huangyhg

reinforcing steel cover
i am a project manager on a waterwater treatment project and the inspector (construction manager in this case) and myself have had some issues over rebar clearence. he thinks more clearance is better.
e.g. on mat slabs, the inspector instructed us to shorten the rebar standees height to create more top and bottom clearence. his comment more clearence is better (i.e. corrosion concerns). i informed him he better check with the structural designer since this affects the streight of the slab.
after convincing him he was wrong he started enforcing crsi tolerance of +/- 1/2". i accepted this tolerance for all area except the bottom mat cast against earth.
we requested a bottom mat of clearence of +1/2" to -1 1/2". i aurgued the grade is irregular and that is why you start with a 3" clearance so that you can get a minimum of 1 1/2" clearance. i asked them to run this by the designer.
he will not run my request by the designer since he has a code (crsi) that says +/- 1/2". he is a by the book inspector with no common sense.
for you information the reason we are having trouble getting a level plan of 3" clearance is the following:
the rodbuster foot traffic slightly shifts the gravel subbase that was level and tested prior to instalation of reinforcing steel.
the rod buster burys a bar (every 4th bar) so you natually get less clearance under those bars. at one point, the inspector has suggested we should hand excavate a trough under the bury bars.
the rebar supports (we are using 8" x 8" bricks) are actually only 2 3/8" high and do sink in the gravel a little (1/2" to 3/4"), but the 2 3/8" works well since the rodbuster is burying a #6 bar. this get the majority of the bars at the right plane (3" above grade).
so my question is: should the crsi tolerance of +/-1/2" for rebar clearance apply to the bottom mat cast against earth or does the design standard of 3" allow for a greater variance.
the requirement you must meet (it appears you are in the us) is aci 318 "building code requirements for reinforced concrete". this code states the cover as a minimum, thus there is no undertolerance as you have referenced for crsi (which, by the way, is not a code).
the reason for the large cover when concrete is cast against earth is not for the undulations and irregular surface, but moreso for the variations in moisture content in soil and the ability for moisture to more easily transcend the interface between the soil and the concrete cast against it, since no formed (thus more dense) surface exists in the concrete.
i have a real problem with inspectors making changes to a reinforced concrete structure's rebar placement. this can be catastrophic if done in the wrong places (column shear heads, reverse moment areas etc.). the structural engineer of record should be informed immediately of this practice so that he can put a stop to it.
thanks ron
the 3" minimum has been discussed, but to which bar. the rod buster is burying a bar to facilitate placement.
but only having + tolerances the design "d" is reduced. so i would think it would be better to sacrifice a little on the cover. this situation is a mat slab on grade.
the reason for the cover is corrosion protection on the rebar. the code (aci 318) states that the cover is measured from the outermost point of the reinforcing steel or ties or stirrups as applicable to achieve the minimum required cover.
only the structural engineer of record should decide variations since he knows the analytical parameters he used and others can only guess (such as the "d" value used).

__________________
借用达朗贝尔的名言：前进吧，你会得到信心!
[url="http://www.dimcax.com"]几何尺寸与公差标准[/url]