How to protect concrete footings and slabs in a high salt environment

[jdmm]

I am building a house in Mexico near the beach. The soil profile consists of fine silty sand to sandy silt in the upper 3 feet over loose medium sand to silty sand that extends to 6 feet where the sand is salt cemented probably from tidal fluctuations. I have noticed that masonry blocks deteriorate quickly when laid on footings in the surficial soil. I am replacing the upper 3 feet of soil with imported sand (compacted to 100%ASTM D698) and founding the footings on the top of the sand. I have recommendations from a Mexican geotechnical engineer to do either:

1. Spread 2kg/m2 lime on the bottom of the excavation and irrigate for three days, or

2. Place a pvc liner on the subgrade before placing the fill.

I was under the impression that sea salt was not a problem for concrete. The site is desert and I don't know if there are other salts such as sulphates. I don't know what the purpose of the lime would be, does anybody have any idea?

 

[Ron]

I would suggest having the soils tested for the type of salts you will encounter. Typical "seawater" salts do not affect concrete a great deal...they affect the reinforcement. Sulphates and certain alkali salts can affect concrete. Get the chemistry first, then make the concrete to deal with it.

Concrete is a very durable material, but it must be good quality concrete. Dense, consolidated concrete having good aggregates, good mix proportions, and properly placed/cured will resist a lot of stuff.

 

[fmathis]

If it's available, it might be necessary to use Type V Portland Cement in the concrete mix. It is designed specifically to resist sufate reaction.

Also, take a look at a publication put out by the Portland Cement Association entitled "Design & Control of Concrete Mixtures". It covers all you'd ever want to know about corrosion-resistant concrete, including the use of sea water in the concrete mix -

 

[TJWATKINSPG]

I THINK THAT WHAT RON SAID MAKES A GREAT DEAL OF SENSE. IT IS DIFFICULT TO KNOW HOW TO PROTECT FROM A CHEMICAL IF YOU DO NOT KNOW WHAT IT IS. A QUICK GLANCE AT YOUR LOCAL SOILS MAP OR GEOLOGIC MAP MIGHT PROVIDE A CLUE AS TO THE MINERAL THAT IS CAUSING THE DECAY. MY GUESS AS TO THE PURPOSE OF THE LIME IS TO PROVIDE A SACRIFICIAL CALCIUM ION. THAT'S PURELY A GUESS THOUGH. HOPE YOU GET IT FIGURED OUT . The truth will set you free. Best of luck. Geodude

 

[ctmtwilliams]

Great project! I've always dreamed of building on the beach in Baja. Based upon 35 years of experiences in Baja I'd like to add a couple of things.

The availability of quality materials in Mexico is a major problem. It may be possible that the degraded block you've seen was not of the greatest quality to begin with. If you look around you'll notice that the quality of the concrete or mortor used is generally pretty poor and degrades rapidly. Typically, Mexican material sources are located less than a mile or two from an project. Construction near the beach commonly use beach sand in mortors (well rounded beach sand grains make poor mortor). Additionally, the precentage of aggregate in concrete is commonly low due to a lack of availability.

Depending upon where you are going to build, it is very possible that the potiential sources of sulfate or alkali salts are comming from the desert. Good luck trying to get any technical (esp.soils) maps for your area. I'm willing to bet they don't exist. I agree with the other posts which suggest you sample the soils on your site. If you know the source of your import, I'd sample it as well. After all, there is no substitute for good data (can you have too much data?).

Use the data with the PCA publication to try and convince the locals what you need, which could be the hardest part.

Good luck,

Craig

 

[jdmm]

tgwatkinspg

The calcium ion makes sense. I will look further in that direction.

ctmtwilliams

I have been working on this project for 3 years so I know exactly what you mean. For masonry blocks the contractors will offer you best quality for one price and next quality for less. The difference is the amount of cement in the blocks. If you pay for the more expensive block you might get the cheap one delivered if you are not careful.

In this case I am not using blocks but building the entire house as reinforced concrete (using a Canadian builder). I just want to be sure that I don't end up with deterioration of the footings and slabs. Using higher cement content and additives to make the concrete sulphate resistant should be enough.

Thanks for the advice

 

[Focht3]

Why not use epoxy-coated rebar?

It's not a perfect solution, but it might offer enough protection from corrosion to make it worthwhile. Given your location, you will almost certainly have salt in the concrete (see excellent comments by ctmtwilliams.) I know of no practical way to get high quality, salt-free concrete in the desert/beach environments of Mexico. The coating is useless if scratched, so handling is a big concern.

Sounds like a great getaway. Good luck!

 

[jheidt2543]

I remember reading an article in Engineering News Record (ENR) years ago about an appartment complex on the beach in Florida that collapsed. They had used beach sand in the concrete mix and the salt ions corroded the rebar. I like the epoxy coated rebar idea, but the installers must remember to touch up the cut ends with epoxy before concrete is poured.

For winter concreting at temperatures below 32 degrees F, the addition of up to 12% (by weight of mixing water) salt (calcium chloride) decreases the setting time of the concrete and increases concrete strength. The draw back is the effect on the rebar, which is why most specifications limit the addition of calcuim chloride to 1% - 2%. {Source: "Materials of Construction" by Lloyd F. Rader}