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Black Rebar vs. ECR or Calcium Nitrite in PCB

Question
State WI
Description Text

MnDOT
needs MwRSF's opinion on the following Portable F-Shape Concrete Barrier
specification (reinforcing bar specification).


We currently specifically the following on our plan (attached):

Reinforcing
steel shall be Grade 60 and shall conform to either of the following:

•Epoxy-coated deformed bars as specified in Spec 3301.

• Spec 3301: deformed and plain billet steel reinforcing bars for use with calcium nitrite
corrosion inhibitor (30% calcium nitrite solution.)

The spec 3301 is a MnDOT specification concerning Reinforcement Bars (attached).

A question has come up from a manufacturer.  They are asking if the epoxy-coated steel reinforcing is required for crash test performance.  They are also asking the same question regarding the alternative of using use of corrosion inhibitor in the concrete

The barrier will not be owned by MnDOT, that is, it will remain the property of the Contractor.  The barrier is required (and inspected) to be in good condition every time it’s placed on a MnDOT project, but it will always be the property of the contractor.  Therefore, the long term durability and barrier condition risk is transferred to the contractor if they choose a less durable construction method for the barrier (provided that it does not risk the barrier safety performance).

A few of our neighboring states just call out the following:

Use Grade
60, ASTM A615 for these bars.

Keywords
  • Temporary Barriers
Other Keywords Epoxy Coated Rebar, Corrosion
Date April 15, 2015
Attachment MnDOT 8337C 2of3.pdf
Attachment MnDOT 3301 Reinforcement Bars.pdf


Response
Response Hello Michael,

I reviewed Minnesota's specs and some additional supporting material regarding corrosion protection for concrete barrier reinforcement.

First, I wanted to highlight a Pooled Fund consulting question answered in 2010 regarding concrete barrier reinforcement specs for the loop bars:

The loop bar steel is the A706 spec because we have found that the small bend diameter can cause reduced ductility and toughness in some grades of steel which compromises the impact strength of the loop. As such, we current specify that the loop steel must have a minimum yield strength of 60ksi, a minimum tensile strength of 80 ksi or 1.25 times the yield strength " whichever is higher, and a minimum % elongation of 14%. A706 and A709 steel can both meet that spec with the correct grade. Others may as well. The bars can be deformed or smooth as long as the steel is within spec. Some of our states prefer smooth, so it is on the drawings that way.

Minnesota's specs are similar for the loop bars with regards to strength. However, I did not see a note for how the loop bars are protected from corrosion. Some states have used galvanization or stainless steel for loop bars. Epoxy-coated loop bars may experience cracking or peeling during installation or impacts, and exposure to the environment may create a galvanic circuit.

Several online resources have proven valuable for evaluating effectiveness of epoxy coated reinforcement (ECR) and generalized corrosion resistant reinforcement (CRR), compared to "black bars" (BB). Those resources are attached.

Based on the available research, I concluded the following:

(1) In the short term, BB has a higher concrete chemical and physical bond than ECR. Thus, use of ECR may decrease reinforcement bond strength slightly. The effect is not particularly pronounced and MwRSF has used both types of reinforcement in concrete barriers with success. The MIT research synthesis indicated  that the recommended factor to account for bond slip in reinforced concrete was 1.35x, and the ACI code recommends a 20-50% longer anchorage length to account for reduced concrete-to-epoxy interface strength. In conclusion, the bond strength of BB is slightly higher than ECR, but in general the effect is not extreme.

(2) Service life of ECR reinforcements have been shown to be vastly increased compared to BB. Although there are still maintenance issues that occur over time,  particularly in harsh environments with frequent freeze-thaw cycles and in environments with large chlorine concentrations such as locations with heavy use of chlorinated de-icers or salts, ECR may increase service life for permanent concrete structures by 50 years or more. Note a case study in the MIT synthesis in which a parking garage in Minneapolis using BB and with actual clear cover of concrete of 1" resulted in extensive repairs within 12 years. Although other ECR structures in Canada, Florida, and Iowa still corroded over time, and the epoxy lost the chemical bond with the steel during exposure and when surface cracks were present, the predicted increase in service life was 40 years when both top and bottom reinforcing mats of the bridge deck were ECR, compared to ECR in the top mat and BB in the bottom mat. Also note that the ECR may be damaged by aggregate when nozzles are used to pour concrete over a form; damage can be reduced and potentially eliminated by keeping the nozzle close to the form.

(3) Benefits of ECR are heavily dependent on the attention paid in construction, as noted by Dr. Hartt at FAU (note Review_Corrosion_Performance_Epoxy_Hartt_2012.pdf). Hartt noted that all CRR structures benefit from high-quality concrete and good clear cover. Reduction in clear cover or excessive water-to-concrete ratios result in a reduced benefit of all CRR. In general, subject to the high quality concrete and clear cover as noted, CRR generally increase service life by approximately 10x with respect to simple BB.

(4) Lastly, calcium nitrite is effective in increasing the threshold of chlorine concentration at which corrosion begins to appear. It does not prevent corrosion if the chlorine concentrations are higher than the threshold. However, the service life over which this increase in concentration builds up without damage to the structure may be significant, particularly in locations such as Minnesota that does not heavily use chlorinated de-icers.

In conclusion, the use of BB in temporary barriers offers a short term increase in reinforcement-to-concrete bond strength compared to ECR, but does not offer any other short or long term benefits. In contrast, CRR does increase service life of the barrier by approximately a factor of 10 if adequate clear cover and good quality concrete is maintained. After an impact, if the concrete cracks, the benefits of the CRR will be significantly reduced. Lastly, if all bars, including loop bars, are BB, it may be appropriate to inspect the loop bars for corrosion and estimate internal damage based on the corrosion of the loop bars. This method may not be perfect as it is impossible to accurately identify which elements corrode faster with the current state of knowledge, but BB loop bar inspection is at least a minimum service inspection that could indicate if additional problems may be present. 

I recommend that any barriers cast with BB only should have a date stamp indicating the casting date. and any barriers found to have corrosion with the same or similar casting date be fully inspected. Both corrosion resistant reinforcement (CRR) and black bars (BB) may be used and will develop the appropriate strength, given consideration for the increase in lap length and anchorage length for ECR.
Date April 15, 2015
Attachment Review_Corrosion_Performance_Epoxy_Hartt_2012.pdf
Attachment Corrosion Resistance of Concrete Reinforcement - MIT 2005.pdf


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