Precast panels, Carbonation and reinforcing coverage article
Concrete carbonation is a chemical reaction that occurs inside concrete, Concrete is porous, it’s like a massive hard sponge and it soaks up water and allows air and C02 to enter through its pores.
This is how the C02 in the environment get inside the concrete to react with hydroxide solution that is contained in the concretes pores to produce carbonic acid.
And now begins the process called carbonation.
This carbonic acid now inside the pores of the concrete reacts with the calcium hydroxide in the microscopic structure of the concrete and causes it to dissolve; this lowers the acidity of the solution and creates calcium carbonate instead of carbonic acid in a process called buffering.
Calcium Hydroxide is the same substance that causes white marks on the exterior of cracks where water has soaked into the panel and has escaped out of the cracks transporting the calcium hydroxide with it and depositing it on the exterior of the coatings. It is easily dissolvable in water as well as in carbonic acid.
The dissolution of the calcium hydroxide by the carbonic acid leaves behind voids in the concrete where it once existed; these voids are microscopic and happen to the concrete in every area.
This process keeps on occurring over and over again until all of the calcium hydroxide in the concrete is used up or carbonized.
This results in the concrete being more porous than it was before and this increased porosity increases the rate that the co2 enters the concrete, as well as also allowing the Co2 to continue to react with the hydroxide in the concretes pores and again create carbonic acid without the buffering by the calcium hydroxide in the concrete because its now, all gone.
The PH begins to drop and become more acidic.
Dark music begins to play.
This entire process is called carbonation; carbonation happens progressively throughout the depth of the concrete from the outside in and can be measured by the PH value and depth of these values inside the concrete.
To test PH we use a chemical called phenolphthalein, which is clear at PH values under 9 and pink above. This test clearly shows how deep the carbonation process has penetrated into the concrete and is quite cool to witness, especially around cracks as it creates a clear line showing the differences in PH values.
This is what we call the carbonation front. Carbonation commences as soon as concrete is exposed to the atmosphere, and can advance at a rate of 1mm to 5mm per year, dependent on the concrete’s porosity and permeability.
Carbonation is the cause of 95% of all reinforcing damage and concrete spalling on buildings and it is the reason that proper reinforcing coverage is prescribed in the Nz standards NZS 3604:2011 for 50mm against form and is to be exposed to the weather.
Almost all buildings constructed with precast panels that Zenith have been asked to survey and repair have had less than 30mm of concrete cover to their reinforcing, this is extremely common in building built in the eights and early nineties.
When new concrete has a high PH, this high PH causes reinforcing steel to create a layer of passivation around it that protects it from rust, passivation is a non-conducting film of oxidizes that prevents reaction to gases and liquids.
Low PH is bad; As the carbonation front passes the embedded reinforcing the PH drops and the reinforcing loses its layer of passivation and starts to rust inside the concrete in exactly the same manner as if it was sitting outside, exposed to the elements.
Rust (Iron oxide) has up to 6 time’s larger volume than the original steel, so as the steel reinforcements rust inside the concrete, this expansion causes the concrete to crack and spall adding to the already crack prone increased porosity carbonated concrete problems.
Zenith have seen a number of buildings that are experiencing wide spread cracking, spalling and delamination to their panels that have been caused by application of coatings without any remedial repairs or repairs to cracks.
This has resulted in entire areas of paint bubbling and falling off as the water found its way into cracks and behind the coatings pushing the paint off.
All concrete structures experience carbonation and to avoid its impacts the best course of action pre construction is to use high strength (denser) concrete and the addition of anti-carbonation coatings or treatments.
Anti carbonation treatments are like paint coatings and are applied as such, they resist carbon dioxide from entering the concrete but allow moisture inside the concrete to come out.
Hydro gels are an interesting treatment for carbonated concrete and they have the effect of lining the concretes pores to stop the reactions as mentioned before. Hydro gels penetrate more that 150mm deep into concrete and line the pores.\
Hydro gels are also used as an anti-insipient anode syndrome treatment after a concrete repair has been performed.
Insipid Anode Syndrome is an interesting thing and relates in many ways to our earlier discussion, it occurs when a concrete repair is undertaken where rusted reinforcing is cut out and replaced as well as damaged concrete being removed and replaced with new mortar.
Months or years after the repairs have been performed areas surrounding the repair start to experience massive reinforcing corrosion, far and above the amount that just time and existing condition would be the cause of.
This is due to the loss of the corroded steel (removed during repair) providing natural cathodic protection to the surrounding reinforcing in much the same way as a sacrificial zinc anode would for an out board motor.
Hydro gel can stop insipient anode syndrome by covering the reinforcing in the concrete and preventing it from reacting, also the use of a galvanic anode embedded in the repaired area can replace the rusted reinforcing as a sacrificial anode in exactly the same way as on an outboard motor.