Reinforcement in Precast - The Basics

My blog this week will focus on the reinforcement used in precast concrete units.  Before I begin, let me make it clear that I am not a licensed professional engineer, and my comments come from a combination of experience, education, and 15 years of practical experience.  (Also, I recently slept at a Holiday Inn Express!)  My past experience includes working as a designer where we were required to make some simple calculations in house as part of the production drawing process.

Let’s start out with the basic question of why we reinforce concrete.  Concrete is a very strong composite material with compressive strengths of usually 5,000 psi or more.  And while concrete can handle such great forces in compression, it is weak in tension, roughly 1/10 of the compression.  Tension is the force of being pulled apart.  A simple concrete beam supported on each end with a force applied downward in the middle will create a natural tendency to “bend” the concrete.  The middle of concrete is the neutral zone.  Above the neutral zone the concrete is in compression, and below the neutral zone the concrete is in tension.

If the concrete was not reinforced, then the concrete would crack in the middle and collapse.  

If the reinforcing steel is placed in the concrete, but it is located in the upper half called the compression zone, it will crack severely, but the reinforcing steel will likely prevent a collapse.  

On the other hand, if the reinforcement is located in the bottom half of the concrete, also called the tension zone, the concrete will bend and until the steel is engaged by the tension forces.  Minor cracking may occur, but nothing serious.  And most importantly, the reinforced concrete section will be ready to continue to perform its designed function.

The location of the steel reinforcement is critical to its function.  Also, the concrete must bond with the steel to transfer the forces.  There are a couple of important facts to consider in placing the steel.  The steel can be affected by the environment and the alkalinity of the concrete can reduce this, but only if there is a sufficient amount of concrete covering the steel.  There is also a minimum amount of concrete covering required for the steel to effectively receive the tension forces.  The second important consideration is the distance that the center of the steel is from the force or compressive load.  This distance is used in calculations to determine the amount of steel required, or the amount of load permissible.

 

Do you see the steel bars?

I know some manufacturers who still “hand place” steel bars in the concrete product after the concrete is poured.  This practice is concerning for several reasons.  Can you be sure of the location and exact placement of the steel?  Does the steel have enough concrete cover to prevent corrosion leading to spalling of the concrete and further degradation?  And did the act of placing the steel create voids within the concrete that can weaken the structure?  In the case of a parking block, just drive around and look at these products in use.  I see a lot of them that were made with “hand placed” steel that look terrible in just a few years.

This is just a quick blog on the importance of the location of reinforcing steel in precast concrete units.  Proper placement will affect the overall quality and longevity of your products.  Your reputation and the reputation of the precast industry will be judged by the perception of quality seen.