Design principles and methods according to EN 1504
6.3 PRINCIPLE 2: MOISTURE CONTROL .1 General
The approach of Principle 2 is to adjust and maintain the moisture content in the concrete within a specified range of values to control adverse reactions. The concrete is allowed to dry, and moisture buildup is prevented. This method is often used to control alkali- silica reaction, sulphate attack, or freeze- thaw damage.
Figure 6.19 Continuous membrane after spraying (see Figure 6.18); afterwards a bituminous protection layer, followed by a sand bed and pavement, is applied.
Table 6.9 Summary of Method 1.8 Method 1.8: Applying membranes.
Principle 1: Protection against ingress.
Approach: A membrane is applied to the concrete surface, which prevents the ingress of adverse agents.
Typical applications: All types of concrete surfaces.
Special attention should be paid to:
• Design: Crack movements, solutions for joints, protective layers, etc.
• Product requirements: Not in the EN 1504 series.
• Execution: Careful surface preparation; concrete surface must have the required wetness; minimum thickness must be ensured.
• Quality control: Adhesion to concrete, membrane thickness, etc.
Durability/ maintenance: Depending on system and use.
Complementary methods: Methods 3.1 to 3.3 for defects in concrete; optionally 1.5; for some systems 1.2 (impregnation) before application of the membrane.
Design principles and methods according to EN 1504 163 According to the informative annex of (DIN) EN 1504-9:2008-11, surface protection systems applied to vertical and soffit surfaces should be permeable to water vapour to allow moisture to escape from the concrete. Upper surfaces of horizontal concrete members, e.g., a suspended floor slab in a parking structure, may have an impermeable surface protec-tion system applied. If the concrete contains extraordinarily high moisture, surface pro-tection systems should not be applied.
To achieve moisture control five methods are available, which are described in the fol-lowing sections. The first three methods, hydrophobic impregnation, impregnation, and coating, have already been introduced for Principle 1, and later it is shown that they are also used for Principle 8.
Regarding control of concrete corrosion, it should be noted that the drying effect of the concrete requires some time. Especially when the concrete is very wet, it may take some months or even years until the corrosion rates are sufficiently reduced to prevent damages.
The fact that corrosion will continue for a certain time has to be taken into account for the design of the repair measure. If corrosion has proceeded so far that limit states like critical cracking will soon be reached, it might be too late to use moisture control, and alternative methods should be used that stop corrosion immediately.
6.3.2 Method 2.1: Hydrophobic impregnation
Moisture control can be reached by hydrophobic impregnation. For this method it is impor-tant to prevent ingress of water and to allow drying out by evaporation through the hydro-phobic layer, as shown in Figure 6.20.
The products for hydrophobic impregnation are standardised in EN 1504-2. In this stan-dard the drying rate coefficient describing the evaporation rate of water from the concrete through the hydrophobic layer, which is important for this method, is given as a perfor-mance criterion. When drying out is attempted to proceed quickly, the drying rate coef-ficient shall be close to 1, which means that evaporation is not hindered by the presence of the hydrophobic layer.
Regarding crack treatment, carbonation and appearance advice is given in Section 6.2.2 on Method 1.1. Besides these criteria, it has to be considered whether water may penetrate from the other sides to the concrete element, e.g., by capillary suction from the soil or other sources. If this cannot be excluded, other methods than 2.1 have to be selected.
To control concrete corrosion processes by moisture, it is necessary that the hydropho-bic treatment is effective over the whole remaining service life. Therefore, inspections and maintenance are required. Most hydrophobic treatments can be repeated when the inspec-tions show that the effectiveness is already reduced. When this method is intended for a long remaining service life, it is recommended to use such hydrophobic agents that can be applied repeatedly. This is not a performance criterion in EN 1504-2, but can be declared by the product manufacturer.
To determine the effectiveness of a hydrophobic impregnation, usually the water uptake of the concrete is measured after the treatment on site or at drilled cores. Actually research is performed on the durability of hydrophobic treatments (Antons et al. 2012). It shows that, e.g., mobile NMR (Nuclear Magnetic Resonance) is able to indicate the thickness of the hydrophobic layer at the concrete surface.
To determine the reduction of the water content within the concrete, sensors are avail-able that can be embedded into the concrete. If they are placed in different depths or multi-probes are used, drying out can be monitored continuously (see Sections 6.10.4 and 8.4) (Table 6.10).
6.3.3 Method 2.2: Impregnation
Moisture control can also be achieved by impregnation of the concrete, which fills the pores in the area of the concrete surface. As preparation of the concrete surface, concrete restora-tion and crack filling have to be carried out if necessary, as shown in Figure 6.21.
As already explained in Section 6.2.3, impregnations are not crack bridging. As soon as new cracks develop in concrete or existing cracks get wider, the impregnation will crack and water can penetrate into the crack counteracting the principle of moisture control.
Table 6.10 Summary of Method 2.1 Method 2.1: Hydrophobic impregnation.
Principle 2: Moisture control.
Approach: Reduction of concrete corrosion rates by drying of the concrete.
Typical applications: Concrete corrosion like alkali- silica reaction, sulphate attack, and freeze- thaw in an early stage.
Special attention should be paid to:
• Design: As drying takes time, corrosion will continue after hydrophobic treatment and slow down slowly, etc.
• Product requirements: According to EN 1504-2.
• Execution: Careful surface preparation; concrete surface must be sufficiently dried out; high penetration depth shall be targeted.
• Quality control: Depth of penetration, hydrophobicity, etc.
Durability/ maintenance: Regular inspections recommended.
Complementary methods: Usually Methods 1.5 and 3.1 to 3.3.
Figure 6.20 Schematic representation of Method 2.1 before and after application.
Design principles and methods according to EN 1504 165
Therefore, it cannot be recommended to use Method 2.2 when crack movements or new cracks are expected. However, as Method 2.2 is predestined to be used on horizontal sur-faces like floors, where often significant crack movements are expected, the fields of applica-tion of this method are limited. If only very few cracks are expected, it has to be considered to use Method 2.2 in combination with crack treatment, e.g., Method 1.5.
The products for impregnation are standardised in EN 1504-2. In this standard the dry-ing rate coefficient is not used as a performance criterion like for the hydrophobic impregna-tion, but the permeability to water vapour. However, for impregnation products, like epoxy resins, it can be expected that the rate of drying out of the concrete is reduced considerably (Table 6.11).
6.3.4 Method 2.3: Coating
Coating systems can also be used for moisture control. Figure 6.22 shows schematically the application of this method. If necessary, as preparation of the concrete surface, concrete restoration and crack filling can be carried out. Compared to Methods 2.1 and 2.2, coatings have the advantage that crack bridging coatings are available. To allow moisture control, the coatings have to be impermeable for water from the outside and open for evaporation of water vapour from the concrete.
The coatings for Method 2.3 are standardised in EN 1504-2. The performance character-istics of the products for Method 2.3 are similar to those for Method 1.3 (protection against ingress), but there are no requirements regarding carbonation rate, ingress of chemicals, or chlorides because Method 2.3 is only focused on reduction of the water content of the concrete.
If drying of the concrete shall proceed quickly, coatings should be selected that allow high evaporation rates. As for all applications, the crack bridging performance should be speci-fied carefully to prevent unexpected cracking (Table 6.12).
Before
Figure 6.21 Schematic representation of Method 2.2 before and after application.
6.3.5 Method 2.4: Erecting external panels
External panels can be erected to reduce the water content of the concrete in front of con-crete surfaces. The buildup looks similar to Method 1.7, but for Method 2.4 it is addition-ally important that the water from the concrete can evaporate through the panel system, as indicated in Figure 6.23.
Before
After
Coating Concrete
restoration Crack
treatment H2O H2O
Figure 6.22 Schematic representation of Method 2.3 before and after application.
Table 6.11 Summary of Method 2.2 Method 2.2: Impregnation.
Principle 2: Moisture control.
Approach: Closing the pores of the concrete surface to reduce the water content of the concrete and the rates of concrete corrosion.
Typical applications: Limited; floors, horizontal surfaces.
Special attention should be paid to:
• Design: No protection when cracks move or new cracks occur; as drying takes time, corrosion rates will decrease slowly, etc.
• Product requirements: According to EN 1504-2.
• Execution: Careful surface preparation; concrete surface must be sufficiently dried out.
• Quality control: Depth of penetration, film thickness, etc.
Durability/ maintenance: High durability, depending on use.
Complementary methods: If required, Methods 1.5 and 3.1 or 3.2.
Design principles and methods according to EN 1504 167
Figure 6.24 shows an example for an external panel system on a building façade. Such systems can also be used to improve the thermal insulation and appearance for the building.
For applications on vertical surfaces like façades, the requirements of watertightness can be achieved much easier than for horizontal surfaces like floors. Rainwater will run down the panels, and the effective duration of the water exposure will be limited. For floors the Table 6.12 Summary of Method 2.3
Method 2.3: Coating.
Principle 2: Moisture control.
Approach: Application of a coating that prevents water ingress and allows evaporation of water from the concrete.
Typical applications: Concrete corrosion like alkali- silica reaction, sulphate attack, and freeze- thaw in an early stage.
Special attention should be paid to:
• Design: Requirements have to be specified according to EN 1504-2.
• Product requirements: According to EN 1504-2.
• Execution: Careful surface preparation; concrete surface must have the required wetness; minimum thickness must be ensured.
• Quality control: Adhesion to concrete, coating thickness, etc.
Durability/ maintenance: Inspections are recommended, depending on use.
Complementary methods: Usually Methods 1.5 and 3.1 to 3.3.
Before
After
External panel Fixation element H2O H2O
Figure 6.23 Schematic representation of Method 2.4 before and after application.
formation of puddles has to be expected, which extend the periods of water exposure and increase the performance requirements for the panel system, which shall additionally be able to allow evaporation.
With special cases it should also be considered to use external panel systems as protec-tive roofs.
An external panel system can also be used for Principle 8, although it is not mentioned there as an extra method (Table 6.13).
6.3.6 Method 2.5: Electrochemical treatment
Method 2.5 is not specified in detail in the EN 1504 series. In recent years electrochemi-cal methods for drying concrete, like electro- osmosis or the electro- osmotic pulse method, have been on the market for this method. However, there are serious doubts whether these methods are able to dry concrete significantly. As long as no scientific reports are available Figure 6.24 Example of an external panel system to control moisture according to Method 2.4.
Table 6.13 Summary of Method 2.4 Method 2.4: Erecting external panels.
Principle 2: Moisture control.
Approach: Installation of a watertight panel system allowing evaporation.
Typical applications: Concrete corrosion like alkali- silica reaction, sulphate attack, and freeze- thaw in an early stage, preferably at façades and roofs.
Special attention should be paid to:
• Design: Additional loads, solutions for details, etc.
• Product requirements: Not in the EN 1504 series.
• Execution: Build up according to specifications.
• Quality control: Tightness, stability, possibilities of evaporation, etc.
Durability/ maintenance: Inspections for leakages.
Complementary methods: Optionally, Methods 1.5 and 3.1 to 3.3.
Design principles and methods according to EN 1504 169 that present not only the assumed theory, but also the effect of these methods on the water content of the concrete quantitatively, these methods have to be treated carefully.
6.3.7 Method 2.6: Filling of cracks, voids, or interstices (not in EN 1504-9)
Filling of cracks, voices, or interstices can also be used for local moisture control to prevent damages due to corrosion of the concrete. This method is not included in EN 1504-9, but may be used in special cases, e.g., in combination with coating systems.
6.4 PRINCIPLE 3: CONCRETE RESTORATION