Most manufacturers of precast concrete products use reinforcing steel in their forms simply because they have been told to do so by a specification or a design engineer, or in many cases simply because the father told him or her to do it. But is it really necessary to put steel bars 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 it can withstand a load of 4,000 lbs of compressive force for every sq in. of surface area. That’s pretty strong stuff! Concrete strength is measured in a compressive test on a concrete cylinder where the sample is squeezed (compressed) between two hydraulic cylinders.
All strength is not the same
Compressive strength is one test, but what happens to the strength of concrete if we pull on the ends of the sample, rather than squeeze them 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 tensile strength is only one-tenth of its compressive strength. Concrete that has an impressive compressive strength of 4,000 psi has maybe 400 psi of tensile strength. 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 addresses only tension steel, the reinforcement most precasters use in their products.
The structural integrity of every reinforced concrete product is dependent upon the following:
1. Grade of steel;
2. Size and spacing of the steel reinforcing; and
3. Location of the steel within the product.
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./ft of reinforcing steel to safely carry the load. And, every foot of product must have the same amount of steel as the foot beside it to ensure 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 every 4 in., three #5 bars need to be placed for every 12 in. of the form. If the steel placer is a little sloppy and places the #5 bars at 5-in. spacing rather than 4-in. 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 4-in. spacing provides a steel area of 0.93 sq in., whereas placing the same bars incorrectly at 5-in. 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. Even if the rodmen space the bars at every 4.5 in. rather than at every 4 in., 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.
“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.”
Pulling the wrong size rebar from the inventory pile can also result in a serious problem. Incorrectly placing #4 rebar at a spacing of 4 in. (rather than the specified #5 rebar spaced every 4 in.) 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, ensure that steel reinforcing bars are properly secured in place, either by welding (use only Weldable Grade ASTM C706 rebar) or by installing suitable wire ties. Rebar cages must be as sturdy as possible.
The spacing of rebar is crucial, so take the time to do it right!