What are the disadvantages of epoxy coated rebar?

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10 Oct,

Cellular concrete consists of a mixture of water, cement, and pre-formed foam. Contractors and manufacturers sometimes refer to cellular concrete as foamed concrete. Because cellular concrete forgoes stone aggregate, and because the foam introduces a higher ratio of air, it has a much lower density than traditional concrete.

Cellular concrete possesses numerous benefits when it comes to certain types of building and restoration projects. Unfortunately, many people still fail to understand just what makes cellular concrete so special. This article seeks to increase your knowledge of the contemporary concrete world by outlining four key advantages of cellular concrete.

1. Lighter Weight

The defining feature of cellular concrete is its relatively high air content. The foam leaves behind evenly distributed cells of air in the hardened concrete, and the air may account for up to 80 percen t  of cellular concrete's volume. That said, a contractor can carefully adjust the precise ratio of foam introduced to control the amount of air and end up with a desired density.

As you can imagine, the more air cellular concrete contains, the lighter it will be. Generally speaking, however, virtually all cellular concrete weighs less than its traditional equivalent. This characteristic gives it a distinct advantage when it comes to certain types of construction or repair tasks.

Cellular concrete allows builders to stay within strict weight management limits for certain types of buildings. Walls and floors constructed from cellular concrete place a much smaller load on a building's foundation, often allowing builders to construct taller structures. The light weight means it can even be used as a roofing material.

2. Better Thermal Insulation

The high air content of cellular concrete also produces a drastic effect in terms of thermal insulation. Just as with density, insulating power relates directly to the proportion of air in the concrete &#; more air increases the insulating power. As a result, structures built using cellular concrete tend to exhibit much better energy efficiency.

Manufacturers test a building material's insulating power according to R-value Higher R-values indicate a greater degree of insulation. One inch standard concrete, which has a density of 150 pounds per cubic foot, has an R-value of just 0.07. Cellular concrete, at the extreme end of the spectrum, may have R-values as high as 2.0 per inch .

3. Improved Fire Resistance

All concrete possesses a natural degree of fire resistance. Yet not all concrete can withstand the same temperatures or for equal lengths of time. The same insulating properties that make cellular concrete such a great choice for energy efficiency also give it an improved ability to resist heat transfer, thus making it harder for fire to spread throughout a building.

Standard concrete that has been reinforced with rebar or other internal steel elements has a greater susceptibility to fire. As the heat from especially intense fires passes into the concrete, the metal rebar expands, creating internal stress. As a result, the concrete often succumbs to spalling damage.

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To combat this weakness, concrete manufacturers often add monofilament polypropylene fibers to the mix. While effective, this strategy can drive up the cost of the concrete. The excellent insulating properties of cellular concrete offers an alternative way to resist spalling. Cellular concrete is also less likely to explode as the result of exposure to a high energy flame source.

4. More Cost-Effective

Most cellular concrete contains an ingredient known as fly ash, which promotes a greater degree of internal strength. Fly ash replaces some of the cement that would otherwise be used in the mix. This replacement tends to bring down the overall cost of the concrete, since fly ash is an industrial waste product that can be acquired for a fraction of the cost of cement.

Cellular concrete has become an increasingly popular building choice, thanks to its wealth of undeniable benefits. For more information about whether cellular concrete would make a good choice for your next construction project, please contact the pros  at Jimenez Concrete Inc.

This particular subject appears to be a little bit of a controversial one. There are many opinions regarding the the longevity and corrosion protection of epoxy coated rebar (ECR), but the purpose of this article is to simply discuss some findings and allow for people to just talk and have dialogue about it. Over the years (whether in the repair or new construction), I've seen quite a bit of ECR installed (within core and shell jobs as well as infrastructure repair work) in the hopes that it will prevent salts, moisture & carbon to hit the surfaces of the rebar and start the corrosion process. And in most cases, that appears to be the case. Here are a couple abstract links from ACI (American Concrete Institute) showing their long term findings and prove that:

* 35-Year Field Performance of Epoxy-Coated Reinforcing Bars (concrete.org) Snapshot - "The corrosion state of bar samples removed in cores was examined as well as bond between the epoxy coating and steel and coating thickness. Chloride profiles from concrete cores were also obtained. Overall, the epoxy-coated reinforcing bars were in excellent condition, with very little active corrosion after up to 35 years of service."

* Assessment of Epoxy Coating on Bridge Deck Reinforcement (concrete.org) Snapshot - "Although the percentage of specimens exhibiting corrosion and cracking of the concrete cover was significantly lower for specimens containing epoxy-coated reinforcement, the results indicated that there was significant variability in the corrosion protection provided by the epoxy coating. Factors influencing the time to cracking of specimens containing epoxy-coated bars are discussed."

There is some research being done by a few DOT departments in the US and Canada that continue to challenge and explore the protection characteristics that ECR will provide over long periods of time. Here's an interesting investigation article:

* Field investigation of the corrosion protection performance of bridge decks and piles constructed with epoxy-coated reinforcing steel in Virginia. (bts.gov) Snapshot - "Abstract Virginia Department of Transportation E. Broad Street Richmond, VA The corrosion protection performance of epoxy-coated reinforcing steel (ECR) was assessed in three bridge decks and the piles in three marine structures in Virginia in . The decks were 17 years old, two of the marine structures were 8 years old, and the other marine structure was 7 years old at the time of the investigation. The deck investigations included visually surveying surface cracks in the right traffic lane and drilling 12 cores randomly located in the lowest 12th percentile cover depth. The pile investigations included removing 1 core at an elevation between high and low tides from each of 30 piles. The evaluation of the concrete in each core included visually inspecting and measuring moisture content, absorption, percent saturation, carbonation depth, and effective chloride diffusion constant. The evaluation of the ECR from each core included visually inspecting and measuring physical damage, coating thickness, adhesion loss and corrosion at damaged sites, and undercoating corrosion at adhesion test sites. The chloride content of the concrete and the carbonation of the ECR trace were also determined for each core. In the majority of bars examined, the epoxy coating has debonded or is debonding from the reinforcing bar. This occurs without the presence of chloride, and its rate is related to concrete moisture conditions, temperature, coating defects, and other bar and coating properties. Based on the results of this field study, epoxy coatings can be expected to debond from reinforcing steel in Virginia's marine environments in about 6 years and from bridge decks in about 15 years. The authors recommend that additional bridge decks be evaluated to confirm these results"

I believe that there are a few variables at play here.....such as the thickness and quality of the epoxy applied to the surface of the bar and the imperfections (holidays) on the surface of the rebar caused during fabrication, delivery, storage, handling, installation, age, etc.

What type of rebar you use is ultimately up to your Engineer and how they feel about their design criteria and anticipated performance. I personally have always been a fan of black or galvanized rebar due to cost, availability, bond to concrete, etc. And I feel that there are so many technologies out there that can enhance the performance of your reinforcement starting with good quality concrete, proper coverage, coating systems, etc. It's not to say that epoxy coated rebar is a bad option but there are options. Epoxy coated rebar is a viable option and I think overall, it will provide you with significant protection from corrosion based on much history and testing. But knowing that the quality of the protection is based on the condition/quality of the epoxy coating on the bar itself is very important.

Another option to consider can be FRP bars (carbon fiber). FRP bars are expensive and their structural characteristics differ from a standard ductile iron bar. The major benefit is the elimination of corrosion all together by removing the iron component. But again, the unique performance characteristics of FRP bar and price are things to consider. Here's a good video by Tyler Ley (TylerLey.com - About Me) regarding this very topic :

I find this topic to be very interesting and no one seems to have the silver bullet answer to it but talk to your Professional Engineers, specialty contractors and ask tons of questions. Educate yourself and make the best possible decision for your building. Most of the time, the need, cost, design criteria, environment and availability will dictate what you use.

This habit alone will allow you to add many years to the life of your building & structure. Share and educate. Restore and protect.

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