Terms You Should Know:
- post-tensioning: a pre-stressing technique where steel strands are tensioned after the concrete is cast
- pre-stressed: concrete that undergoes internal stresses from reinforcing steel strands to offset tension stress of future loads
- pre-tensioning: a pre-stressing technique where steel strands are tensioned prior to the concrete being cast
- rebar: nickname for reinforcing bar that is used to increase the tensile strength of concrete
- reinforcing bar (rebar): steel rods, strands, or metal fabric placed in concrete slabs, beams or columns to increase their strength
- reinforced concrete (RC): a composite of two materials: concrete and reinforcing steel (bars and mesh) using the best of both properties
Mechanics of Materials
The mechanics of materials is a term used to describe how different types of materials behave under stress. This article focuses on how concrete behaves under compressive and tensile stresses. We will also review some of the techniques that are implemented to counter the material’s weaknesses, which, as a result, makes concrete strong and thus a common material used as a structural component in commercial buildings.
Standard concrete responds well to compressive stress but poorly to tensile stress; therefore, reinforcement is used to improve the strength of the material. The concrete resists compressive stress, and the reinforcement provides strength against the tensile stress.
NOTE: Concrete expands or stretches when subjected to tensile stress, and it shrinks or shortens when subjected to compressive stress.
Concrete is generally considered to be a brittle material; thus, without reinforcement, it will experience brittle fracture as a mode of failure. Brittle fracture is a tensile mode of failure, meaning that prior to complete loss of strength, the material exhibits little to no signs that something is wrong. The ultimate failure is relatively sudden. The reinforcement in concrete changes the brittle fracture mode of failure to ductile fracture; therefore, prior to complete loss of strength, cracks will become visible. Hence, there is a visible warning before ultimate failure.
The mechanics of concrete tells us that concrete on its own does not make a good structural material, especially since concrete in service is subject to significant tensile stress and varied loads. So, all concrete is reinforced to resist applied tensile forces and to control the development of tension cracking under load.
Reinforced concrete (RC) is a composite of two materials: concrete and reinforcing steel (bars and mesh). The reinforcing steel, also called rebar, is embedded into the concrete so that the two materials can resist the applied forces together. Note that reinforcing steel installed in this manner is often referred to as conventional or regular reinforcement.
Conventional reinforcement is a form of passive reinforcement such that the reinforcing steel doesn’t resist tension until it stretches, which often means that the concrete must crack before the reinforcing steel can resist the tensile stress. In other words, the cracking may activate the strength of the reinforcing steel, so deflection of the concrete may be present but manageable for the material. The reinforcing steel is often placed at the top and bottom of slabs.
Conventionally reinforced concrete may also be augmented with pre-tensioning or post-tensioning steel rebar strands. When these techniques are implemented, the material is collectively referred to as pre-stressed concrete. This is a form of active reinforcement, which, as the name implies, means that the concrete is pre-stressed before being placed in service. It is pre-stressed by means of stretching (tensioning) steel rebar strands.
The two pre-stressing techniques are described below:
- Pre-tensioning: The concrete is cast around pre-tensioned steel rebar strands. These strands are stretched across a concrete framework between two end-anchoring points. The concrete is bonded to the steel strands, and once the concrete reaches a specified compressive strength, the steel rebar strands are released. In this technique, once the concrete cures and the pre-tensioning steel rebar strands are released, the tension is transferred inside the concrete as compression through friction with the reinforcement.
- Post-tensioning: The concrete is cast around sleeves or ducts, and the pre-tensioning steel rebar strands are threaded through them. Once the concrete reaches a specified compressive strength, the steel rebar strands are stretched with hydraulic jacks and permanently anchored at each end. The sleeves or tubes are usually filled with grout. Post-tensioning is also achieved by providing steel rebar strands the freedom to move within the concrete to some degree. In this case, the steel rebar strand is greased with corrosion-resistant lubricant and insheathing. This is referred to as unbonded post-tensioning. In this technique, permanent compression is applied to the concrete when the steel rebar strand is permanently anchored.
In both pre-stressing techniques, the stretching of the strands is a form of stressing that compresses the concrete. This, in turn, introduces internal stresses that counter the tension stress from future service loads. To sum up, pre-stressing increases the tensile strength of the concrete because future service loads must negate the compressive pre-stress. Pre-stressed concrete is often incorporated into civil engineering projects, like bridge decks, as well at the following elements of commercial buildings: balconies, lintels, floor slabs, beams, foundation plies, and parking structures.
Common Reinforced Concrete Defects
Cracks are a common and easily visible defect found in reinforced concrete. Inspectors should note that not all cracking observed may negatively affect the structural integrity of concrete elements. One type of cracking is referred to as plastic settlement, and it usually forms above and aligned with steel reinforcement. Another type of cracking is referred to as corrosion of reinforcement, and it forms above the reinforcement, as well. Some defects appear within a few hours of the concrete’s curing process, while others take years to develop. In any case, inspectors should report on signs of cracking according to their location and characteristics.
Does Concrete as a Structural Component Need Reinforcement?
Without reinforcement, standard concrete is not suitable as a structural component in commercial buildings because it has poor tensile strength and develops tension cracking under load. Naturally, concrete responds well to compressive stress; thus, reinforcement is used to provide strength against tensile stress and to suppress cracking (and complete failure).
That said, concrete that experiences significant applied loads should have reinforcement. But although reinforcement makes concrete stronger, some concrete structures and elements may not have or need reinforcement. This includes residential driveways, garage floors, and steps.
The decisions for which materials to use in the construction of various types of commercial structures are made in the preliminary engineering stage. Concrete structural components may include beams and columns, frames, diaphragms, and/or shear walls. It’s important for commercial property inspectors to understand the basic mechanics of common materials and techniques, including pre-stressing of concrete, in order to competently inspect and report on most commercial structures.
Concrete structures and the techniques implemented for their construction may be quite complex. Commercial property inspectors should have a professional engineer or concrete repair and maintenance specialist on their team of specialty consults. Some engineers spend their entire career studying and specializing in concrete construction techniques.