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TechTalk with Mel
Why Rebar Spacing is CRUCIAL
Most manufacturers of precast concrete products use reinforcing steel in their forms simply because they have been told to do so by a specification, by a design engineer or maybe his/her father said to put it in. But is it really necessary to put reinforcing steel in concrete? If so, why?
In the construction industry, we are all familiar with the term “concrete strength”, which actually refers to concrete’s compressive strength. For example, a concrete strength of 4,000 psi means that the concrete can withstand a load of 4,000 lbs of compressive forces for every sq. in. of surface area. That’s pretty strong stuff! We measure concrete strength in a “compressive test” done on a concrete cylinder, where the concrete sample is “squeezed” (compressed) between two hydraulic cylinders.
All strength is not the same
But what happens to the strength of concrete if we pull on each end of the concrete sample, rather than “squeeze” the ends together? In other words, what if we put the concrete in tension, or “stretch” it, in what is referred to as a “tensile test”? Now, we find the strength is only 1/10th of its compressive strength! Concrete that has an impressive compressive strength of 4,000 psi, but a tensile strength of only about 400 psi. Not so strong in tension!
Since reinforcing steel can withstand much higher tension (or “stretching”) forces than concrete, we use steel to withstand the tensile stresses that build up in the product when it is loaded. The steel is located in those parts of the product where the concrete is forced to “stretch” or “bend” under service loading. In some design situations, compression steel is also required, but this article will address tension steel, only, which is the reinforcement most NPCA precasters use in their products.
The structural integrity of every reinforced concrete product is dependent upon the:
- Grade of steel,
- Size and spacing of the steel reinforcing, and
- Location of the steel within the product.
In this discussion, we will focus on the importance of ensuring accurate spacing of the steel.
Calculating the amount of steel needed
When a civil engineer designs a reinforced concrete component, the cross-sectional area of reinforcing steel required for every foot of product length must be calculated. All reinforced concrete designs are based on the required number of sq. in. of reinforcing steel per foot, to safely carry the load. And, every foot of product must have the same amount of steel as the foot beside it to ensure that the product has uniform strength throughout.
If the steel rebar placers do not maintain correct spacing in the forms, the product strength is comprised. For example, if the designer calls for #5 rebar spaced at every 4in., three - #5 bars need to be placed for every 12in. of the form. If the steel placer is a little sloppy, and places the #5 bars at 5in. spacing, rather than 4in. spacing, the strength of the product will be reduced by 20%. Yes, concrete’s structural integrity can be compromised just that easily!!
Placing #5 rebar correctly at 4in. spacing provides a steel area of 0.93 sq. in., whereas, placing the same bars incorrectly at 5in. spacing, will reduce the steel area provided to only 0.74 sq. in. – 20% weaker!
It is very possible that this difference in spacing will be missed if the QC Inspector does only a quick visual inspection of the rebar spacing. Consider that even if the steel placers space the bars at every 4.5in. rather than at every 4in., the strength is reduced by 10%, which is still a very significant error. QC inspectors must take the time to accurately measure rebar spacing as part of their pre-pour inspections.
Pulling the wrong size rebar from the inventory pile can also result in a serious problem. Incorrectly placing #4 rebar at a spacing of 4in. (rather than the specified #5 rebar spaced every 4in.) will result in 35% less reinforcing than is needed for structural strength. When hiring new employees for the steel yard, take the time to familiarize them with rebar sizing and the importance of using the specified size for the job. At a quick glance, the difference between #4 and #5 rebar is not clearly noticeable, especially with some deformation patterns.
Because spacing is critical, care must be taken to ensure that steel reinforcing bars are properly secured in place, either by welding (making sure to use Weldable Grade ASTM C706 rebar) or by installing suitable wire ties. Rebar cages must as sturdy as is possible.
The spacing of rebar is crucial, so take the time to do it right!!www.precastconcretebc.com