Welding of Rebar 8

[LPPE]

Can anyone tell me why welding rebar results in a brittle weld? What does AWS D1.4 say about welding rebar?

 

[CWIC]

The alloying elements present in rebar (primarily carbon) makes the welding operation somewhat more critical than that of plate/shape steel products. The relatively high amount of carbon requires preheat and interpass temperatures higher than that of "mild steels".

Preheat/interpass temps. and electrode selection are typically determined by the carbon equivalent value determined from the chemical composition of the material noted in the mill test report (MTR).

Factoids:
1.) There are no prequalified joints or welds in this code. There are code approved weld and joint details. All weldments and personnel require qualification by testing.
2.) Workmanship and technique are critical to welding of reinforcing steel.
3.) Welding of intersecting bars is almost always prohibited.
4.) Bend radii of these bars is also critical to the materials performance.
5.) Essential variables are more restrictive than those of the D1.1 code.
6.) The D1.4 code does not encompass impact testing of welds.

Others can provide more insight.

 

[IFRs]

I believe that there is special rebar designed for welding, or at least I remember reading about it somewhere. Possibly in England?

 

[AJUK]

It is common practice to weld rebar in the UK. It is particularly useful when producing beam cages for ground beams. The cages are delivered pre-formed to site and the only manual fixing required is for the splices.

However, the cages only require spot welds to hold them together and I have never heard of any problems. If you need to join long runs of bar, it is always best to use a mechanical splice.

Regards
AJUK

 

[dik]

CWIC is essentially correct. There is weldable rebar, and if rebar is spec'd to be weldable, then preheat/interpass temps should be spec'c.

Tack welding of cages can be a real problem. A crack can initiate at one of the tack welds.

Generally the steel used for rebar can be a mishmash of Volkswagons or Amtrack rail...

 

[Paperpete]

I saw a building site in Europe where a man walked around with a clamp (spot) welder and used this in place of metal wire. He was moving right along so it was probably more a question of time. I believe that the reinforcement mesh is made this way (spotwelded). I would be concerned to depend on a weld in the rebar to absorb any significant loads. The steel is really low grade (recycled metal cans and cars) However spotwelds to hold it together and make the installation easier may not be a problem. How strong is a metal wire? The only potential problem could be if you weaken or embriddle the rod while welding but if all you are doing is spotwelding then I don't think that this is a problem.

 

[CWIC]

ASTM A706 is considered "weldable" rebar. There are many types of reinforcing steel that are also weldable, but the 706 has a distinct marking of "W" (weldable) stamped into the deformations for identification. Or a "W" and "S" denoting compliance with multi-grade bars (A706/A615). Many think this is the only type that is weldable and will not spec anything else.

The mesh used for reinforcement is spot welded. But, depending where it is being used may require it to be inspected regularly during manufacturing by the building departments (via ICBO ES or equivelant watch-dog organization).

There is a rebar mill here in So Cal that manufactures rebar from old cars (as noted by dik), and lots of confiscated firearms.

I have samples of rebar with cracks in the tack welds and plain old cracked welds. All were taken from jobs where the welder(s) said they knew how to weld rebar. Have also been involved in an investigation regarding faulty rebar welding.

While in Taiwan a few years ago I watched a tack welder run around tacking #57 bars without any preheat. He was trying to stay ahead of the pump pouring grade beams for a high rise. We walked down to the site and noticed about 1/2 of the tack welds had cracked and/or completly broke in two. He did give it his best effort though... I took numerous photos after the big one there a couple of years ago where rebar had failed cleanly through the welds. This was noted all about the structures where the epicenter of the quake occurred.

Rebar is not handled and welded in many countries the way we do right here in the US.

 

[AJUK]

The grade of rebar is controlled in the UK otherwise how could you design consistently with rebar that is variable grade? However, Paperpete is correct that the spot welds are used to keep the rebar in position until the concrete is placed, after that the welds are irrelevant.

 

[LPPE]

Thanks all for the input. My questions about welding rebar comes from this situation-
A precast double T parking garage has flange clip connections every 8' o.c. max. These connections are all coming "unzipped". The connection is 2 lengths of rebar embedded into the concrete T flange, then a vert. stainless steel plate is welded to the ends of the embedded rebars. The T's are erected in the field, then the stainless plates are connected by welding a piece of rebar inbetween them. There have been fractures of the welds from the embedded rebar to the stainless plates, and the welds from the plate-to-plate rebar connection.
My first thought was "No wonder they're coming unzipped. A) Never weld rebar, and B) Welding dissimlar materials is never a good idea, especially when rebar is one of the materials".
Am I on the right track with thinking that was a terrible idea from the start, or is that generally industy practice?

 

[dik]

If the rebar is weldable, then with care, there shouldn't be a problem.

It's common to weld clip plates to rebar for precast. The PCI manual used to show this (I don't have a copy here to confirm that this is current). My preference for connectors is using 'headed studs'.

With a proper procedure it is possible to weld many dissimilar materials, including stainless, cast, etc.

You may want to check the rebar (might be metallurgical testing) to determine if it is weldable. It appears that the weld itself is failing and the problem may be with the procedure used. Find out from the erector what method was used for welding and qualifications of welder. Also if you can post a note on what electrodes or type of welding was used. If inert gas and exposed to outside breeze, etc. then may be problems with the welding equipment.

 

[CWIC]

pylco:
You have shed new light on this subject. An assumption was made on my part that your application was rebar to rebar.

Rebar to SS is another situation alltogether. As noted by dik, the welding process, electrodes/filler metals, shielding and workmanship/technique are paramount. It is feasible, but much is dependant on the tradesman. The metallurgical considerations increase with joining a relatively high C steel to a low C/high CR/Ni material.

Dissimilar metals are commonly joined. This may not be the case with regards to structural-field welding. I assume the fabricator/erector submitted a procedure (WPS) and test results (PQR) prior to the field/shop welding? It really sounds like it may be a procedural issue to me. (Or lack thereof.)

AJUK:
Here in the US we have an ongoing trend of permitting multi-designation of materials. The other day I saw a piece of A36/A572 Gr.50/A992/A709 material (there were a few more specs., but this is what I remember). I asked the fabricator to identify this material in accordance with ASTM A6. He asked: "Which color?" I responded with "...whatever specification it is intended to be used for..."
[The shotgun method of compliance? Micro-alloying? Availability? Cost?]

 

[wsiladi]

pylco:

Perhaps your concern does not arise solely from a metallurgical issue. There is another source of concern on my behalf that I would look at and it is "why are the welds unzipping."

The use of double tee flange connectors is required to develop a diaphragm in an untopped (or pretopped) parking garage and this type of connection is commonplace. There should be two types: flange to flange connectors to transfer shear and chord connectors at the extreme ends of the tees to develop the chord forces through the deck and emulate monolithic diaphragm behavior. The latter requires full length reinforcement from chord to chord to emulate a rigid diaphragm and not a series of stacked plates.

It has been my personal experience to see the incorrect application of engineering principles to the design of these structures. By far the main concerns are seismic load transfer and unintended thermal restraint. Sounds like you may have experienced the latter.

Since the description of the condition is "unzipping" it sounds as if you have developed stiffness on both sides of the tee to tee detail that may be separating the joint. The other mode of failure that I have seen is a poorly detailed flat or round bar as the connecting mechanism between the two connections. This fails as a result of wheel impact load due to the traveling wheel load from stiff section (double tee web) to more flexible section (flange to flange).

My first approach would be to look at unintended stiffness since thermal restraint generates ridiculously high forces and connection failure is a method to release the unintended restraint. Good luck!

 

[LPPE]

Another engineer was contracted initially to fix the problem. I just got on board, and my scope of work on this project is to make sure the repairs work and are installed correctly. But in my own field observations and reading the other engineers report, I was disheartened by the apparent lack of judgement by the precaster and the overall lack of quality control when the project was being constructed.
The rebar was called out as ASTM A615M, which I understand is not easily welded, especially in the field. The field electrodes were called as E308-L-16.

 

[dik]

I struck out... I know nothing about E308-L-16 electrodes or procedures that use them, except that the electrodes are used for stainless... I don't know what Chromium or Nickel contents are stipulated; for some of the exotic (and I'm not sure these fall into that category) welding materials, the percentages of Cr and Ni in the base material have to be balanced (in some fashion) with the electrode. Welding procedure and as wsiladi pointed out, operator skill would be critical. I've posted a querie to my programmer buddy... he's a tekkie welder, too regarding the electodes and procedures...

 

[CWIC]

dik:
The E308L-16 is an all-position stainless steel SMAW, low carbon (AC/DCRP) electrode. Because it is specified as a low carbon, there is a C maximum content requirement.
Typical chemistry:
Cr = 18-21%
Ni = 9-12%
C = 0.04 max.

Pylco has not stated what type, grade or specification for the SS used in the structure. So, for the sake of not making any more assumptions, more details would help.

Wsiladi:
I try to maintain faith that the designer used better judgment than the fabricator/erector did. This is typically the situation although it very well could be a design issue, welding issue, or a combination of both. The odds are not the the welders' favour. I still would like to see the WPS/PQR/WPQR. This would provide much needed insight to the production welding.
1. What was the preheat? (critical to rebar and certain ss)
2. Welding parameters? (compatible with both materials joined)
3. Electrode diameter vs position? (3/16" and larger are not all position)
4. Was there Special Inspection? (we know what happens if you don't keep an eye on em') Note: Approved fabricators do not always provide approved weldments.

pylco:
This is getting interesting.

 

[dik]

Just heard from my programmer buddy... In response to my question on the weld rod, and without advising him of the problems that pylco had encountered, he had the following information to add (hoping I got it correct):

The E means that it's a stick electrode and the 308 is the alloy. It is meant for welding of 300 series stainless to stainless, typically 303, or 304. It is not to be used for welding 300 series to ferrous metals. For this, E309 electrodes should be used.

The L denotes low carbon; this causes fewer defects in the weld due to the reduction in carbon content.

With E308 electrodes, the weld is dependent on the skill of the operator with a heavy emphasis on the manufacturer of the electrode; there is a significant difference in the ease of weldability for the various manufacturers of rod. Most operators have a particular preference for the manufacturer.

He said that cleaning of the welds should only be done with a stainless steel wire brush to minimize rusting of the surface. Also, the surface can be passivated using a chemical cleaner.

He mentioned that E312 and E321 electrodes could be used and that the higher the number, the more Cr and Ni present as well as other alloys. As the number increases, there is a reduction in hot cracking. E321's are good for this and even have Titanium as one of the alloying agents.

TIG and MIG welding are a better method, TIG being less porous and slower and MIG having a problem for achieving high strengths; there are weakness of the weld joint due to 'cold starts'. Both methods can be set up to minimize problems. They often use TIG for the more critical root pass and then use stick for filler.

Of real interest (he had no information on pylco's problem) he stated that because the weld material was not for ferrous metal, that there was a good likelihood that the weld would crack.

 

[LPPE]

Wsiladi - It appears expansion/contraction has contributed greatly. The top deck is experiencing the majority of the weld fractures.

The report I have mentions nothing of the type of stainless plate used, except a 1/4" plate. A chemical analysis and grain size was performed, though. They indicated nothing that would adversely affect the weldability provided the correct stainless electrode was used. I can list the chemical analysis % if desired. I also have no information on the welding procedures to be followed, or the inspection program. I'm guessing both were poor, at best.

Overall, it appears that the cause of the fractures are related to the quality of the welds performed, the procedure followed and the base metal used.

Since day one of my engineering tutelage, I was told "Never weld rebar" (unless absolutely neccessary, and then with extreme care). This situation seems to be a text book example of why welding rebar is a bad idea. Welding it correctly probably would have increased the cost of labor 200%.

Why not use A36 rods, or all stainless rods/welds?? That was my initial thought if I was originally asked to design this (which I wasnt! So I guess it doesnt really matter what I think...). Was my initial thought incorrect, or was I taught bogus information?

 

[dik]

Pylko:

You may want to check to see if E308 welding material is appropriate for stainless to regular steel.

 

[CWIC]

pylco:
I would not reject the idea of rebar welding as it is commonly performed (at least here in So Cal) daily with satisfactory results. The "savings" of eliminating welding of rebar has cost sevaral contractors I have dealt with turned out to be an expensive undertaking. Had these bars been welded to begin with, the couplers (and failure of couplers not properly installed) was just as much a factor as poor welding-workmanship.

Reinforcement of grade beams often requires welding to existing column flanges. Mechanical couplers and flash welding (of bar-butt joints) are also routinely performed and tested mechanically to verify weld integrity. I respectfully disagree that welding of reinforcing steel is not recommended.

I would insist on the required documents (WPS/PQR/WPQR's) for the project. (Sorry for repeating myself.) But, these are code-required records which should have been submitted for approval prior to the start of work. A friend of mine is at a fabricator who performed faulty welds on a large truss, and now the fabricator cannot get the welders to pass performance or the procedure testing which were required from the start of the project! What a mess.

dik:
I would not recommend the use of GTAW (MIG) or GMAW (MIG) as these both require gas shielding. Not the first choice for field welding which is why FCAW-S and SMAW are the processes of choice for the field. Weld initiation/termination discontinuities are inherent to most arc welding processes, this is why workmanship/technique are paramount (and detailed in the WPS).

I respectfully disagree that these welds are suceptable to cracking. Particularly if a qualified WPS and PQR were implemented. I have been on projects where rebar to SS has been performed successfully using SMAW. The usual qualification process was performed without issue other than some of the welders could not pass due to the discontinuities you mentioned at weld terminations (in mid-weld locations).

Several architectural stuctures I have worked on were fabricted entirely from SS. Tying in the structure to the reinforcement was required. This was done by joining the rebar to the ss at various locations. RT of 10% of field joints were specified to verify the welders' activities (even after the WPS/PQR were qualified.)

 

[CWIC]

P.S. dik:
I have test samples sitting here on my display that have been welded sucessfully with E308L-16 electrodes. The materials joined are 316L ss to A500 Gr.B and 316L ss to A572 Gr.50. The tests included bends, tensiles and macroetching (for the PJP welds). We also did a full scale mock-up tensile of one joint. It is possible.

We qualified procedures for several reasons:
1.) Dissimilar materials joined - falling out the the scope of the SS and rebar codes.
2.) All the welding suppliers and sales people told my clients (the RDP) it could NOT be done unless an E309L electrode was used.

One electrode rep. mentioned "...we got lucky..." Of course we did not use his brand of electrodes for procedure qualification.