking garages and other multi-story structures.

Concrete buildings can fail because of improper design, improper installation and weather. Concrete restoration is vital to maintaining a structure’s (and the concrete’s) ability to resist the forces that cause failure. Concrete restoration includes the components of vertical construction, including columns, slabs, balconies, beams and walls, as well as roads, sidewalks and driveways.

Concrete failure in Florida starts with surface cracking. Once moisture and salt penetrate the building façade through cracks and mix with oxygen, corrosion (rust) starts to eat away at the reinforcing metal rebar and rods. As the rebar rusts, its volume is increased by up to 10 times its surface area, which introduces tensile stresses into the concrete and widens cracks, separates pockets of concrete around the rebar, a process called spalling, allowing more water and salt to enter. Concrete will eventually fail if the mixture of water, salt, air, corrosion and cracking is allowed to continue. Failed concrete affects the structural integrity of a building.

Sound Testing

Sound testing is the most common method for finding hidden concrete damages in buildings. There are two types of sound testing: Hammer sound testing and the chain drag.

Hammer sound testing involves tapping the concrete with a hammer or the head of a golf club. If the concrete is good, tapping it will result in a ringing sound. If the concrete has cracks, the sound from the hammering will be drum like. This method cannot detect defects that are deep in the building or under overlays.

Chain Drag

The chain drag involves dragging a heavy chain across the surface of the concrete while the sound is recorded with microphones. The chain drag method can be more effective because it can cover a larger area and can be mechanized. This method provides results when defects and deteriorations are in advanced stages.

Echo Location

An echo method locates cracks, voids and delaminations when a solenoid impactor mechanically taps the concrete. Low frequency sound waves spread into the structure and are reflected by external surfaces and flaws. The depth and magnitude of partials cracks, voids and flaws are recorded in wave form.

Pinpointing Flaws With a Drone

Using a drone prior to the sound testing, helps the structural engineering firm pinpoint cracks and suspected and actual areas of spalling in a building’s envelope. When structural flaws are identified, a structural engineering firm will establish ascope of work and budget for the restoration project.