Deterioration Mechanisms

Concrete

The significance of each of the forms of concrete deterioration listed below are assessed for each element in the structure the Durability Plan is being prepared for.

Reinforcement Corrosion

There are various potential causes of reinforcement corrosion but the primary ones are chloride ingress and carbonation.

Chloride Induced Corrosion

Chlorides change the nature of passivation of steel in concrete such that reinforcement corrosion initiation can occur even at high alkalinity. Chloride ions can be incorporated in the mix (i.e. in admixtures, mixing water or aggregates) or penetrate from the environment (i.e. ground water, wind blown).
The Durability Plan sets out calculations for the ingress of chlorides based on sorptivity, diffusion, permeability and transpiration. Mitigation of corrosion by high resistivity concrete is also included.

Carbonation Induced 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 and the passivation afforded reinforcement  by the concrete’s normally high pH is eliminated.
The rate of carbonation of a 40MPa concrete, for example, is estimated to be 25mm at 100years. Provided this is less than the reinforcement cover corrosion due to carbonation should not be an issue.

Chemical Attack

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 the Durability Plan. 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.

Sulphate Attack

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 and the replenishment rate. The resistance of the concrete is a function of the cement system composition and the concrete impenetrability. AS3972-1997 specifies cement by performance.

Sulphate resisting cement is no longer defined as 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.

Where sulphates are present in the groundwater the Engineering Manager will assess the concrete mix requirements as set out in the Durability Plan.

Mechanical Attack

Abrasion resistance

Abrasion resistance requirements apply specifically to floors subject to pedestrian traffic. The Design Manger will estimate the expected usage and hence wear requirements for each floor. He will then assess performance requirements based on the floor covering to be used as set out in the performance requirements to be included in the Durability Plan.

Impact Resistance

Impact resistance requirements are a part of the standard design procedures for concrete sleepers. This will be the only reference to impact resistance in the Durability Plan.

Thermally Induced Distress

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.

Heat of Hydration

Maximum Temperature

High temperatures (>70oC)can induce micro cracking and a chemical change to the cement hydrates such that the concrete strength is reduced by up to 30%. Delayed Ettringite Formation can also lead to long term deterioration. Hence, the maximum temperature of the concrete during the first 28days shall be limited to 70oC

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 is to limit the thermal strain. For all structures the temperature differential across any section shall be limited to that shown in Table 9.

Table 9 - Limit On temperature differentials

Aggregate Type	Allowable Temperature Differential
Siliceous Gravel	20 oC
Granite	27 oC
Basalt	32oC
Limestone	38 oC

As a rough estimate the predicted maximum temperature rise (TR) towards the centre of the pour can be calculated from Figure 2.
The approximate maximum temperature (TM) and maximum temperature differential (TD)can then be calculated from:
TM = TC+ TR
TD = TM - TA
Provided TM and TD are less than 90% of allowable no further assessment or measurement is required. Where this is not the case a more detailed heat flow analysis is required to show what temperatures are expected. If the detailed analysis shows that the allowable temperatures will not be exceeded then the only testing required is to monitor one pour for the temperature 25mm below the concrete surface and at the point of maximum temperature to confirm the accuracy of the prediction.

If the analysis or testing shows that the allowable temperature will be exceeded then measures to reduce the temperature will be required and the calculations and testing repeated.

Figure 2 -  Predicted Temperature Rise
Temperature Differentials between Pours
The largest risk of cracking is when one concrete pour is cast against another concrete pour eg wall on top of a concrete foundation. In general the new pour is likely to be fully retrained by the old pour. Creep in the old concrete will be minimal over the period of concern.
Where concrete is poured up against hardened concrete the risk of cracking will be specifically assessed. If cracking outside of that allowed by the Durability Plan might occur mitigation measures will be instigated as set out in the Durability Plan.

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.

Cracking

Plastic Cracking

Plastic 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 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.

The Construction Manager is to prepare a Construction Execution Procedure applicable to all concrete slabs which outlines the procedure for assessing if plastic cracking is a risk, the pre-pour procedures to be undertaken where plastic cracking is a risk, and the method of dealing with plastic cracking should it start to occur.

Plastic Settlement

Plastic Settlement occurs when the concrete solids settle and water rises. The solids settling results as the mix grading is imperfect and gaps between the particles are not filled. Settlement is generally only noticeable as the  bleed water on the 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.

Bleed is normally limited to 3%. This is suitable to most ground slabs no more than 300-400mm thick and is even suitable for larger pours where the pour rate is not rapid. For this project, unless it can be shown that alternative requirements are more appropriate, the maximum  pour rate shall be calculated from:

Placing Rate  (m/hr) < -1.5ln(bleed %) +2.5 The allowable placing rate is shown as the grey area in Figure 3. As the placing rate slows and the bleed reduces plastic shrinkage may become an issue (refer to section 4.5.1)

Figure 3 - Maximum Placing Rate
The Construction Manager shall include on the pre-pour check list an item that checks the proposed pour rate conforms with this based on the measured bleed rate of the proposed concrete mix.

Detailing

Detailing will be specifically considered based on the actual design as part of the Durability Plan. The requirements will include, for example,
Wherever cover might be reduced due to details appropriate provision shall be made to offset the increased durability risk. For example where drip grooves are provided, thereby increasing the difficulty of achieving the cover required, the surface of the drip grove shall be coated. The coating will provide added protection.
Ponding of concrete surface leads to accelerated deterioration. Hence the upper horizontal surface on all elements shall be laid to a fall to ensure water shedding. Where this is not possible for  structural or operational reasons the exposure class shall be lifted by one category.
Congested reinforcement can prevent proper placing and compaction of the concrete. The Engineering Manager shall review reinforcement details for each element and verify how allowance has been made for concrete placing and compaction.

Construction

Concrete Supply

Many construction problems arise because the materials supplier is given inadequate information about the purpose to which the materials will be  put. The Engineering Manager shall require that the concrete supplier confirms that the mix to be supplied is fit for the purpose specified by the client and will provide the following details to the concrete supplier for each concrete pour:

• Application:
• Expected Usage:
• Environmental exposure:
• Pour thickness (m):
• Pour size (m3):
• Rate of Placing (m thick/hr):
• Proposed Curing Method:
• Proposed Finishing Method:

Curing

Curing is essential to producing concrete of the quality expected for the designated grade. It is also an aspect that is often not carried out to the level required because of the lack of supervision.

Enforcement of curing requirements is the responsibility of the Project Engineer who shall include in the pre pour check list that materials for curing are available for application. Subsequent to the pour it is not practical for the Quality Manager to check curing is being applied and hence he will establish a reliable system using the site management team to verify that curing as required is applied. The Quality Manger will undertake random checks. Where concrete is found that is not being cured appropriately it shall be raised as a non-conformance.

Waterproofing Considerations

The Durability Plan will outline the water proofing requirements of walls and roofs based on the actual overall design

The Durability Plan will allow base slabs to be waterproofed using integral waterproofers. The design crack width will be 0.2mm max. Where the concrete crack width does exceed 0.2mm all cracks will be required to be repaired in a manner to be specified. The Construction Manager will be required to prepare a Construction Execution Procedure for repair and approval by the Durability Consultant. Requirements for cracks less than 0.2mm wide would be defined based on the environment.

Concrete Coatings

Concrete coatings will be specified in the Durability Plan based on the application. Most coatings will be for decorative purposes and will be specified based on film thickness, elongation at break and adhesion.
Coatings will also be specified for internal surfaces where water run of is considered a possibility. Depending on the risk the coatings may range form a silane to a epoxy based coating.

Joints Durability Considerations

Expansion Joints

Expansion joints are often a point of leakage and raise durability issues for the joints and components that might get wet as a result of their leakage.  Continuous construction and integral abutments should be used wherever possible.

Where expansion joints are unavoidable the drainage design should be such that water drains without contaminating the concrete surface. The Engineering Manager shall include in the Durability Assessment Report details of all joint types. Previous experience of the joint type and materials shall be included to demonstrate the performance.

Tunnel/Station box joints to allow differential vertical movement.

Construction Joints

Construction joints are a point of weakness in regards water penetration. The Durability Plan will consider the water pressure and the aggressiveness of the groundwater and provide requirements for treating construction joints
Requirements will start at the lowest level with standard requirements for internal construction joints i.e.

• water blasting the joint to remove laitance. Unlike mechanical means this does not fracture the aggregate.
• Soaking the surface for 2-3hrs before construction with removal of all surface water 20-30minutes before construction to give a saturated surface dry condition
• Ensuring the concrete surface is not contaminated by a hardened grout from the concrete as it is placed before application of the fresh concrete.

At the highest level construction joints will be required to be sealed with a double water sealing method eg treated with a membrane on the outside face and contain an approved water stop.

Segment Joints

Segment joints are a major source of potential leakage. Jointing to prevent leakage under a 15m pressure head will be particularly difficult. The Durability Plan will require that the jointing system has a proven track record over at least 5years in a similar environment. Details of the installation method on previous projects and evidence of proven performance will be required.

Tile Adhesive Considerations

Tile failures are generally related to the performance of the tile adhesive. Tile adhesive performance will be specified such that materials and workmanship have to meet strict requirements. Testing will be required to show that insitu performance requirements will be achieved.

EXPOSED Steel Durability Considerations

Stray Currents and Interface Currents.

Reinforced Concrete

Stray currents are those that have deviated from their intended path. They may be DC or AC depending upon the source. They deviate from their intended path primarily because the resistance of the unintended path is lower than that of the intended path, or the parallel combination of the two allows part of the current to take the unintended path. Anodic current that is picked up from an external source, such as an electrified transit system, will cause metal loss where it leaves the metal. For instance, the current returning to the traction power substation along the rail may stray from its normal or intended path.

For example it may flow into the soil, where it may be picked up by a gas main and discharged back to the soil and then to the rail near the traction power substation. The points where the current leaves the steel structures and goes into the soil or the concrete surrounding the steel are where metal loss occurs. In some cases, underground structures will pick up current at some point remote from the traction power substation and discharge the current to the soil and then to rail near the substation. At the point of discharge, the corroding structure acts as the anode in the corrosion cell.

While stray current can produce corrosion on nearby structures, corrosion is also very likely to occur on the transit system's properties as well, i.e. when stray current leaves the structure or rail.

Exposed Steel

Stray currents and cathodic protection interference currents create the most aggressive corrosive environment for the underground steel elements and for some components of the above ground metallic appurtenances, such as ground rods, railings and anchors. Stray currents can originate from grounded dc-power supply systems, such as DC rail transportation (trains), welding plants, and high voltage DC power transmission.

Interface currents can originate at impressed-current cathodic protection installations of other (foreign) underground structures. The rail bridge infrastructure can pick up such currents by crossing or closely paralleling the foreign structure, for example, gas and water pipelines.

Inspection

An inspection of the sites in question and the presence/absence of Alinta Gas and Water Corporation pipelines or other facilities protected by a cathodic protection system is to be undertaken but is outside the scope of this project. The Durability Plan will require that the Engineering Manger shows in the Durability Assessment Report that a suitable interference survey has been undertaken.

Bacterial Corrosion

The most frequently encountered bacterial corrosion in the rail tunnel service environment will probably be caused by the sulfate reducing bacteria (SRB). Such bacteria can cause corrosion of steel piles, steel reinforcement and cables in both  anaerobic soils (dense clay-type soils) and water usually in the pH range of 5.5 to 8.5.

The Durability Plan will identify, based on existing data, where additional testing is required.

Stress Corrosion Cracking

Although many metallic components have residual manufacturing stresses or applied stresses, in the rail infrastructure, they are seldom exposed in environments that induce stress corrosion cracking (SCC).

Hydrogen Stress Cracking (HSC) usually takes place where hydrogen enters the stressed metal. In the main, this phenomenon is more likely found on stainless steel fasteners in contact with galvanized steel or galvanized iron. The galvanic coupling generated the hydrogen on the stainless steel cathodes.  The Design Manager will identify any instances of stainless steel/galvanised steel couple present on the proposed structures and how corrosion is mitigated in the Durability Assessment Report.

Steel Components

Soil Nails

Soil nails are particularly vulnerable as they have a limited cross section. Corrosion protection requirements will be specifically considered based on their usage in the design.

Piles

Piles that are subject to varying soil types or soils with differing oxygen concentrations are usually subject to differential aeration cells where corrosion can occur at those locations low in oxygen tension. The presence of mud or silt in contact with the steel piles further enhances this effect.  The likelihood of Concentration Cell effects can be predicted by Redox potential test results.

Pile systems are not so sensitive to local corrosion and being more robust the likelihood of failure is reduced compared to soil nails.

The environment assessment will be based on Table 5 and Table 6. For a 100year life, even with a lower consequence of failure, double protection maybe  required.  The method to be used shall be detailed by the supplier and approved by the Durability Consultant. The ground will be specifically assessed based on the fill material in accordance with Table 5.

Pipes

Steel pipes are not considered as none are being used. Other utilities surrounding the Project structures may be affected by stray currents from the returning rail current or Project reinforced concrete and steel components may be affected by stray currents from cathodic protection of utility company pipes.

Above Ground

Requirements will be included in the Durability Plan based on types of element to be used.

Dissimilar Metals

Galvanic metallic couples are likely in the rail bridge infrastructure. Although bare metallic components are necessary to provide electric safety through grounding, metals in various positions on the galvanic series are used because of strength, conductivity, formability, and cost requirements. Hence the combination of carbon steel with stainless steel, or stainless steel with galvanised steel is quite likely with corrosion to the more anodic material occurring.  Galvanic corrosion may also take place between the embedded rail bridge infrastructure and other inadvertently connected foreign metallic structures, such as copper, copper clad ground rods.

A galvanic couple of stainless steel to mild steel is not significantly different to mild steel to mild steel and would therefore not constitute a durability issue. Where other galvanic contact of dissimilar metals exist the Engineering Manager shall specifically consider them and note in the Durability Assessment Report the analysis of how corrosion has been prevented.

Protection Method Standards

The protection measures listed in Table 6 shall be applied to the following standards:
Zinc Rich Coating – AS3750.9, AS2312
Fusion Bonded Epoxy Coating – AS3862
Heavy Duty Epoxy Coating – AS2312
Galvanized coatings – AS4680, AS4791 & AS4792
Cathodic Protection – AS2832 series

Mechanical and Electrical COMPONENTs & Systems

By thorough and clear definition of the operating environment and atmosphere for the types of equipment/materials to be used the Durability Plan will ensure the Design Life of mechanical and electrical equipment is assured.  Due consideration will be given to:

• Construction methods
• Corrosion assessment
• Strength, creep and fatigue performance
• Cathodic and other electronic requirements
• Coatings and protection
• Maintenance
• Commercial and intangible aspects

The operating environment will determine specification requirements in order to assure durability of materials.  When specifying the materials all relevant Australian/NZ will be invoked providing clear direction on the constitution, method of fabrication/production, physical property determination and testing methods required.  Performance driven specifications will be prepared for inclusion with the equipment and components procurement documents. The Durability Plan will require that the Engineering Manager include in the Durability Assessment Review evidence that that suppliers have met these specification requirements.

The Durability Plan will include the necessary technical information to define the

• Environment
• Equipment performance criteria in relation to durability
• Standards for materials and construction
• Durability and quality documentation to be provided by suppliers
• Warranty and defects response schedules

Once the equipment types and locations have been defined by the Engineering Manager the Durability Plan will consider their integration into the construction and environment and will define documentation, inspection, maintenance and monitoring requirements.