IJR,
I have specified them before in cooling towers where there is a continuous exposure to water. Most of the U.S. Departments of Transportation use epoxy coated rebar in bridge decks, bridge abutments and piers, and in reinforcing in highways.
The idea is to coat the rebar so air and moisture cannot begin the process of oxidation (rusting). The Concrete Reinforcing Steel Institute has been a promoter of it but a few years ago one of their engineers put out a report that questioned the total benefit of the coating based on tests he'd studied. There was a big "war" of words at the time over his report.
They provide a level of corrosion resistance and most reinforced concrete structures in these environs, exposed to de-icing salts use them. Also prevents 'staining' of concrete due to corrosion products.
There is some indication that anodic corrosion of the rebar can occur and be accelerated at blemish locations. In any event, it slows the corrosion problem down.
The 'pros' -- as JAE and dik have mentioned, increased resistance to corrosion.
The 'cons' -- The coating may need repair, after placement of bars but before placing concrete or the actual level of protection may be compromised.
-- The study by Concrete Reinforcing Steel Institute mentioned above provoked a lot of discussion but, I do not believe the answers are in.
My own 'con' -- I have had the opportunity to test a significant number of old and new structures, to include highway bridges. Some of the concrete used for bridge decks on the U.S.A. Interstate Highways in the 1960's into the 1980's has undergone very high 'wear', allowed high salt intrusion with rebar corrosion and experienced, in my mind, excessive surface delamination. However,2 bridges constructed about 1935 still had servicable bridge decks but, had to be replaced as the structures were too narrow.
The main difference is the older concrete was placed in a manner which created and maintained a less permeable concrete mass, protecting the rebar. The salt intrusion was very small.
I am not saying the old is better than the new but, the newer concretes and additives have created very 'strong' concrete but techniques for producing 'durable' concrete have been neglected. The epoxy coated rebar tends to focus on maintaining the epoxy coating and the old methods are neglected. I believe a combination of the older methods, combined with new and usually better constituants, would serve us better.
Have done a lot of work with parking garages (20 or so) and if they're like bridges, there are some additional reasons for corrosion:
Designs tend to be tighter, causing a reduction in reinforcing as well as bar sizes. The economics has also had an impact on concrete cover which has a direct bearing on corrosion resistance.
Because labour costs are more intensive, there is likely more salt being used today.
I've had good results with epoxy coating, but there hasn't been enough elapsed time to really tell, but I use it in conjunction with increased cover and higher strength concrete just like I used to with normal rebar and I haven't experienced any problems with those either.
Any demolition work that I've seen with where epoxy coated material has been used, shows it holds up well, and occasionally an area where bars have pitted are found.
IJR..
As emmgjld stated, there are a few "cons", the most imposing of which is damage to the coating that is not detected at the time of construction. This leads to concentrated corrosion at the breach in the coating, which can result in localized failure of the bar. This is a particular problem if the rebar is bent on site as the coating often breaks at the bends.
The performance of well-coated, carefully placed epoxy coated bars has been good. Equally, there are a few horror stories.
I am still of the mindset that rebar corrosion is best controlled by quality, dense, concrete; appropriate cover; and a recognition of the corrosion potential so that appropriate compensation can occur if necessary. I am much more in favor of allowing generalized corrosion to occur at slow rates than risking the potential of concentration of the corrosion in critical areas in a short period of time.
And from a design point, the epoxy coating has a reduced bond strength and can require greater bending radii for hooks, etc. which may cause problems with detailing.
Yes, the diminished bond is a problem. Furthermore the coating creeps at high temperatures attained by bridge decks, and some states in hot areas have forbidden altogether its use.
ITS BEEN AN IN THING TO USE EPOXY COATED REBARS IN MOST OF THE MIDDLE EAST COUNTRIES, SPECIALLY FOR PILES ON WHICH MOST OF THE BUILDING STAND HERE IN THE UAE AND OTHER GCC COUNTRIES, AS THE WATER TABLE IS JUST 1 -2 METRES DOWN. AND MOVEROVER DUE TO THE SULTRY AND HUMID CLIMATIC CONDITIONS.
Just to add:
1. Epoxy coated bars are often used in marine structures. Totally epoxy coated reinforcement was used on some marine structures in Hong Kong;
2. Epoxy coating gives the Design Engineer the get out as "something is seen to be done about corrosion, otherwise checking Engineers always pick up on the issue". Epoxy coated bars are also considered to be useful in sections where sufficient steel-cover cannot be provided.
3. Stainless steel rebar is becoming more economical nowadays, and epoxy coated rebar is being replaced by stainless steel reinforcement. One must note that we can not mix ordinary steel rebar with stainless steel due to the bi-metallic action.
Is stainless steel rebar really feasible? Where do you use it? in bridges?. Why did someone think of using stainless rebars from the beginning?. You guess what! I am simply curious
This thread started with a question about epoxy coating and then went to stainless
steel. Most of the corrosion in concrete literature in the United States is related to Federally funded efforts to stop bridge deck corrosion. In the 1970's the "answer" was to use galvanized steel, primarily because it seemed to have worked in Bermuda. The research on this was very questionable and it didn't prove a good idea--mainly because if concrete is "good" no corrosion occurs and if it's "bad" it will leach calcium minerals which have a high pH (are basic or caustic) and are very corrosive to zilnc and galvanized steel.
The epoxy coating people wanted the bridge deck market, so they produced a product that would match the cost of galvanized steel. Unfortunately that meant that they had to cut the quality of the epoxy coating--using a very thin layer, not inspecting, etc.--something they would never do on e.g. a pipeline. Lots of lousy epoxy coatings were placed, usually in the top layer of rebar on bridge decks, but also elsewhere. It didn't work, except where the concrete was "good" and would have prevented corrosion of carbon steel rebar with no coating.
At about the same time (1970's--early 1980's) a British company placed stainless clad rebar on the Cadillac Bridge in Detroit. It worked, but the "buy American" people pushed for expoxy coating. Now that the limitations of the epoxy coatings applied to rebar are being shown, Uncle Sam (and several state highway departments) are supporting research and demonstration projects on stainless steel rebar. It will work, but it isn't cheap.
Once again, good concrete (proper low water cement ratio, properly compacted, adequate depth of cover, etc.) will protect steel from corrosion. There are hundreds of thousands of successful applications. The contruction industry problem is that it's hard to insure quality control on a construction site. Stainless is one technically feasilbe way to overcome this lack of quality control.
SuqlainUK: Under what conditions can't you mix stainless and black rebar? We're looking at a bridge job that has black rebar in the substructure and stainless rebar in the deck. The dowels from the substructure into the deck are specified as stainless but they will undoubtedly be tied to black rebar.
I work in a Chemical factory and would like to share with you my experience vis a vis epoxy coated rbars.
Our old buildings did not use epoxy coated rbars and I noticed severe corrosion of such bars leading to failure of the concrete structure. Eversince we have started using epoxy coated rbars there is no such failure even after about 10 years. All I can say is rbars are strongly recomended for longer life of structures eventhough they may add a little to your initial cost
What is the design bond strength to be followed for epoxy coated rebars since all codes give bonstrength between rebar and concrete.Is there any code or standard available for this?
ACI 318 addresses the bond issue in their development lengths:
epoxy coated rebar w/ cover less than 3bar diameters or clear space less than 6 bar diameters: multiply by 1.5
all other epoxy coated rebar: multiply by 1.2
standard hooks, epoxy coated: mulitply by 1.2
All the details are in ACI 318 12.2.4 through 12.5
