By Gert Schmidt
Hindle Mason Projects (HMP) understands spalling and, therefore, how to “cure” this concrete “cancer”. If left untreated, spalling will spread to a point where it jeopardises structural integrity.
Spalling appears as cracks and red dust. In extenuating circumstances, concrete sections completely break away from reinforcing bar and exposing it to the elements. Just like cancer, as spalling advances, extensive and costlier treatment is required. Therefore, it is important to consult a concrete repair and corrosion protection specialist, such as HMP, for advice timeously.
HMP addresses spalling root causes
HMP successfully addresses spalling root causes and not only the symptom. In this way, we can prolong the lifecycle of concrete assets. If the primary cause of spalling is not addressed, the problem will persist and even become progressively worse.
One of the main causes of spalling is concrete carbonation. This occurs when carbon dioxide and other acidic chemicals in the air and rainwater neutralise calcium hydroxide in cement. They enter concrete via its many interconnected pore structures which are created when bleed water evaporates during curing. This carbonation front starts at the surface and gradually makes its way to the reinforcing bar. Once the alkalinity at rebar decreases, corrosion will start. Bear in mind that the high alkalinity of concrete passivates rebar surfaces, providing an excellent barrier to oxidation. The corrosion products take up eight times the volume of the original steel. The expansion within, therefore, exerts a significantly greater force than concrete’s flexural strength, causing cracking around the reinforcement. These provide further entry for deleterious agents causing more concrete breakdown and accelerating deterioration.
There is also another form of corrosion that causes spalling, namely exposure to chloride ions. These occur in marine environments and also enter concrete via its interconnected pores. Chloride-ions interrupt the steel surface passivation even if concrete still has high alkalinity. Nodules of rust that form on the rebar create internal stresses that crack the surrounding concrete. As with carbonation, the cracks expose rebar to more deleterious agents which accelerates corrosion and exacerbates spalling.
Even well designed and built concrete structures need to be safeguarded against carbon and chloride-ion attack. Generally, reinforced concrete structures are designed for a service life of between 50 and 100 years when maintained by experts. This is according to Euclid Chemical – South Africa.
HMP’s spalling expertise
HMP’s spalling expertise can also prolong the lifecycle of poorly designed and constructed concrete buildings and infrastructure. These are especially vulnerable to corrosion in the very phases of their design lives.
One of the design and construction issues that can lead to concrete spalling includes faulty concrete specification or design. Incorrect placement of reinforcing bar/mesh is another as it leads inadequate concrete cover. Rebar can also be pushed too close to the surface during pouring as a result poor site supervision. Sound concrete design and construction practices entail specifying a minimum concrete cover according to the exposure classification. Good concrete curing technique will also achieve the maximum concrete design strength. To ensure this is met, projects must have adequate site supervision. Considering skills shortages, this can be a problem on many worksites. In some instances, contractors may have not applied an anti-carbonation or chloride-ion resistant surface coating correctly or at all. This leaves structures exposed to these deleterious agents.
How does HMP repair spalling?
So how does HMP repair spalling and protect concrete against corrosion?
Firstly, we remove the loose, delaminated concrete until we reach sound material, undercutting the corroded rebar by about 20mm. We keep the shape of the prepared cavity as simple as possible, usually square or rectangular. Edges are saw-cut perpendicular to the surface to a depth of 12mm to avoid feather-edging the repair material.
We ensure that the final surface texture is rough with an amplitude of about 6mm. Thereafter, we remove residual dust, debris and any containment which impedes proper bonding by water-jetting at a 250MPa minimum pressure.
Using wire brushing, abrasive blasting or needle scaling, we remove corrosion from the reinforcing steel. Thereafter, we apply a protective coating.
Prior to applying cementitious mortars, we pre-wet substrates to a saturated, surface dry condition. This prevents excessive loss of moisture from the mixed mortar at the bond line which will weaken adhesion.
HMP uses quality spalling repair materials
HMP only uses quality spalling repair materials as an approved applicator of these systems.
The type of cementitious material that we select for reprofiling is based on many factors. This includes the placing method, for example, hand-patching, formwork casting or shotcreting. These are determined by the volume of repairs and the required physical strengths of the placed mortars. We will also consider the required properties for compressive and flexural strengths and compressive modulus when selecting a reprofiling material. This is in addition to its resistive properties, such as water permeability and chloride-ion diffusion. Another important consideration is mortar characteristics. These include their permitted application thickness, flow, shrinkage compensation, non-bleed and rate of strength gain. MAPEGROUT T40 | Mapei is an example of quality cementitious material that is applied by hand. MAPEFILL GP | Mapei is used in formwork. MAPEGROUT GUNITE | Mapei is used in shotcreting applications.
HMP then ensures uninterrupted curing protection before stripping the formwork. The recommended curing period is usually seven days.
We then apply a protective coating to the structure. 1_02146_mapelastic-guard_en_9db6341200494268a2c3d1810c1ce58d.pdf is an example of a protective render. Elastocolor Paint | Mapei provides excellent protection against carbon. Planiseal WR | Mapei is a hydrophobising and migrating protective treatment.
Learn more about HMP. www.hindlemason.co.za
Schmidt is a HMP Director
