huangyhg

concrete corrosion in partial saturated condition
hi,
appreciate if anyone has knowledge or where information on concrete corrosion in partial saturated condition. see, i have a retaining wall subjected to sea tides. when the tide is low, the outside wall will be dried. but it is protected with 2 layers of epoxy coatings. unfortunately, the inside wall is without any coating protection. the inside face of the wall will likely be damp as the tide water will take some time to drain off completely. by the time, the next tide will come in. we know wetting and drying will accelerate the corrosion of the concrete surface. but what about dampness or moisture environment? thanks for yout contribution
you should probably post it on marine/ocean engineering. install anode i guess? thats just a wild guess. usually they use it for steel piles.
if the inside face has ready access to oxygen (probable, if drying expected), you have the worst possible option regarding reinforcement corrosion (wetting, drying, chlorides). it would be better if the wall remained soaked, oxygen starvation would minimise the corrosion rate. active or passive cp would reduce problems.
does concrete itself actually corrode?
actually i am more concerned with the protective layer ie the concrete. once the concrete spalls, the next easy meat is the reinforcement.
the wall, i am mentioning is a foreshore beach retaining wall. the front surface can be coated but unfortunately the back (ie soil side) is not. there are weepholes to allow water to seep out and at the same time, seawater to seep in during high tide. when the seawater seeps out during low tide, the face of the wall will experience wetting and drying and hence will the concrete protection spalls and then exposes the reinforcement?
ozziz....concrete doesn't corrode. further, it doesn't significantly deteriorate under wetting/drying cycles, with the exception of increased carbonation and enhanced cracking.
having said that, both of those can contribute to a greater tendency for the reinforcement to corrode. exfoliation of the reinforcement is what causes most spalling issues (unless freeze-thaw is an issue). protect the rebar and your protect the concrete. protect the concrete and you generally protect the rebar.
in most marine applications, the better performance from concrete comes from the placement of dense, well-consolidated concrete and from increasing the cover on the rebar for extra protection. a significant study done in the 1970's by a several groups in germany concluded that the two most significant parameters for concrete performance in a marine environment were concrete quality and rebar cover. increase both and you will enhance the life of the concrete.
at the risk of wading into dangerous waters...salt water has no chemical effect on the integrity of the paste; you can cast concrete utilizing sea water as your mix water, disregarding corrosion of any embedded metal. in the absence of effective air entrainment however, eventual scaling and progressive material loss in the concrete will occur due to the progressive increase in salt crystalline formation within the matrix of the concrete inducing a form of physical attack, not unlike freeze-thaw damage. the hygroscopic property of the salt will continue to aggravate salt crystal build-up within the pore structure, leading to eventual scaling.
we generally do not rely on coatings to compensate for poor design. we rely on high performance materials, such as silica fume modified concrete. epoxy coated steel would be a minimum level of protection; consider cathodic protection, stainless steel bars or even frp. extending costs over 50 years, your capital cost becomes more attractive when maintenance and repairs are incorporated into long-term life cycle costs.
cheers!
pinehurst,
we intend to epoxy coat the outside face of the concrete wall and not the reinforcement. they always do it later in life as part of the maintenance. but it is the inside face of the wall in contact with the soil.we have drainage holes through the wall to relieve the hydrostatic pressure.will the inside face of the wall scale at the highest monthly tide level? for a day in a month, it will be wet at that level and then the level of the tide will drop and will rise again to thhe level the next month. the important question is the soil moisture content helps to prevent the crystallisation of salt in the concrete matrix?
the recommendations made above to use higher quality, low permeable concrete while providing adequate cover are the two most important provisions to enhancing the durability of a reinfoced concrete structure, especially for this exposure.
i question the use of an epoxy coating on one face of the wall for several reasons: it is a backward means to reduce moisture and chloride ingress, when the primary resisitance can be better provided by the concrete itself; the use of a non-breathable epoxy coating on a submerged wall can be problematic unless carefully (i.e., expensively)installed due to potential build-up of signifcant osmotic pressures within the wall that can wreek havoc and blister the coating; why add the burden of continual maintenance obligations (i.e., dollars) to the owner by using a coating, when a small extra intial investment can result in lower overall costs?
put your efforts and money in the concrete and design, not in a coating!
the epoxy coatings could have been applied for asthetic reasons; realistically, they were probably meant to protect the seawall. as previously mentioned, the coatings (particularly, on just one side) are making matters worse. a remedial solution might be to completely remove the coatings and apply a deep penetrating, crystalline forming waterproofing. since concrete is porous, it will absorb water; along with any water-borne contaminates. the crystalline structure that develops within the concrete matrix becomes so dense that it prevents the passage of water molecules. both, the concrete and the reinforcing steel will be protected. let us hope that the steel has not yet been affected by the salt water attack.
my previous comments were intended for existing concrete structures; thus my suggested 'remedial' solution. for people that are considering new concrete structures, the same crystalline technology can be 'batched' into fresh concrete. i have first hand knowledge of concrete improved by this crystalline technology. the protection is created within the concrete, permanently; guarding the concrete in a way that coatings can't and without the risk of coating failure. furthermore, this technology permits the passage of vapor, which means that there is no longer entrapped moisture.

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