Waterproofing Concrete
When a concrete structure is designed, its nominal service life must be taken into consideration, which varies according to the type of structure. Apart from complying with the requirements of applicable norms and standards, guaranteeing higher durability also means reducing maintenance costs and, therefore, the demand for new infrastructures. Hence, the aim to maximize the tightness of the concrete to prevent the passage of any harmful substance is very important to achieve a durable structure.?
The impermeability of a material is its capacity to withstand the passage of fluids. With concrete, in particular, this means its capacity to prevent the passage of water within its structure. In fact, constructing impermeable structures is important not only to protect buildings against leaks but also, and above all, to reduce harmful, aggressive agents to enter and extend the service life of the structure itself.
The entering of water into the pores inside the cementitious matrix, a potential vehicle for aggressive ions, and the passage of gases such as carbon dioxide and oxygen, are the main causes of deterioration in concrete structures. It is very important, therefore, to define the transport mechanisms that regulate the kinetics of deterioration phenomena. The following mechanisms may be identified inside concrete:
? Diffusion
? Permeation
? Capillary Absorption
? Electrophoretic Transport
HOW CAN YOU REDUCE PERMEABILITY?
The technologies available to reduce the permeability of concrete deal with the separate or combined use of three different approaches:
- Appropriate mix design
Capillary voids are the main cause of permeability in concrete and their formation is connected to the evaporation of water which, as it rises through the cement paste, leaves inter-communicating pores along its path. The mix design, therefore, must aim at reducing the amount of water added as much as possible in order to minimize the formation of intercapillary channels.
- Extended curing
Curing is the period of time after the concrete has been laid during which the surrounding temperature and level of humidity must be controlled so that the properties of the concrete develop correctly. A correct curing cycle is fundamental for both the development of the concrete’s strength and to improve its characteristics, which in general means its impermeability, volumetric stability, durability, and strength integrity.
- Permeability reducing admixtures
The classification proposed by the American Concrete Institute divides permeability reducing admixtures into two main categories: those that improve resistance to water in concrete not subjected to hydrostatic pressure and those, on the other hand, that increase resistance to the penetration of water in concrete subjected to hydrostatic pressure.
Admixtures in these two categories are made up of a vast range of raw materials: from water-repellent substances such as vegetable oils, derivatives of long-chain fatty acids, mineral oils, waxes, and bitumen emulsions for hydrophobic admixtures, right up to inert powders or chemically active powders, such as colloidal silicates, for admixtures commonly known as densifiers. The most widely used hydrophobic admixtures are stearate-based which, by reacting with the lime in the concrete, form a layer of non-soluble calcium stearate on the walls of the pores which reduces permeability by capillary absorption
The use of hydrophobic admixtures depends on the service conditions of the concrete. If water under pressure is not foreseen, the use of hydrophobic admixtures is more than sufficient to solve the problems connected to this phenomenon. Hydrophobic admixtures are generally aimed at the light prefabrication sector, particularly the production of blocks and tiles. Hydrophobic admixtures, however, may also be used for ready-mixed concrete, particularly when used to build walls and for exposed surfaces. Crystallizing admixtures, unlike hydrophobic admixtures, need water to make them react and produce hydrated calcium silicate and other inorganic compounds that precipitate inside the capillary pores and microcracks. The crystalline deposits that accumulate inside the capillary pores become an integral part of the concrete mass, reducing its porosity and improving its resistance to the action of water under pressure.
MAPEI SOLUTIONS
- Hydrophobic admixtures are salt-based liquid additives that are used to increase concrete’s resistance to rising damp. IDROCRETE DM is MAPEI admixtures specifically formulated to protect the surface of the concrete from water absorption. IDROCRETE DM is added in mass to concrete to form a thin hydrophobic film around the pores. This technology is not used to fill the capillary pores, but rather to create a water-repellent layer that removes particles of water when they collect on the surface of concrete and form a “droplet effect” on horizontal surfaces. IDROCRETE admixtures are added at a rate of between 0.2% and 0.8% of the weight of the cement for the more concentrated DM. Its hydrophobic effect may allow the ingress of water, including at low pressures, but its beneficial effect on concrete may be quantified by measuring the coefficient of absorption described in the EN 13057 test method.
- Crystallizing admixtures, the technology of making concrete more impermeable by means of a crystallization process have been adopted for many years and allow the permeability of concrete to be reduced by adding a special admixture to the concrete mix. IDROCRETE?KR?1000 is a MAPEI admixture with the capacity to make concrete totally impermeable by means of a progressive crystallization process. When IDROCRETE?KR?1000 comes into contact with water, it reacts to form hydrated calcium silicates and other solid precipitates that are deposited inside the capillary pores, becoming an integral part of the concrete mass and improving its resistance to the permeability of water.