EFFECT OF MORTAR SPACERS OF DIFFERENT GRADES ON THE PERFORMANCE OF CONCRETE
MOHAMED ALI MOHAMOUD
A project report submitted in partial fulfillment of the requirements for the award of the degree of Master of Engineering (Structures)
Faculty of Civil Engineering Universiti Teknologi Malaysia
JANUARY 2017
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This project report is dedicated to my beloved family;
Father, Mother, Siblings and Relatives.
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ACKNOWLEDGEMENT
In the name of Allah, the Most Gracious, and Most Merciful. Praise be to Allah S. W. T, Peace and blessings of Allah be upon His Messenger, Muhammed S.A.W, and all his family and companions.
I would like to express my sincere appreciation to my supervisor, Associate Professor Dr. Abdul Rahman Mohd. Sam for his encouragement, guidance, critics, friendship and help during the development of this project. Thanks to all other lecturers for helping me either directly or indirectly in the completion of this report project.
My special appreciation goes to my father, my mother and siblings, whom their love and care have brought me to this level. Their substantial encouragements and support have helped me to succeed in finishing my program here at Universiti Teknologi Malaysia.
Last but not least, I would like to express gratitude to my colleagues and seniors their help is really appreciated and will be remembered forever. Also, I would like to thank the staff of Structure and Materials Laboratory, Faculty of Civil Engineering, University Teknologi Malaysia for their assistance in the experimental works.
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ABSTRACT
Rebar spacers are crucial component in reinforced concrete structures. Their purpose is to provide and maintain cover for the reinforcement bars. Since rebar spacers are permanent part of the member their quality must be taken into consideration, failure to do so will jeopardize the durability of the whole reinforced concrete member. In this research project, the effect of mortar spacers on the durability and strength of concrete is investigated by examining the penetration of moisture and other corrosive agents such as chloride through the mortar spacer or through the concrete-spacer interface, assuming that the concrete is in good condition dense and durable. For lab experimentation, cube and prism specimens with different curing ages and conditioning were cast, the specimens were embedded with mortar rebar spacers and tested in terms of strength and durability. Strength tests have been conducted such as compressive strength and flexural, also durability tests for cube specimens were conducted including water absorption, expansion and shrinkage, chloride penetration and carbonation. Results showed that the presence of rebar spacer in concrete increases the ingression and transporting of moisture and other corrosive agents, especially at the concrete-spacer interface and when the quality of the spacer is lower compared to the quality of the concrete. Moreover, the spacer will negatively affect the flexural strength of prism specimens by initiating cracks. The spacers might seem small and hold low value during construction, but because they are permeant part of the structure and they are placed every ≤1 m along the rebars, their overall effect on ingress of corrosive agents might be significant, and that must be considered by the concrete designers and practitioners.
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ABSTRAK
Alas rebar adalah komponen penting dalam struktur konkrit bertetulang. Tujuan mereka adalah untuk menyediakan dan mengekalkan perlindungan untuk bar tetulang. oleh kerana alas rebar adalah sebahagian tetap ahli kualiti ia perlu diambil kira, kegagalan berbuat demikian akan menjejaskan ketahanan keseluruhan anggota konkrit bertetulang. Dalam projek penyelidikan ini, kesan alas rebar mortar pada ketahanan dan kekuatan konkrit disiasat dengan memeriksa penembusan kelembapan dan agen menghakis lain seperti klorida melalui alas rebar mortar atau melalui antara konkrit-alas rebar, dengan anggapan bahawa konkrit adalah dalam keadaan baik padat dan tahan lama. Untuk uji kaji makmal, kiub dan spesimen prisma dengan peringkat umur pengawetan yang berbeza dan pengawetan dibuat, alas konkrit dimasukkan ke dalam sampel dan diuji dari segi kekuatan dan ketahanan. Ujian kekuatan telah dijalankan seperti kekuatan mampatan dan lenturan, juga ujian ketahanlasakan untuk spesimen kiub telah dijalankan seperti penyerapan air, pengembangan dan pengecutan, penembusan klorida dan pengkarbona tan. Hasil kajian menunjukkan bahawa kehadiran rebar dalam konkrit meningkatkan serapan dan pengangkutan kelembapan dan agen menghakis lain, terutama di antara muka konkrit-alas rebar apabila kualiti alas rebar adalah lebih rendah berbanding dengan kualiti konkrit. Selain itu, alas rebar juga akan memberi kesan negatif terhadap kekuatan lenturan spesimen prisma dengan memulakan retak. Alas rebar mungkin kelihatan kecil dan tidak kritikal semasa pembinaan, tetapi kerana mereka akan menjadi sebahagian daripada struktur dan diletakkan setiap ≤1 m di sepanjang tetulang, kesan keseluruhan mereka pada kemasukan agen menghakis mungkin menjadi penting, dan perlu dipertimbangkan oleh pereka dan pengamal konkrit.
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TABLE OF CONTENTS
CHAPTER TITLE PAGE
DECLARATION ii DEDICATION iii ACKNOWLEDGEMENT iv ABSTRACT v ABSTRAK vi TABLEOF CONTENTS viii LIST OF TABLES xii LIST OF FIGURES xiii LIST OF ABBREVIATION xiv
1 INTRODUCTION 1 1.1 Background of the study 1 1.2 Problem statement 2 1.3 Research Objectives 3 1.4 Research Scope 3 1.5 Significance of Research 5
2 LITERATURE REVIEW 6 2.1 Introduction 6 2.2 Spacers Development 7 2.2.1 Spacers Types 8 2.2.2 Use of spacers 14 2.3 Effect of Rebar Spacers on the Microstructure and Properties of Concrete 18 2.4 Conclusion for Literature Review 25
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3 METHODOLOGY 27 3.1 Introduction 27 3.2 Experimental Program 28 3.3 Experimental Framework 29 3.4 Material Used 30 3.4.1 Cement 30 3.4.2 Fine aggregate 31 3.4.3 Coarse Aggregate 32 3.4.4 Water 33 3.4.5 Silver Nitrate 33 3.4.6 Phenolphthalein 34 3.5 Mix Proportions 35 3.5.1 Concrete Mix Design 36 3.5.2 Mortar Mix Design 36 3.6 Specimen Preparation 37 3.6.1 Molds 37 3.6.2 Concrete and Mortar Mixing 38 3.6.3 Number and Types of Specimen 40 3.6.4 Specimen Details and Specifications 42 3.6.5 Curing Condition 44 3.7 Experimental Tests 45 3.7.1 Sieve Analysis 45 3.7.2 Slump test 46 3.7.3 Density of Hardened Concrete 47 3.7.4 Compression Test 48 3.7.5 Ultrasonic Pulse Velocity Test 50 3.7.6 Water Absorption Test 51 3.7.7 Chloride Penetration Test 52 3.7.8 Carbonation Test 53 3.7.9 Expansion and Shrinkage Test 54 3.7.10 Flexural Test 55 3.7.11 Modified Compression Test 56
4 RESULT AND DISCUSSIONS 58 4.1 Introduction 58
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4.2 Sieve Analysis 58 4.3 Slump Test 60 4.4 Density of Hardened Concrete 61 4.5 Compressive Strength Test 62 4.5.1 Mode of Failure 64 4.6 Ultrasonic Pulse Velocity Test for Cubes 66 4.7 Water Absorption Test 68 4.8 Chloride Penetration Test 69 4.9 Carbonation Test 72 4.10 Ultrasonic Pulse Velocity Test for Prisms 73 4.11 Expansion and Shrinkage Test for Prisms 74 4.12 Flexural Test 78 4.13 Modified Compression Test 80
5 CONCLUSION AND FUTURE RECOMMENDATION 84 5.1 Introduction 84 5.2 Conclusion 85 5.3 Recommendations 86
REFERENCES 87
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LIST OF TABLES
TABLE NO. TITLE PAGE
2.1 Summary of various codes of practice on spacer classification and placement 12 2.2 Causes of concrete cover defects and their influence on spacers 20 3.1 Concrete mix proportions according to DoE method 36 3.2 Mortar mix proportions 37 3.3 Number of prisms 40 3.4 Number of cubes 41 4.1 Fine aggregate sieve analysis data 58 4.2 Coarse aggregate sieve analysis data 59 4.3 Slump test data 61 4.4 Average density of hardened cubes 62 4.5 Average compressive strength of cubes 63 4.6 Average pulse velocity for cubes, (km/s) 67 4.7 Water absorption for different types of specimens 68 4.8 Average chloride penetration depth for the specimens 70 4.9 Average pulse velocity for prims, (Km/s) 73 4.10 Expansion and Shrinkage results 76 4.11 Flexural strength for prism specimens 78 4.12 Compressive strength of prisms 81 4.13 Compressive strength for Prism and cube specimens, (MPa) 83
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LIST OF FIGURE
FIGURE NO. TITLE PAGE
2.1 Examples of clip-on spacers: a) A-spacers, b) wheel spacers and c) tower spacers 8 2.2 Some types of cementitious spacers: single spacers with centre opening for tie wire (a & b) and line spacer (c) 10 2.3 Steel wire chairs types: (a) single chair, (b & c) continuous chair and lattice (d) circular chair 11 2.4 Placing of plastic spacers 17 2.5 Causes of concrete cover defects observed in 25 construction sites in the UK 19 2.6 Local chloride induced corrosion adjacent to plastic spacer location 21 2.7 Corrosion at the interface between the mortar spacer and the concrete, the red circles indicates the locations where the corrosion started 22 2.8 Significant corrosion spotted at the plastic spacer location (left) but no corrosion spotted near the mortar spacer (right) 23 2.9 A-shaped spacer still attached to reinforcement with no sign of corrosion after 36 years of use in car park 24 2.10 Close look at reinforcement steel bar after ten years of weathering 24 2.11 Difference between A shaped plastic spacers used in pervious study 25 3.1 Flow chart of research stages 28 3.2 Ordinary Portland cement 31 3.3 Fine aggregate (sand) 32
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3.4 Coarse aggregate 32 3.5 Water 33 3.6 Sliver nitrate and distilled water used 34 3.7 Measurement of the depth of carbonation with phenolphthalein 35 3.8 Steel molds used a) cube molds b) spacer’s molds c) prism molds 38 3.9 (a) Mixing machine, (b) vibration table 39 3.10 Cubes and prisms after casting 39 3.11 Cube specimens and prisms 41 3.12 (a) mortar spacers used (b) mortar spacers inside the cube molds (c) mortar spacers inside the prism mold 42 3.13 Spacers embedded inside the cube and prism specimens 43 3.14 Cube specimens 43 3.15 Prism specimens 44 3.16 Sieve analysis set up 46 3.17 Slump test 47 3.18 Measuring scale 48 3.19 Compression machine 49 3.20 Positioning of UPV transducers (a) Spacer side (b) Normal side 50 3.21 (a) UPV for cube (b) UPV for prism 51 3.22 Water absorption test 52 3.23 Chloride penetration test steps (a) cubes inside chloride solution (b) splitting the cubes (c) spraying sliver nitrate and measuring the chloride depth using digital caliper 53 3.24 Concrete cube tested for carbonation 54 3.25 Demec studs and the digital reader used for the expansion and shrinkage test 55 3.26 Flexural test 56 3.27 Modified compression test 57 4.1 Grading curve for fine aggregate 59 4.2 Grading curve for coarse aggregate 60 4.3 Slump test 61 4.4 Compressive strength development of different specimens 64
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4.5 Mode of failure for cubes (a) concrete cube (b) concrete cube with spacer (c) Mortar cube grade 20 (d) Mortar cube grade 30 65 4.6 Concrete cubes (Control) 65 4.7 Concrete cubes with 20 MPa spacers 66 4.8 Average pulse velocity for cubes 67 4.9 Water absorption rate for different type of specimens 69 4.10 Chloride penetration depth 70 4.11 Chloride penetration into cubes (a) M20 (b) M30 (c) Concrete (d) C+S 20 (c) C+S 30 71 4.12 Cube specimens tested for carbonation 72 4.13 Average pulse velocity for different prisms 73 4.14 Expansion for prisms 75 4.15 Shrinkage for prisms 77 4.16 Flexural strength for prisms 79 4.17 Flexural failure for prism 80 4.18 Flexural failure started at the location of the spacer 80 4.19 Compressive strength of prisms 82 4.20 Failure of prisms (a) concrete prism, wet curing (b) concrete prism with spacer (c) mortar prism, dry curing (d) mortar prism, wet curing 82 4.21 Compressive strength of cube and prism 83
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LIST OF ABBREVIATION
ASTM - American Standard Testing Materials BS - British Standard EN - European Standard OPC - Ordinary Portland Cement UPV - Ultrasonic Pulse Velocity Test
CHAPTER 1
1 INTRODUCTION
1.1 Background of the study
One of the ways to increase and provide durability to the reinforced concrete members is to provide concrete cover for reinforcement. Concrete cover for reinforcement is required to protect the rebar against corrosion and to provide resistance against fire. The thickness of cover depends on environmental conditions and type of structural member. The minimum thickness of reinforcement cover is indicated in the drawings, or shall be obtained from the relevant code of practice. The correct quality and depth of concrete cover to the reinforcement has a great importance, both for the durability of reinforced and pre-stressed concrete structures and for their fire resistance (concrete cover for reinforcement, 2016).
Correct cover to reinforcing steel not only affects the durability of the concrete but also the extent and severity of cracking under working loads. In condition of fire the amount of cover also governs the time scale before damage to the reinforced concrete elements occurs. The general requirements for concrete cover must be followed, so that minimum required cover is achieved. The amount of nominal cover is dependent upon the size and type of the structural member, the environmental conditions to which it will be exposed (including fire), the class of concrete, the type and size of reinforcing bar which will be used (South African RC Engineers, 2016).
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Therefore, rebar spacers are required during construction to maintain cover before and during concreting and they must achieve serviceability and durability of the structural member after concreting. A rebar spacer is a device that secures the reinforcing steel or rebar in reinforced concrete to maintain the concrete cover, as the rebar is assembled in place prior to the final concrete pour. The spacers are left in place for the pour to keep the reinforcing in place, and become a permanent part of the structure (Rebar spacers, 2016).
The rebar spacers can be divided into three raw materials categories namely, concrete spacers, plastic spacers and metal spacers. The concrete spacers have the advantage of the same raw material which will improve the water tightness and strength of the concrete. Plastic spacers have the advantage of the low cost production and fast processing (Rebar spacers, 2016). The use of spacers is particularly important in areas with high earthquake activity in combination with corrosive environment. While preparing the rebar spacers two things must be taken into consideration, firstly the quality of the spacer in terms of strength and durability, secondly the type of the spacer.
1.2 Problem statement
Usually during construction and concreting of reinforced concrete members, the quality of the rebar spacer in terms of strength and durability is neglected, or not much attention is paid to it due to poor workmanship. This will result in low quality rebar spacer that will affect the durability of the whole reinforced concrete member.
Chloride, sulphate and other corrosive agents such as moisture may penetrate through the location of the rebar space. Therefore, the corrosion of steel reinforcement is believed to start from the location of the rebar spacer due to its low quality. Therefore, the strength and durability of rebar spacers must be taken into consideration.
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In this research, the effect of mortar spacers on the performance of concrete in terms of strength and durability were examined. Concrete specimens with different grade of mortar spacers, tested in terms of durability and strength to check whether the mortar spacers will be a weak point that will negatively affect the performance of the concrete or not. The concrete specimens with mortar spacers were exposed to chloride solution, to determine the penetration rate of chloride ions and their effect on durability of the concrete. Also, water absorption rate and other durability and strength tests were conducted for both the mortar specimens and the concrete.
1.3 Research Objectives
In this research, the aim is to examine and test mortar spacers with different grades and their effect on the performance of the concrete in terms of strength and durability. The following objectives are targeted to be achieved.
I. To assess the durability and strength of mortar spacers with different grades. II. To determine the effect of mortar spacers with different grades on the durability of concrete. III. To determine the effect of mortar spacers with different grades on the strength of concrete.
1.4 Research Scope
The research was based on lab experimentation; there are many types of rebar spacers, concrete, plastic and mortar. In this research, the mortar spacers only were tested. Durability and strength tests were conducted to a number of mortar spacers and specimens of concrete prisms and cubes embedded with mortar spacers. The parameters that were considered are as follows:
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Different types of specimens (cubes and prisms)
Different grade (strength) of mortar spacers
Different types of durability tests conducted on concrete and mortar specimens
Different types of mechanical tests on mortar and concrete specimens.
The purpose of testing mortar spacers and concrete specimens is to examine their durability in terms of water absorption rate, especially at the rebar spacer location and check the water absorption rate for mortar spacers as well. In addition, the uniformity of the mortar and concrete specimens with spacers were assessed by using ultrasonic pulse velocity test. Moreover, the strength development was assessed through mechanical testing (flexural and compression) throughout the curing period of concrete and mortar specimens.
The water absorption rate for cube specimens was determined by water absorption test. Some specimens were immersed inside chloride solution to determine the level of chloride ion penetration inside the specimen. All the intended tests have been conducted in accordance to standards, British and ASTM. The project scope can be summarized as follows:
This study focused on the use of one type of spacers, which mortar spacers of different grades 20 and 30 made in the laboratory.
All concrete specimens had the same concrete grade of 30.
There were no reinforced concrete specimens, due to time constrain. Only concrete specimens embedded with mortar spacers.
Various strength and durability test were conducted for this study. The tests were all done according to British Standard.
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1.5 Significance of Research
In construction, rebar spacers of different types are used for reinforced concrete members to provide cover for the reinforcements. The concrete used for construction must have adequate durability to withstand deterioration during service life, also great care is taken when designing the concrete mix of the reinforced concrete. However, the rebar spacers that are permanent part of the members are usually neglected in terms of quality and durability during construction. Rebar spacers with low durability may act as a weak point and jeopardize the durability of the whole reinforced concrete member.
In previous researches and studies, there were no literature on the effect of rebar spacer on the durability and strength of concrete. In this research the effect of mortar rebar spacers on the durability and strength of concrete is assessed and determined. This research will assess the hypothesis that weak rebar spacer will negatively affect the durability of the whole concrete member.
REFERENCES
Alzyoud, Wong, H. S. and Buenfeld, N. R. (2016). Influence of reinforcement spacers on mass transport properties and durability of concrete structures. Cement and Concrete Research, 87, 31– 44. American Concrete Institute (2004). Sp-66(04): Aci Detailing Manual, American Concrete Institute. BS EN 13670: (2010): “Execution of Concrete Structures”. London: British standard Institution. BS EN 12390-5: (2000): “Method for determination of flexural strength”. London: British Standards Institution. BS 7973-1: (2001a): Spacers and Chairs for Steel Reinforcement and Their Specification. Product performance requirements. London: British Standards Institution. BS 7973-2: (2001b). Spacers and Chairs for Steel Reinforcement and Their Specification. Fixing and application of spacers and chairs and tying of reinforcement. London: British Standards Institution. BS EN 12504-4: (2004): “Determination of ultrasonic pulse velocity”. London: British Standards Institution. BS EN 12390-3: (2009): “Compressive strength of test specimens”. London: British Standard Institution. BS EN 12350-2: (2009): “Slump test”. London: British Standard Institution. BS EN 12390-7: (2009): “Testing Hardened Concrete. Density of hardened concrete”. London: British Standards Institution. BS 1881-122: (2011): “Method for determination of water absorption”. London: British Standards Institution. BS EN 12390-11: (2015): “Determination of the chloride resistance of concrete, unidirectional diffusion”. London: British Standards Institution.
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Chemrouk, M. (2008). Influence of Poor Workmanship and Construction Defects on the Durability and Strength of Concrete. Concrete Durability: - achievement and enhancement, Dundee, Scotland. IHS BRE Press, 153-164. Clark, L. A., Shammas-Toma, M. G. K., Seymour, D. E., Pallet, P. F. & Marsh, B. K. (1997). How Can We Get the Cover We Need? Structural Engineer, 75. Concrete cover for reinforcement. Available form:
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Levitt, M. and Herbert, M. R. M. (1970). Performance of Spacers in Reinforced Concrete. Civil Engineering, 1-5. Mackey, G. and Januszke, R. (1995). Influence of Bar Chairs, Spacers and Blocks on Concrete Structures. Max Frank (2015). Frank: Spacer Technology. Germany: Max frank. Meck and Sirivivatnanon. (2003). Field indicator of chloride penetration depth CSIRO Manufacturing and Infrastructure Technology, North Ryde, New South Wales 2113, Australia. Poulsen, E. and Mejlbro, L. (2010). Diffusion of Chloride in Concrete: Theory and Application, Taylor & Francis. Rayner, R. (2005). Concrete Spacers. Concrete, 39, 31-31. Rebar spacer. Available from: < https://en.wikipedia.org/wiki/Rebar_spacer>. [8 November 2016]. Rostam, S. (2005). Service Life Design of Concrete Structures-a Challenge to Designers as Well as to Owners. Asian Journal of Civil Engineering (Building and Housing), 6, 423 445. Santos, P. M. D. and Julio, E. N. B. S. (2007). Correlation between Concrete-to- Concrete Bond Strength and the Roughness of the Substrate Surface. Construction and Building Materials, 21, 1688-1695. Seymour, D., Shammas Toma, M. and Clark, L. (1997). Limitations of the Use of Tolerances for Communicating Design Requirements to Site. Engineering Construction and Architectural Management, 4, 3-22. Shaw, C. (2007). Cover to Reinforcement-Getting It Right. Structural Engineer. Shaw, C. (2008). Long Term Durability of Plastics Spacers in Reinforced Concrete: A Case Study. Concrete Durability: - achievement and enhancement, Dundee, Scotland, UK. IHS BRE Press, 259-269. Shaw, P., Materialrontgen, A. and Kutti, T. (2003). Field Measurement and Experience of Chloride Induced of Reinforcement in Submerged Structures. Goteborg, Sweden: AB Fardig Betong. South African Reinforced Concrete Engineers' Association, Correct Concrete Cover to Reinforcement, available from: < http://www.sapta.co.za/coverblocks.php>. [8 November 2016].
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Tang, L. and Utgenannt, P. (2009). A Field Study of Critical Chloride Content in Reinforced Concrete with Blended Binder. Materials and Corrosion, 617 - 622. Vik, O. T. K. (2002). The Role of Bar Supports in Combatting Corrosion in Reinforced Concrete. In: Engineering, C. S. f. C. (ed.) The sixth International Conference on Short and Medium Span Bridges, Vancouver, Canada.