Not only San Marco, in Venice, needs to be "saved" from water. Its value is undoubted, but when the tide reaches 160 cm in the lagoon, then something is wrong.
Moreover, water is "heavy". Suffice it to say that in a 100 m2 terrace, exposed to a rainfall of 200 mm, they drain 20 thousand liters of water weighing 20 tons. This mass, in addition to its own weight, generates relevant thrust effects in its movement, moves objects, makes it float over others and forcefully drags everything it finds.
These events can sometimes have devastating effects on construction works, and on their "content". Is it possible to do something in advance? What solutions do Venetians adopt against high water? Especially since cement and brackish water do not really agree …
Water (high), what solutions?
Let's consider concrete, the material of choice for building. Its impermeability is one of the essential prerogatives for the durability of structures over time, and is comparable in nature to a compact natural stone. The water introduced into the concrete mixture, due to the hydration and the workability required by the installation, leaves a network of dense burrows in the concrete matrix, after curing, creating a porosity of cement paste, made up of gel pores and capillary pores.
Read also: Water infiltration in buildings: how to read the signs
Maybe not for Venice, but i osmotic cements with capillary penetration they are materials supplied in powder form, which are mixed with water in precise proportions to form a grout to be applied to fresh or mature concrete surfaces. By means of water and humidity, they penetrate into the porosities of the material and then produce irreversible chemical reactions, with the formation of insoluble crystals inside the pores.
Generally the depth of penetration of the product is in the order of a few centimeters and the process is irreversible and definitive. They can be applied to hardened concrete both in thrust and in thrust. In operation, they withstand high water column pressures, up to 200 m (20 bar). They do not need any additional protection on the surfaces.
They are used successfully in situations of continuous water presence, though marina. They cannot come into contact with oils or solvents, but are very suitable for contact with drinking and drainage waters.
>> This information is taken from Waterproofing in construction by Marco Argiolas
The counter-pressure waterproofed concrete is permanently in contact with the soil moisture. This fact has no negative effect on its durability, which is approximately equal to that of the dry concrete itself, waterproofed in thrust.
"It can be concluded, therefore, that the risk of corrosion in permanently immersed structures it is from engineering point of view practically zero".
Prof. Ing. Luigi Coppola, Professor at the Faculty of Engineering and Applied Sciences of the University of Bergamo.
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Innovative materials and UNI EN 206-1 standard
The standard UNI EN 206‐1 introduces, in point 3.1.23, the concept of "addition", defined as a finely divided material used in concrete for the purpose of improving certain properties or obtaining special properties.
This standard considers two types of inorganic additions: practically the additions aggregates (type I) and additions pozzolanic or with latent hydraulic activity (type II). In point 188.8.131.52.1, the concept of the k value is also inserted in the same standard (not to be confused with the homonymous parameter of permeability). The concept k referred to additions, allows the additions of type II to be taken into account by replacing the term "water / cement ratio"(Defined in 3.1.31) with the term" water / cement ratio + k addition ", in the minimum cement dosage requirement (see 5.3.2). The actual value of k depends on the specific addition.
I'm additives based on the peculiar catalytic action in the rheological field of the concrete mix design, dosed for about 1% by weight with respect to the weight of the cement. Allow it to seal gaps and micro-cracks up to 400 microns, through a capillary crystalline reaction that exploits the mineral compounds still present after the main reaction of the concrete, working in combination with the water and with the humidity present in the cement matrix.
These are innovative technologies that in addition to determining drastic reductions in the permeability of concrete and "its" hygrometric shrinkage, already in the first 28 days of maturation, promote a real "self-healing" capacity of the cement matrix. The reactive processes mentioned do not require specific or particularly reduced water / cement ratios, since their effectiveness is in any case ensured even with A / C values of the order of 0.50 / 0.60, decidedly more usual on site.
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High water in Venice, what solution besides MOSE?
Historic masonry and water: the case of Venice is exemplary to expose the serious problem of chemical-physical degradation. The walls immersed specially in the banks and facing the canals are subjected to very heavy conditions, both for the chemical-physical actions of water, both for those of character physical-mechanical due to wave motions.
What determines the stability and durability of buildings is therefore the compatibility of the whole water / masonry / soil system.
In Venice, i bank models they respond very well to the needs of the city, even if there is a strong need for periodic maintenance, what in recent years seems to have been lacking (in favor, it is rumored, of a single macro-intervention called anachronistically Electromechanical Experimental Module, alias MOSES).
>> If you want to know more about MOSE read: Mose di Venezia
The use of slabs of Istrian stone has always had very precise functions, which can be lost when the boundary conditions change, with the increase of the water level and with the waveiness (such as frequency, intensity, height) as known by the builders. There is no doubt that for the walls the weak element of the system is constituted by the bedding mortars.
Sea water (high), what direct damage?
Certainly they are a function of water level same.
One can thus distinguish:
– the area where the wall is always immersed and therefore always wet;
– the area subjected to shoreline with aperiodic wet action;
– the upper part, in which thewater action is discontinuous.
The capillary rise of water
It manifests itself instead in indirect way through ascent process, logically very active and intense in the walls immersed in water, such as those of buildings, and more reduced in the banks of the shore, where the water with a certain frequency wets all or almost all the masonry, avoiding or at least limiting this phenomenon .
Indeed the capillary rise to manifest itself it needs porous systems like masonry, but to carry out a negative effect theevaporation, which determines a certain accumulation of salts present in the absorbed water that migrates into the masonry.
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The latest surveys carried out on masonry in Venice have shown that the main behavioral processes following the action of sea water are:
– chemical processes on the brick, very reduced and without consequences for the material;
– processes of physical degradation.
Distinguishing the shoreline area from the upper one, we find in the mid-sea level part an action of abrasion on the capillaries of the exposed bricks which leads to heavy losses of matter – the porosity also gets to double – but the phenomenon concerns only the first centimeters thick. On the other hand, the deterioration in the highest parts where the phenomenon of capillary ascent persists is much more intense, involving a serious mechanical action due to the high presence of salts in the masonry, which through known mechanisms of crystallization pulverize the bricks.
The direct relationship between quantities is also significant salts present in the masonry is porosity growth. These processes should be studied further by assessing the various situations, in particular thewave action, both for the mechanical / abrasive effect and for the extension in height of the surface subject to these phenomena. Even the action of high water deserves to be evaluated in relation to its frequency.
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Waterproofing in construction
Marco Argiolas, 2017 Maggioli Editore
Waterproofing has always been a particularly critical element of construction, where risks are significant, errors are very frequent, and damage is sometimes important. Often the correction of defects becomes very expensive, also because it is not limited to the pure and …