Concrete Durability Design
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A contract was provided to John Holland by the NSW Government for the construction of a new metro railway deep under the harbor of the Sydney. The contract mainly focuses on the construction of 15.5km tunnels. Construction and design of “Sydney Metro City” & “Southwest Tunnel station Excavation” contract comprises tunnels from Chatswood to Sydenham which is traveling under the CBD and Sydney Harbor with the use of five tunnel boring machines (TBM). Between the Temporary Access shaft and the tunnel passage, approximately every 240 meters, there will be 57 cross passages. There will be digging earthwork for construction of six new metro stations located at Victoria Cross, Crow’s Nest, Barangaroo, Pitt Street, Waterloo and Martian Palace. There is a huge manufacturing precast concrete around 99,000 for lining the tunnels. This Sydney Metro in Australia is a public transport metro. The 8.3 billion dollar project is an extensive computer based project that is connecting the North West of Sydney. The work will begin with the manufacturing of the precast concrete segment that will be placed inside the lining of the tunnels. Concrete is the main construction material to be considered here. The construction material should allow appealing curves, graceful curves and durable strength is required for the construction of the Sydney tunnel structure. The concrete provides the advantages. With the advancement of the boring machine it places the segment of the precast tunnel in the rear position. The lining of the concrete is grouted that provides soundness, structural stability and waterproofing in the inside of the tunnel. The automated which is operated by computer driverless train that requires a waterproof, dependable, stability of tunnel configuration. It is the concrete segmental lining that is responsible for the working of the tunnel and the precast concrete that provides the style. In Sydney, these new precast concrete segments are used in the tunnels (Lee and Kwon, 2018). The internal and external radius of the tunnel is 2.975m and 3.54m respectively. Both horizontal and circular reinforcement are used. There is 50mm thickness of shotcrete layer at the external surfaces. A similar kind of structure that was constructed 50 years ago that is located nearby. That structure reveals cracks in concrete, steel corrosion and surface spalling. This deterioration of the structure is due to acid and sulfate attacks, “alkali silica reaction” (ASR), due to the penetration of chloride ions there will be corrosion of the structure.
Long term durability
When we are designing any project, we should focus primarily keeping the “long term durability” in mind. The Sydney’s metro that will be integrated to Sydney’s infrastructure therefore should be able to withhold minimum outlay requirements. The corrosion in the concrete occurs when the air and water penetrate inside the structure and reaches to the inside of the steel bar. As the water reaches the reinforcement inside it, the chemical reaction that takes place is called corrosion. When the corrosion of steel begins, the volume of the rust occupies more volume than the volume of the original steel. This leads to spalling or cracking as tensile stresses start developing in the concrete. Chloride ion reaction and concrete carbonation are the main reason for corrosion in reinforced concrete structures. Carbonation of concrete occurs only when carbon dioxide gets inside the concrete and reacts with the coating of calcium hydroxide on the steel bar. The reaction that takes place is:--
CO2 + Ca (OH) 2 = CaCO3+ H2O
The result of the reaction produces water surrounding the steel bar which leads to the corrosion. In the similar way if chloride ion penetration occurs inside the concrete somehow , then it will corrode the inside steel bar (Yang et al. 2021). The mechanism of chloride ion reaction is that when it reacts with a steel bar it reduces the pH value. That again results in the corrosion of steel bars.
It is a property of the concrete that determines the amount of effort required to operate a freshly mixed concrete with minimum loss of homogeneity. Workability of concrete depends on the amount of compaction done in order to get the desired strength of the concrete. Depending on this project, desired workability is required. A low workability concrete will not be capable of covering and compacting fully over the reinforcement, which may leave air bubbles in the concrete and hence reducing the strength. The concrete required for this project will be prestressed with various types of reinforcing steel, so for the full compaction surrounding the steel, it is necessary to acquire high workability of the concrete.
The consistency and workability of fresh concrete is determined by the slump value. The slump of the concrete is affected by cement content, water content and aggregate size (Rahimi, 2020). As per the specification of the project the concrete should have a slump value of 130mm.
The role of water cement ratio is significant in determining slump value of concrete. Too high value of W/C ratio will produce a very low value of slump as the concrete will be runny. The mix design of the concrete allows a W/C ratio to be approx 0.4 in order to acquire the required strength while maintaining the “minimum slump value”.
The strength of the concrete is its compressive strength, it measures the compression that the concrete can withstand without cracking. There are many factors that affects the strength of the concrete:--
- Compaction: It defines the density of the concrete. The compaction of concrete is affected by the air bubbles and using a high quantity of water in the concrete mix. The more the density, the more will be the strength and durability of the concrete.
- Curing: It is a process that ensures that the concrete will remain hydrated for a longer period of time. This permits the concrete to reach to its maximum strength and also no formation of surface cracks.
- Cement mix: The ingredients used in the concrete are the main factor for achieving the desired strength in the concrete (Wang et al. 2019). Grade of the cement used, size and type of aggregate, W/C ratio are the factors that will affect the strength of the concrete.
Prevention of corrosion: The construction of structure in coastal areas includes major problems of corrosion, therefore focusing on most appropriate design is the key. There are so many methods for stopping corrosion or slowing the rate of occurrence of corrosion. Many initiatives can be taken to prevent the corrosion of structure in the project of Sydney Metro. This process includes keeping a low water cement ratio, application of steel bar that is galvanized, in the cement mix using clean water, grouting and application of supplementary materials of concrete.
Due to effective reduction of corrosion the galvanized steel is widely known. The occurrence of chloride ion penetration inside the concrete but the “galvanized steel bar” will assist in putting off the corrosion occurring due to penetration of chloride ion. The galvanization provides huge protection by coating the surface of the steel with a layer of zinc on the surface of the steel bar. This zinc layer then reacts with any existing corrosive material present and forms zinc carbonate, this layer further protects the underneath steel from corroding. However, since the electropositivity of zinc is more than the steel, this galvanization process can still prevent the steel from corrosion even if the layer of zinc carbonate is destroyed (Bakhshi and Nasri, 2019). According to the study of “American Galvanizers Association” it states that “reinforced concrete structures have significantly longer life even if they are subjected to an aggressive environment in comparison to bare steel bars”.
Water: The quantity of water used in the concrete mix can also lead to the occurrence of corrosion. The water that will be used in the construction process might not be acidic in nature (sea water) or contain carbon dioxide and bicarbonate ions because these may contain a huge amount of salts like chloride and sulphate. So, using filtered and clean water can prevent sulphate or acid attack and penetration of chloride ions. Another step that can be focused concerning this is the “water cement ratio”. In the concrete mix, a minimum amount of W/C ratio should be there in order to attain the sufficient workability and strength of the concrete mix design. The reason is that water used more than desired level can permit easy entry of chemicals inside the concrete.
Supplementary Cementitious Materials (SCM)
The advantage of using SCM is it alters the basic characteristics of the concrete that is suitable for that particular project. In comparison to using plain cement, the use of fly ash cement can prove to be beneficial. The major advantage is using fly ash reduces the permeability of the concrete. This further reduces the percolation of soluble materials, thus reducing the quantity of carbonation. Therefore better protection from corrosion is provided to the steel bar.
The structure's strength, stability and waterproofing can be controlled and improved by the process of grouting. Grouts are applied for filling the “precast concrete slab” present in the lining of the tunnel walls. The grout acts as a water repellant or sealant when it is injected as a fluid in the concrete. This assists in selling the tunnel from the surrounding environment thus prevents percolation of any unwanted substances inside the tunnel.
Cracking: The formation of cracks in the concrete is unfamiliar. A map pattern concrete is observed, so it is important to find out the reason for why and how the cracking of the concrete happens and how to reduce its effects. Plastic shrinkage is the main cause for cracking of concrete. This happens when the top of the concrete hardens quickly and the inside remains still wet (Farzampour, 2019). The other reason for cracking of concrete is due to thermal expansion during the setting phase and expansion because of alkali silica reaction.
The process of curing is to ensure that the concrete is properly hydrated during the phase of setting. This makes sure that the concrete sets evenly and eliminates the occurrence of plastic shrinkage thus allowing the concrete to acquire its maximum strength. This project involves the use of steam curing (Ekolu, 2022). Steam curing enables the slab of precast concrete to achieve its maximum mechanical strength, less susceptible to shrinkage and creep, thus better sulfate resistance and thus leads to less chances of “map patter cracking” that develops in the concrete in both long and short term.
Alkali Silica Reaction (ASR)
The reaction between various types of silica that are present in the aggregate and alkali hydroxide or sodium in concrete is called the Alkali silica reaction. This reaction causes the formation of gel which causes swelling of size due to absorption of moisture from the surrounding of the concrete. If the gel swells enough that will lead to excessive expansion force in the concrete and results in the loss of mechanical strength of the concrete and cracking occurs (Zhao et al. 2020). The concrete mix consisting of fly ash and silica fume along with the use of non- reactive aggregate can totally eliminate the damage of the concrete due to ASR.
Figure 1: Sydney Metro Construction
Material specification must be put in place that requires that the final project built must satisfy the minimum project requirement in the plan of the project. A major concern related to this project is the location where it is placed, that is the coastal area which contains chemical materials along with sewerage. A similar project that shows the formation of map pattern cracking due to sulfate/acid reaction, ASR and penetration of chloride ion, so important measures to be taken to restrict the formation of the same in the “Sydney Metro Project” of tunnel construction (Ferdous et al. 2020). The measures need to be taken to ensure it is minimum concrete cover, water, appropriate curing, aggregates, binder specification and admixtures.
In order to eliminate Alkali- silica reaction, the aggregate used in the concrete must be chemically neutral. The use of porous rocks in the aggregate must be avoided to reduce the expansion of the concrete and also eliminate corrosion. To test the suitability of the concrete a test is performed known as petrography test (Jiao et al. 2018). The concrete used must meet the understated requirements (“CONCRETE BASICS A guide to concrete practice, 2010).
- Strong and hard: That provides adequate strength to the concrete. Flaky or crumbly rocks should not be used.
- Chemically inactive: to make sure that aggregate itself is not reactive to the cement or the surrounding environment. The chances of cracking and corrosion can be limited. Increases the durability of the structure.
- Well graded: Aggregate must consist of a mixture of both fine and coarse aggregate. Concrete should be more workable.
Aggregates are the best way for finely regulating the concrete to the particular design specifications. Due to the properties of the “Polycarboxylic Ether Superplasticizer”(PCE) in self compacting, even a small amount of it can improve the workability of the concrete. This improves the workability of the concrete along with maintaining the strength of the concrete. The “High Range Water Reducer” (HWR) is combined with the concrete to reduce the amount of water that is required in the mix (Paul et al. 2020). This reduction of water content will help in reducing shrinkage and bleeding that ultimately limits the cracking of the concrete. The minimum cover of 45mm is recommended. Keeping durability in mind, a cover thickness of 50mm will further limit the possibility of corrosion.
Supplementary Cementitious Materials (SCM)
The concrete is made by the mixing of materials like water, cement, admixture and aggregates in order to get a concrete that acquires all the necessary properties required for the project. In this project it should be taken care that concrete slab lining for the canal should resist the map pattern cracking of the concrete, spalling of the surface and corrosion of steel caused due to attack by acid/ sulfate, ASR and penetration of chloride ions.
When it comes to the design of concrete, cement is the “basic building block”. The cement is in powder form, when it is mixed with water it forms a paste. The paste acts as a glue material which binds the aggregate and admixtures (Varghese et al. 2021). In the final concrete mix, the cement will constitute even less than 25% of the total mass of the concrete. The rest of the 75% consists of “Supplementary Cementitious Material”.
Figure 2: Fiber reinforced concrete
Water is the main ingredient that combines and mixes all the ingredients into one. The mixing of cement with water to form a paste, that assimilates all other aggregates and admixtures. It is necessary that the water should be clean and must be devoid of unwanted chemical substances.
Fly ash consists of the residue of burnt coals supplied from the power station. Along with Plain cement concrete if the admixtures are used this will provide several benefits. In the setting phase, the use of “fly ash concrete” can increase the workability and reduce the hydration temperature of the concrete. Hence, the final slab of concrete that is made from the mix design will be posing more durability, UTS and reduces the possibility of corrosion and prone to chemical attack through permeability reduction.
Fly ash majorly eliminates the need of water in the concrete mix thus enhancing the workability and increasing the strength of the “Fly Ash Concrete” (Alexander, 2018). This also causes reduction in the speed of the concrete hydration process while setting phase, thus resulting in less evolution of the heat. This property of fly ash makes it possible to set evenly and thus less prone to formation of cracks.
The mixing of aggregate in the concrete has a vital role in the durability, workability and strength of the mixed concrete. The total mass occupied by the aggregate in the concrete mix is 75%, therefore the aggregate that is used must be clean and must comprise both coarse and fine aggregate.
The construction of the tunnel occurs in places from soft clay till hard rock. The construction of the tunnel depends upon factors such as the condition of groundwater, condition of the ground, diameter and length of the tunnel drive, the tunnel depth, planning that supports excavation of the tunnel, final shape and use of the tunnel and suitable risk management (Geiker et al. 2019). Tunnel construction involves underground construction. The basic types of tunnel construction used are:-
- Cut and cover tunnels that are constructed in a shallow trench.
- Bored tunnels are constructed in situ without the removal of the above ground. Usually their cross- section is horseshoe or circular in nature. Some underground process of mining is applied (Melchers et al. 2020). The new modern technique of Shotcrete that is used in the Australian tunneling method, uses Tunneling shield or “Tunnel Boring Machine”(TBM). But still construction of tunnels occurs with the help of shoring and pit props and support of the timber is also provided. Barrel vault technique is also helpful.
- Tunnel tubes are immersed, sink into the water body and buried or lay just under its bed.
Due to the development and the construction, there should not be any crack formation in the tunnel structure. For this, impact assessment should be performed at the design stage itself. Across the development stages of construction, there should not be any increase in the existing cracks that are visible. The cracks should not increase by 300mm total in length or increase by .2mm in width. The composition of visible cracks should not lead to spalling of concrete or hampering the safe operation of the structure. If occurrence of seepage is observed through the visible cracks, then those cracks should be ceased properly by the process of grouting until the seepage stops permanently (Cai et al. 2019). The presence of existing cracks that are visible on the infrastructure must be taken into account during the engineering assessment and analysis. The monitoring plan keeps a check on the existing cracks that are present in the structure and crucial structural elements.
Based on the design of construction of the structure, a risk assessment must be issued that supports the developed application, before commencement of any work. The upgraded risk assessment consists of any modifications in the design and the effect these are causing on the identified risks. “Safe Work Method Statement” (SWMS) must be prepared that include the following:
- According to the risk assessment plan, the detailed work procedure must be implemented.
- A response plan for emergency purposes
For getting approval for the Sydney Metro Project, the developer has to submit the upgraded report of risk assessment and SMWS.
Benefits and impacts of the proposal
This proposal will lead to the delivery of the Sydney Metro Project. During the operation and construction of the proposed project, it will enhance the economic and social benefits by providing employment. The proposal will be managed and designed to get the desired efficiency and to reduce the impact of local communities and surrounding environment (Ventura et al. 2021). The benefits of Sydney Metro is that this will support customers in their daily tasks which may be going to work, education, meetings, sports, for a day out and of course getting home.
The precautions and the steps recommended for this project must be implemented to ensure long life of the Project of Sydney Metro. Failure in the application of these design considerations can cause major impact and also leads to cracking and corrosion as seen in the similar structure seen earlier. Again the use of fly ash cement concrete as recommended along with the “supplementary cementitious material” to ensure that they must meet all the design specifications mentioned above. The life of the surface or activities in the ground will not be disturbed with the construction of the tunnel. The method is made possible with high speed of construction and lesser consumption of power. Reduction of noise pollution. In the areas of high altitudes, this method is free from iceberg and snow hazards.
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