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RE: Pins Supporting Rebar

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Rich,
It is a very common practice. It is a bad practice. That said, I can not site any structural problems that I have ever observed that were caused by the use of pins. The truth be told, I used them myself in my days as a rod buster.

There are many reasons that you don't see problems:
1. They are used in grade supported foundation concrete. It is not easily visable unless you are excavating after years of service. 2. The foundation concrete is generally not as exposed to corrosive elements
3.  It is a point location and not as prone to spalling the concrete
4.  You have a lot of cover to the main flexural steel

If you can get rebar accessories at all, you can get dobies. Dobies are pieces of precast concrete that have wires cast into them. They are placed at the bottom, and sometimes at the sides of grade formed concrete to provide the proper clearance without using pins. I usually have rebar assemblies placed using dobies and standees, and I can stabilize almost any rebar assembly in grade supported reinforced concrete elements. I prefer the adjustable base standees (CRSI #25). They allow for field adjustment.

In my field observation work I will have them use dobies, but I generally don't have them tear out pins that are already placed. If they use pins after I have told them to use dobies, I will have them tear out the pins and replace them with dobies.

Sometimes the use of pins is unavoidable. It is one thing to stand rebar up. It is another to try to keep it stable when you are placing and consolidating concrete. Ever wonder why anchor bolts have a tendancy to wander?

I wish every engineer could have the opportunity to actually build what they design at least once in their career.

Regards,
Harold Sprague





From: "Rich Lewis" <seaint03(--nospam--at)lewisengineering.com>
Reply-To: <seaint(--nospam--at)seaint.org>
To: <seaint(--nospam--at)seaint.org>
Subject: Pins Supporting Rebar
Date: Fri, 19 Aug 2005 09:54:16 -0500

I have a question regarding rebar support in foundation construction. It is
very common in the area I work to drive #3 bars into the ground and tie
rebar up on the pins instead of on chairs or blocks.  This occurs primarily
in grade beams and column footings.  My gut feeling is to reject this.  I
was wondering if there have been any studies done on the affects of this
regarding rebar corrosion.  Is it possible the pins create a path for
moisture to eventually penetrate and corrode the foundation reinforcement,
or am I over reacting to this?  Is this common in other areas of the
country?





Thanks.



Rich




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