Assessment Task 3 - Concrete Durability PDF

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2020

Assessment Task 3 – Concrete Durability

48352 SOIL BEHAVIOUR

Table of Contents 1.0 Introduction

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2.0 Design Considerations

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3.0 Durability Characteristics

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4.0 Supplementary Cementitious Materials

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4.1 Slag / Cement Binder

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4.2 Fly ash / Cement Binder

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5.0 Material Specification

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6.0 Bibliography

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1.0 Introduction The following report has been created to analyse the concrete mix designs presented in the previous report for the construction of the precast concrete segments to be used in the Sydney Metro Tunnel. The Sydney Metro project is set to cost $8.3 billion that will connect Sydney North West with the city. The project implements revolutionary computer operated driverless trains that will run every few minutes, delivering world class public transportation. Concrete has been chosen as the main construction material because of its favourable characteristics, especially in a tunnel environment. The two concrete mixes that have were proposed in the previous report were designed to have the following requirements as a minimum:

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Characteristic compressive strength of 55MPa

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Minimum slump of 130mm

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Precast concrete segments will be steam cured

This report will further discuss in detail the durability characteristics of each mix, how the supplementary cementitious materials used affect some of the characteristics and finally, and further additives that have been considered to improve durability in coastal environments.

2.0 Design Considerations There are many considerations to be made when designing a concrete mix that is to be used for the construction of precast concrete segments in a tunnel environment. As such environments are very harsh, the concrete is subjected to a variety of different chemicals and contaminates that will destroy the structural integrity of the concrete if not designed or constructed correctly. Portions of metro line will tunnel below Sydney Harbour. These conditions greatly increase the risk of corrosion due to the high alkali and acid concentrates in the area. Exposed to these environments, the precast segments are placed under increased stress and any imperfections will be exploited and potentially resulting in segment failure due to severe corrosion. Surface spalling and steel corrosion are typical defects that will be seen in such environments and although these defects are typically seen through improper placement of the concrete, the mix design plays a significant role in ensuring that the mix is workable. The mix must be designed in a way to eliminate the chances of poor placement due to poor concrete characteristics. Slump, set times, and set rates must be accounted for to ensure correct placement within the segment moulds. The Sydney Metro project has very strict design requirements for the precast segments. Including, minimum compressive strength of 55 MPa, minimum slump of 130mm and a strong resistance to cracking to minimise the chance of corrosion. Furthermore, each of the segments are to be built in accordance with Australian standards and meet the minimum requirement of 100 years before induced concrete cracking should occur.

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3.0 Durability Characteristics The following two concrete mix designs have chosen as potential mixes for the construction of the precast concrete segments. Each have been specifically configured to address the minimum requirements of the project as well as ensure that the effects of corrosion are minimised.

Material

Unit of Measure

Cement Fly ash GGBFS 20mm 10mm Coarse Sand Fine Sand Admixture (WR) Admixture (HWR) Water Total

Kg/m3 Kg/m3 Kg/m3 Kg/m3 Kg/m3 Kg/m3 Kg/m3 ml/m3 ml/m3 litre/m3 Kg/m3

SSD Batch Weights (Mix 1 – Slag / Cement) 275 0 275 735 315 200 375 2475 1000 185 2410

SSD Batch Weights (Mix 2 – Fly ash / Cement) 420 140 0 600 330 270 430 0 2000 180 2415

The durability of concrete is defined by its ability to resist weathering, chemical attack, and abrasion while maintaining its desired engineering properties. Concrete durability has been the main focus for both mix designs. As the Sydney Metro is located in both coastal areas as well as below sea level it is critical that the proposed mixes will have the capability of providing a 100-year design life. Both mixes are also capable of meeting the minimum requirements for the project.

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4.0 Supplementary Cementitious Materials 4.1 Slag / Cement Binder “Slag cement or otherwise known as ground granulated blast-furnace slag (GGBFS) is produced by quenching molten iron slag to produce a glassy product which is then grounded into a fine powder. It is added to concrete mixes as a supplement for cement that gives the concrete different characteristics.

Figure 1: Effect of slag on Portland cement

Although a straight cement mix will before better during the initial stages of curing, the addition of slag gives the following improvements:

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Better workability of the concrete by prolonging the initial setting and reducing heat in the mix which in turn allows the concrete to be finished easier. Higher compressive and flexural strengths at 28 days when mix ratio is around 50% slag cement. Depending on the application of the concrete a higher ratio will increase the strength of the concrete at 7 and 28 days. Lower permeability. This characteristic is extremely beneficial for the construction of the Sydney metro as portions of the tunnel are below the seabed and the ability to resist water is desired. Improved sulfate resistance. Elements in the slag react with CA(OH)2 to create calcium silicate hydrate gel which is the primary reason for strength is cement based materials. Producing a more consistent plastic and harden properties. The precast segment is required to be constructed with little to no defects. With the addition of slag, the chance of this is reduced.

Figure 2: Effect of slag on compressive strength

Overall, the addition of slag into a concrete mix design is effective is reducing permeability and increasing the durability of the concrete as well as reducing the environmental impact of concrete mixes.

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4.2 Fly ash / Cement Binder Fly ash, like slag is used as a supplementary cementitious material (SCM) in the production of Portland cement concrete. Fly ash is a powder that resembles cement and has been used in concrete since the 1930’s. Fly ash is produced through coal-fired electrical generating stations. As the crushed coal burns away in the boiler, the non-combustible inorganic minerals melt together and form tiny molten droplets. Once cooled, these droplets form glassy particles which are then grounded to produce fly ash. Like slag, fly ash alters the characteristics of the concrete in both its fresh and harden states. These changes are explored below: -

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For fresh concrete, the workability is improved, and water demand is reduced. Additionally, there is less segregation making is easier to pump and bleeding at high replacement levels is significantly reduced. The initial set time for the concrete is extended based on how much fly ash is added to the mix. Care must be taken as adding to much fly ash can severely retard the setting times. This is increased in cold conditions. Early age strength of the concrete is reduced depending on the amount of fly ash added to the mix, especially at 1 day. Hence, if being used in a precast factory it would not be wise to have a ratio of >30% as you would have to leave the segments in the moulds for longer, reducing production rate. Long term strength is increased. Although it may reduce the early age strength, adding fly ash to a concrete mix will increase the strength the older it gets. Permeability and chloride resistance are increased with increasing amounts of fly ash. The expansion due to alkali-silica reaction and sulfate resistance is reduced and completely supressed with the correct amount of fly ash. Figure 3: Effect of fly ash on compressive strength

Overall, fly ash is a beneficial additive for concrete and the optimum amount will vary of application. It also offers environmental advantages by re-using industry waste products. It is important to completely understand the requirement of the concrete when using fly ash, as incorrect doses could significantly impact the performance of the concrete depending on its application.”

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5.0 Material Specifications

- General Materials used in the production of the concrete must conform with the guidelines specified in the Australian Standards AS 1379

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Cement

All cements including supplementary cementitious materials (SCMs) used in the works must conform to the specifications outlined in the Australian standards and the RMS 3211 specification.

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Admixtures

Chemical admixtures must conform with the requirements outlined in the Australian Standards AS 1478.1. They must not contain calcium chloride. Where there are two or more admixtures proposed, their compatibility must be certified by the concrete manufactures.

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Aggregates

All aggregates used in the works are to conform with the Australian Standards AS 2758.1

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Testing

Tests must be carried out in laboratories accredited by NATA, unless it is the testing on slump where that is conducted on site before the mix is placed. Testing frequencies must be adhered to as specified in the Australian Standards and the RMS B80 specification.

- Pre-Pour Inspection Prior to the placement of precast segments, all aspects of the product are to be checked for any nonconformances or defects that could jeopardise the structural integrity of the segments.

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6.0 Bibliography -

Wilson. W, Kavanagh. G, McLellan. G, Melander. J. (2015) Benefits of Slag Cement in Concrete, Available at: https://aci-ga.org/images/news/Southeast_Slag_Assoc__Slag_Cement_In_Concrete_GA_ACI_Luncheon.pdf [Accessed 12/06/2020]

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Cement Australia. (2020) Slag, Available at: https://www.cementaustralia.com.au/products/slag [Accessed 15/05/2020] Accessed 12/06/2020]

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Thomas. M, (2007) Optimising the Use of Fly Ash in Concrete, Available at: https://www.cement.org/docs/default-source/fc_concrete_technology/is548-optimizingthe-use-of-fly-ash-concrete.pdf [Accessed 12/06/2020]

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Cement Australia. (2020) Fly Ash, Available at: https://www.cementaustralia.com.au/products/fly-ash [Accessed 15/05/2020] Accessed 12/06/2020]

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Road & Maritime Service (RMS). (2019) Concrete Works for Bridges B80 Specification, Available at: https://www.rms.nsw.gov.au/business-industry/partnerssuppliers/documents/specifications/b080.pdf [Accessed 12/06/2020]

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PCA Cement, (2019), Durability, Available at: https://www.cement.org/learn/concretetechnology/durability#:~:text=Durability%20of%20concrete%20may%20be,exposure%20env ironment%20and%20properties%20desired. [Accessed 12/06/2020]

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