Sweet lake waves

 Here’s a nice exaple about enviromental conditions seemingly not particularly aggressive with decades can significantly disrupt and erode the concrete.

These photos were shot at Lake Como and show the foundation curb the wall of the lake-side walk. The waters are sweet so there are no chlorides that might damage the armor. But there’s winter frost. A rather insidious frost. At first I didn’t notices but thanks to the rise of the lake level typical of the winter season the foundation beam is located right in the area constantly hit by the waves or their spray so it is constantly moist and subjected to freezing. Yes, because during the winter it’s quite easy to encounter snow and freezing conditions on the shores of lake Como. The concrete then finds itself facing one of its worst enemies in the most unfortunate condition: it is soaked with water.
The effects are manifest.

wpid-IMG_20130820_091745.jpg wpid-IMG_20130820_091722.jpgA comparison with the stones used right above is merciless: while it is true that they are not subject to the splashes therefore the action of frost is less destructive they do not show any sign of degradation.

I had no way to know which kind and quality of the concrete has been used, but given the age of the building the design specifications most likely hardly required only the amount of binder (cement (( they called it “composition” ready-mix concrete )) ) and seldom a mass higher than 300 kg/m³ were prescribed . Then add the fact that in a such concretes plasticizer additives were not used unless specifically requested and paid for((and perhaps even they didn’t existed when the wall were built )), and that often unsupervised workers gladly add water to bring the mixture slump to fluid consistency (S4 ) or super fluid (S5). Such mixes have a water content definitely higher than 200-210 l/m³ raising the water cement ratio to values around 0.66-0.7, that are way too high to achieve any durability. In fact all the mixing water over the “stoichiometric amount” ((we cannot strictly speak of stoichiometric water when referring to the amoun of water reacting with cement because it is not a well defined chemical specimen but it is a mixture of different components)), generally estimated at 0.28 times the mass of the cement, all the volume of water in excess has the sole function to make the mass in the fresh concrete fluid enought to be casted and compacted. Once the concrete has set, hardened, and dried all the volume previously occupied by water turns into porosity concentrated in the cement paste . All this additional water lower the mechanical resistance of the concrete while making it extremely vulnerable to frost.

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