Construction Materials - Concrete Report GW 13193913 PDF

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CONCRETE MIX DESIGN PROJECT – GEORGINA WALKER 13193913

Georgina Walker - 13193913 AUTUMN 2020: 48352 CONSTRUCTION MATERIALS

CONCRETE MIX DESIGN PROJECT – GEORGINA WALKER 13193913

Table of Contents 1.

Introduction: ................................................................................................................................... 2

2.

Concrete Mix Design ....................................................................................................................... 2

3.

4.

2.1

Prefabricated Concrete Segments .......................................................................................... 2

2.2

The British Method of Mix Design .......................................................................................... 3

2.3

Sulphate Resisting Portland Cement....................................................................................... 3

2.4

High Alumina Portland Cement .............................................................................................. 5

Mechanical Strength ....................................................................................................................... 5 3.1

Sulphate Resisting Portland Cement / Slag Mix...................................................................... 5

3.2

High Alumina Portland Cement .............................................................................................. 5

Slump .............................................................................................................................................. 6 4.1

Sulphate Resisting Portland Cement: ..................................................................................... 6

4.2

High Alumina Portland Cement .............................................................................................. 6

5.

Curing Method ................................................................................................................................ 7

6.

Material Specification: .................................................................................................................... 8

7.

Bibliography .................................................................................................................................... 8

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CONCRETE MIX DESIGN PROJECT – GEORGINA WALKER 13193913

1. Introduction: This report aims to outline and detail the concrete mix design, curing, strength, durability and material specifications necessary to complete the construction of the new transport system put in place by NSW transport connecting the North West of Sydney. This system employs the use of precast concrete segments to line the inside of the metro tunnels, providing many advantages during the construction stage. As each boring machine advances, the concrete lining segment are able to be grouted into position; providing structural stability, soundness and waterproofing the inner tunnel. Two mix designs will be included within the report; one utilising fly ash and the other slag/cement.

2. Concrete Mix Design The design of the concrete’s mix is crucial in creating a sound and strong foundation for all infrastructure (Concrete Supply Co. 2020). Considering the nature of construction of the transport tunnel, it is especially important to ensure there is no leakage or external compromise to the structural integrity of these concrete sections, as they also self shore the earth surrounding them. As outlined by UTS online, it is evident that “similar structures located nearby constructed 50 years ago showed map pattern cracks of concrete, surface spalling and steel corrosion” (School of Civil and Environmental Engineering Faculty of Engineering and Information Technology University of Technology Sydney 2020). Thus, it is important to consider the Figure 1: deterioration of an old concrete tunnel exposure the tunnel will receive to alkali silica constructed in Sydney 50 years ago (UTS online) reactions, sulfate/acid attacks and chloride ion penetration due to being within a coastal area when designing the concrete mix. As stated by Fu and his team in their research findings, fly ash and slag are useful additives that are capable of effectively resist chloride penetration (Fu et al. 2019).

2.1 Prefabricated Concrete Segments This report will put forward two separate concrete designs suitable for this project; a Sulfate Resisting Portland Cement (SRPC) and a High Alumina Portland Cement (HAPC). The SRPC is a slag based aggregate supplemented concrete supplementing sedimentary sand and rocks whilst the HAPC mix utilises fly ash as its main aggregate. It is important when designing such concrete mixes to incorporate the correct ratio of supplementary cementitious material to achieve desired strength and workability. Thus in this design and coinciding with Cement Concrete & Aggregates Australia’s ‘moderate´ range of SCM requirements, the mixes will be created using 30% fly ash and 60% Slag respectively (Cement Concrete & Aggregates Australia 2018). -

It is important to note that both designs will be created to meet the following criteria: o Compressive strength of 55MPa o Minimum slump of 130mm

Fly ash is a by-product obtained at coal burnt power stations through electrostatic precipitation. It is used as a supplementary cementitious material (SCM) used in concrete mixes to improve later-age compressive strength, workability, resistance to chemical attack and resistance to chloride ingress amongst other things (Cement Australia 2020). Such properties allow the precast sections of the 2

CONCRETE MIX DESIGN PROJECT – GEORGINA WALKER 13193913 tunnel to resist the complications faced in the previously mentioned tunnel located nearby. Fly ash is rich in aluminous and siliceous material due to chemical reactions occurring during the electrostatic precipitation which proves advantageous to increasing permeability to water and aggressive materials (National Precast Concrete Association 2020) Slag or ground granulated blast furnace slag is a by product of iron production. The residue is cooled and milled to mimic the size of cement particles. Slag demonstrates long-term performance enhancements, including increased strength, increasing durability and reducing thermal cracking, whilst allowing designers to reduce their environmental footprint (Slag Cement Association 2017)

2.2 The British Method of Mix Design The British method of mix design is an industry accepted method of calculating required material ratios to a set of compressive strength requirements and is calculated as follows: 1. 2. 3. 4. 5.

Determine the free water/cement ratio Determine free water content Determine the cement content Determine the total aggregate content Determine fine and coarse aggregate content

2.3 Sulphate Resisting Portland Cement As previously mentioned, for the SRPC sulphate resisting cement (SR) will be the chosen coarse aggregate (of 20mm), additionally the age of concrete used to calculate will be the standard 28 days. To complete the first step in the British Concrete Mix design process first the free water/cement ratio must be determined. This is done using the table 1.From table 1, the compressive strength of the chosen concrete with an initial w/c ratio of 0.5 will be 45MPa. Moving forward the free water content has to be adjusted to suit the brief. Thus the w/c to reach a compressive strength of 55MPa is done in accordance with table 2. As can be viewed the new w/c ratio is 0.44. Free water content depends on the type and size of the aggregate to ensure the concrete is of the right design specifications. In accordance with table 3 and the slump specification of 130mm, the free water content for this design will be 200Kg/m3. Step three can be calculated using the following equation:

Table 1: Relationship between Compressive Strength and Water/Cement Ratio

Table 2: approximate compressive strengths (MPa) of concrete with a free water/cement ratio of 0.5

3

cement content = free content / free water/cement ratio cement content = 200/0.44 cement content = 455 Kg/m3 However, it is to be noted that 60% of this cement content is to be slag. Therefore the concrete will contain 455 x 60% = 273Kg/m3 Slag and 182 Kg/m3 SR cement.

CONCRETE MIX DESIGN PROJECT – GEORGINA WALKER 13193913 Wet density of fully compacted concrete is employed to determine the total aggregate content required for the concrete design. Following table 4 whilst assuming average specific gravity of 2.7 (crushed aggregate), the wet density of the concrete is found to = 2420Kg/m3. Table3: Approximate free-water content(kg m-3) for various levels of Total aggregate content is then workability calculated by subtracting free water content and cement content from the wet density of concrete.

Total aggregate content = 2420-455-200 Total aggregate content = 1765Kg/m3 Finally, we use the maximum aggregate size (which was determined by workability, free w/c ratio and percentage of fine aggregate through a 600-micron sieve). Following table 5 the percentage of fine aggregate can be interpreted at 37% and the fine aggregate content can be calculated by: Table 4: Wet density of fully compacted concrete

Fine aggregate content = total aggregate content x proportion of fine aggregate Coarse aggregate content = total aggregate content – fine aggregate content Fine aggregate content = 1765 x 37% Fine aggregate content = 653 Kg/m3 Coarse aggregate content =1765-653 Coarse aggregate content = 1112Kg/m3

Table 5: proportions of fine aggregate determined from fieness modulus

Therefore the proportions of the SRPC cement in kilograms per meter cubed are as follows:

Sulphate Resisting 60% Slag Binder Concrete design: Material: Proportion (Kg/m 3): 182 Crushed Sulphate Resisting Cement Slag SCM 273 Water content 200 653 Fine Aggregate Coarse Aggregate 1112 Total: 2420

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CONCRETE MIX DESIGN PROJECT – GEORGINA WALKER 13193913

2.4 High Alumina Portland Cement

High Alumina Portland, 30% Fly Ash Concrete Design: Proportion (Kg/m 3 ): Material: Crushed Portland 379 Cement 162 Fly Ash SCM 200 Water content Fine Aggregate 581 1078 Coarse Aggregate Total: 2400

Following the same steps as outlined in section 2.3 of this report, the same design parameters as in section 2.2 whilst maintaining a 30% ratio of Fly Ash SCM the following proportions were calculated:

3. Mechanical Strength Mechanical strength is defined as the “ability to withstand the stress of physical forces” (Mechanical strength dictionary definition | mechanical strength defined n.d.) and is essential to project success by accurately incorporating into the design criteria. For instance, if these precast concrete tunnel sections were built with too low a mechanical strength, the tunnel would unable to withstand its own weight or the weight of the earth around, resulting in structural failure. Therefore, this section will analyse the effect the different SCM’s within the design mixes have on the project’s mechanical strength through exploring their benefits and draw backs.

Figure 2: Precast concrete segments for metro tunnels

As indicated through the relationships outlined in table 1 and experimentally in Chen, Hang & Zhou’s report (Chen, Huang & Zhou 2012), mechanical strength is directly related to the free wate/cement ratio i.e. the lower the ratio the higher the mechanical strength. However, it is also known that the particular cement and SCM additives also have a substantial and varying influence on the strength of concrete designed. These are outlined in sections 3.1 and 3.2 below:

3.1 Sulphate Resisting Portland Cement / Slag Mix This first mix design employs the use of sulphate resisting cement to prevent the corrosion and ion attacks presented inherent with building in a close proximity to a coastal region. This as shown in table 3, increases the overall early set mechanical strength. Additionally, slag increases overall mechanical strength of concrete by minimising the amount of Calcium hydroxide (Ca(OH)2) (a byproduct of the hydration of Portland cement that doesn’t contribute to overall strength). Furthermore slag generates more calcium silicate hydrate which in turn acts as a glue providing additional strength to hold the concrete together

3.2 High Alumina Portland Cement The second mix design incorporating a high alumina fly ash SCM as reflected in the calculations throughout section 2 requires a significantly lower water content to mix 1 and other typical cement mixes. This reduction in water leads to a notably higher ultimate compressive strength. However fly ash produces a slow set time with low early strength, particularly in 30%+ supplementations of fly ash for Portland cement. This issue can be overcome with the addition of accelerators, plasticisers, or additional condensed silica flume. In succession to this, certain curing methods such as wet coverings, steaming ponding and fogging can aid in increasing early strength gain in concrete.

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CONCRETE MIX DESIGN PROJECT – GEORGINA WALKER 13193913

4. Slump Slump is a critical element to ensuring a correct workability of cement specific to a certain project. as stated in the Boral book of concrete (Boral Concrete 2017), projects that require a low slump and majority dry mix concrete include the construction of roads, employing a slump of 025mm. Low workability mixes of the range 10-40 mm are most commonly used in large foundations with light reinforcement. Medium workability mixes with slump 50-90 mm are used for normal Figure 4 : Cross section and reinforcement detail of concrete reinforced concrete placed where vibration is metro tunnel required. Finally, projects with slump > 100 mm such as the construction of these large prefabricated concrete sections where tight and intricate reinforcing is used. Figure 4 highlights how within the tunnel section reinforcement is placed at 400 centres each way. Furthermore, the sections of tunnel are long to reduce construction and assembly costs. Therefore, both designed concrete mixes employ such a fluid workability (130mm slump) to ensure the concrete does not honeycomb (figure 3) and the structure remains sound.

Figure 3: Honey combing of a structural concrete column due to too little workability and insufficient vibration of concrete. Sourced from http://constructioncost.co/honeycombi ng-in-concrete.html

Slump is dependent on a variety of factors including water content, cement paste content and aggregate size. When there is a low cement paste content the slump is lower, when the aggregates used are larger in size a higher slump is produce. However the main influencing factor on slump is the water to cement ratio or water content within the mix. When there is a higher water content in the cement, the mix is more fluid and therefore has a larger slump value. However, the amount of water required for different admixtures varies, due to the physical and chemical properties of such SCM’s.

4.1 Sulphate Resisting Portland Cement: Within the SRPC mix the base cement of sulphate resisting cement has little to no effect on the overall workability of the mix as it’s maximum particle size is equal to that used in the typical Portland cement mixture. However the use of slag in the mix design does pose some challenges to maintaining a slump of 130mm. Ground Granulated Blast Furnace Slag (GGBFS) as investigated by Xiaolu Guo, Huisheng Shi and Kai Wu (Guo, Shi & Wu 2014) used alone or as an admixture produced a lower slump than their Portland cement counter parts. Such an observation was investigated and interpreted through the use of X-ray diffraction through the concrete samples.

4.2 High Alumina Portland Cement However, the SCM of fly ash increases workability of cement mixtures or requires less water to reach the same level of workability as typical cement or GGBFS mixes. “Since fly ash particles are spherical and in the same size range as Portland cement, a reduction in the amount of water needed for mixing and placing concrete can be obtained.” (National Precast Concrete Association 2020). This allows for better workability pumpability, cohesiveness by reducing bleeding and segregation within concrete mixes. 6

CONCRETE MIX DESIGN PROJECT – GEORGINA WALKER 13193913

5. Curing Method “Curing is the maintenance of satisfactory moisture content and temperature in concrete for a period of time immediately following placing and finishing so that desired properties may develop” (Design and Control of Concrete Mixtures 2020) (figure 4). Curing determines properties of hardened concrete including durability, strength, waterproofing ability, abrasion resistance, volume stability and resistance to freezing and thawing and deicers. Thus it is of the upmost importance to the success of the metro tunnel to employ a suitable curing method. When portland cement is mixed with water, the chemical reaction known as hydration takes place (equation 1) 2𝐶 𝑆 + 11𝐻 → 𝐶 𝑆 𝐻 + 𝐶𝑎(𝑂𝐻)

Figure 5: curing should begin as soon as the concrete stiffens enough to prevent marring or erosion of the surface. This image shows burlap sprayed with water. Image sourced from http://www.ce.memphis.edu/1101/notes/concrete/PCA_m

Equation 1: Hydration of slow set concrete using dicalcium silicate

Surfaces or exteriors of prefabricated concrete sections within this project are especially susceptible to inadequate hydration due to water from the surface of the concrete evaporating resulting in plastic shrinkage and thermal cracking. This is evident in figure 6, highlighting how concrete that is kept moist during curing improves compressive strength over time in comparison to other methods of curing. Figure 6: Effect of moist curing time on strength gain of concrete. Imaged sourced from Tension Tests of Plain Concrete,” Major Series 171, 209, and 210, Report of the Director of Research, Portland Cement Association, November 1928, pages 149 and 163.

Figure 7: A typical stmospheric steam-curing cycle. Image from http://www.ce.memphis.edu/1101/notes/concrete/PCA_ manual/Chap12.pdf

The method that is to be used during this project is steam curing. Due to the fact that the product being constructed is portable and made off site before assembly, it is possible to live steam at atmospheric pressure (autoclaves cannot be used due to the sheer size of these sections). Such steaming is most commonly conducted in an enclosed space to minimise moisture and heat losses. This can be done in a warehouse or using tarpaulins. Steam pressure is maintained at roughly 60°C until the desired strength within the precast sections has been reached as seen in figure 7. 7

CONCRETE MIX DESIGN PROJECT – GEORGINA WALKER 13193913

6. Material Specification: No.

Material

Role

1 2

Portland Cement Sulphate Resisting Cement

3

Slag

4

Fly Ash

5

Fine Aggregate

6

Coarse Aggregate

7

Mixing Water

Concrete binder that hydrates rapidly a t a low temperature used in a concrete mix A type of Portland cement in which the amount of tricalcium aluminate is restricted – restricting the sulphate salt formation and therefore lowing the possibility of a sulfate attack on the concrete (Rahman 2020) A supplementary cementitious material (SCM) used to increase compressive strength, produce a higher resistance to chemical attacks, and reduce permeability. By prod...