Concrete Defects Flashcards
(19 cards)
What concrete defects are you aware of?
- Carbonation.
- High alumina cement concrete.
- Sulphate attack.
- Chloride attack.
- Alkali silica reactions.
What are the causes and effects of concrete carbonation?
Carbonation in concrete is a naturally occurring process where concrete absorbs carbon dioxide (CO2) from the atmosphere, leading to the formation of calcium carbonate. This process reduces the alkalinity of the concrete, which can eventually initiate corrosion of embedded steel reinforcement.
DWL’s answer:
- Atmospheric carbon dioxide reacts with calcium hydroxide within the cement paste which produces calcium carbonate.
- The reaction of carbon dioxide and calcium hydroxide can only occur in solution, and as such, there needs to sufficient moisture within the concrete for carbonation to occur.
- Concrete carbonation reduces the alkalinity of the concrete whereby if the pH falls below 8.6, the passive layer to the reinforcement steel is lost.
- Oxygen can initiate surface corrosion of the reinforcement steel which decreases steel strength and increases the volume occupied by the steel causing cracking and spalling within the concrete.
How can you identify concrete carbonation?
Concrete carbonation can be identified by observing colour changes in concrete after applying a phenolphthalein indicator solution. The indicator solution will turn pink in areas where the concrete is uncarbonated (alkaline) and will not turn pink in areas where the concrete has carbonated (loss of alkalinity).
- Phenolphthalein indicator solution can be used to determine the depth of concrete carbonation. The solution can be applied to a freshly exposed section of concrete such as a core sample or fractured surface. The indicator turns pink above pH 8.6 and remains colourless below pH 8.6 suggesting concrete carbonation has occurred.
- Alternatively, petrographic analysis under microscope can determine the depth of carbonation.
How can you treat concrete carbonation?
- The reaction of carbon dioxide and calcium hydroxide can only occur in solution and so minimising moisture ingress can control carbonation i.e. repair leaking pipework etc.
- Removal of any corrosion from the exposed steelwork by grit blasting and treat the steelwork with an anti-corrosion coating (zinc-rich paint or cementitious paint system).
- Remove cracked, loose or spalled concrete and patch repair.
- Apply an anti-carbonation coating to the face of the concrete which prevents water and CO2 ingress but allows water vapour to escape.
1) What is high alumina cement concrete, why was it used and what are the defects associated with its use?
- High alumina cement concrete contains a higher composition of calcium aluminates whereas Portland cement concrete contains a high composition of calcium silicates.
- High alumina cement concrete develops high early strength, high temperature resistance, sulphate attack resistance and weak acid resistance. High alumina cement was also relatively fast to manufacture.
- High alumina cement concrete was used in structural concrete to form pre-stressed, pre-cast concrete ‘X’ or ‘I’ beams. The structural concrete was commonly used in floor beams, roof beams or marine applications.
- High alumina cement concrete is prone to a crystalline rearrangement called conversion. The rearrangement can create voids within the cement matrix which reduces strength of the concrete, increases porosity leading to increased steel reinforcement corrosion and decreased resistance to sulphate attack.
In what age of building would you be most likely to find high alumina cement concrete?
High alumina cement first developed in the UK by LaFarge in 1925. However, high alumina cement was used most commonly in the 1950’s, 1960’s and 1970’s. High alumina cement was effectively banned for structural use as of 1976.
How would you identify high alumina cement concrete?
- The age of the construction may be indicative as high alumina cement was developed in 1925 but used most commonly between 1950 – 1976 when it was banned for structural use.
- The type of construction may be indicative as high alumina cement was often used to form pre-stressed, pre-cast concrete ‘X’ or ‘I’ beams for use as floor beams or roof beams.
- The Building Regulation Advisory Council (BRAC) published a 3-stage assessment framework for high alumina cement (commonly known as the BRAC Rules).
Stage 1 (identification)
- Visual inspection is not very reliable. Concrete can appear dark grey with a brownish tinge.
- Rapid chemical test in sodium hydroxide solution and oxine. Presence of yellow precipitate indicates high alumina cement.
- Petrographic analysis.
Stage 2 (strength assessment)
- Determine the structural capacity of the concrete (accounting for reduced ‘conversion’ strength) to withstand the applied loading.
Stage 3 (durability assessment)
- Determine the durability of the concrete to withstand chemical attack and steel reinforcement corrosion.
- Petrographic analysis of alkalis and sulphate. Visual inspection of the steel.
How can you treat defective high alumina cement concrete?
If the BRAC analysis deems the high alumina cement concrete to be unsatisfactory with regards strength and/or durability, the following remedial work may be undertaken:
- Removal of any corrosion from the exposed steelwork by grit blasting and treat the steelwork with an anti-corrosion coating (zinc-rich paint or cementitious paint system).
- Remove high alumina cement concrete completely.
- Install secondary support systems to structural high alumina cement concrete.
- Carbon plate bonding which involves bonding sheets of carbon fibre to the underside of the high alumina cement concrete member which increases the structural strength of the concrete beam.
- or, in some cases, complete removal and replacement of the affected sections.
What are the causes and effects of sulphate attack in concrete?
- Sulphate attack often occurs in concrete ground-bearing floor slabs or cement mortar due to contamination with water soluble sulphate salts (such as gypsum or magnesium sulphate) from sources such as groundwater, seawater or brickwork.
- Sulphate salts react with tricalcium aluminate to form ettringite.
- Ettringite leads to the formation of crystals leading to significant expansion in the concrete or mortar and resultant bowing and cracking.
How can you identify sulphate attack?
- Sulphate attack in cement mortars is characterised by horizontal and vertical cracking along the mortar joint.
- Sulphate attack in restrained concrete floor slabs is characterised by upward bowing, map pattern cracking and possible displacement of brickwork or blockwork at slab level.
- Petrographic analysis under microscope can identify the presence of gypsum or ettringite.
- White Crystalline Deposits (Efflorescence): These deposits, often appearing as white salts, can be a tell tale sign of sulphate reaction.
How can you treat sulphate attack in concrete?
- Minimise moisture ingress which will be transporting sulphate salts into the concrete (such as repairing leaking pipework).
- Ensure ground-bearing concrete slabs are laid over a damp-proof membrane to prevent the migration of sulphate salts into the concrete.
- Remove and replace significantly damaged concrete using a sulphate-resisting concrete which has a low concentration of tricalcium aluminate.
- Consider removal of external renders which may crack and trap water against brickwork causing sulphate salt migration out of the brickwork and into the mortar.
What are the causes and effects of chloride attack in concrete?
- Chloride attack occurs in concrete due to contamination by chloride ions which can be introduced as an accelerant during the mixing process, introduced from unwashed marine aggregates within the concrete or introduced externally by de-icing salts or seawater.
- Chloride ions can cause the passive layer to the reinforcement steel to be lost in localised areas.
- Chloride items can also increase the conductivity of electrical conductivity of concrete.
- This can cause rapid corrosion in localised areas of the reinforcement steel which leads to pitting within the reinforcement steel and loss of cross-sectional area.
How can you identify chloride attack?
- Cracking or delamination of concrete may occur.
- Localised pitting to steel reinforcement.
- Deformation of concrete may occur due to loss of structural strength following steel corrosion.
How can you treat chloride attack in concrete?
- Limit contamination with chloride salts such as de-icing salts.
- Removal of any corrosion from the exposed steelwork by grit blasting and treat the steelwork with an anti-corrosion coating (zinc-rich paint or cementitious paint system).
- Removal of significantly pitted steelwork.
- Patch repair or full replacement of cracked or highly contaminated concrete.
- Surface treatment to prevent further ingress of external chloride salts.
What are the causes and effects of alkali silica reactions?
- Alkali silica reactions are a form of alkali aggregate reaction.
- Silica within the aggregate can react with alkaline hydroxides to form a gel which expands as it absorbs water leading to significant expansion in the concrete and resultant cracking.
How can you identify alkali silica reactions in concrete?
- Minimise moisture ingress into the concrete (such as repairing leaking pipework).
- Removal or patch repair of cracked or delaminated concrete.
What is Regents Street disease?
- Refers to corrosion of steel framed buildings built largely between 1900 – 1950.
- The buildings were tightly clad in stone, brick or terracotta and voids between the cladding and the frame were filled with low-grade mortar.
- Water ingress can cause steel corrosion and the steel can then occupy a volume of up to 7 times its original thickness.
- The steel corrosion causes cracking to the cladding owning to its close proximity and mortar infill to the voids.
- Cracking to the cladding can lead to further water ingress and risk of cladding detachment.
How you would identify Regents Street disease?
- Knowledge of the form of construction and age of construction can indicate the building may be at risk of Regents Street disease.
- Cracking which mirrors the arrangement of the steel frame.
What remedial methods are available for Regents Street disease and what are the associated advantages and disadvantages of these methods?
- Firstly, it is important to remove all visible sources of water ingress to the steel frame (i.e. replace faulty rainwater goods, repair damaged roof coverings, repoint degraded mortar joints etc.).
- There are then two prominent remedial methods available.
- Remove the external cladding and full investigation of the steelwork - Removal of any corrosion from the exposed steelwork by grit blasting and treat the steelwork with an anti-corrosion coating (zinc-rich paint or cementitious paint system). Alternatively, remove and replace the steel if deemed structurally inadequate. Reinstate cladding but maintain an air gap between the cladding and the steel frame. This method is expensive, highly disruptive, does not address all steelwork (unless the entire envelope is removed) and may be unfeasible on listed properties.
- Cathodic protection -reverses the direction of the electric currents associated with corrosion. The technique involves an external power source connected to the steel frame and to sacrificial anodes (discrete rods or titanium mesh). The technique ensures the steel frame acts as the cathode and so does not continue to corrode. Method is much cheaper and less invasive. However, can not rectify previous corrosion damage to the steel and requires ongoing of the power supply current.
