Table of Contents
supplementary Cementitious Materials
Sydney also known as the Emerald city is a coastal metropolitan city with over five million residents which makes it the largest city in Australia. The city is full vibrant places including luxurious landmarks like Sydney Harbor Bridge and the famous Sydney opera house. People from all over the world come to this city which makes it the biggest and attractive destination for tourists, students, emigrants in Australia. Due to the need for the increase in the transport infrastructure, a proposal for the fully automated rapid transit metro was raised for the first time in 2001 and later in 2008 to have a rapid transit fully underground railway system.
Australian government is providing a revolutionary measures in transportation sector for its citizens by initiating an $8.3 billion project which is entirely an automated train system without a driver and that connects the north west of Sydney. It will cover more than 66 and having 31 metro stations will prove to a big revolution in Sydney transport system. To have an underground tunnel for connecting that is one of the most emphatic and challenging task involved in the project, precast concrete segments are chosen as the most appropriate material to be used in this metro rail system. The specialized concrete factory named as Marrickville has successfully generated 99,746 concrete segments that will be utilized to line the 15.5 km from Chats wood to Sydenham that also includes Sydney harbor. This is a twin metro railway station. A tunnel boring machine is used for this purpose to install six concrete segments that will make up one ring of tunnel. It is estimated that 16,451 rings will be used to line each tunnel.
This report will be based on the reasons of why precast concrete is used in tunnel linings and the essential requirements needed in concrete mix design in order to avoid any damages to the location of tunnel across coastal area from the sewages and deterioration issues and maximum durability is to be ensured.
In this particular project, fiber-reinforced precast concrete linings will be used as they are more suitable now a days in soil boulders and similar tunnel projects. Precast concrete segments lining will be placed at specific positions in the tunnel with the help of Tunnel Boring Machine. In order to tighten the segments bolts impact wrenches are used. Tapering of linings rings is used to achieve the curved alignment and all rings are tapered in order to have the desired curvature by making necessary adjustments in the orientation of the rings. The concrete segments are checked thoroughly to make sure that there are no damages or cracks before taking them into the tunnel.
Water tightness is an extremely important factor in the durability of the tunnel lining. For this purpose a double gasket system is used having elastomeric gasket and specially designed water sealing made from hydrophobic material. The gaskets are put into the grooves which are found at the edges of precast concrete segments. The use of steel moulds, gaskets along with highly précised concrete segments are helpful to maintain durability and water tightness in the tunnel structure. The basic advantage of hydrophobic seals is that when they come into contact with water, they seals may expand up to 250% of its original size.
In order to have protection against fire and have adequate safety measures as trains caught fires commonly these days, an extra thickness of concrete covers diameter of 75mm is to be provided over the steel reinforcement. This is a mandatory precaution that cannot be neglected. To ensure the failure in the ductile phase if it occurs, under reinforced concrete segments can be the best option.
The process of grouting will be done in line with the tunneling. Tunnel boring machine has inbuilt ports which are used for the grouting of voids that are found between excavation profile and on the outer structure of the precast concrete ring. Grouting will be done up to 3 bars having (0.3 MPA) pressure. Till the tunnel lining is not ended, excavation will not be started. Secondary grouting will be done within 14 days. There will be a pressure of (0.3 MPA) to every third ring during the grouting process. The purpose of secondary grouting is to ensure that all the voids are filed up that may remained during the primary process due to shrinkage. This will help to prevent any water and materials that can deteriorate the concrete structure.
The construction process of tunnel can be significantly divided into two steps. First one includes removing of earth and other includes curing of concrete wall and slab. Curing of concrete is one of the main factor which in turn shapes up the durability factor of overall structure. Improper and poorly measures taken in this regard results in weak and porous material near the surface. During the tunnel lining the main focus is to maximize the quality measures and deterioration is too be avoided at any cost. Any lacking in this manner is strictly checked and prevention from ingress of different substances which is harmful for the structure is ensured.
Curing starts right after the concrete placing and finishing. It must be kept in mind that adequate temperature and suitable moisture should be maintained for some extended time period. This must be done both at the depth and near the surface. Following factors must be kept in mind which affect the duration of suitable curing time of concrete:
For the project of structural concrete like this tunnel project, it is suitable to have a curing time of seven days above 40 degrees Fahrenheit. This is in accordance with the American Concrete Institute (ACI) Committee as it will give approximately 70 percent of the specified compressive strength. This level of strength can be achieved at higher temperatures and also when specific combinations of admixtures are used. The following minimum curing periods must be followed as ACI Committee recommends them.
The material requirements and specifications for concrete lining of tunnels needs to be examined with great care as they differ from normal concrete works. For this all the requirements for fine and coarse aggregates must be in accordance with standard specifications. A concrete pump is to be used for placing the concrete. Thin sections of concrete, rock profile irregularities and also where curvature is needed high precision is to be maintained. Slump of concrete is managed not less than 10 cm and sand content have to more than that specified in IS 456-1964. In case where concrete is to be placed directly such as inverts and kerbs, the slump can be reduced to 5cm. in order to improve the pump ability of concrete it is recommended to use natural aggregates sand in particular.
Size of aggregates is preferred to be not more than 40mm but in some cases it may reduce depending on specific areas. Seepage water issues are also needed to be monitored carefully and it should not be mixed with green concrete lining.
The timing of placement of concrete lining is dependent on the state of rock tunneled inside. It is advisable in this project to allow adequate time for rock to dilate as it will ensure that concrete will not be failed to cracks developed on the surface because of heavy load and external forces.
In this case where heavy external load is to be applied on the structure, a thin layer of concrete is to put between the steel ribs which prove to be a support for the entire surface of rock. Blocking concrete is suitable to be placed with least time that is possible after excavation. Better quality rock is advisable to be used as it will help for the concrete lining to come at some appropriate time. The shape, size and rock strata and what equipment is used, type of formwork are the main factors which decide for the sequencing of concrete placement.
Following sequences can be adopted:
A 25 mm thick layer of mortar is advisable to cover the surface of old concrete. Concrete pump should be used for placing of concrete or a pneumatic placer is suggested. To avoid segregation, the discharge end of the concrete pipe is to be kept inside the concrete that is placed freshly. It is essential to use batching plant and mixing plant to manufacture concrete for the lining for large tunneling project like this particular one where initially outside the tunnel , Concrete is to be mixed in standard batching and mixing plant and then mixed concrete is taken inside the tunnel at the placement site. Adequate amount of cement is preferred to be added.
Fly ash, silica fume, rice husk ash and water glasses are one the prominent industrial by-products (IBP) that are effectively utilized in the concrete industry as supplementary cementitious materials. These materials have pozzolanic properties that are useful for the performance of Portland cement based materials. In an underground tunnel project, high –performance non-shrinking grout is considered as a high strength material that is best suited for precision in grouting and general construction applications.
Sulfate attack is a common example of concrete deterioration. It occurs when concrete is in contact with the water containing sulfates (SO4). These are found mostly along the arid conditions like in sea water and waste treatments plants and underground structure. In this project, some immediate maneuvers needs to take to overcome this issue. Sulfates are primarily responsible for the disintegration of some cement hydration products. Following factors affects the amount and severity of sulfate attacks includes permeability of concrete, Ca (OH)2 content and C3A content. It is advisable to minimize the alumina content by reducing the C3A content in portlnd cement . Use of slag cement is one of the most effective method that can prevent the damage done by sulfate attack.
First of all, it is suggested to check whether the sulphate attack comes from internal or external sources. Internal sources are less often, they normally originate from concrete-making substances like hydraulic cements, fly ash, aggregate or admixtures. It is observed that presence of natural gypsum in natural aggregate, admixtures also have some amounts of sulphate or Portland cement might be over-sulphated are the main reason of attacks.
Externally, due to the presence of high-sulphate soils and ground waters are main reasons of sulphate attacks. Other than this atmospheric and industrial water pollution also causes sulphate attacks.
Keeping in view the previous issues of deterioration, few necessary steps should be taken to avoid such attacks in this tunnel structure.
1. A low permeable concrete is suggested as it proves to be effective against sulphate attack.
2. High cement content
3. Appropriate concrete thickness
4. Proper curing time and compaction
5. Low water to cement ratio
Further, sulphate resisting cements materials are to be recommended against this issue.
This is normally termed as the cancer of concrete as this is highly toxic reaction that happens in concrete with the passage of time between alkali cement paste and silica compounds that are found in aggregates. This reaction normally triggers when adequate moist conditions are provided. Water plays the primary role in these kind of reactions not only as a solvent for reaction but also as a medium of transport for the reaction of dissolved species. In fact, it also acts as a reagent after the reaction too.
The best possible solution for this is to repair the damages structure possibly but as a usual safety measure, a water tight membrane is recommended to be installed that helps to slow down the reaction from evolving the deteriorating process. Because of this, cutting slots across the structure is also advisable as they may release some pressure which is helpful for the functioning of the structure. Few preventive and necessary standards needs to follow.
1. Non- reactive aggregates should be chosen.
2. Using of low alkali (LA) cement
3. Water membrane is to be used
4. Limiting the contact of water filtrations with the concrete structure.
Chloride attack is basically linked to the durability of the concrete structure. Almost 40% concrete failure happens due to this attack. This attack immensely deteriorates the reinforcement by causing corrosion in it that effects the durability of structure. This attack can be originated either from inside or outside of the concrete structures. In the construction projects like this Sydney metro project which is near the coastal area, chlorides may exist in concrete at the time of casting process due to use of sea water, use of calcium chloride as additive during the setting time, use of aggregates having unwashed chlorides and also the aggregates having chloride content more than limit. Externally, chloride enters the concrete due to exposure to sea water, use of salt for melting of ice etc.
This attack is extremely disturbing for the strength of concrete structures as it will be effect the reinforcement of steel which put all the structure in great susceptible state.
Few of the essential methods are to be adopted in this case for the durability of structure.
1. By increasing the cover of the reinforcement bar is the most suitable way for the prevention of this chloride attack. This will be helpful to increase the life period of tunnel structure
2. Use of stainless steel-clad-rebar can be operated.
3. Using a rebar coated with epoxy having cathodic protection.
4. Low permeable concrete is advisable.
1. Adam D. Neuwald (2010) at https://precast.org/2010/05/supplementary-cementitious-materials/
2. Bao Lu, ... Tung-Chai Ling, in Carbon Dioxide Sequestration in Cementitious Construction Materials, 2018 at https://www.sciencedirect.com/topics/engineering/chloride-penetration
3. David Burroughs Mar 24, 2020 at https://www.railjournal.com/passenger/metros/tunnelling-for-sydney-metro-city-and-southwest-complete/
4. Desirée Rodríguez-Robles, ... Nele De Belie, in New Trends in Eco-efficient and Recycled Concrete, 2019 at https://www.sciencedirect.com/topics/engineering/supplementary-cementitious-material
5. Halvorsen, G T,Kesler, C E,Paul, SL in University of Illinois,Urbana-Champaign (1975-78)
6. Jorge de Brito, ... José D. Silvestre, in New Trends in Eco-efficient and Recycled Concrete, 2019 at https://www.sciencedirect.com/topics/engineering/chloride-penetration
7. Michael John McCarthy, Thomas Daniel Dyer, in Lea's Chemistry of Cement &Concrete (Fifth Edition), 2019 at https://www.sciencedirect.com/topics/engineering/acid-attack
8. R.K. Goel, ... Jian Zhao, in Underground Infrastructures, 2012 at https://www.sciencedirect.com/topics/engineering/acid-attack
9. T.G. Nijland, J.A. Larbi, in Non-Destructive Evaluation of Reinforced Concrete Structures: Deterioration Processes and Standard Test Methods, 2010 at https://www.sciencedirect.com/topics/engineering/acid-attack
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