Deterioration Mechanisms

BCRC have expertise in most concrete deterioration mechanisms. When preparing Durability Plans and when assessing the cause of defects the exposure and structural configuration are considered in relation to these mechanisms to determine a suitable durability design or remedial approach. Key aspects of the deterioration mechanisms are out lined below.

Abrasion

Abrasion is where the concrete surface is worn away by materials rubbing across the surface. Areas where abrasion are more commonly a problem include

  • Invert of pipes and culverts
  • Roads and pavements

BCRC specify appropriate mix designs and toppings to limit abrasion damage.

Alkali Aggregate Reaction (ASR)

Alkali from cement can react with the silica in aggregate to form a gel that imbibes water. The resultant swelling can lead to cracking of the concrete. Table 8 of Report No T47 “Alkali Aggregate Reaction in Concrete” 1996 sets out a procedure for aggregate assessment. This procedure, and associated testing detailed in the report are generally specified in BCRC Durability Plans. Only aggregate that has been shown to have a low risk of becoming innocuous in the proposed mix should be used. The specification should require that all concrete suppliers confirm the means and results by which the aggregate is deemed to be innocuous.

Carbonation Initiated Corrosion

The reaction of atmospheric carbon dioxide and calcium hydroxide released by Portland Cement results in the formation of Calcium Carbonate. This chemical reaction lowers the alkalinity of the concrete. Once a carbonation front reaches the reinforcement the passivation afforded reinforcement  by the concrete’s normally high pH is eliminated. Carbonation is generally only an issue in the following circumstances:

  • Poor quality concrete -
  • Floors with high bleed and SCMS
  • Low strength silica fume concrete
  • Wet internal areas
  • Enclosed car parks
  • Automobile tunnels

BCRC's analysis will include calculations of the rate of carbonation based on the exposure, concrete mix and concrete quality. An assessment of the recoat interval for carbonation barriers can be provided by including the diffusion rate of carbon dioxide through the coating, its applied thickness and its oxidation rate.  

Chloride Initiated Corrosion

When the chloride concentration at the reinforcement reaches a certain level (activation level) the natural passivation of the reinforcement is broken down and the reinforcement will start to corrode. There are various potential sources of chlorides

  • Maine - Marine structures are subject to high to very high chloride levels from splashing and wind blown
  • Coastal Exposure - Chlorides can be blown 50km inland and can build up on the concrete surface to quite high levels in coastal areas.
  • Groundwater - Groundwater, including stream and river water, can have chlorides ranging from low to very high.
  • Process water - Chlorides in plant process water may contaminate the concrete when used as wash down water or where the process water is circulated across the concrete.
  • Concrete - Chloride ions can be incorporated in the mix (i.e. in admixtures, mixing water or aggregates).
  • Industrial chemicals - Many chemicals used in industrial processes contain chlorides and these can lead to corrosion when the concrete is exposed to them.

BCRC's analysis typically includes calculations for the ingress of chlorides based on sorptivity, diffusion, permeability and transpiration.

Delayed Ettringite Formation (DEF)

This is an internal form of sulphate attack caused where temperatures during curing are excessive. It results in cracking many years from construction. The main elements at risk are:

  • Precast sections that are steam cured
  • Thick sections where there is a high heat of hydration

BCRC specification limit maximum curing temperatures relative to cement type to eliminate the risk of DEF.

Evaporative Concentration

Where water evaporates from a concrete surface it leaves behinds any salts it contained. These salts can build to much higher levels than the concentration in the original water would suggest. Ultimately they can for a wide range of deterioration. Areas where this is more commonly an issue are:

  • Capillary rise areas
  • At leaks in lining
  • Where water runs across a concrete surface

BCRC limit this by removal of the water source, use of water replants or by appropriate coatings.

Sulphate Attack - External

Sulphates in the ground water can attack cement paste in two principle ways. In high concentration acid attack takes place while at lower concentration an expansive reaction occurs. The aggressiveness of the environment is a function of the sulphate concentration , the form of the anion and the replenishment rate. The resistance of the concrete is a function of the cement system composition and the concrete impenetrability. 

Sulphate resisting cement is no longer defined as cement with a low C3A content. Cement is defined as sulphate resistant if the expansion of a mortar bar is within specified limits when subject to a 5% sodium sulphate solution. Typically 65% slag, 25% Fly ash or 5% silica fume blended with a GP cement can be classed as a sulphate resisting cement.

BCRC specify speciality cement systems, low water : cement ratios and coatings for sodium and magnesium sulphate attack problems.

Impact

Impact damage results where concrete is subject to repeated sudden localised loads. The principle method of preventing impact damage is to increase strength and ductility. Fibre reinforcement is particularly beneficial.  

Plastic Shrinkage Cracking

Plastic shrinkage cracking occurs when the rate of evaporation exceeds the rate at which bleed water arrives at the concrete surface. Evaporation water is then drawn from below the surface causing a reduction in concrete volume and cracking while the concrete is still plastic.

Evaporation is increased by high temperatures, low air water contents and high wind speeds. Bleed is a function of the mix grading. Fines material, particularly silica fume, has a dramatic effect on bleed such that at 10% silica fume the bleed is negligible and at 5% is about half of that of a similar GP mix. BCRC are able to estimate the risk of plastic cracking by calculating evaporation rates and the rate of water rising to the concrete surface.

Plastic Settlement Cracking

Plastic Settlement occurs when the concrete solids settle and water rises. Settlement is generally only noticeable as bleed water on the concrete surface. The settlement of the surface is generally too small to detect visually. Allowable bleed after screeding should not exceed 2mm if plastic settlement  is to be acceptable. Plastic settlement is consequently more of a problem on deep pours where, if concrete is placed rapidly, even small bleed rates can result in a high overall settlement.

BCRC can calculate the maximum allowable placing rate based on the bleed of the concrete and pour height.

Thermal Cracking

Thermal distress can result from high temperatures or high temperature differentials during the early life of the concrete. The temperatures can arise from hydration of the cement or heat curing. Typical issues include:

  • Temperature Differentials Within One Pour - Where the body and the surface of a concrete pour heat up and cool down at different rates temperature differentials can lead to cracking. The stress development is complex as the concrete properties are changing rapidly during the initial few days (i.e. when temperature differentials are at their peak). A simplified approach to restrict cracking due to temperature differentials to limit the thermal strain.
  • Temperature Differentials between Pours - The largest risk of cracking is when one concrete pour is cast against another concrete pour, e.g. wall on top of a concrete foundation. In general the new pour is likely to be restrained by the old pour. Creep in the old concrete will be minimal over the period of concern.
  • Maximum Temperature - High temperatures can induce micro cracking and a chemical change to the cement hydrates such that the concrete strength is up to 30% lower than standard cured concrete. High curing temperatures can also lead to DEF. 

BCRC have a range of software for predicting the insitu concrete temperatures and these are used to determine the risk of unacceptable cracking.

Steam Curing

The requirements for steam curing are set out in the Austroads Bridge Code however the method of monitoring temperatures is not. The Durability Plan will detail this if required. It is likely to require that the precaster prepares a Construction Execution Procedure showing how the steam curing will be undertaken and includes calculations that show the maximum concrete temperature allowed in the Durability Plan (section 4.4.1) will not be exceeded.

On the first pour the concrete temperature will be monitored to check that the  maximum temperature allowed is not exceeded. If the results show that the maximum temperature is not exceeded then the steam curing method shall be accepted. Checking of the maximum temperature achieved shall be undertaken at approximately monthly intervals.