EFFECTS OF PVA FIBER ON BOND STRENGTH IMPROVEMENT IN GEOPOLYMER CONCRETE
THESIS (RC-142501)
EFFECTS OF PVA FIBER ON BOND STRENGTH
IMPROVEMENT IN GEOPOLYMER CONCRETE
KEFIYALEW ZERFU 3115 202 701 SUPERVISOR: Dr.Eng.JANUARTI JAYA EKAPUTRI,S.T,M.T MASTER PROGRAM DEPARTMENT OF CIVIL ENGINEERING FACULTY OF CIVIL ENGINEERING AND PLANNING
INSTITUT TEKNOLOGI SEPULUH NOPEMBER SURABAYA 2017
THESIS (RC-142501)
EFFECTS OF PVA FIBER ON BOND STRENGTH
IMPROVEMENT IN GEOPOLYMER CONCRETE
KEFIYALEW ZERFU 3115 202 701 SUPERVISOR: Dr.Eng.JANUARTI JAYA EKAPUTRI,S.T,M.T MASTER PROGRAM DEPARTMENT OF CIVIL ENGINEERING FACULTY OF CIVIL ENGINEERING AND PLANNING
INSTITUT TEKNOLOGI SEPULUH NOPEMBER SURABAYA 2017
DECLARATION
I hereby declare that this research has been carried out under the supervision of Dr.Eng.Januarti Jaya Ekaputri, Civil Engineering Department, Institut Teknologi Sepuluh Nopember as part of Maters of Science program in Structural Engineering. In addition, I declare that this research is my original work, the findings presented are not found in any other previous work.
Kefiyalew Zerfu Civil Engineering Department
Institut Teknologi Sepuluh Nopember December 2016
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ACKNOWLEDGEMENTS
First of all, I would like to praise and give thanks to the Almighty God who gave me wisdom and strength from the inception until completion of this research. I would like to express my deeper gratitude to my adviser Dr.Eng.Januarti Jaya Ekaputri for her valuable suggestions, time, commitment, encouragement and support throughout my thesis. I would also like to acknowledge KNB scholarship council for covering all my study costs. My deepest thanks also goes to academic and technical staffs of civil engineering department at Institut Teknologi Sepuluh Nopember. I would also like to pass my deep gratitude to Kamaji Inti Utama, PT. Petrokimia Gresik and SBI Surya Beton Indonesia companies for providing materials for this research.
I would like to acknowledge all my friends in LBE research group for their continuous support. My sincere thanks also goes to Mr. Basar Ismail, Uwitije Pierre Damian and Eskinder Desta for assisting me during my experimental work. Special thanks for my families, Aynalem Asmare, Andualem Gebre Medihen for their continuous support and encouragement.
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EFFECTS OF PVA FIBER ON BOND STRENGTH
NAME : KEFIYALEW ZERFU
ID : 3115 202 701 SUPERVISOR : Dr.Eng.JANUARTI JAYA EKAPUTRI,S.T,M.T,
ABSRACT
Several researches were conducted to understand the bond behavior between reinforcing bars and surrounding concrete in Portland cement concrete. However, few results were presented on bond strength of geopolymer concrete (GPC). In this research, the effect of polyvinyl alcohol (PVA) fiber on bond strength in geopolymer concrete was studied. The main aim of this study was to investigate how bond performance affected by varying the amount of PVA fiber content. As a result, PVA fiber of 0%, 0.2%, 0.4%, 0.6% and 0.8% by volume of concrete were applied. Results showed that application of PVA fiber improves the bond resistance between reinforcing bar and concrete matrix. It has been investigated that using of PVA fiber in geopolymer concrete improves up to 25.9% bond strength from concrete without PVA fiber. From different percentage of PVA fiber used, the specimen with 0.6%PVA fiber resulted both maximum compressive and bond strength. A comparative study reveals that geopolymer concrete shows higher bond strength than OPC concrete. Furthermore, it has been studied that FEA result confirms the validity of the experimental pullout test for geopolymer concrete without fiber. The bond strength from experimental result deviates by 0.38% from the ultimate shear stress obtained in FEA analysis.
Key words: Bond strength, Finite element analysis, Geopolymer concrete, Pullout test,
PVA fiber.
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PENGARUH SERAT PVA TERHADAP PENINGKATAN
KUAT LEKAT BETON GEOPOLIMER
NAME : KEFIYALEW ZERFU
ID : 3115 202 701 SUPERVISOR : Dr.Eng.JANUARTI JAYA EKAPUTRI,S.T,M.T,
ABSTRAK
Beberapa penelitian telah dilakukan untuk memahami perilaku lekatan antara tulangan dan beton dalam beton semen Portland (OPC). Namun, beberapa hasil pada penelitian ini, disajikan dalam kuat lekat beton geopolimer (GPC). Dalam penelitian ini, dipelajari pengaruh serat alkohol polivinil (PVA) pada kuat lekat beton geopolimer. Tujuan utama dari penelitian ini adalah untuk menyelidiki bagaimana pengaruh kinerja dari variasi jumlah kandungan serat PVA. Sebagai hasil, diterapkan serat PVA dari 0%, 0,2%, 0,4%, 0,6% dan 0,8% berdasarkan volume dari beton. Hasil penelitian menunjukkan bahwa penerapan serat PVA meningkatkan ketahanan lekatan antara tulangan dan matriks beton. Berdasarkan investigasi juga menunjukkan bahwa menggunakan serat PVA di beton geopolimer meningkatkan kuat lekat hingga 25,9% dari beton tanpa serat PVA. Berdasar presentasi serat PVA yang berbeda, spesimen dengan 0,6% serat PVA menghasilkan kondisi maksimum baik kuat tekan dan kuat lekatnya. Sebagai fakta, studi perbandingan menunjukkan bahwa beton geopolimer menghasilkan kuat lekat lebih tinggi dari beton OPC. Selain itu, dilakukan studi validitas dari hasil permodelan elemen hingga (FEA) dengan eksperimen uji tarik untuk beton geopolimer tanpa serat. Kuat lekat dari hasil eksperimen memiliki deviasi sebesar 0,38% dari tegangan geser ultimate yang diperoleh dalam analisis FEA.
Kata Kunci:
Kuat lekat, Permodelan elemen hingga, Beton geopolimer, Uji tarik, Serat PVA.
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TABLE OF CONTENTS
LIST OF FIGURES
LIST OF TABLES
CHAPTER-1.
INTRODUCTION
1.1 Background For last many decades, Concrete is the most widely used construction material.
Ordinary Portland cement (OPC) is used as binding agent in concrete mix. The problem behind to OPC is, it requires large burning fuels which causing carbon emissions to the atmosphere. Production of OPC is currently exceeding 2.6 billion tons per year worldwide and growing at 5% annually. Five to eight percent of all human-generated atmospheric carbon-di-oxide worldwide comes from the cement industry. Among the greenhouse gases, carbon-di-oxide contributes about 65% of global warming (Sathish et al. 2012).
As a result, it is appropriate to search environmentally friendly binding agents for concrete. Currently geopolymer concrete (GPC) is introduced to reduce the above problem. This concrete made from geopolymer binder, which is an inorganic alumino- silicate materials like fly ash, metakaolin and blast-furnace slag (Majidi 2009). Geopolymer cements don’t need to be fired in production nor do they give off much CO during curing as OPC. The energy required to produce GPC is considerably less
2
than that required for OPC mixes, resulting up to 80% reduction in carbon dioxide emissions(Castel & Foster 2015).
In previous researches, low calcium fly ash (class F) has been investigated as suitable material for geopolymer concrete binder because of its wide availability and its less water demand. Low-calcium fly ash based GPC had shown excellent mechanical Properties and has good durability properties in short and long term tests (Rangan 2010).Currently few studies are done to investigate an important property of hardened GPC, which is its bond with reinforcing steel bars and concrete matrix. Some attempts were reported in some literatures to assess steel–geopolymer concrete bond. Research using the pull-out test shows that geopolymer concrete generally provide
2015 better bond strength than OPC concrete ).
Researches shows that PVA fiber improves the mechanical properties of geopolymer concrete such as flexural strength, impact resistance, toughness, and to shift failure mode. The use of short fiber is very preferable due to the simplicity and economical nature in fabrication. Considering the brittle characteristics of hardened GPC, polyvinyl alcohol (PVA) is incorporated to improve the ductility of hardened GPC. The addition of PVA fiber changes the impact failure mode of GPC from a brittle pattern to ductile one, resulting in a great increase in impact toughness (Yunsheng 2008). Another recent research in OPC shows that addition of PVA fibers increases split tensile strength up to 200% from original split tensile of bare paste without fiber. However, another publication states that addition of PVA fibers decreases the workability (Ekaputri 2015).
This research investigates mainly the effect of PVA fiber on bond strength of reinforced geopolymer and OPC concrete. Under this research, pullout test is also conducted to study the bond resistance between concrete matrix and reinforcing bars. In addition, compressive test for each variations are done with regard to standard codes and specification. Afterward, based on the experimental analysis, results are compared accordingly:
Frist, the maximum stress from experimental analysis are compared with approximate bond strength estimation by using compressive strength of the concrete, cover, diameter of bar, and bond length.
Second, the pullout result for geopolymer concrete is compared with the control specimen, i.e. OPC concrete.
Third, the result verification of maximum stress from 3D-finite element model analysis were conducted with experimental pull out test results.
Most importantly, in this research the distribution of stress inside the geopolymer concrete matrix and reinforcing bar were investigated from finite element analysis. To investigate the effect of PVA fiber on bond strength, PVA fiber of 0%, 0.2%, 0.4%, 0.6% and 0.8% by volume for both geopolymer and OPC concrete specimens were used. Reinforcement bar with diameter of 16mm, without lateral confinement will be used by providing bond length of 8cm, i.e. five times the diameter of bar. Class F fly ash were used as a binder in geopolymer concrete and alkaline solution of sodium silicate to sodium hydroxide ratio of 2.5 by mass ratio were also used. Analytical methods from high strength Portland concrete were used to predict the test results.
Concrete specimen of 150x150mmx150mm cube was used.
In addition to experimental tests, finite element analysis (FEA) was applied by using ANSYS software to confirm maximum stress that occurs around bond. Finally the bond stress versus slip relationships is constructed for each variation, which allowed to estimate bar development length for reinforced concrete structures. Test specimen for pull out test of 150mm x150mmx 150mm concrete cube for the chosen reinforcing bar were used according to Indian Standard Methods of Testing Bond in Reinforced Concrete (IS : 2770 1967).
1.2 Objectives
The main objectives of this research are:
To investigate the effect of PVA fiber on pull-out strength in PVA fiber blended class F fly ash geopolymer concrete by using different PVA fiber content.
To determine the optimum PVA fiber content for geopolymer concrete mix that resulted optimal bond strength.
To conduct comparative study for the pullout results with bond strength prediction formulas in ordinary Portland cement concrete.
To conduct Finite Element Analysis to validate the results from experimental test and study stress distribution under concrete matrix.
1.3 Problem statements
In structural design of any kind, there are basic design requirements that should be taken into consideration. These requirements depend on design philosophy and the design code to be used. It is obvious that in condition where there are well defined design code, building materials and design philosophy available, structural design will be safe and easily to handle .However, currently design of structures using GPC is very rare due to few researches done on structural requirement as OPC. Therefore, research questions are:
How much PVA fiber will be applied in the geopolymer concrete mix to obtain optimum bond strength?
How much maximum load can be resisted by the concrete before failure for each variation of PVA fiber?
How bond strength and slip varies for different PVA percentage content?
How stresses are distributed under concrete matrix during pullout loading?
1.4 Purpose
The benefits of this research is to study the bond improvement by adding PVA fiber into concrete mix. So that it deliver an important information about bond performance criteria on which designers can rely. This study provides:
Appropriate PVA fiber percentage which can be applied into concrete mix, so that the concrete matrix and re-bars have optimal bond. The ultimate Load that breaks the adhesive bond between concrete interface and rebar Results showing how bond stresses distribute along the bond length. The failure mechanism and precise compressive and tensile stress distribution from FEA result.
CHAPTER-2.
LITERATURE REVIEW2.1 Introduction
Since 1824, after discovery of cement by Joseph Aspdin, Portland Cement Concrete is one of the most widely used construction materials over the world. As the demand for concrete as a construction material increases, the demand for Portland cement also increases. Currently the usage of Ordinary Portland cement (OPC) as the primary binder to produce concrete is increasing due to many infrastructure construction. The problem with Ordinary Portland cement (OPC) concrete is that it requires large kilns burning fossil fuels to produce Portland cement causing air pollution and carbon emissions(MCCAFFREY 2002; Djwantoro Hardjito, Steenie E. Wallah, Dody M.J, Sumajouw 2003).
According to Davidovits, the production of geopolymer cement does not require more calcination of calcium carbonate, unlike ordinary Portland cement (OPC), which results from the calcination of limestone (calcium carbonate) and silico-aluminous material reaction. Furthermore, he coined that production of 1 tone of OPC directly generates 0.55 tons of CO In contrary, the production of 1 tone of geopolymer cement 2. generates 0.184 tons of CO from combustion carbon-fuel, compared with 1.00 tons of
2 CO for Portland cement. That shows geopolymer cement generates 5-6 times less CO
2 2 during manufacturing than Portland cement (Davidovits 2002).
2 Figure 2:1 CO emissions during OPC manufacture for countries (Davidovits, 2002).
In the same paper Davidovits had stated that different countries have different CO
2
emission rate and China is the leading country by CO emission from Portland cement
2
factory ash shown in Fig. 2-1. Still now, the use of Portland cement as binder material in construction is ongoing due urbanization and big infrastructure projects in different countries. Hence suitable ecofriendly alternative cement binder would needed to overcome environmental pollution. Currently geopolymer concrete had drawn the attention of researchers because of its lower carbon foot print and potential superiority over OPC in terms of its mechanical properties (Hardjito et al. 2005; Raijiwala 2011)
2.2 Geopolymer Concrete
2.2.1 Definition Geopolymer is inorganic materials firstly developed by Davidovits in the 1970s.
According Raijiwala, geopolymer is a type of amorphous alumino-Hydroxide product that exhibits the ideal properties of rock-forming elements, i.e., hardness, chemical stability (Raijiwala 2011). The chemical composition is similar to the naturally occurring zeoltic materials, but their microstructure is amorphous instead of crystalline (Hardjito et al. 2005). According to Davidovits, the polymerization process involves a chemical reaction under highly alkaline conditions on Al-Si minerals, yielding polymeric Si-O-Al-O bonds and he proposed that an alkaline liquid could be used to react with silicon (Si) and aluminum (Al) in a source material of geological origin to produce binders (Jaarsveld & Deventer 1997; J.Davidovits 1991). Therefore, because the chemical reaction that takes place is a polymerization reaction, the term geopolymer was used to represent this reaction (J.Davidovits 1991);(Damian 2011).
The two main constituents of geopolymer are the source materials and the alkaline liquids. The source materials for geopolymer based on alumina–silicate should be rich in silicon (Si) and aluminum (Al).These could be natural minerals from kaolinite, clays. Alternatively, materials such as fly ash, silica fume, slag, rice-husk ash and red mud could be used as source materials for geopolymer concrete production(Damian 2011) . Factors such as availability, cost, type of application, and specific need of the users determine choosing of source material for making. Alkaline liquids are used to make geopolymer paste, which are usually sodium or potassium based. The most common alkaline solution used in geopolymer concrete is a combination of sodium hydroxide (NaOH) or potassium hydroxide (KOH) and sodium silicate or potassium silicate (Wallah 2006).
The number of scientific papers dealing with geopolymer science and technology before twenty five years ago are very few. But it shows that there is strong increase in geopolymer research during last five years. Figure 2:2 shows the evolution of the number of geopolymer papers published from 1991 onwards up to 2009 (Geiger 2011).
Figure 2:2 Geopolymer research progress two decades (Geiger, 2011)
2.2.2 Engineering properties
Due to good mechanical properties, geopolymer matrices have attracted significant attention in recent years. Geopolymer as novel cementitious materials, compressive strength is an important factor. According to previous publications, geopolymer concrete has high early strength, low shrinkage, freeze-thaw resistance, sulfate resistance and corrosion resistance (Majidi 2009; Kannapiran et al. 2013; Ariffin 2013). It also indicated that the compressive strength in GPC was higher over controlled OPC as shown in
Fig 2:3. Even though, the mechanical properties of
geopolymer concrete are affected by pore contents, H2O, and alumino silicate minerals, it results better mechanical properties as compared to conventional concrete (Triwulan 2011; Palomo & Fernández-jiménez 2011).
Figure 2:3 Mechanical strength of fly ash geopolymer (Palomo & Fernández-jiménez 2011).
2.2.3 Factors that affects engineering properties
a) Alkaline ratio
Previous researches stated that Sodium silicate to Sodium hydroxide ratio between 2.0 to 3.3 provides better compressive strength of up to 71 MPa (Hardjito et al. 2005). More recently Sourav, stated that a mass ratio of sodium silicate to sodium hydroxide of 2.50 gives the optimum compressive strength (Das et al. 2014).
b) Molarity of alkaline solution
Molarity of alkaline solutions (NaOH or KOH) contribute an important role in the strength improvement of geopolymer concrete. With a higher concentration of NaOH solution, a higher compressive strength can be achieved (Das et al. 2014). NaOH concentration of 8M had been used in various researches papers. Increasing of molarity for alkaline solution results fast setting time as compared to OPC concrete (Triwulan
2011). However, 12M concentration of NaOH is ultimately the most recommended solution even if it gives slightly less strength compared with 14M and 16M, in which the later gives denser and less workable paste (S. V. Joshi and M. S. Kadu, 2012).Recent study by Raijiwala also show the increase in molarity of alkali solution has siginificant impact on compressive strength of geopolymer concrete as shown in Fig 2:4 (D.B. Raijiwala, 2011).
Figure 2:4 ; Strength gain with variation in molarities
(D.B. Raijiwala, 2011)
c) Fineness of Fly Ash
The fineness of fly ash is also a major factor on the strength of the geopolymer concrete. It had been studied that fly ash with good fineness values results up to 80 MPa of strength at 24hrs continuous curing at 90 C (Naganathan & Linda 2013). According to Naganathan & Linda, lower fineness value of fly ash decreases the strength of geopolymer concrete.
d) Curing temperature
In geopolymer concrete curing temperature is an important factor to obtain a better strength. The main polymerization process or the chemical reaction of geopolymer concrete takes place with the temperature imposed during the curing and it may attain almost its 70% strength with in the first 3 to 4 hours of hot curing (Anon 2014; Kong & Sanjayan 2010). As shown in
Fig 2:5, longer curing time enhanced the
polymerization process and results in a higher compressive strength (Hardjito & Rangan 2005). According to Sourav, The rate of increase of strength is rapid in the initial 24 hours of curing beyond that the gain of strength was moderate so the specimens should be cured for 24 hours only which will sufficient enough (Das et al. 2014) . As stated in previous researches, the curing which is done for geopolymer is hot steam curing or normal hot curing in oven with in a temperature of 60 C-90 C for 24 hours The curing temperature of 60 C is more effective than other temperature or can be said is the optimum curing temperature. Beyond 60 C it doesn’t affect the polymerization process significantly (Das et al. 2014).
Figure 2:5 Influence of Curing Time on Compressive Strength
(Hardjito & Rangan 2005)
e) Aggregate content
The proper selection of all aggregates and its ratio of fine aggregate to total aggregate content improves modulus of elasticity and Poisson’s ratio of geopolymer concrete(Joseph & Mathew 2012). At the same time, the split and flexural tensile strength increased for the total content varied from 60% to 75% (with a constant fine aggregate to the total aggregate ratio of 0.35). In addition, higher and more stable strength values can be achieved when there is an optimum surface area for interfacial bonding between the geopolymer matrix and the aggregate (Isabella et al. 2003).
2.2.4 Economic Benefits
According to previous researches, Class F fly ash geopolymer concrete yields several economic benefits over Portland cement concrete. Loyad and Rangan under their publication stated that, very little drying shrinkage, the low creep, excellent resistance to sulfate attack, and good acid resistance offered by low-calcium fly ash- based geopolymer concrete may yield economic benefits when it is utilized in infrastructure applications. The overall prices of geopolymer concrete is almost 10% to 20% less than the same quantity of fly Portland cement concrete. In addition, In addition, the appropriate usage of one ton of fly ash earns approximately one carbon- credit that has a redemption value of about 10 to 20 Euros. Based on the data given in this paper, one ton low-calcium fly ash can be utilized to manufacture approximately 2.5 cubic meters of high quality fly ash-based geopolymer concrete, and hence earn monetary benefits through carbon-credit trade (Wallah 2006).
2.3 Polyvinyl Alcohol Fiber (PVA)
Polyvinyl Alcohol Fiber has a significant importance according to previous researches. Plastic shrinkage cracking due to volumetric changes present in a cementitious matrix during the drying stage cannot be avoided but can be reduced
(Juarez et al. 2015). The following sections will discuss how PVA fibers improves engineering properties in concrete work.
2.3.1 Reduces plastic shrinkage cracking
According to American Concrete Institute manual, there are two causes that creates plastic shrinkage cracking in concrete structures. The first one is, the loss of water that occurs after concrete casting and its exposure to ambient temperature produces deformation due to volumetric changes. And the second one is, the water consumption due to hydration reactions of Portland cement leads to a reduction of the solid’s volume, which generates internal deformations in the concrete. These cracks can be reduced by using PVA fiber. The effective use of PVA fiber can result about 93% reduction in plastic shrinkage cracking (Juarez et al. 2015).
2.3.2 Improves flexural resistance
Experimental work done by Jang, resulted that employing of PVA fiber improves the elasticity property of concrete. The loss of compressive strength and flexural strength from the freezing and thawing action reduced by utilization of PVA fiber (Jang et al. 2014). In addition, the addition of PVA fiber enhances toughness, and prevents the sudden brittle failure of concrete. This due to very strong fiber–matrix bond resulting from high chemical bonding caused the micro-fibers to rupture instead of being pulled out. Larger ductility may be achieved by fiber pullout rather than rupture (Hamoush et al. 2010; Luo et al. 2013).
2.3.3 Improves compressive and splitting tensile strength
Addition of short fibers in concrete mix increases the compressive and splitting strength. About 30% splitting tensile strength can be achieved by using PVA fiber than non PVA concrete (Noushini et al. 2013).
2.4 Bond Strength
Bond strength of reinforced concrete is the shear resistance between a bar and the surrounding concrete matrix. This bond between concrete and reinforcement is one of the most important contributing factors in modern concrete structures. Because concrete has such a low tensile strength the proper functioning of any concrete system that contains any significant tensile forces relies completely on the strength of the bond between itself and the steel reinforcement (Damian 2011). Due to longitudinal force transfer from the reinforcement to the surrounding concrete, the force in the reinforcing bar change along its length, as does the force in the concrete embedment. Wherever steel strains are differ from concrete strains, a relative displacement between the steel and the concrete (slip) does occur. But thus lack of compliance is also the effect of highly localized strains in the concrete layer closest to the reinforcement (Tastani & S. J. Pantazopoulou 2002).
The resistance mechanisms upon which steel-to-concrete bond in Portland cement concrete are already well known due many research tests. But researches done on steel- to-concrete bond in geopolymer concrete are very few. Some researches show that geopolymer concrete has better bond resistance than OPC concrete (Ginghis Maranan, 2015).
The factors that affect the bond strength are many and their interactions are complex. Almost any variation in the chemical or physical characteristics of either the concrete or the steel bars is likely to have some effect on the ultimate strength of the bond.
However, the most significant factors relating to the bond strength developed between the concrete and the reinforcement are; concrete strength, Rebar diameter and cover, chemical adhesion, friction and mechanical Interlock (Damian 2011)
2.4.1 Factors related to the bond strength development
a) Concrete strength The bond strength increased with reduction of the w/c ratio of concrete mixes, resulting in higher bond strength in the case of high strength concrete. Concrete strength is the most important factor to control bond strength. Splitting failure takes place when the tensile hoop stress exceed the tensile strength of the surrounding concrete. As a consequence of splitting failure, bond stress in the bar is lost abruptly and the residual bond strength is practically zero (Muñoz 2011).
According to previous studies, it is possible to estimate the bond strength by using concrete compressive strength. Some of the approximate formulations are given below.
0.5
(1) f = 2.5 ∗ f
b c ′
Where; f is the concrete compressive strength (MPa)
c
0.6
c d
b ′
0.55
τ = (0.672 ∗ ( ) + 4.8 ∗ ( )) (f ) (2)
max c
d l
b d
Where; = ℎ ( ), =
( ), =
′
ℎ ( ), ℎ ℎ ( ) Equations (1) and (2) are recommended expressions to estimate the bonding stress from (ARAÚJO 2013; Asl, Farhadani 2012),respectively
b) Rebar Diameter and cover
Research result shows that by increasing concrete cover to bar diameter ratio, bond strength increases and slip decrease. An increase in confinement offers more resistance to longitudinal splitting cracks and reduces the uneven bond stress distribution along the embedment lengths. In high strength concrete using conventional steel with ordinary profile and ordinary profile and ordinary cover, may cause premature longitudinal cracks (ACI-318). Pull out test result analysis shows when the increase of slipping with increase of concrete strength and bar diameter (Hameed et al. 2013).
c) Chemical adhesion and Friction
Chemical Adhesion comes from the weak chemical bonds between the steel and the concrete. This weak adhesion is the first resisting mechanism for small bond stresses. Once the bond stress becomes too great for the weak chemical adhesion to resist and slip commences, frictional forces take over to take up the additional bond stress. Frictional adhesion depends upon the surface roughness of the reinforcement bar and the strength of the surrounding concrete. A stronger concrete will obviously result in a stronger bond to the steel. The quality of the concrete and the amount of voids it contains is also a factor that will increase the frictional adhesion. Fewer voids mean more surface area contact with the reinforcement bar and therefore a greater frictional adhesion that will hold to the steel longer. Frictional adhesion is the primary resisting mechanism for smooth reinforcement bars(Damian 2011).
d) Mechanical Interlock
Mechanical interlock is the primary resisting mechanism for standard steel reinforcement bars and is by far the greatest contributor to bond strength. This mechanical interlock can be caused by two actions. It can be the bearing stress against the face of the lug or the shearing stress on the concrete surface developed between the lugs as shown in the figure (Damian, 2011).
2.4.2 Bond failure types
The average bond stress is calculated at any stage during the pull out test until one of the following occurs; the reinforcement bar yields, the enclosing concrete splits or a slippage of at least 2.5mm has occurred at the loaded end. Aside from the first case which does not reveal any useful information about the ultimate bond strength, it can logically be concluded that upon reaching either of the other cases that the bond stress calculated is no longer equal to the applied force on the reinforcement bar being tested, therefore the last calculated average bond stress the instant before failure should be taken as the ultimate bond strength. Furthermore, the loads at which the slippage or splitting occurs should define the point at which either bond failure by slippage or splitting occurs (Damian, 2011).
There are two types of bond failures, pull out and splitting failure. Pull out failure is likely to occur when the concrete in between the reinforcing steel bar ribs is weak and surrounding concrete is strong. In case of splitting type of failure large compressive stress occur on the contact point in front of the rib. These stress are inclined from the out wards the rib and the rib exerts equal and opposite stresses (ACI 318-08 ,2008) (Damian, 2011)..
2.5 Pullout Test
According to researchers conducted on reinforcing bars pull out tests, the steel- concrete bond is very complex phenomenon. Connection between the reinforcement bars and concrete which prevents slipping between each other depends on the surface of bars. For smooth bars, the bond is mainly consists of chemical adhesion between the cement paste and the bars where as in deformed bars, the strength in slipping is mainly due to the mechanical action between concrete and the ribs. Pull-out tests are usually used to study of bond performance of steel reinforcing bars and concrete. Pull out tests have been widely adopted because they offer an economical and simple solution for the evaluation of the bond performance of reinforcing bars (Tastani & S. Pantazopoulou 2002)
The pull-out tests is applied to investigate bond performance between reinforcing bar and concrete. Even though the stress distribution is not exactly uniform around bond length, bond stresses are assumed uniformly distributed along the bonded length (l b ) for analytical calculation. Bond stress ‘τ’ can be calculated by dividing the measured bond force by the bonded surface area of the deformed steel bar as shown in Eq.3 (Hameed et al. 2013) .Hence, the bond stress can be expressed as:
P max
(3) u =
πd L
b d
ℎ = , = ℎ ℎ
According to previous researches, the interaction between concrete and rebar subjected to pull out test force is characterized by four different stages as shown in
Fig.2:6 (FIB Bulletin 10, 2000, (Muñoz 2011)).
Figure 2:6 Steel-concrete local bond stress slip law
FIB Bulletin 10 (Muñoz 2011)
Stage I (uncracked concrete): In this stage bond strength is mostly assured by
occurs. The bar slip chemical adhesion and low bond values ≤ = (0.2 − 0.8)
1 is considered as negligible. However, highly localized stress occur close to lug tips.
Stage II (first cracking): First cracking experienced in this stage. And for higher
, the chemical adhesion breaks down; in deformed bars the bond stress values >
1
lugs induce large bearing stresses in the concrete and transverse micro-cracks originate at the tips of the lugs allowing the bar to slip as shown in
Fig. 2:7 (a). However, the wedging action of the lugs remains limited and there is no concrete splitting.
(a) Bar-concrete slip and wedging action of the bar (b) Transverse cracks and splitting (Muñoz 2011)).
Figure 2:7 Crack formation in during pullout loading (Muñoz 2011)).
ℎ ; ∗ ℎ ∗ ∗ ℎ ℎ
Stage III: In this stage as shown in Fig 2:7(b), for this stage also higher bond stress
, longitudinal cracks (splitting cracks) spread radially, the values, i.e. > (1 − 3) wedging action also enhanced by the crushed concrete. And the outer ward component of the pressure is resisted by the hoop stresses in the surrounding concrete. In this stage gradually more or less sudden failure occurs depending on the transverse confinement. In a case of heavy transverse reinforcement or large concrete cover, through splitting is prevented by their confining action and concrete splitting remains limited to a cracked core around the bar (stage IVc, Pull-out failure).
Stage IVa: This stage immediately follows the breaking of adhesive bond in plain
bars. Force transfer is provided by friction and strongly affected by the transverse pressure, in which concrete shrinkage and bar roughness favor friction.
Stage IVb: In this stage deformed bars confined by light-to-medium transverse
reinforcement. The longitudinal cracks (splitting cracks) break out through the cover and the bar spacing, and the bond tends to fail abruptly. But if sufficient transverse reinforcement (stirrups) is provided, bond can be assured in spite of concrete splitting, because of the confinement action developed by the reinforcement.
Stage IVc: In this case the force transfer mechanism changes from rib bearing to
friction. Deformed bars confined by heavy transverse reinforcement, splitting does not occur and bond failure is caused by bar pull-out.
Sample study on direct pullout study were summarized in
Table 2:1
21 Table 2:1 Summary of sample pullout study from previous researches No.
References Parameters Test specimen/mold configuration Experimental/Schematic Pullout test setup Sample Bond strength versus slip plots
S. J.
1 (Tastani &
Pantazopou lou 2002) Normal strength OPC concrete.
Compressive strength of concrete in a range of 27.4-33MPa 2 (Hameed et al. 2013) Hybrid fiber-reinforced Concrete Compressive strength of concrete in a range of 42-50MPa Diameter of pullout bar-12mm (deformed) Bond length 5d b
- GFRP-glass fiberglass-reinforced polymer
22
3 (Maranan et al. 2015) Geopolymer Concrete Compressive strength of concrete in a range of
33MPa *GFRP bar diameter- 12.7,15.9, and 19.0mm Bond length 5,10, and 15d b
4 (Castel & Foster 2015)
Geopolymer concrete and OPC Concrete (control) 12mm diameter bars (smooth and deformed) Bond length 5d b Cross-sections of specimen 100×100 mm
[2] in Table 2:1. Table 2:2 summarizes the study parameters, experimental setups and sample result from this study.
Table 2:2 Short summary from current study References Parameters Test specimen/mold configuration (unit in mm) Experimental/Schematic Pullout test setup Sample Bond strength versus slip plots
Current study (Kefiyalew)
Geopolymer Concrete Main variable-PVA fiber content Compressive strength of concrete in a range of 36.08-43.16MPa Diameter of pullout bar-16mm (deformed).
Bond length 5d b
23 Hence, the current study was developed from pervious researches considering the experimental setups as given in reference
24
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CHAPTER-3. METHODOLOGY
START LITRACTURE REVIEW MATERIAL PREPARATION Fine Aggregate Coarse Aggregate Fly Ash (class F) Portland Cement Steel Bar ( Ф16mm) PVA fiber (0%,0.2%,0.4%,0.6% & 0.8%) by concrete volume NaOH 8M ( l) Na 2 SiO 3 Prepare Mix design
Prepare Compressive test specimen for OPC and GPC using 20cm high and dia_10cm cylinder Test the specimens at age of 28 days Strain guage small Large, prepare new mix design OK! If the deviation of compressive strength between OPC and GPC is Prepare Pull out test specimen Pull out test, test specimens at 28 days age Result and Discussion FINSH Finite Element(FE) Analysis Compare FE analysis with Experimental Results, if the result is….. OK! Not OK! Repeat Conclusions Do material testing according to standards Use available material test data’s
Direct tensile test Test for Modulus of Elasticity and poison’s ration Cure(wet) the specimen for 28 days Figure 3:1 Flow Chart
3.1 Introduction
In this section, mainly constitute materials for geopolymer concrete specimen, Preliminary tests for constitute materials, mix design, pull out test preparation, experimental set up, direct cost of the research and schedule were discussed.
3.2 Preparation of Materials
In order to study the bond performance of reinforcement bar in geopolymer concrete, different materials were used for specimen preparation. Materials that are used for making fly ash based geopolymer concrete specimen are:
- Fly ash(class F fly ash)
- Cement Coarse and fine aggregates
- Reinforcement bar
- Alkali activator (NaOH & Na
2 SiO
3 )
- Polyvinyl Alcohol(PVA)
- Strain gauge
- PVC
3.2.1 Fly ash Fly ash was used as a binder material in this research was class F fly from PT.
Petrokimia Gresik, Indonesia. It is available with low lime content and greater combination of silica, alumina and iron.
3.2.2 Cement Ordinary Portland cement was also used as a binder material for control specimen.
Ordinary Portland cement from PT. Semen Gresik, Indonesia with reference to the quality standards ASTM C 150 were used.
3.2.3 Coarse and fine aggregates
Local aggregates, coarse and fine aggregates, which were used in this research, were from PT.Surya Beton Indonesia. Coarse aggregate from crushed stone with maximum size less than 14mm and fine aggregate of sand in saturated dry condition were used.
3.2.4 Reinforcement bar
Deformed reinforcing bar with diameter of 16mm was used in order to perform pull out test to investigate and compare bond performance of geopolymer concrete with OPC concrete.
3.2.5
2 SiO 3 ) Alkali activator (NaOH & Na
The alkaline solution was used from combination of sodium silicate solution
2
and sodium hydroxide solution. The sodium silicate solution (Na O= 18%,
2 SiO =36%, and water=46% by mass) was purchased from a local supplier in
bulk. The sodium hydroxide (NaOH) with in flakes form was also purchased from a local supplier in bulk. The sodium silicate to sodium hydroxide ratio of 2.5 by mass is used.
3.2.6 Polyvinyl Alcohol (PVA)
PVA Fibers (polyvinyl alcohol) was used in this research because of wide variety application in concrete such as superior crack fighting properties, high modulus of elasticity, excellent tensile and molecular bond strength, and high resistance of alkali, UV, chemicals, fatigue and abrasion. The unique characteristics in their ability to create a molecular bond with mortar and concrete that is 300% greater than other fibers, PVA fiber is preferred for this research. Figure 3:2 shows the type of PVA fiber used as main parameter in this study.
Figure 3:2 PVA fiber The physical characterizes of the PVA fiber is given in Table 3:1.
Table 3:1 PVA fiber properties
Tensile Flexural Melting Length