1. WHAT IS SPALLING CONCRETE? WHAT CAUSES IT?
“Spalling concrete” refers to the phenomenon of surface patches of concrete breaking up and delaminating in the absence of immediate external influences such as accidental impact loads or fires.
Spalling may be caused by several factors, including:
(a) Corrosion of rebars, the process of which results in the steel bars expanding and forcing the concrete apart.
(b) Sulphate attack on concrete.
(c) Alkali-silica reaction.
(d) Freeze-thaw cycles.
This article focusses on (a), the spalling of concrete caused by corrosion (and expansion) of embedded steel reinforcements.
2. WHAT IS CARBONATION? HOW DOES IT LEAD TO CONCRETE SPALLING?
Fresh concrete is highly alkaline with a pH of about 12. This alkalinity provides passivation protection to the steel rebars and wards off corrosion. However, over a passage of time, the alkalinity drops and when it reaches a pH value of about 9 or less, the passivation protection is destroyed.
The decline in alkalinity is caused by a process called carbonation. The high alkalinity of fresh concrete is due to the presence of calcium hydroxide (common name, hydrated lime), a by-product of OPC cement hydration. Penetration of moisture containing dissolved carbon dioxide into the concrete matrix results in a reaction which converts the calcium hydroxide to calcium carbonate (an insoluble salt). As more and more of this conversion takes place, the calcium hydroxide content in the concrete decreases, and with it, the alkalinity.
When we look at the carbonation process described above, the following points become clear:
• The agents of carbonation i.e. moisture and carbon dioxide, are present everywhere but more so in open, exposed environments.
• In non-defective, high-grade concrete, carbonation is a slow and gradual process and it takes a long time for the reduction in alkalinity to drop to pH 9 at the rebars.
• A number of factors allow faster rates of carbonation progression; these include the use of low-grade, high-porosity concrete, and the presence of defects such as honey-combs, voids and cracks (in low- or high-grade concrete).
• Engineers design reinforced concrete with a specified cover for the rebars, depending on the design criteria. In simplified terms, a concrete cover of 30 mm provides 25% less carbonation protection for the rebars than a cover of 40 mm. Therefore, mistakes in construction which result in reduced concrete cover must be rectified in order to preserve structural durability.
3. HOW CAN CARBONATION BE CONTROLLED?
We have seen that the phenomenon of carbonation, in the absence of protective measures, leads to rebar corrosion and thence to spalling of concrete. Therefore, in order to preserve the service-life of structures and obviate the need for expensive and disruptive repairs, it is important to control or slow down the process of carbonation.
Considering the points listed in (2) above, we are able to identify the important measures for doing so:
4. REPAIR OF SPALLED CONCRETE AND PROTECTION MEASURES
When spalling due to steel corrosion has become visible on the concrete structure, carbonation would have already reached an advanced stage and a methodical, multi-step, repair method has become necessary.
The essential repair steps for a typical repair situation are -
(i) Breaking out the spalling concrete: Remove loose, delaminated concrete until the substrate consists of sound concrete. The removal should include undercutting (i.e. cutting behind) the corroded steel reinforcements by approximately 20 mm. Keep the shape of the prepared cavity as simple as possible – generally square or rectangular in shape. Saw-cut the edges of the repair areas perpendicular to the surface to a depth of 12 mm to avoid feather-edging the repair material.
(ii) Substrate preparation and cleaning: The final surface texture should be rough, with an amplitude of approximately 6 mm. Remove residual dust, debris, and any contaminant that may interfere with proper bonding. This may be done by water-jetting at a minimum pressure of 250 MPa.
(iii) Treatment of exposed reinforcements: Completely remove corrosion from the reinforcing steel by wire brushing, abrasive blasting or needle scaling as necessary. If the cross-sectional area of the reinforcing steel has been significantly reduced, a structural engineer should be consulted on the permitted limit of reduction (typically 20%) and the strengthening measures, if any. Apply a protective coating on the steel after it has been cleaned. (Ref. MAPEFER 1K, a re-alkalising protective coating for rebars.)
(iv) Substrate pre-wetting: In general, absorbent substrates (e.g. concrete) need to be pre-wetted to SSD (saturated, surface dry) condition prior to application of cementitious mortars. This is to prevent excessive loss of moisture from the mixed mortar at the bond line and thus, to weakened adhesion. (SSD condition is achieved when the substrate is saturated but there is no free standing water.)
Note: SSD surface is not required when a polymer bonding agent is used. Follow the manufacturer’s instructions when using polymer bonding agents.
(v) Material selection and re-profiling: Select the cementitious mortar for the re-profiling works based on the following considerations –
(a) What is the placing method? By hand-patching? Or formwork casting? Or shotcreting? (This decision is in turn dependent on the volume of repairs and the required physical strengths of the placed mortar.)
(b) The required physical properties for compressive and flexural strengths and compressive modulus.
(c) Special resistive properties such as water permeability and chloride ion diffusion.
(d) Characteristics of the mortar such as permitted application thickness, flow, shrinkage-compensation, non-bleed, rate of strength gain, etc.
When the material has been selected, it needs to be applied according to the manufacturer’s instructions and the engineer’s approved method statement.
(Refer to the MAPEGROUT series for hand-patched mortars, the MAPEFILL series for formwork casting, and the MAPEGROUT GUNITE series for shotcreting repairs.)
(vi) Curing and protection of repair works: Provide uninterrupted curing protection for all cementitious repair materials for the recommended curing period, normally seven days. Formwork may only be stripped after the minimum waiting period specified by the manufacturer.
In all cases, refer to the manufacturer’s instructions for curing methods and waiting periods.
(vii) Protective coatings: In view of the known degradation suffered by the structure in its environment over the service period to-date, it is common for engineers to specify protective treatment over the repaired structure in order to enhance its durability. The properties provided by these treatments may include resistance against…
Coloured or pigmented coatings may be selected to enhance the aesthetic appearance of the structure.
(Refer to MAPELASTIC GUARD protective render, ELASTOCOLOR range of anti-carbonation coatings, and PLANISEAL WR range of hydrophobising and migrating protective treatments.)
(The method described above is a typical repair situation but It may not be representative of repair requirements for all project sites. Please call MAPEI Technical Service for assistance with assessing your structure and making appropriate recommendations for repair and protection.)
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