Realkalization of Concrete Structures?

In my new role, I've been doing a fair bit of "literature review" of concrete condition assessment and concrete renewal options.

From what I've seen, there are two situations.

The first is where the low pH front has not yet reached the re-inforcement. In those cases, the preference (in articles from Japan, Brittain, North America and others) seems to be removal of the deteriorated concrete and application of a complete new overcoat.

The second is where the pH front has already reached the re-inforcement and you are now wishing to arrest the deterioration. The re-alkalization methods and examples cited all seem to have a high set-up cost, and shorter life than the original concrete, but most can be performed with the item still in service.

This all assumes that the problem that you are seeing is due to propagation from Carbon Dioxide reaction with the cement. In theory, if you applied a surface layer of high pH material that stopped the incursion of CO^2 into the structure, then the pH front would halt, but if it's already reached the re-bar, then corrosion will happen regardless of the surface treatment.

My experience (I've now tested around 50 items ranging from 5 years to 85 years) has been that this effect is overtaken by other environmental activity and even the quality of the original mixing materials. Hydrolysis, salt (both in the mixing water and environment) temperature cycling, surface moisture, lichen growth, aggregate selection and specification all have cumulative impact. [By comparing samples from different portions of the same pour, internal and external walls, above and below ground, and different pours at the same site (poured within 100 days 80 years ago.).

This is only to be regarded as indicative. There are others out there who's life experience is concrete and they will be able to substantially expand on this.

By the way, how have you determined the current extent of the deterioration? Have you taken core samples, drilled sample holes or done surface hardness tests?

An Australian product call Crystalfix is said to be effective (but expensive) in waterproofing of deteriated, poorly made, cracked, dusty, unbonded layers, cold joints or porous concrete and will protect the re-bar.

Apparently SA WAter Corporation use this product on dam walls

A Recipe I found to water proof concrete with porosity or holes etc, is the following mixture measured by weight.

54 parts Lime (CaO)

37 parts Portland Cement.

8 parts Salt (Sodium Chloride NaCl)

1 parts Soda (Sodium Carbonate Na2CO3 )

I have some rusty galv tanks which were then poorly plastered (me-an-na-mate), and have recently got the above ingredients to "av-a-go-ad-it" Has anyone herd of these or know their effectiveness?

Cheers

Tim on Kangaroo Island (downunda even "downunda")

Why would you want low pH on rebar in concrete?

Lime will leach through concrete over time in the presence of water. So it might work to raise the pH a little bit, if you can get the Lime to penetrate. The precipitation of lime could be an issue though, it is not very soluble. Maybe a more soluble hydroxide would provide better results (as long as it doesn't react with the concrete to mobilize or weaken it). Though not sure how any of this will address removing the chloride ions, which are the problem. As I understood it, the electrical field in the process of realkylization is to cause chloride ions to migrate away from steel. You could try to precipitate the chlorides i guess and thus neutralize them. Lead, Mercury and Silver can all form insoluble chlorides. Maybe one of those. Is it really worth the extended life expectancy?

The idea is to get "high pH" at the rebar. The reason is that if pH can be maintained somewhere above 9, then corrosion of the rebar ceases and there is no expansion of the rebar. This means that the concrete doesn't crack and thus the structure remains functional for longer.

As regards to cost versus life expectancy, it depends whether it's a footpath in the city, or a structural member of a bridge in a 6 lane highway.

What has me surprised is that "Kenedy Space Centre" is the source of the question. If it's for personal understanding, then all is well. If that group need our advice on issues such as this, then .........

I didn't see the OP talking of chloride ions, as there is a totally different treatment for that concern relating to applying a reverse polarity voltage to the rebar with the surface wetted to help the ions migrate away from the rebar.

Your idea about precipitates instead has merit. I haven't seen any critique of that in the literature, however I'm sure that people involved with city water supply would be very reluctant to use Lead Silver or Mercury as the stabilizer for the concrete water tanks.

Actually this question is more of a " does anyone have experience in using this method to increase pH at re bar depth?" We have an understanding of conventional methods, the results, the cost and the pitfalls of current induced fixes.

It has been suggested that a high pH Lime Wash will penetrate to that depth and restore the passivating layer? I would prefer to see a level of 11.5 and not 9, and Chloride Extraction is not an option due to cost.

The priest at this facility in the N.E. US has no budget, has no testing but has seen this process serve well in third world countries and insist it does work. "Impressed Immaculate Protection?" May be a fitting name for it? I just asked- Don't shoot the messenger!

My question is, how long does it take for the migration through the concrete. mm/hr?

Also is the use of Caustic Soda (NaOH) an option?

We have a problem in my lant where free fatty acids penetrate the concrete over time and some of our concrete is in a poor state of repair with some explosion of the rebar.

Concrete has a permeability of typically less 10^-8 cm/s just for water to infiltrate (so for water to penetrate under saturated flow conditions and 1 cm of head to a depth of 2 inches would only take about 30,000 days). Then you have transformation and retardion effects, like precipitation and sorption to contend with. The transport of Sodium Hydroxide to reinforcement depth would likely be less impacted by these factors than the lower solubility calcium. Though at very high pH it could adversely effect silica and other minerals in the sand and gravel component of the concrete, making them soluble. Since most concrete has excess lime in it when installed that leaches out over time. Use of calcium hydroxide should not cause much of a problem, assuming most of the gravel has reacted and is stabilized (some minerals can experience expansion when exposed to lime solution). However, Calcium Hydroxide reacts relatively rapidly in air when most to capture CO2 and form lime, which is pretty insoluble, and you have at least 1.5 inches that the solution has to move to reach the steel (probably more like 2 to 3 inches). On the plus side, the lime becomes sacrificial to any incidental acids that come in contact from that side of the concrete, neutralizing them before they get to steel depth.

Sorry if it sounded like shooting the messenger. Was never the intention.

As I said earlier, only have done document study on the deterioration of concrete. Testing time would be years.

The idea though sounds feasible if the re-bar hasn't yet been compromised. A high pH layer between the air (where the CO2 is coming from) and the parent concrete to my understanding would halt the progress of the low pH front through the concrete. It might give local reversal near the surface where it soaked in, but not at depth unless the material was particularly porous.

I sugges that the priest approach a local expert for clear diagnosis of the issue. If the symptoms are alread visually apparent, then it's probably too late. Note that in Northern US, the problem could relate to temperature cycling, frost and other environmental effects.

Repeated washing/coating with the lime wash might hide symptoms of more serious structural issues. A proper daignosis is worth the time.

So a sacrificial layer, if the acidity is arising from the exposed surface and not from subsurface water rising from shallow water tables in acidic soils. I think it would take many years to leach lime down into concrete at steel depth.