SIZE AND SHAPE EFFECTS ON THE COMPRESSIVE STRENGTH OF HIGH STRENGTH CONCRETE
AUTHOR
MONTH / YEAR
Declaration of Originality
I certify that except where due acknowledgment has been made, the work is that of the author alone; the work has not been submitted previously, in whole or in part, to qualify for any other academic award; the content of the thesis is the result of work which has been carried out since the official commencement date of the approved research program, and, any editorial work, paid or unpaid, carried out by a third party is acknowledged.
Name:
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Acknowledgments
All praises, Glory, and Honor to the creator and sustainer of the universe. Gratitude and thanks to Almighty for the gift of life, peace of mind, and good health throughout the period of undertaking this project.
My sincere thanks goes to my family, relatives, and friends for their much love and unwinding support from the beginning of this project up through to its completion. Gratitude to all, your efforts are much appreciated.
I would now take this humble opportunity to express my deep and sincere thanks and gratitude to my thesis supervisor (NAME) for his continuous guidance, dynamic supervision, enthusiastic encouragement, and invaluable suggestions throughout various phases of this research development. His valuable advice and interest in the topic was the source of my inspiration
While working on the project, I collaborated with many of my colleagues for whom I have unmatched regard and respect and I would wish to extend my warmest gratitude to all who have helped me complete my project
EXECUTIVE SUMMARY
In structural constructions, concrete has proved to be the main material for the completion of infrastructural development projects where high-quality strength is of the essence. Concrete is a versatile material that is made from proportionately mixing ingredients such as Portland cement, limestone, water, admixtures, coarse or fine aggregate depending on the specification and the purpose of the prepared concrete. All these constituents of concrete are mixed in accordance with predefined codes and standards of concrete constructions and these codes and standards vary from one country to another. It is important to adhere to the recommended proportion ratios since to a great extent they influence the strength of concrete made. However, there are two other crucial aspects of concrete that influence the level of compressive strength of concrete. These two parameters are the size and shape of the prepared concrete. This project presents a detailed analysis of how the size and shape affect the compressive strength of concrete. The project is organized into chapterwise arrangements from the introduction of the subject of analysis to an extensive literature review focusing on the previous experimental analysis relevant to the topic of study. The main methods of analysis applied are the experimental analysis and critical review of published journals to obtain relevant information that informed the conclusion of the research study. Generally, the shape and size of concrete are ideal in determining the overall compressive strength of prepared concrete as further elaborated in the result and discussion section of this report.
Table of Contents
Declaration of Originality. ii
L I S T O F A B R E V I A T I O N.. ix
1.2 Background of the Project 11
1.7 Research Justification. 17
2.2 Compressive Strength Tests of Concrete. 20
2.3 Cubic and Cylindrical concrete. 22
2.5 Factors affecting the strength of concrete. 31
2.5.3 Cement-Aggregate ratio. 34
3.2 Description of Material Used. 38
3.2.2 Coarse and Fine Aggregate. 39
4.3 Tests on hardened concrete. 45
4.3.1 Compressive strength tests. 45
4.3.2 Splitting tensile strength test 48
Project time management plan. 49
List of figures
Figure 1: Essence of concrete compressive strength on structures. 7
Figure 2: Typical comparison of compressive strength on cubes after 7 days. 11
Figure 3: Concrete cylindrical specimen. 20
Figure 4: Concrete cubes specimen for analysis. 20
Figure 5: Correction factors for various specimens. 23
Figure 7: Conversion factors between various types of specimens. 27
Figure 8: Strength of concrete Vs water-cement ratio. 29
Figure 9: Non-air entrapped and air entrapped concrete. 30
Figure 10: Air Entrainment on concrete’s strength. 31
Figure 11: Effect of aggregate cement ratio on concrete. 32
Figure 12: The graph of W/C, Aggregate and Compressive strength. 33
Figure 13: Ordinary Portland cement 36
Figure 14: Coarse aggregate. 37
Figure 15: Fine coarse aggregate. 37
Figure 18: Compressive strength test machine. 43
Figure 19: Capped cylindrical specimens. 44
Figure 20: Cylinders samples under splitting tension test 44
Figure 21: Project planning diagram.. 46
Figure 22: Project schedule. 47
list of abbriviations
ACI American Concrete Institute
AASHTO American Association of State Highway and Transportation Officials
ASTM American Society for Testing and Materials
BS British Standards IS Indian Standards
UPV Ultrasound Pulse Velocity
PSI Pound Per Square in.
LGED Local Government Engineering Department
REB Rural Electrification Board
PCC Portland Composite Cement
FM Fineness Modulus
chapter one
Introduction
1.1 General
Concrete has numerous applications in the civil and construction industry. Universal structural concrete is made with a compressive strength of approximately 20 MPa to 36 MPa (Del Viso et al, 2017). In recent years the use of high strength concrete in major construction projects in high rise buildings and supportive members of bridges involving different shapes and sizes. It has become an issue in using the compressive strength values in control specimen shape and size analysis are quite different from country to another. Given the significance of compressive strength of concrete and due to the fact that the structural elements of different sizes and shapes are used, it is proposed to investigate the effect of size and shape of the specimen on the compressive strength of concrete (Ahmed et al, 2016). In this work, specimens of plain as well as Glass Fiber Reinforced Concrete (GFRC) specimens are cast to carry out a comparative study.
Figure 1: Essence of concrete compressive strength on structures
1.2 Background of the Project
Concrete structures are considered one of the most durable and strong construction material. The strength of concrete is often measured through the compressive strength test, the test is the well-accepted measurement often used to determine the performance of a given concrete mixture (Ravindrajah et al, 2018). Since loads subjected to concrete reduce their size, the text gives the ability of the concrete to withstand such loads. Another important concrete test is the tensile strength test, which shows the ability of the concrete to resist cracking or breaking when subjected to a substantive tension (Goldman, & Bentur, 2013). The tensile strength test on concretes shows the size and shape of cracks on the concrete structures and the extent to which they occur. Generally when structural tensile forces exceed tensile and comparing traditional concrete has significantly lower than the compressive strengths.
A compressive strength test is the most conducted test on concrete construction materials. A compressive strength test is most common since it is simple and easy to conduct.
In the past three decades, concrete technology has tremendously improved leading to the development of high-performance concrete. Some of the improved techniques are self-compacting concrete SCC (Tsai, 2018). The advantage of the self-compacting concrete in non-segregation and highly workable that can reach up to the corners thus facilitating the filling of the remote corners without using vibration efforts. For the past decade, self-compacting concrete has been used in the underwater, bridges, and construction if heavily reinforced concrete structures (Lue et al, 2017). The self-compaction concrete test is composed of conventional concrete cement admixture, water, and mineral addition and aggregate, however, the final components are different. Conventional self- compaction strength tests were made using finely crushed limestone, coarse aggregate with abundant water. The mixing proportion of the concrete preparation influences the properties of the prepared in its hardened state. Properties of the freshly formed concrete such as the hydration process are influenced by the modifications in the initial compositions (Xiao, & Wu, 2020). The shape and size of concrete are quite crucial in evaluating the mechanical, behavior, and structural performance of the concrete structure. Again accuracy of the estimated mechanical properties of concrete structures is very important since it is through mechanical properties that determine the application of concrete materials (Brand et al, 2015, Kwan, 2020).
The tensile and compressive strength tests are more essential in determining the strength and other related properties of concrete that are used by design engineers to design standard concrete structures. The addition of fiber enhances the engineering performance of non-structural and structural concrete. Fiber-reinforced concrete is of special interest for seismic and retrofit structural designs (Ozcebe et al, 2019).
The incorporation of metallic fibers can be problematic in some situations, especially when the fiber volume is high and the FRC is cast in sections with a moderate-to-high degree of reinforcement. The fiber content, length, aspect ratio, and shape play an important role in controlling the workability of FRC. Such concrete presents greater difficulty in handling and requires more deliberate planning and workmanship than established concrete construction procedures (Cui, & Sheikh, 2015). The additional compaction effort required for such concrete contributes to the increase in construction cost. To provide sufficient compaction, improve fiber dispersion, and reduce the risk of entrapping voids, the FRC is often proportioned to be fluid enough to reduce the need for vibration consolidation and facilitate placement (Abo-Qudais, 2015). An extension of this approach can involve the use of SCC to eliminate or greatly reduce the need for vibration and further facilitate placement. A truly fiber-reinforced self-compacting concrete (FRSCC) should spread into place under its weight and achieve consolidation without internal or external vibration, ensure proper dispersion of fibers, and undergo minimum entrapment of air voids and loss of homogeneity until hardening (Abosrra et al, 2017). Lack of proper self-compaction or intentional vibration and compaction can result in macro- and micro-structural defects that can affect mechanical performance and durability.
Generally, the compressive strength of concrete increases with the increase in the concrete size, therefore the size of the concrete is proportional to the size of the specimen concrete prepared. The compressive strength also decreases with a decrease in size, hover the decreasing rate remains constant beyond the specified limit depending on the concrete mixing proportion. Considering different shapes of concrete, study reveale4d that cube-shaped concrete materials have higher compressive strength as compared to cylindrical-shaped concrete hence the ASTM specification standards gives a correction factor for concrete compressive strength between 15 Mpa to 43 MPa to compensate the strength in case the specimen ratio is less than 2.0 (Johnson, & Ramirez, 2019). The CEB-FIP specification standards cite the ratio of 150mm by 300mm cylinder strength to 150mm cube strength. However, these standards requirements do not give elaboration non how they are applied or modified to accommodate the correction factors for the lightweight compressive strength of compressive.
1.3 Curing of Concrete
Concrete curing comes in numerous dimensions as provided by various scholars and researchers. It is a process aimed at regulating the loss of moisture from concrete is it upon placement in position or even during the process of manufacturing products of concrete hence offering time for hydration of cement to take place. Cement hydration goes on for days or an even week hence curing is carried out for some time supposed concrete is to attain the potential durability and strength (Tokyay et al, 2017). Curing may as well include regulating the temperature as this has effects on the rate of hydration of cement. Proper curing of concrete is of importance for the development of proper strength, durability as well as imperviousness of concrete. When proper curing is done, the concrete gains more strength, resistance to stress, abrasion, thawing as well as freezing besides becoming more impermeable (Del Viso, Carmona & Ruiz, 2007). The improvement tends to be spontaneous at the initial stage but goes on more slowly thereafter for a period that is not defined hence the role of curing is to ascertain the concrete does not undergo premature drying but instead retains the moisture so it might build up the strength alongside gaining resistance to wear and durability (Poon et al, 2020).
Figure 2: Typical comparison of compressive strength on cubes after 7 days
1.4 Problem Statement
Since the adoption of high strength concrete has turned out to be a widely adopted practice in different applications for numerous decades mostly for high rise buildings, bridges, long-span as well as works involving repairs and rehabilitation, there is need to gain confidence in applicability as well as the suitability of prevailing practices for testing. The 28 days concrete compressive strength is establishing using a standard uniaxial compression strength that is universally accepted as a general index for concrete strength.
Among the numerous parameters, size, as well as the shape of the specimen under test, tends to be the parameters that bear the highest importance in influencing the outcomes of concrete compressive strength as a result of characteristics of fracture. There are ideally two shapes of specimens that are often using in the testing procedure: cylinder and cube that play a basic role in the determination of concrete compressive strength. The British approach adopts a 150mm cube to be the standard specimen even as the cylinder used is often for the American approach (Ahmed, Mallick & Hasan, 2016).
The shape as well as the sizes of specimens of concrete for the compressive strength test tends to vary from a region to another. Cylinders are commonly adapted in Australia, USA, France, and Canada even as cubes are commonly used standard shapes within the United Kingdom, Germany alongside numerous other European countries. Still, there are numerous countries in which both cylinders and cuboids are used in the tests. The standard sizes that are used include 150mm and for the cubes as well as for the centers. Nevertheless, the benefits for instance easy of handling, minimal use of concrete, the need for lower capacity machines alongside others provided by smaller specimens have resulted in them being used in most cases.
There are no extra additives or even materials that are often applied in keeping the strength of concrete. The compressive strength of 10mm size aggregate, as well as 20mm size aggregate, tends to be the major parameter under study, the test used ion the evaluation of compressive strength being Nondestructive test with the adoption of Schmidt hammer test as well as ultrasound pulse besides destructive tests on the 28th day that is implementable on given sample alongside cubes of concrete. The two tests are conducted as per the provisions of British Standard BS 1881 (Barbieri, Biolzi, Bocciarelli & Cattaneo, 2016).
1.5 Research Question
Numerous past experimentations and exploratory have been directed in request to discover how changing example shape and estimate could impact outcomes. Besides, past looks have likewise analyzed the impact of relieving conditions on transformation factors. The focus of this examination is to decide transformation elements of blocks of cement restored in air and water at 7 days and 28 days of ages
- What are the features of compressive strength as well as splitting tensile strength from specimens of various sizes and shapes?
- How does compressive strength and splitting tensile strength compare to the same concrete mix with changing shape as well as the size of the specimen?
- What are the factors of size and shape for changing concrete compressive strength calculated from various specimen types to standard cylinder sample and 150mm for the case of cube strength?
1.6 research Objectives
The research study presented in this publication was aimed at the following objectives:
- To evaluate and establish the correlation that exists between concrete compressive strength with shape and size of the concrete, shape to be considered in the study analysis cylindrical concretes
- To determine and evaluate the correlation between the concrete compressive strength with the shape and size of a cubic concrete material
- To examine the overall impact and effects of concrete’s shape and size on the ultimate compressive strength and how these parameters influence the application of concrete materials in different construction projects
- To determine a conversion factor between concrete compressive strength of cubes and cylinder specimens and the applicability of various expressions for the conversion factor of a cylinder and cubic samples used as a specimen.
1.7 Research Justification
Just like other construction materials, concrete needs to have perfect and desirable quality for construction purposes. In the last two decades, concrete has been the darling of construction activities in high rise constructions and construction of pies and bridge pillars due to its improved strength (Zuo, & Darwin, 2019). However compressive strength of concrete is affected differently with various factors among them being shape and size of the concrete, ingredient mixing proportion, curing process, and other properties of the materials used to make the concrete.
Several experiments have been conducted to verify and establish how these parameters affect the compressive strength of concrete and how to improve the quality and strength of concrete for construction activities. Among the previously conducted experimental analysis included the test and experiment to determine the designated value of resistance of concrete against a load. However, among these parameters, the most influencing factors are the size and shape of the prepared concrete that holds a lot in terms of the concrete’s compressive strength.
Therefore this project focusses on the analysis of how these two parameters are the shape and the size of concrete effects and influence its compressive strength property. Split tensile and compressive strength tests are the most important tests that directly influence the durability of concrete materials to sustain and withstand loads of structures. Therefore this project is ideal as it examines the shape and size factors and how they influence the compressive strength of concrete thus giving accurate recommendations on what is the best shape and size to be adopted in construction to ensure that high strength concrete is obtained in construction activities.
1.8 Project Scope
The research study was organized into sections through a thorough and detailed literat8ure review on the previously conducted tests on other parameters of concrete. The available published literature was critically analyzed and useful information drawn from them for accomplishing this study. The research and study will be limited to the scope defined below:
The study just handles axial compressive strength as well as splitting tensile strength from various test specimens at day 7 and day 28 with limitation in the variables to the type of specimen as well as concrete strength level
The test specimens used in this investigation are cubes of dimension 150mm and cylinders measuring. It is anticipated that compressive strengths as well as splitting tensile strengths to be in the range of 550 kg/cm2 and 1000kg/cm2 which is standard cylindrical strength noticed at 4 weeks. The specimens will be subjected to tests by the uniaxial load to point of achieving ultimate load capacity.
chapter two
Literature review
2.1 Introduction
Compressive strength and hardness test on concrete to establish and determine the hardness of concrete materials is one of the most crucial and necessary experiments conducted widely nowadays. There are several methods to accomplish this, however, the most commonly used method for determining the compressive strength of concrete has been then experimentation of casting concrete samples and crushing them through the use of relevant testing machines and equipment (Nagashima et al, 2012). After series of experimentation on the compressive strength on concrete, the studies established that the compressive strength is affected by diverse factors such as specimen’s shape, their sizes, the applied casting mold, rate of load application, and curing conditions.
Thailand is one of the numerous countries that adopt both cube and cylinder as the standard test specimens in establishing the characteristic compressive feature. Owing to the variations in the aspect ratio, shape as well as the related end restrain offer by the machine platen, cylinder, as well as cube strengths acquired from the same concrete batch, tend to be different where cubes provide higher strength. Research carried out at various centers of research all over the world have shown that the factor that links cube strength to cylinder strength is never constant for all the provided concrete grades (Akçaoğlu, 2017).
Concrete is the main material for construction in Saudi Arabia for nearly all the building types alongside other structures. A good number of structural elements of concrete are erected using a compressive strength that falls in the range of between 20MPa and 35MPa. There is an increase, of late, in the adopting of high strength concrete in most construction projects for instance high rise buildings as well as bridges. The development in the concrete production quality control is allowing ready mixed, pre east, as well as prestressed concrete plants to attain enhanced strength, concrete (Bayrak, & Sheikh, 2017). The characteristic compressive strength of concrete is often measured depending on 150mm cubes locally even though in the design practice, the design compressive strength is often a factor of standard cylinders. Using cylinders has gained greater acceptance within the local region with the increase in the need for testing high strength concrete. This is anticipated as most of the machines used for testing locally tend to have a full capacity of about 1300MPa thus testing a standard specimen with a compressive strength of 80MPA could need a test machine having a capacity that is more than 1300MPa (Montgomery, & Wang, 2016).
2.2 Compressive Strength Tests of Concrete
The compressive strength test is by far and large the most commonly used test in conducting tests on concrete. The major reason for gaining an understanding of the fact is that this type of test tends to be easy and quite affordable to conduct. Testing standard requirements deploy various geometries of specimens in the determination of compressive strength of concrete. Cylinders having slenderness that is equivalent to two alongside cubes are the most commonly adopted geometries. The effect of shape on compression strength has been studied extensively and various relationships between compressive strength affirmed for such geometries have been suggested, majorly from a technical point of view (Azhdarpour, Nikoudel & Taheri, 2016). Such an approach acknowledges the fact there tends to be a direct correlation between propagation and nucleation of the process of fractures and specimen failure. Actually, experimental observations affirm that the localized micro-cracked region develops when the stress is at a peak. For such reason, compressive failure tends to be suitably analyzed by the means of Fracture Mechanics.
In preceding decades, concrete technology has rendered it highly possible to get to higher strengths and high strength concrete has seemed to be new material for construction. This volume of compressive strength ignites that the standardized specimen for normal strength concrete for instance cylinder may go beyond the capacity of provided standard laboratory machines and equipment. To counteract such a drawback, high strength concrete mixtures are normally estimated with the aid of cylinders that as well meet the needs of ASTM C39. The obtained strength of the specimen tends to be higher in mean in comparison with the one obtained using cylinders. Such an effect of size on the compressive strength shows that the specimen does not behave in a simple manner that it looks to be behaving. The effect of size indeed is often understood as the reliance of the nominal structure of a building or structure on the size when a comparison is made against another structure that is geometrically similar to it.
The strength of concrete is a factor of numerous parameters hence a thorough comprehension of concrete alongside skills in the same is highly needed by the designers, suppliers, manufacturer’s specifications as well as contractors. Their skills would play a significant role in the determination of the quality of concrete building or structure that is to be erected. The compressive strength of concrete is affected by the material type, age, alongside the curing process, the ratio of water to cement, aggregate type, and aggregate size alongside other numerous parameters. Focus and attention are given to the parameters that revolve around the coarse aggregate size alongside its effects on the strength and workability of given concrete.
2.3 Cubic and Cylindrical concrete
Commonly used molds in testing concrete compressive tests are cubic and cylindrical, though they have various differences; both of these two models are popularly used in the experimentation of the compressive strength test on the concrete materials. Cubic (150 mm by 100 mm) specimens are mostly used in European countries including the United Kingdom and Germany while cylindrical (150 mm by 300 mm) specimens are often used in Australia, New Zealand, United States, and Canada (Bayrak, & Sheikh, 2017). For an experiment involving each of these shapes follows different experimentation codes and standards depending on the region of the experimentation, for example, the United Kingdom applies the ASTM standards and codes in cubic shape experimentations. The difference in experimentation involving these two shapes is that cylindrical shapes need capping before loading. The capping of the cylinders needs to be done with either mortar, cement paste or Sulphur mortar to provide the required plain loading surface. While cubic shaped specimens that do not demand any form of capping since they are turned y sides upon loading (Wasserman, & Bentur, 2016). On the other hand, cylinders are tested in the direction of casting giving them the testing advantages while cubic specimens show an improved comprehensive strength than cylindrical specimens hence they require higher capacity testing equipment and machines.
Figure 3: Concrete cylindrical specimen
Figure 4: Concrete cubes specimen for analysis
2.4 Scholarly Review
Scholars have conducted quite a sever researches in the past to clarify and better understand the shape size effects of concrete specimens and their corresponding compressive strength on testing and their results analyzed for further conclusions. In 1998 Planas and Bazant experimented to establish the shape size effect on the compressive strength of concrete and they established that size effects can be verified by altering the size of the concrete member. They added that by altering the shape, the concrete’s normal strength was also affected even though the specimens used were of the same shape, they concluded that the same relationship could be true with the shape effects when normal strength of concrete members depends on their shape.
Other than parameters of normal strength, other concrete properties differ in their results about the concrete specimen’s shape and size. Some of the parameters are patterns and trends of stress-strain curve and fracture or cracking pattern. When it comes to the studies geared towards overcoming the effects of the shape and size, conversion factors have been proposed and established regarding various conditions. on this account the first study on the size and shape effect was carried out as early as 1925 by scholar Gonnerman. He used different standard cubes of 8 inches and 6 inches with different cylinders (Sheikh et al, 2014). After a series of experiments and tests of varied ages, Gonnerman established that the average cube-cylinder ratio was 0.85 to o.88.
Plowman in 1974 conducted an experimental test to establish the effects of conversion factor on cubes/cylinders on different curing conditions. Plowman investigated the effects of conversion factor unde4r different conditions, at different times and established that the correction factor varies with shape and size of concrete material and eventually reflects the effects on the value of the compressive strength of the concrete material (Ozbakkaloglu, & Idris, 2014). The investigation on the effects of size and shape on compressive strength on high strength concrete was carried out by Jakolosai in 1995 and from the experiment, they proposed different conversion factor of 0.83 for the cylindrical specimens (150 mm by 300 mm) and cubic specimen (150 mm) and when the different cylindrical size of 200 mm by 100 mm with a cubic specimen of 150 mm the conversion factor was established to be 0.87. the whole study conducted by Jakolosai concluded that mixed design parameters, also change the strength ratio of cylinder/ cubes (Rashid et al, 2019).
The values of compressive strength of cylinder and cube specimens were studied by Felekoglu and Turkel (2005) at two various levels of strength with various sizes and suggested transformation coefficients. Concrete mix designs at two various strength grades (normal strength concrete and high strength concrete) were prepared. A cumulative of 144 specimens of 4 various kinds (150mm & 200 mm cubes, cylinder specimens) from normal strength concrete and high strength concrete mixed adopted in collection of nine in compressive strength tests at days 7 and 28 were made. Standard curing was used on such specimens until rest time (Kozul, & Darwin, 2017).
The findings of the test demonstrated values of compressive strength increases when specimen size is increased. Such behavior is contradictory to literature and was thought to be a result of the wall effect. The maximum size of aggregate of concrete mixtures is often 25mm that resulted in wall effect as well as low compactness, especially for cylinder specimens. Such conditions present the need for selection of the type of model which is suggested by the standards for a specific maximum size of aggregate (Scott et al., 2015).
The experimental analysis confirmed that cylindrical specimens often offer lower values of strength in comparison with cubic specimens. In a bid to change the strength of various kinds of samples to standard cylinder as well as a standard cube, 150mm, strengths (Chen et al, 2014), suggestions were made on the coefficients as demonstrated in below table
Figure 5: Correction factors for various specimens
Some studies have been carried out in a bid to propose equations for changing the compressive strength of various specimens for each one of them. For instance
L’Hermite’s equation
Cylinder strength/cube strength=0.76+0.2*log10 fcu/2840
In which fcu defines cube compressive strength in psi
Another yet widely known formula and law regarding size effect have been suggested by Bazant. The size effect rule explains briefly by increasing the size of the specimen, the compressive strength of the specimen of similar mix design lowers. The formula of such law is:
Where defines concrete tensile strength, B and d0 are constants even as d defines characteristic dimension which is the size of the specimen.
Weinbul (1951) suggested a similar theory. The weakest link hypothesis states that bigger specimens tend to be more ready to contain defects as well as anomalies in themselves that may fail at lower stresses. On another hand, as per the theory of summation as provided by Tuckers, specimen strength, as opposed to the least strength particle, is equal to the sum of the strength of every part of the individual (Silva, De Brito & Dhir, 2015).
In summary, it may be said that the findings of various sized concrete specimens in various circumstances are controlled by various factors among them various strengths of particles as well as defects within them. There is as well a difference between the patterns of fracture of cubes and cylinders. A major fracture surface tends to be nucleated for the case of cylinders even as there is a breakage of lateral sides for the case of the cube and there is destruction as a result of crushing. Such shape effect may as well be noted in
The impacts of shapes as well as the size of specimens have as well been studied regarding the tensile strength of samples of concrete mostly on outcomes of splitting tensile strength test. During a study conducted by Kadlecek et al. (2002), the splitting tensile strength of different samples of concrete cubes, prisms as well as cylinders were calculated. A general formula was suggested in the research that relates the area of fracture of every specimen to the relative splitting tensile strength of specimen based on the basic size of the specimen. The suggested formula is as shown below
Rt, tr=200.A-0.128
In which Rt, tr defines relative splitting tensile strength in % while A is an area of fracture given in cm2
Besides, another study has as well established that to the tune of a certain value, by increasing the size of the specimen, there is a decrease in splitting tensile strength even though after point nature tends to have deviated from the law trend of the size effect. The reason for such a result may be both, as a result of not enhancing the length of splitting fracture by enhancing diameter or as a result of failure mechanism change through the increasing size of the specimen.
Investigations have also been carried put on the effect of shape and size on high strength concrete demonstrating the effect of size tends to be stronger in cubes in comparison with cylinders. One of the parameters that alter the conversion factors is the grading of aggregates that illustrate itself via wall effect. Such an effect shows the amount of mortar needed in filling space between aggregates of concrete tends to be lower than the amount need in filling spaces between the wall of molds and aggregates (Jin, Ding & Du, 2018).
Figure 6: Wall effect (Lim et al 2014)
The additional mortar between mold walls and aggregates results in enhanced compressive strength of the specimen. It is as well more notable in specimens having large volume to surface area ratios and results in changes in the conversion factor of cylinder or cube.
There has been an elaborate study carried out on the wall effect. One such study was conducted by Zheng and Li 2002 in which they suggested a three dimensional model in the simulation of aggregates density within concrete specimens. The corresponding graphical representation of the model is in a manner that by moving to the inner zone from sides of the concrete specimen, the density of aggregate, at first has a rising trend to a certain peak then upon a small decrease, density gets to a constant amount. Still, the peak point of the model graph gets high by having more fraction of aggregates (Kodur, & Mcgrath, 2013).
Turkel and Ozkul (2010), in a bid to remove the effect of wall effect, sawed specimens of concrete during a study from casted specimens. In the study, it was established that size effect tends to be more dominant in concrete samples having higher compressive strength that may be attributed to more brittle features of such grades. Still, it was established that side effect is a factor of maximum aggregates sizes of concrete for the medium as well as high compressive strengths in a similar way.
With an increase in the adoption of high strength concrete, it is of utmost importance to have requisite confidence in applicability as well as the durability of prevailing practices of testing. Refinement, as well as verification, might be required for the factors which may influence outcomes of the compression test (Li, Wu & Hao, 2015). For the case of normal strength concrete, the impact of foregoing parameters has been extensively researched ad reported. For the case of high strength concrete, there have been reported relatively few investigations regarding such aspects. Imam et al. (1995) study significant factors including the shape of the specimen, mold material, size of the specimen that might affect outcomes of compressive testing of high strength concrete. Compressive tests were carried out on 6 various types of specimens with a cumulative of 360 specimens cast from about 18 various high strength concrete mixtures. From findings, they concluded that there is a decrease in compressive strength by about 5.8% of 150 mm cube specimen more in comparison with that of cylinder specimen. Still, conversion factors were as provided in below table
Figure 7: Conversion factors between various types of specimens (Mansur et al, 2018)
An experimental investigation was conducted by Mansur and Islam (2002) on the impacts of various concrete specimen types on compressive strength and came up with a correlation between their strengths. Each of the cumulative sets of test data in investigation composed of five values of strength for five various kinds of test specimens. Every strength value was determined by finding the mean of this strength of not less than three identical specimens. The 150mm cube compressive strength in the analysis was picked as a reference while the determined strength values for every kind of specimen were changes to corresponding standard strength of cube with aid of suitable expressions derived from linear regression analysis (Li, Wu & Liu, 2018).
The effect of specimen shape and size on the compressive strength of high strength concrete was studied by Tokyay and Ozdemir (1997). The experimental investigation was carried out on various cylinder sizes as well as cube specimen cast from three various concrete mixtures with compressive strength levels of about 40MPa, 60MPa, and 75MPa. The findings demonstrate that strength values of a diameter 75mm cylinder and cube specimens of size 75mm and 100mm were lower in comparison with larger specimens adopted.
2.5 Factors affecting the strength of concrete
Some numerous factors and parameters influence the compressive strength of concrete in one way or another. Among these factors, most are independent and some are correlated with other underlying features. Some of these factors are briefly discussed in the following sections.
2.5.1 Water-Cement ratio
The ration of mixing water and cement affects the curing process thus influences the strength of the prepared concrete. The compressive strength is inversely proportional to the water-cement mixing ratio as illustrated in the figure below
Figure 8: Strength of concrete Vs Water-cement ratio
The water-cement mixing ration is a very crucial factor in concretes porosity that eventually reflects the strength of concrete. In 2014, Neville established that increase in temperature increases the rate of exothermic hydration reaction thus enhances the strength of the concrete with time. The water-cement ratio is essential in obtaining a high concrete strength (Wasserman, & Bentur, 2016).
2.5.2 Air Entrainment
The process of incorporating air bubbles into the concrete through the use of air-entraining cement or air-entraining admixture is referred to as air entrainment. Within concrete there are two forms of air, that’s entrained air and entrapped air. These two forms of air affect the strength of concrete in that entrained air reduces the level of compressive strength when compared to non-entrained concrete. It is found that as the amount of entrained air increases, the demand for mixing water and sand reduces at particular cement content. However, when the cement content increases the reduction in the demand for mixing water decreases (Wasserman, & Bentur, 2016) Thus the reduction in compressive strength associated with air-entrained concrete can be somewhat compensated by making air-entrained concrete with lower water-cement ratios
Figure 9: Non-air entrapped and air entrapped concrete (Wasserman, & Bentur, 2016)
Figure 10: Air Entrainment on concrete’s strength (Wasserman, & Bentur, 2016)
2.5.3 Cement-Aggregate ratio
Previous studies showed that when constant cement water ratio is used in concrete preparation, a lean mix results in a higher strength concrete. The main impact of the cement aggregate ratio lies in the overall volume of voids in the formed concrete. When there are plasters on the concrete, then the porosity of the formed concrete is reduced thus high compressive strength. However, this fact remains relevant as long as the voids in the gravel or aggregate are ignored.
Figure 11: Effect of aggregate cement ratio on concrete
2.5.4 Aggregate Size
The size, shape, surface texture, grading, and mineralogy of the aggregate, stiffness, strength, and the and the optimum size of aggregate as illustrated in the figure below.
Figure 12: The graph of W/C, Aggregate and Compressive strength
Gilkey (2013) and Monteiro and Mehta (2015) in their studies concluded that the size and texture of aggregate affect the overall strength of the concrete materials except for the normal concrete strength. This is due to the fact that normal concrete is much stronger than the cement paste matrix and the transition zone. Monteiro and Mehta (2015) added that transition zone and the paste matrix would fail before the aggregate and this nullifies the overall strength of used aggregate. In 2015 Kosmatka and associates conducted studies on aggregate strength and they had the following suggestions. They mentioned that aggregate strength is not, in any case, a factor of consideration in the normal strength concrete as a failure in normal concrete is determined through the cement paste aggregate bond. Never the less, many studies relating to the bonding of aggregates to strengthen concrete. Brooks and Neville (2017) concluded that coarse aggregates give higher strength concrete than the fine coarse aggregate. They also observed that rough aggregates tend to exhibit better bonding than smooth aggregates. Jones and Kaplan (2017) made similar observations as Neville and Brooks and Neville aggregates would crack at higher stress compared to smooth aggregates
chapter three
Research Methodology
3.1 Overview
Various high strength concrete mixtures will be used in this examination. The anticipated cylinder compressive strength of various mixes of concrete are 550 kg/cm2, 700 kg/cm2, 850 kg/cm2 and 850 kg/cm2. The design, control as well as treatment of concrete mixes were done under similar conditions. The cement adopted in all mixes was Portland cement of type I and natural river sand is used as fine aggregates and limestone as coarse aggregates. The mineral admixture used is silica fume. A superplasticizer which is in aqueous form is as well adapted to aid in attaining the workable mixes with needed strength as well as quality (Tabatabaeian, Khaloo, Joshaghani & Hajibandeh, 2017).
3.2 Description of Material Used
3.2.1 Cement
For this experimental analysis, just normal Portland cement of grade 53 as per IS12269:1993 was used in concrete preparation. Sand from the river was used as fine aggregate and crushed granite was used as coarse aggregate for the experimentation. The analysis involved strength analysis for both with and without fiber. For the Portland cement used, had normal consistency of 37% with 64 minutes and the initial setting time and 42i minutes as the final setting time. The fineness of aggregate was determined through sieving methods
Figure 13: Ordinary Portland cement
3.2.2 Coarse and Fine Aggregate
Coarse aggregate was obtained through crushing granites into particles of approximately 20 mm size and was downgraded to approximately 5.0 mm from the local crushing plant. For the purpose of experimental analysis, the following properties were assumed for the coarse aggregates. The specific gravity of 2.68, fineness modulus of 7.77, void ration of 0.88, the weight of aggregate to be 17.0 kN/m3, and porosity of 0.47.
Locally available sand from the local river bed was used as a fine coarse aggregate for the study investigation. The sand from the river had about 4.76 mm size then was downgraded to 150 microns from the nearby crushing plant. The following properties were assumed for the fine coarse aggregate for analysis. The specific gravity of 2.5, fineness modulus of 2.33, void ration of 0.578, the weight of aggregate to be 12.75 kN/m3 and porosity of 0.366
Figure 15: Fine coarse aggregate
3.3 Mix Design Procedure
The experimental mix design was established as per the American Concrete Institute method of mix design of (ACI-211.1). the ingredients were mixed in the ratio of 1: 1.63:2.65 with cement water ratio being 0.5. the cement water ratio simply implies that for every cubic meter, 400gs of cement was used while that of fine and coarse aggregate being 1056kgs and 645kgs respectively and water proportion was 200 liters per m3
chapter four
Results and discussion
4.1 Experimentation
Eight trial mixtures of high strength concrete were conducted depending on the very materials using four various water-binder ratios in a bid to attain adequate information in the design of final proportions of mix. The design of trial mixes is anticipated to generate an avalanche of strengths which include target strengths. 4 test cylinders for every trial mixture are prepared and tested on day 7. From trial mix test findings, high strength concrete mix proportions are customized with the assumption of compressive strength at day 7 being 80% of strength when at day 28. The water-binder ratio, contents of superplasticizer as well as the water of every mix are customized to generate compressive strength almost anticipated strength as well as to enhance the workability of fresh concrete. The slump values for every mix in the study are anticipated to be greater than 200mm to facilitates concrete compaction
Table 3: Mix proportions for high-strength concrete
The uniaxial compression tests were conducted on specimens cast from four various mixtures of high strength concrete. four various types of specimens having dimensions as illustrated in the diagram below were adopted in a bid to calculate shape as well as side effects. All specimens were taken through tests at 2 ages that is day 7 and day 28. The calculation of concrete mixture strength for each sample, type of specimen as well as the age of specimen was dependent on the mean of four specimens (Woode, Amoah, Aguba & Ballow, 2015). The various types of molds list of specimens, as well as strengths of concrete, are as shown in the table below
Table 4: Number of the specimen with various types of molds & concrete mixes
The same conditions were maintained during mixing as well as the casting of concrete for every batch used in the experiment.
Batching of concrete mixtures was done using a pan mixer. Every mixture was divided into four equal batches owing to limitations in the amount of concrete mixer. Mixing of cement, silica fume, as well as aggregates, was done in dray state for approximately 60 seconds to ascertain their homogeneity
There was a gradual superplasticizer and mixing water that was done at the same time during the process of mixing. All contents underwent mechanical mixing for about 120 seconds. The conventional slump test was used in determining the consistency of fresh concrete (Wu, Zhang, & Yang, 2015)
70 specimens were cast from every mixture of concrete. Consolidation of concrete is done using an integral vibrator when placement of concrete is going on to ascertain full compaction
After 24 hours, all test specimens underwent remolding and then subjected to endless curing in water pond to time of preparation and testing of the specimen
The uniaxial testing is carried out using test procedures once specimens have undergone curing for 7 and 28 days. The concrete specimen underwent tests for splitting tensile strength as well as cylindrical compressive strength as per provisions of ASTM C39 even as cubical compressive strength alongside splitting tensile strength as per provisions of BS EN 12390-3.
4.2 Tests
Workability test: The tests that would be conducted on fresh concrete will be Vibe and slump test. The experiments will be carried out as per BS EN 12350-2:2009 as well as BS EN 12350-3:2009 in that order. The performance of two tests is as shown in the figure below
| Figure 16: VeBe test | Figure 17: Slump test |
4.3 Tests on hardened concrete
4.3.1 Compressive strength tests
Specimens of concrete were selected from various sizes as well as shapes for conducting of compressive strength test, various standards were observed. For compressive strength measurement of cubes, the provisions of BS EN 12390-3:2009 were adhered to (Wu, Shi, He, & Wu, 2016). The provisions of ASTM C39/C39M were followed in conducting a compressive strength test on cylindrical specimens. There is an extra stage of capping during testing of the compressive strength of cylinders. Cylinder samples cappers 150x300mm areas demonstrated in the figure below.
Figure 18: Compressive strength test machine
The loading speed in the experiment was mostly maintained at 0.4MPa/s or even 0.5MPa at other times for each of the specimens during the test involving compressive strength. It is worth noting that some of the specimens of concrete were as well selected for plotting of loading versus deformation curve when loading speed was to be 0.05MPa/s (Yoo & Banthia, 2016)
Figure 19: Capped cylindrical specimens
Figure 20: Cylinders samples under splitting tension test
4.3.2 Splitting tensile strength test
The splitting test was as well conducted on cubes alongside cylinders at the age of 4 weeks. The specimens were extracted from the curing tank at the time of testing and a line marked on specimens to ensure the application of load was axial (Yoo, Banthia, Kang & Yoon, 2016). Proper placement of specimens was done into the machine in readiness for testing.
chapter five
Project time management plan
Figure 21: Project planning diagram
5.2 Project Schedule
5.3 Resources required
- weighing scales
- compression test machine
- 100mm cube
- 150mm cube
- cylinder
- cylinder
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