Concrete Mix Design
Discuss about the Materials Design Project for Mix Design and Steel Selection.
This report involves analyzing materials design of a 100m span steel arch bridge construction project. The main elements of the report include concrete mix design, steel selection and material specifications. This project is very informative and useful in engineering practice because it provides essential knowledge and skills on how to select suitable materials for different engineering applications. It helps in understand various factors to consider when selecting materials for a project and how material properties can be manipulated to improve these properties.
The concrete used in this project comprise of cement, coarse aggregates (gravel or broken stones), fine aggregates (sand) and water. The main difference in composition is cement. The available options are: flyash-cement and slag-cement. The concrete designed here will be reinforced with steel components. Some of the factors that have been considered when designing the two mixes include: the water-cement ratio has to be moderate; air content has to be moderate; permeability has to be low; ultimate strength has to be at least 40 MPa. These factors are among the most important ones when designing concrete mix for bridge construction (Xi, Shing and Xie, 2001).
Flyash-cement mix design:
The chosen fly-cement mix design is as follows: water: flyash-cement: fine aggregate: coarse aggregate = 1: 2.5: 5: 6.25
Water-cement ratio, flyash proportion of cementitious material and total cementitious material were assumed to be 0.4, 30% and 450 kg/m3 respectively
Proportion of all ingredients per m3 of concrete is as follows:
Cement = (70/100) x 450 = 315 kg/m3
Flyash = (30/100) x 450 = 135 kg/m3
Water = (1 x 450)/2.5 = 180 kg/m3
Fine aggregate = (5 x 450)/2.5 = 900 kg/m3
Course aggregate = (6.25 x 450)/2.5 = 1125 kg/m3
The chosen fly-cement mix design is as follows: water: cement: flyash: fine aggregate: coarse aggregate = 1: 3: 5: 5.5
Water-cement ratio, slag proportion of cementitious material and total cementitious material was assumed to be 0.4, 50% and 450 kg/m3 respectively. The slag proportion has been selected based on recommendations of making mass concrete (Slag Cement Association, 2004).
Proportion of all ingredients per m3 of concrete is as follows:
Cement = (50/100) x 450 = 225 kg/m3
Slag = (50/100) x 450 = 225 kg/m3
Water = (1 x 450)/3 = 150 kg/m3
Fine aggregate = (5 x 450)/3 = 750 kg/m3
Comparison Between the Two Mixes
Course aggregate = (5.5 x 450)/3 = 1825 kg/m3
Based on the two mixes, both of them are capable of attaining the desired concrete strength. In relation to heat of hydration, flyash-cement reduces the rate of heat generated when making and placing concrete, which reduces rise of concrete’s internal temperature (Thomas, 2007). During massive concreting, heat loss rate is usually low hence peak temperature rise depends on composition and quantity of flyash-cement used. Therefore increasing the volume of fly ash in cement reduces temperature rise of concrete. This is very useful in mass concrete because it lowers chances of cracking, which results from high heat gradients.
Slag-cement reduces concrete’s excessive temperature rise that is related too risk of cracking and internal stress of concrete. Nevertheless, this is achieved by use of large proportions of slag (at least 50%). Construction of the bridge will require substantial amount of concrete to be made and placed at once and therefore the mix design used should reduce the large amount of heat that will be produced as a result of hydration process. In this regard, slag-cement is the recommended mix for this project.
The location of this structure puts it at risk of sea water and/or chemical attacks, specifically chloride corrosion. These risks can be prevented by considering different design factors aimed at improving durability of the structure. Some of these factors are:
Use of quality and adequate concrete cover to protect reinforcing bars. The concrete used should be of high quality, which also means low ratio of water to cementitious material. This helps in reducing penetration rate of chloride salts besides inhibiting growth of carbonation. The ratio of water to cementitious material should not exceed (National Ready Mixed Concrete Association, 1995). This can be achieved by increasing the amount of cement in concrete; using higher quantities of cementitious materials (such as slag); reducing the quantity of water in concrete by use of super plasticizers and water reduces. Most importantly is that the concrete must be properly consolidated and cured.
Adequate concrete cover – the concrete cover depth of the structure should be sufficient to reduce diffusion rate, porosity and cracks. This will reduce the possibility of chloride ions penetrating through the concrete. In this project, concrete cover should be at least 75 mm (Kepler, Darwin and Locke, 2000).
Use of mineral admixtures – these materials contain silica, which prevents chloride corrosion by reacting with chloride hydroxide. These materials also improve concrete workability and durability, and reduces heat hydration. Mineral admixtures that can be used for this purpose include blast-furnace slag, flyash and silica fume. Using flyash as a cement replacement has been proved to be very effective in reducing chloride carrion (Bargaheiser and Butalia, 2003). Flyash-cement mixes reduces rates of chloride ingress and permeability (Bouzoubaa and Foo, 2005), as long as the right proportions of flyash-cement are used and the concrete is cured properly. Flyash also boosts chloride binding, thus increasing the structure’s resistance against chloride penetration.
Use of concrete overlays – they create a layer of low permeability over concrete especially on bridge structures. These overlays will block chloride ions from reaching concentration levels that can cause corrosion. Overlays with low permeability also reduce penetration of water into the structure, thus decreasing mobility of chloride ions. Low-slump and silica-fume are the most suitable overlays for this project (Kepler, Darwin and Locke, 2000).
Use of concrete sealers – they reduce reinforcement corrosion by averting capillary action, thus inhibiting penetration of chloride ions and water into the concrete.
Use of corrosion-resistant steel reinforcing bars. These bars have two microstructures. The bars are manufactured by adding elements such as nickel, phosphorous, copper and chromium, which create a protective layer against chloride corrosion (Sudhir and Anurag, 2012).
Last but not least, it is also important to ensure that the concrete used has a design strength of at least 35 MPa with a maximum water-to-cement ratio of 0.4. These are the recommended values for any structure that is exposed to external chlorides and moisture (Portland Cement Association, 2016).
The main materials for this project are concrete and steel. For the structure to perform its intended purpose effectively, these materials should have desired properties and join flawlessly to create an integrated system. Several criteria should be used for material selection and specification. The selection for different materials is as follows
Aggregates – since high-strength concrete is required for this project, the aggregates need to be strong and durable. Their strength and stiffness should be compatible with cement paste. The most suitable coarse aggregates are those with smaller maximum size. Fine aggregates need to be coarser considering that the total percentage of fines in cementitious materials is high (National Ready Mixed Concrete Association, 2001). The nominal sizes of aggregates should be optimal so as to balance between the amount of cement used and overall concrete strength. Also, both fine and coarse aggregates should be measured by weight instead of volume so as to get more accurate results.
Water – this is a very important ingredient in concrete making. Besides using the right proportion of water, the water must be of good quality. It has to be portable so as to mix with concrete properly. Nevertheless, non-potable water can be used as long as the water (and its impurities) does not affect the properties of concrete.
Cementitious materials – all these materials should be obtained from reliable sources. Recommended cement for this project is Portland cement. Quality flyash should be used so as to achieve the desired strength and durability.
Chemical admixtures – the level of chlorides (if any) in these materials should be very low. The admixtures should also not affect useful properties of other materials when they react.
Steel – mild steel is the most recommended steel because of its high strength and durability properties. If necessary, the steel shall be specially manufactured by adding extra layers aimed at improving its strength and corrosion resistance. All steel components, including reinforcing bars, should have high strength, bond well with concrete, and have high tensile strain and have adequate thermal compatibility.
Material selection is aimed at ensuring that the materials used for this project have adequate abrasion resistance, compressive strength, chloride ion penetration resistance, modulus of elasticity, scaling resistance, shrinkage, freezing/thawing durability, etc. (Caldarone et al, 2005). Most importantly, all these materials should be tested before being used.
Fabrication of main arches is a very important process in this project. This is because the process will determine the ultimate characteristics of the steel arches. This process will take place before assembling the steel components to form one integrated system.
The fabrication process of main arches starts with preparation. Here is where basic steel sections and plates are taken to the fabrication factory. The materials are selected based on the design specifications of the bridge arches. The suitable sections for this structure are plate girders. These sections comprise of a web plate and two flanges that have been fabricated together by welding (SteelConstruction.info1, 2015). Their design allows them to resist high applied forces at very low self-weight.
On arriving at the factory, the materials go through shot blasting so as to achieve a clean and suitable finish for subsequent processes. This is followed by cutting of the materials so as to get the desired shape or size. The cutting can be done by circular saws, plasma cutting, and flame or gas cutting. The materials are then drilled and punched so as to make it easier for quick bolting and connections with other components on site. This is followed by an interesting process called bending, which creates the desired bends and curves. The bending can be done by section bending, plate bending, or tube bending. After bending, what follows is press breaking. This entails pressing the steel components along their length, for lengthier components. Thereafter, the components go through tee splitting (if necessary) then tubular sections are profiled so that they can fit perfectly into curved surfaces.
The next and very crucial process is welding. This is used for preparation of joints on the components, which facilitate quick and accurate connections on site. The type of welding suitable for this project is submerged arc welding (Steel-Insdag, 2000). This process is effective in welding long steel structural components that are to be assembled to form high strength systems. After welding, the components go through non-destructive testing to check whether any unacceptable defects, including inclusions and cracks, were created as a result of welding.
Lastly, fabrication process ends with hot dip galvanizing or paint coating. This last process provides fire resistance, corrosion resistance, aesthetic finish, and also saves time and cost on site (SteelConstruction.info2, 2015). All fabrication processes are done in accordance with design specifications of the components.
The different requirements for steel used for hot dip galvanized steel components are as follows:
Chemical composition – the steel should be of particular grades so as not to affect the performance of hot dip galvanized steel components. It is recommended that the yield strength of steel should not exceed 460 MPa (ArcelorMittal Europe, 2010). This is largely determined by the percentage of carbon, phosphorous and silicon in steel.
Mechanical properties – these include elongation at fracture, reduction of area, uniform elongation, Young’s modulus of elasticity and tensile strength, which collectively determine yield stress of steel. The steel should be adequately ductile so as to avoid excessive deformation during hot dip galvanizing process.
Weldability – the steel should have good weldability so that the process does not affect its toughness and strength. This is usually determined by the purity of steel during production. Therefore the steel should be sufficiently pure.
The fabrication process should also not have affected toughness or brittle fracture of steel. If these two factors are affected, the steel becomes susceptible to defects during hot dip galvanizing due to additional stresses.
Steel components with finer grains – these components have improved structural properties than those with coarser grains. The size of grains influences weldability and strength of concrete.
Casting process used – hot dip galvanizing also works well with steel components cast by continuous casting process instead of ingot casting. Additionally, the best components are those that went through refined rolling processes, including self-tempering, quenching and thermochemical rolling.
The zinc coating used should also be of the right type and adequate thickness so that the hot dip galvanized steel structural components can effectively protect the system against corrosion. For this to be achieved, the corrosive environment has to be analyzed first before determining the most suitable zinc coating and thickness.
In general, the following types of steel are suitable for hot dip galvanizing: plain carbon steel (below 1100 MPa), hot-rolled steel, cast steel, cold-rolled steel, weathering steel and stainless steel (American Galvanizers Association, 2015).
To test the impact characteristics of the steel used in hand rails, various samples of hand rails would be taken to the lab for testing. These tests would be carried out so as to determine the resistance of hand rails to failure. This would be done by applying force(s) on the samples then measuring corresponding impact energy or strength. The results would provide impact energy or strength just before failure of the hand rail samples, which would represent the impact characteristics of the steel that was used in hand rails.
The test that would be used is Izod test. This is a destructive test that would fracture the hand rails and determine their toughness or quantity of energy that the hand rails would have absorbed by the time they were fracturing.
As stated before, this test would be done by taking hand rail samples to a materials testing laboratory. The values obtain would provide different characteristics of steel used in the hand rails. Some of these include: fracture mechanism, ductility, yield strength and transition from ductile to brittle, (AZom.com, 2005). The test would also help in understanding the static strength of steel, impact performance and/or resistance, post-breakage characteristics, and the amount of loads the hand rails can support. Thus it would be easier to determine whether the steel used in hand rails meet the requirements of performance standards, codes, specifications and norms.
There are four main types of steel, which are classified based on their composition. They include carbon steels, stainless steels, alloy steels and tool steels. Carbon steels are the commonest type of steel. Carbon steels are also sub-divided into different groups: low carbon steels (easy for shaping and lower hardness), medium carbon steels (strong, ductile and low ear & tear properties), high carbon steels (very strong), and ultra-high carbon steels (extremely strong and more brittle). Stainless steels include austenitic, martensitic, duplex, precipitation hardening and ferritic stainless steels. These steels have higher ductility, work hardening rate, hardness & strength, cryogenic toughness, corrosion resistance, hot strength and moderate magnetic response (AZoM.com, 2001).
Alloy steels comprise of alloying elements, such as silicon, manganese, chromium, copper, aluminium, titanium and nickel. These elements manipulate the properties of alloy steels, including ductility, formability, strength, corrosion resistance, weldability and hardenability (Bell, 2016). Tool steels are usually suitable for making drilling and cutting equipment. They contain cobalt, tungsten, vanadium and molybdenum so as to increase their durability and heat resistance.
Hot dip galvanizing is a very important process for increasing damage resistance of steel. Some of the specifications for this process are: the proportion of carbon, phosphorous, manganese and silicon in steel should be below 0.25%, 0.04%, 1.3% and 0.04% respectively; products to be galvanized should be delivered, stored and handled properly; coating applicator must be certified; the process should be done in accordance with set guidelines, the surfaces have to be prepared by pre-cleaning before actual hot dip galvanizing process, the actual process has to follow acceptable guidelines, the coating must provide acceptable adhesion and surface finish properties, finished products have to be inspected and tested, and damaged coatings should be repaired (American Galvanizers Association, 2002).
For high strength bolts to perform optimally, they must comply with certain specifications. Some of these specifications are as follows: they should have high tensile strength, must undergo proof load tests and have high proof strength, full size bolts must undergo wedge tests, zinc coating thickness must be specified and checked if the bolts are galvanized, they must undergo rotational capacity tests, must be of the right property class, and must have high yield strength.
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Xir, Y, Shing, B, & Xie, Z, 2001, Development of Optimal Concrete Design Mix for Bridge Decks. Boulder, CO: University of Colorado.
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