There is no limit to the use of concrete. It is a fundamental component of the developing world with a variety of uses in civil and structural work. However, there are prerequisite requirements both for the structural aspects as well as formation domain of concrete. All these requirements depend on the type of structure to be constructed. Furthermore, the designer has to ensure that the maintenance costs are at a minimum without interfering on the design life.
There are various distresses that can be displayed by the concrete element because of inefficiency in design. Distresses are mainly caused by an inadequacy in design and lead to higher maintenance costs and inadequacy of the element to meet the functional as well as the structural requirements. The common form of distress is cracking which may be caused by improper mix proportions as well as inadequacy in the maintenance measures implemented.
Regardless, there are a number of technological improvements that have focused on increasing the design life of concrete and reducing the impact on the environment. The incorporation of fly ash into the mix design is one of the methods that may be used to ensure that the life of the concrete is prolonged and the effects of concrete on the environment are reduced. Moreover, there is an improvement in the quality of fly ash that is produced from power plants as well as other industries and as such, advocates for the incorporation of fly ash into concrete mix designs.
Fly ash, silica, kaolin, and slag may be used to improve the mechanical as well as the structural properties of concrete. The incorporation of these admixtures ensures that the use of concrete is kept at a minimum while the aforementioned properties of concrete are achieved. However, the mix design has to ensure that the various concrete components are optimized which in turn will ensure that the concrete produced is workable.
In this report, the admixture that is under scrutiny is fly ash. Fly ash can be incorporated in concrete mixtures in different proportions in order to determine the properties of the improvised concrete. The experimental set up is based on different proportions of fly ash on concrete with the main aim to determine the optimal mixture. Moreover, the analysis is based on a comparative analysis with plain concrete that has the same mix design.
The most likely outcome from the research is that there will be an increase in the structural strength of the fly ash concrete mixtures than plain concrete. However, different proportions of fly ash concrete are likely to have different properties. In this, the most likely outcome is that an increase in the fly ash content will increase the strength of the concrete mixture to an optimum and thereafter there is likelihood the properties will dwindle. It may, therefore, be used alongside other journals and articles to determine the best mix design and the type of fly ash that can be used.
Concrete is one of the most important elements that is used in construction. Some of the other elements used in construction include steel and wood but their usage is not as defined as concrete. Concrete is commonly used because of the engineering properties, the availability and the cheap nature (McCormac & Brown, 2014). However, in the recent past, there has been an increase in the demand for concrete as well as stringent measures towards reducing the impact of concrete on the environment.
The use of Portland concrete has had an increasing effect on the environment because of the use extended use of Portland cement. However, this increases the carbon footprint of concrete. According to research, there was about 35.7 billion carbon dioxide produced in the year 2014 with 8% of this coming from cement production (Olivier, et al., 2014). The use of fly ash reduces the amount of Portland cement that is used and more to this improves the mechanical as well as the structural properties of the concrete. In this, there is a drastic reduction in the quantity used which reduces the carbon footprint (Yang , et al., 2013)
Tests to determine the structural, as well as the mechanical properties of the high volume fly ash concrete mixture, are done after 14 days, 28 days and 56 days (American Concrete Institute (ACI 318-08), 2008). However, the tests are focused on determining the structural suitability of the mix design with the reference serving as the guiding principles. The properties of the concrete mix may furthermore be altered by the use of superplasticizers (Jayasree & Gettu, 2008). Fly ash used alongside plasticizers in concrete mix design improves the mechanical, the sustainability as well as the durability of the concrete by a far higher margin
Finally, considering that mix design of concrete considers the water to binder ratio, a change in the amount of the Portland cement has a direct impact on the amount of water used (American Concrete Institute (ACI 318-08), 2008). Water used in the case of a fly ash Portland cement has to consider the properties of the cement as well as that of the fly ash and this is one of the fundamental principles of the design. The water used has to ensure that the concrete is workable and more to this, the structural and functional integrity of the desired class of concrete should be achieved.
The following sections of this report provide the detailed information on high volume fly ash concrete mix design. The mix design is an important part of the whole construction process. As such, the sections that follow include the literature review, the methodology, the experimental setup, the results and analysis and the conclusions. The report presents both the theoretical and statistical evidence that emphasizes the use of fly ash in the concrete mix design.
There is an increase in the demand for concrete majorly because of the advance in technology as well as the increase in population. In the year 1880, the demand stood at 2 million tons while in the year 2017, the demand was 407 million tonnes (Mehta, 2009). These two factors have led to an increase in the innovation levels leading to the design of high rise buildings to accommodate the increasing population as well as compensate for the shrinking land. Furthermore, increase in concrete prestressing has also led to an increase in the pressure regarding the mix designs. The mix design has to consider the severity of exposure as well as the strength of the concrete (American Society for Testing and Materials, 2009).
Fly ash is a byproduct that is usually derived from the combustion of coal that has been pulverized (American Coal Ash Association (ACAA), 2010).In between the processes, the mineral products of the coal fuse as suspensions and thereafter leave the chamber with the exhaust gas. The minerals of clay that fuse during the process include clay, feldspar, shale and quartz and is the fusing process that leads to the formation of fly ash (Shi , et al., 2011). The collection procedure is mainly achieved through the use of electrostatic precipitators.
The increased strength of the high volume fly ash concrete mixture is brought about by a chemical reaction between the fly ash and the calcium hydroxide that is usually produced when the Portland cement and the water in the mix design react (Soto-Pérez & Hwang , 2016). The reaction between the fly ash and this calcium hydroxide leads to the formation of crystalline substances that increase the compactness of the cement (Bentz , et al., 2008). The major difference between the water-cement reaction and the fly ash-water reaction is the slow rate of reaction which means that the time that is required for the high volume fly ash concrete to cure is prolonged (Valendia, et al., 2016).
During the concrete design mix, the designer has to ensure that he/she gets the right ratios as per the class of the concrete required. There are two classes of fly ash that depend on the calcium content. To begin with, class C is made up of fly ash that has a very low content of carbon while the calcium content is very high. In this, the carbon content may be as low as 2% and this makes the proportion that is used in concrete to vary between 15% and 40% (Awanti & Harwalkar, 2016).On the other hand, class F fly ash has a very low content of calcium and the carbon available may also be less than 5%. Therefore, the class F fly ash is used in the concrete mixture in the ratios 15% and 25% of the total material (Marlay, 2011). The difference in the proportion of fly ash used is mainly dependent on the type of fly ash which determines the level of reaction (Mehta & Monteiro , 2014).
As per this research, the main aim is the high volume fly ash concrete. This is concrete that has fly ash content that ranges between 15% and 35% (Dinakar, et al., 2008). However, this is the percentage of the total mass of the cementitious materials that are used in the concrete. It, therefore, contains fly ash in much higher proportions than the typical fly ash concrete.
The quality and type of fly ash used, the type of element to be constructed and the design life directly influences the type of design mix used (Manjunath, 2016). Mix design is defined as the science that is centered on the relative proportioning of the different components of concrete. This proportioning is done in order to produce concrete of the desired strengths in the most economical way. Some of the qualities that determine the proportioning include durability, strength, cohesion, and workability.
Concrete is made up of 5 basic elements: cement, water, admixtures, coarse aggregates and fine aggregates (American Concrete Institute (ACI 318-08), 2008). Therefore, the basic building blocks are usually the same but the difference in proportioning is the reason why concrete has different strength as well as workability levels. The strength of concrete ranges between 10Mpa and 100Mpa while the slump test, which determines the workability, produces a range of between 0 and 150mm (Bentz, et al., 2009). Moreover, this mix design has to consider the two stages that the concrete will undergo before the eventual curing. The two stages that concrete undergoes are the plastic stage and the hardened stage.
Following the proper mix design methodology has various advantages when it comes to the economics as well as the quality of the product. Cement is one of the major requirements of the concrete and by minimizing the usage, the savings are numerous. According to research, mix designs can help the minimization of cement usage by about 15% in M20 concrete (International ASTM., 2009).
In summary, there is an increasing urge to use fly ash in the concrete mix design. The usage of fly ash is geared towards reducing the amount of Portland cement used which means a reduction in the carbon footprint of concrete (Rodriguez, 2018). Current mixtures have fly ash percentages of between 20% and 30% but there is an increasing pressure to incorporate more fly ash into the concrete mix design (Marthong & Agrawal, 2012). However, increasing the amount of fly ash in the concrete mix design has an effect on the various properties of concrete. The rate at which the concrete will develop strength as well as the durability of the concrete is mainly affected by an increase in the fly ash percentage to about 60% and 80% (Durán-Herrera, et al., 2010). Therefore, this study aims at investigating the effect of the addition of fly ash on the properties of the concrete.
The research questions will be centered on the advantages of high volume fly ash concrete. All this will be determined by the relative comparison of cement concrete and fly ash concrete. The research questions and the hypothesis are as follows:
|What is the optimum mixture that can be used to achieve a high grade of high volume fly ash concrete?||An increase in the fly ash content increases the compactness as well as the strength of concrete.|
|Which admixtures are required alongside fly ash to provide a suitable mix?||Addition of fly ash into the mix design reduces the heat of hydration and alters some of the desirable cement concrete properties.|
|What properties of high volume fly ash concrete overshadow those of cement concrete?||There is an increase in the strength of concrete with the increase in fly ash into the mix.|
Goals and sub goals
- To determine the admixtures that can be incorporated in the mix design to ensure that the properties of high volume fly ash concrete are better than those of cement concrete.
- To determine the percentage of fly ash that can be incorporated in the mix design so that the properties of the concrete are optimum.
- To determine the properties of high volume fly ash concrete that are better than those of cement concrete.
Mix design is the proportioning done to the different components so that the strength, workability, creep, the flexural strength of the eventual concrete properties are within the allowable ranges. However, incorporating fly ash into the mix design has the effect of altering these properties. In no more instances is this more pronounced than in high volume fly ash concrete. High volume fly ash concrete uses enormous amounts of fly ash in the design which means that the resulting properties are highly altered.
Mix designs act as experimental setups to determine the best combination of the individual concrete components. It is a way through which the design team will arrive at the best combination as per the project requirement. The strength, creep, flexural strength among other properties are all affected by how the different components interact. Fly ash improves the properties of the concrete because of the cementitious crystalline materials formed when the fly ash reacts with the calcium hydroxide byproduct of water and Portland cement reaction. However, there are optimum as well as undesirable results that may are produced during the mix design. An increase in the fly ash component is likely to increase the properties of the concrete to an optimum point from where the properties may begin to fall.
High volume fly ash concrete mix design has to, first of all, consider the individual components of the concrete. Such concrete is made up of water, fine aggregate, coarse aggregate, fly ash and a plasticizer. To begin with, the concrete has to undergo the various cement tests such as consistency, soundness, fineness, specific gravity, the initial and final setting times, the strength after 14 and 28 days among others. On the other hand, both the fine and coarse aggregate should be analyzed mainly through sieve analysis to determine the particle size distribution. Finally, the fly ash to be used has to be well defined as per the class under which it falls In this study, the fly ash used falls under class C because of the high calcium content.
The analysis will involve selecting the best percentage of plasticizer to be used considering that the sump test cannot be conducted without this component. Furthermore, the cement will be replaced by the fly ash in different proportions which in turn will provide a clear picture of the effect of the addition of fly ash to the concrete properties. The water used in the experiment will have to be pure and as such, tap water with low calcium content can be used.
Two set ups are to be used in the research. One set up is the control while the second is the experimental one. With standards outlining mix designs, the two set ups will be centered on the design of concrete with a strength of 60Mpa.The control set up has water, cement and aggregates while the second set up will have fly ash incorporated in the mix design.
The study takes the water-cement ratio as 0.38 while fly ash is added to the trial mixture at an increasing rate of 10%. The percentage of fly ash that provides the greatest strength is taken as the optimum. The compressive strength of concrete is tested after 14, 28 and 56 days.
The major limitation is to the number of tests that can be conducted on the High volume fly ash concrete designed. Limitations are brought about by the number of equipment available for conducting the tests. The creep, cracking, durability among others are some of the tests that may be limited by the number of equipment available.
Last, but not least, the variability of admixtures such as superplasticizers may on the general properties of the concrete may not be determined. The price of superplasticizers among other admixtures may be too exorbitant for the purchase and more to this, the wide range of superplasticizers provides a challenge to the selection.
Project planning and Gantt chart
The demanding part of the project is the design phases. Some of the requirements include sieve analysis, cement tests, fly ash selection among others. However, literature and books are used as reference points on a weekly basis and more to this, consultations conducted. The following Gantt chart is used to measure the progress of the project.
Results, outcomes and relevance
Considering the analysis on the concrete after 14, 28 and 56 days, the table below presents the different strength of the high volume fly ash concrete over that specified period. Moreover, there is the column that represents the addition of the superplasticizer.
|Percentage of fly ash used||Percentage of the plasticizer||Compressive strength after 14 days-N/mm2||Compressive strength after 28 days-N/mm2||Compressive strength after 56 days-N/mm2|
Looking at the above table, the optimum fly ash composition is 60%. This has been demonstrated by the compressive strength that is achieved after 14, 28 and 56 days. Moreover, the experiment demonstrates that once the optimum composition is achieved, the compressive strength of the concrete begins to fall.
Comparing the HVFAC to the normal concrete, the following deductions can be made.
|Compressive strength in N/mm2||61.26||64.45|
|Flexure in N/mm2||4.31||7.22|
|The elastic modulus in N/mm2||5.12*102||5.12*104|
As indicated in the table, the properties of the high volume fly ash concrete are far much superior to those of the normal concrete. The three tests serve as a smokescreen through which the designer can view the adoption of fly ash concrete.
The tests have established that the properties of fly ash concrete are better than those of cement concrete. As such, they indicate the suitability of high volume fly ash concrete to specific building requirement. Structures requiring higher compressive and flexural strengths should opt to use high volume fly ash concrete instead of the ordinary cement concrete. It presents a more environmental friendly and economical concept of concrete use.
There is an increase in the demand for better construction techniques. Concrete has been widely used in the construction industry and demand is on the rise. However, one of the major disadvantages of concrete is the fact that it uses humongous amounts of cement. Cement is one of the major pollutants in the world mainly due to its carbon footprint.
Incorporating fly ash into the mix design is one of the methods that may be used to reduce the amount of cement used and presents an eco-friendly option. Furthermore, the fly ash improves the qualities of the concrete because of its cementitious property. Therefore, it reduces the carbon footprint of concrete and improves the properties as well. Tests conducted on an experimental set up indicates that the high volume fly ash concrete has improved flexural as well as compressive strengths.
American Coal Ash Association (ACAA), 2010. 2010 Coal Combustion Product (CCP) Production & Use Survey Report.”, Aurora, Colorado: ACAA.
American Concrete Institute (ACI 318-08), 2008. Buiding Code Requirements for Structural Concrete and Commentary., Detroit, Michigan: American concrete institute.
American Society for Testing and Materials, 2009. ASTM C31/C31 M-09 “Standard Practice for Making and Curing Concrete Test , West Conshohocken, Pennsylvania.: s.n.
Awanti, S. S. & Harwalkar, A. B., 2016. Mix design curves for high volume fly ash concrete. International Journal of Civil and Environmental Engineering.
Bentz , D. P., Sant, . G. & Weiss, . W. J., 2008. Early-age properties of cement-based materials: I.Influence of cement fineness.. ASCE Journal Material and Civil Engineering.
Bentz, . D. P., Peltz, . M. A. & Winpigler, . J., 2009. Early-age properties of cement-based materials: II influence of water-to-cement ratio.. ASCE J Mater Civ Eng.
Dinakar, . P., Babu, . K. & Santhanam , . M., 2008. Durability properties of high volume fly ash selfcompactingconcretes. Cement concrete composition.
Durán-Herrera, A., Juárez, C. A., Valdez, P. & Bentz, D. P., 2010. Evaluation of sustainable high-volume fly ash concretes. Cement & Concrete Composites.
International ASTM., 2009. Annual book of ASTM standards.. West Conshohocken,PA: s.n.
Jayasree, C. & Gettu, . R., 2008. Experimental study of the flow behavior of superplasticized cement paste.. Materials Structural.
Manjunath, B. H., 2016. HIGH VOLUME HIGH PERFORMANCE FLY ASH CONCRETE MIX DESIGN FOR PAVEMENT OVERLAYS FOR SUSTAINABLE DEVELOPMENT. inrternational journal of civil engineering and technology.
Marlay, K. M., 2011. Hardened Concrete Properties and Durability Assessment of High Volume Fly Ash Concrete. s.l.:s.n.
Marthong, C. & Agrawal, T. P., 2012. Effect of Fly Ash Additive on Concrete Properties. nternational Journal of Engineering Research and Applications.
McCormac, J. C. & Brown, R. H., 2014. design of reinforced concrete. s.l.:John wiley and sons.
Mehta, P. K., 2009. Global concrete industry sustainability. Concr International.
Mehta, . P. K. & Monteiro , P. J., 2014. Concrete, Microstructure, Properties and Materials.. s.l.:s.n.
Olivier, . J. et al., 2014. Trends in global CO2 emissions. Report. PBL Netherlands Environmental Assessment Agency., s.l.: s.n.
Rodriguez, J., 2018. Uses,Benefits and drawbacks of fly ash in construction. [Online]
Available at: https://www.thebalancesmb.com/fly-ash-applications-844761
Shi , . C., Fernández-Jimenez, . A. & Palomo, . A., 2011. New cements for the 21st century: Thepursuit of an alternative to Portlandcement.. Cement Concrete Resources.
Soto-Pérez, L. & Hwang , S., 2016. Mix design and pollution control potential of pervious concrete with non-compliant waste flyash. Journal of Environmental management.
Valendia, D. F. et al., 2016. Evaluation of activated high volumeflyash systems using Na2SO4, lime and quicklime in mortars with high loss onignition flyashes, s.l.: White rose university consortium.
Yang , K. H., Song , J. K. & Song, . K. L., 2013. . Assessment of CO2 reduction of alkali-activated concrete. Journal of clean production.