Introduction

The construction of structures in the presence of groundwater has always historically been resolved using so-called "wrapping", or rather waterproofing the structure was guaranteed by applying an external application of bituminous or PVC sheathes, self-adhesive membranes, cement-plastic systems applied with a brush or spray, liquid bituminous sheaths, polyurethane systems, etc. These building techniques have the disadvantage of having to be installed by specialist operators to ensure their laying,  and therefore their efficiency, is state of the art and require waiting time from formwork placement to backfilling, with possible prolonged completion times in the event of poor weather conditions. Furthermore, since these materials are "outside" the cast, they require further management of the warehouse items, waste, packaging to dispose of, etc.

A different approach to the problem of waterproofing underground structures, which for some years now has been forcefully imposing itself as the main building technique in this market segment, is the implementation of intrinsically waterproof structures, created with concrete made impermeable using the addition of appropriate additives in powder, to be conceived as the real and proper installation of the structure and the hydraulic seal integrated within.

The walls and the bottom in waterproof concrete should however be adequately supported by products and systems that also guarantee the joints, the connections and the spacers in the formwork forming part of the overall system are waterproof.

Permeability and impermeability of the concrete

In general, the permeability is the property of material to enable the passage of fluids (liquids, in the case in question) without altering its structure. Materials are defined as permeable that allow passage of relatively high quantities of liquid, while they are impermeable where the flow of liquid crossing them is negligible. The speed with which a fluid crosses a solid body depends on the type of substance composing the body, from the pressure of the fluid to the temperature. To be permeable, material must be porous, i.e. it must have empty spaces, pores, capable of absorbing liquid. The pores are also connected to a network of gaps that allow the fluid to cross the solid substance. To be impermeable, on the contrary, material must have a dense and compact structure, free of communicating gaps.

The impermeability of concrete is one of the essential prerogatives for the durability of structures over time. The concrete or stone conglomerate is similar in nature to compact natural stone, therefore the impermeability to water of compact marble, for example, corresponds to that of concrete with an W/C ratio = 0.48. The water introduced in the concrete mix, by hydration and for the workability requested for use, after use leaves the concrete matrix with a network of dense tunnels determining the porosity of the cement mix, composed of gel pores and capillary pores. The "capillary pores", mostly regulating the "intrinsic" permeability of the conglomerate, depend on the water/cement ratio and the hydration level and can vary from 0 to 40% in volume, based on the volume of the cement mix. With a water/cement ratio over 0.38, the permanence of the capillary pores is practically inescapable even after complete hydration, unless specific intervention is taken using "reactive fillers".

The capillary pores are not visible unless you are using an electronic microscope. Their diameter is in micron (between 0.1 and 10 micron). They have a variable structure and form a continuous and interconnected channel within the matrix: the permeability of the concrete therefore is not a simple function based on its porosity, but also depends on the dimensions, distribution, physicality and continuity of the pores. The empirical formula Vp = 5.9 α + 42 (1 - α) provides indications to assess the volumetric entity of the capillary pores based on the level of hydration and the total water in the set mix (where Vp is the volume of the capillary pores, α is the hydration level, variable from 0 to 1).

The widely diffused presence of "trapped air" in fresh concrete is generally added to the capillary porosity in terms of pore connection, which should be expelled by correct compacting of the conglomerate and which creates macro-vacuums (from approximately 1 mm to a few dozen mm).

A further variable, capable of greatly increasing porosity as well as interconnection of the pores is found in the "transition area", i.e. that part of the cement mix (often a few or dozen micron) in direct contact with the stone aggregate; the transition zone can be significantly more porous than the adjacent cement matrix based on the "bleeding" (collection of water on the concrete surface) which, when rising, remains partially trapped under the bigger stone aggregates.

The greater or lesser presence of vacuums (capillaries) intercommunicating between the surfaces of opposing cast surfaces, "continuous porosity", means a flow of water can occur due to the difference in hydrostatic pressure, representing the "permeability" of the concrete and, as already mentioned, very much depends on the properties of the concrete itself as well as the correctness of laying it, care and damp maturation, as well as any manifestation of micro or macro cracks due to plastic and hygroscopic shrinkage.

During the hardening process of concrete, climatic phenomena such as temperature, relative humidity and ventilation can determine the more or less sudden loss of the mixing water. In the absence of adequate care and humid maturation measures, significant qualitative decline can occur, also involving permeability. 

Obtaining impermeable concrete

As already mentioned, the permeability of concrete is in close relationship with the porous microstructure of the hardened cement, in turn closely related to the water/cement ratio. This means that concrete can have various levels of impermeability, according to how it is packaged and set in place. The factors influencing this characteristic are the same as those determining other properties: composition, work and subsequent treatments. In theory, there is no particular difficulty in obtaining impermeable concrete, more pragmatically you should consider that "truly" impermeable concrete requires effort and attention different from normal building site habits. On a technical-design level, it is indispensable to consider this impermeability as being relative and non-absolute. Accurate design, careful packaging and adequate laying are in fact indispensable to obtain impermeable concrete, without forgetting the indispensable care and curing treatment which must be effective and efficient, on the contrary to procedures "often" apparently implemented on many building sites.

In practical terms, you firstly need to reduce the water/cement ratio to the minimum compatible with adequate working; you must use aggregates of suitable nature and particle size; you need to prevent drying from being too quick of the casts to avoid formation of external and internal cracks due to shrinkage; during casting, you need to avoid sedimentation of the concrete, preventing it from loosing uniformity obtained through mixing. With a water/cement ratio over 0.38, the persistence of a non-negligible quantity in the capillary pores is practically inevitable. Even following complete hydration, specific intervention with the addition of "reactive fillers" can be unavoidable.

Innovative materials and standard UNI EN 206-1

The standard UNI EN 206‐1 introduces, in point 3.1.23, the concept of "addition", defined as the material finely divided used in concrete to improve certain properties or to obtain special properties. This standard covers two types of inorganic additives: practically inert (type 1) additives and pozzolanic additives or latent hydraulic additives (type II). In point 5.2.5.2.1, of the same standard, the concept of value k is also added (not to be confused with the permeability parameter of the same name). Concept k referring to the additives enables type II additives to be taken into consideration, replacing the term "water/cement ratio" (defined in 3.1.31) with the term "water/cement ratio + additive k", in the minimum dosage requirement for cement (see 5.3.2). The actual value of K depends on the specific additive.

For "pozzolanic additives” (such as MICROSIL 90), note that their ideal quantity should be between 7%-12% of the weight of the cement used (minimum 330 kg/m³ of 42.5R or 360 kg/m³ of 32.5R). The availability of "specialities" with high technological content, such as "pozzolanic additives" is therefore recognised as technologically and terotechnologically adequate to build waterproof works using a combination of "intrinsically waterproof concrete", specific controls and adequate building techniques.

The advancement of terotechnological purchases has recently made available innovative products, better known as "crystallizing agents", based on peculiar catalytic action in the rheologic environment of the concrete design mix, dosed per approximately 1% in weight compared to the weight of the cement. These additives enable the vacuums and the micro-cracks up to 400 micron to be sealed, using a capillary crystalline reaction that takes advantage of the mineral compounds still present after the main reaction of the concrete, working with the water and humidity in the cement matrix. It is innovative technology that not only determines the drastic reduction in concrete permeability and "its" hygrometric shrinkage, already in the first 28 days of maturation, promoting the real "self-healing" capacity of the cement matrix. The reactive processes mentioned do not require specific or particularly reduced water/cement ratios, since their efficiency is however also ensured with W/C values around 0.50/0.60, decisively more usual on a building site.

Hermetic controls and accessories

All of the above describes how to prepare and lay concrete works defined intrinsically "impermeable". The overall structure, however, inevitably presents the volumetric discontinuity necessary for its installation. This discontinuity, which is easily subject to the passage of pressurised water, is for example the connections between the base and the elevation walls, the casting and movement joints, the formwork spacers, the passing tubes, etc.

To prevent water passage from all these discontinuities, this system comes with: waterstop kerbs in sodium bentonite or hydro-expansive rubber, waterstop in PVC, hydro-expansive sealant cartridges, hydraulic seals for formwork spacers, both in "blade" and tubular format in PVC, polypropylene fibres to reduce micro-cracks due to plastic shrinkage, hydro-expansive pastes to hermetically seal passing tubes, etc.