I am working on a project with pretty tight tolerances for the placement of rebar. The rebar ranges from #7 to #9 vertical bars (dowels), so it's fairly thick stuff. Anyway it has to be placed within 1/2" of where it shows on the plan so that it is embedded far enough but not too far from the surface of the wall. The nature of the project has everyone being very particular about the rebar placement.
My thought was to set up control lines via hub & tack outside of the building and then have the guys set up a cross line laser with a laser plummet over directly over the stake and align the laser to the corresponding line stake on the other side of the building. Then they could use a sensor to verify line along the wall. I was picturing something like this:
http://www.engineersupply.com/pacific-laser-systems-pls90-system-pls-60534.aspx
Obviously it cannot be set on line with the rebar because the rebar will be in the way once installed. I'm thinking I'll stake the control line some even number of inches off line. Any thoughts or suggestions?
Batter boards and string lines, then use a plumb-bob to transfer to the ground. You can specify on-line, 6" offset, 1' offset or whatever your needs are.
Sometimes technology is the answer and sometimes it isn't. Would a mason's string line and measuring laths cut to the precise offset do the job?
We tend to wait until the base steel is in and wedged or otherwise fixed against sliding.
Then put a wrap of electrical tape around a top mat bar with a marker pen line showing the (concrete) face of wall. They can then set up a string if they want precise results and measure the cover to the rebar off the string. Again, if it's a precise job I would set marks quite close and tie the string on myself (tight!).
The carpenters can also use the same line if the are hanging a kicker (upstand at the base of the new wall).
I would only ever mark the face of wall, not rebar, not some offset off rebar or face of wall.
Reason - possible confusion.
It's funny, my initial response when asked was to use a string. That's how I've seen it done on pretty much every other job, but they didn't like that answer because it's a pretty long stretch that has to be done at once (about 180'). That's what brought about the laser discussion.
I'm assuming the re-bar is being placed in the footing now and your thinking the bar protruding vertically would control the face or the wall because of clearances.
Contractors are used to using pocket tapes and string lines. You would be hard pressed getting them to utilize any method other than what has worked in the past for them. Usually the plans will show a lay out line. I would suggest sticking to staking that line. Let the contractor figure offsets from the lay out line and where the rebar should be placed. I usually give them the LOL, then stake the corners of the forms once they have them built.
If the forms are right the re-bar will be close to right.
It is the contractor's responsibility to place the re-bar in the correct position.
Your right about the protruding rebar having to be right. That's what the concern is. Maybe I will go back to my original advice of using the string. If it ain't broke don't fix it.
Honestly, I just want to layout the building box and column lines and let them figure everything out off of that.
fix the string on the top mat every couple of metres, they can't go far wrong then.
> Your right about the protruding rebar having to be right. That's what the concern is. Maybe I will go back to my original advice of using the string. If it ain't broke don't fix it.
>
> Honestly, I just want to layout the building box and column lines and let them figure everything out off of that.
Right, I often point out that they do not want the land surveyor getting lost in the detailed and confusing offsets and dimensions. Rather let us stake out the lines per the foundation plans, period. We can do that well without getting or causing unwanted confusion.
> Your right about the protruding rebar having to be right. That's what the concern is. Maybe I will go back to my original advice of using the string. If it ain't broke don't fix it.
>
> Honestly, I just want to layout the building box and column lines and let them figure everything out off of that.
I am probably in a unique situation because I am responsible for the staking AND the form/re-bar inspection, so I kind of see the process from start to finish.
Once the forms are sturdy and in a fixed position, I will usually provide a mark on the top of the form at either end and at intervals along the wall. I stick to the LOL even if it is not convenient for them because of re-bar obstructions. They will simply offset it themselves where it is convenient to stretch a string. The forms can shift with the guys walking around on the re-bar, so I make sure and have permanent marks outside construction that can be used to check the LOL if a dispute arises. I usually set something in concrete. I take lots of photos for evidence of what I set and discuss the stability of the forms. It's up to them how to utilize the reference marks (string, line of sight, lasers, ect.).
Once the footing is poured, I come back in and give them something on top of the footing concrete. Fun stuff! It's really fun when there are shop bent curved pedestrian hand railings that have to fit on top of the wall and the anchor plate/bolts have to fit in between rebar in a narrow wall.
> Right, I often point out that they do not want the land surveyor getting lost in the detailed and confusing offsets and dimensions. Rather let us stake out the lines per the foundation plans, period. We can do that well without getting or causing unwanted confusion.
:good:
I don't build buildings...If they can't build it to specs off half a dozen column lines staked and offset in each direction then the problem is they need smarter contractors 😉
> I don't build buildings...If they can't build it to specs off half a dozen column lines staked and offset in each direction then the problem is they need smarter contractors 😉
That's my take too. Give them major corners or some kind of column lines and let them have at it. I only set one BM for FF too, instead of grading all the points for sub-grade, foundation jogs, and the like.
> fix the string on the top mat every couple of metres, they can't go far wrong then.
Wrap insulation tape around the bar and then mark with a marker the face of concrete as previously stated NEVER rebar
Believe me, I just want to stake it out the normal way, but I work directly for the construction contractor as an employee. They are concerned about getting this job done right, so I offered up the laser suggestion as an alternative to string lines. I must say these guys do excellent work. When I stake something out and come back and check the work they have done I am always impressed by how close it is to what I staked. I know from all the time I spent with surveying/engineering consulting firms that not all contractors are that good.
> ...then the problem is they need smarter contractors 😉
>...but I work directly for the construction contractor as an employee
LOL
