Concrete is mostly used as a construction material in world wide. Major construction material of infrastructural facilities in the 21st century is concrete. The basic ingredients are used in concrete mix is sand, gravel (aggregate), cement (binding material) and water. Freshly mixed concrete can be moulded in to any shape and any size.
Concrete is strong in compression but there are numerous drawbacks such as very less tensile strength, nature of brittle failure, low crack resistance, etc. The weakness in tension of concrete can be reducing by the use of conventional reinforcement bars and to some extent by the addition of a sufficient volume of certain fibres.
Fig-1.1 Behaviour of concrete member
Because of the chemical reaction between cement and water, the resulting material is like a stone structure. This stone like material, Concrete is relatively strong in compression but very weak in tension and tends to be brittle. Thus in concrete, cracks will develop at tensile end under small loads. These cracks gradually extended towards the compression end and finally, the member breaks. In the concrete the formation of cracks can also occur because of drying shrinkage. Due to drying shrinkage, cracks are developing as micro cracks. These cracks extended and increasing magnitude as the time is passed and the finally the concrete breaks. The main reason for the failure of the concrete is formation of cracks.
The weakness in tension of concrete can be overcome by the use of conventional reinforcement bars and to some extent by the addition of a sufficient volume of certain fibres. However, steel bar reinforcement in concrete provides local tension only. In reinforced concrete members, cracks extend freely until encountering are bar. Therefore we need multidirectional and closely spaced steel reinforcement but that cannot be practically possible with use of reinforcing bars.
Fig-1.2 Behaviour of concrete with and without reinforcement
So for solution of this problem and to increase the tensile strength of concrete we can use Fibre reinforcement. These fibres prevent the formation of the cracks. These fibres are dispersed and distributed randomly in the concrete during mixing, and Thus improve concrete properties in all directions, which named as fibre reinforced concrete. The main reasons for using fibres in concrete is to improve the post-cracking response of the concrete for improve its energy absorption capacity, ductility, and to provide crack resistance and crack control. Also, it increases structural integrity.
Fig-1.3 Reinforcement bar v/s Fibres
Fibre Reinforced Concrete:
In Fibre reinforced concrete (FRC), concrete is containing fibrous material, thousands of small fibres are dispersed and distributed randomly in the concrete during mixing, thus improve concrete properties in all directions and increases its structural integrity. So we can say that FRC is a composite material of cement concrete or mortar and discontinuous discrete and uniformly dispersed fibre. Fibre is small piece of reinforcing material which has some characteristic properties. There are many fibres available in market but not all will be effective and economical. Fibres are available in circular, triangular or flat in cross-section.
FRC has been successfully used in construction due to its excellent flexural-tensile strength, resistance to spitting, impact resistance and excellent permeability and frost resistance. Using fibres in concrete is an effective way to increase toughness, shock resistance and resistance to plastic shrinkage cracking.
Fig-1.4 Behaviour of concrete with and without fibre
Fig-1.5 Behaviour of fibres in concrete member
In FRC, fibres are mostly added to the concrete by volume fraction.
Classification according to volume fraction as follow,
Low volume fraction (<1%) Moderate volume fraction (between 1 and 2%) High volume fraction (> 2%)
Low volume fraction
For low volume fraction of fibre, it can be used to reduce shrinkage cracking. These low volume fraction fibres are used in slabs and pavements which have large exposed surface leading to high shrinkage crack. Disperse fibres having various advantages over steel bars and wire mesh to reduce shrinkage cracks:
The fibres are uniformly distributed in three-dimensions and all directions making an efficient load distribution.
The fibres are less corrosive than the reinforcing steel bars.
The fibres can reduce material cost, labour cost and time of placing the bars and wire mesh.
Moderate volume fraction
The fibre at moderate volume fraction increases the modulus of rupture, fracture toughness, and impact resistance. This fibre content can be used in construction methods such as shotcrete and in structures that require energy absorption capability, spalling, and fatigue.
High volume fraction
The fibres used at this volume fraction lead to strain hardening of the concrete member. Because of this improvement in behaviour of concrete, these composites are known as high-performance fibre-reinforced composites (HPFRC). In the last few years, even better composites were developed and that are known as ultra-high-performance fibre reinforced concretes (UHPFRC).
Role of Fibre Size
Short fibres: To bridge the large number of micro cracks in the composite concrete member under load and to avoid large strain localization it is necessary to have a large number of short fibres. The uniform distribution of short fibres in concrete element can increase the strength and ductility of the composite.
Long fibres: Long fibres are used to bridge macro cracks at higher loads; however the volume fraction of long fibres can be much smaller than the short fibres. The presence of long fibres significantly reduces the workability of the mix because of its size.
Fig-1.6 Behaviour of small & large sized fibres in concrete
1.3 Types of Fibre:
The most commonly used fibres are as follows:
‘ Steel fibre
‘ Glass fibre
‘ Carbon fibre
‘ Natural fibre
‘ Synthetic fibre
Steel fibre is one of the most commonly used fibres from recent years in construction. Mostly fibre having round cross-section are used. The diameter of steel fibre varies from 0.25mm to 0.75mm and it has high modulus of elasticity. It has tensile strength up to 800Mpa. This fibre sometimes gets rusted and loses its strength. But investigations have proved that fibres get rusted only at surfaces not at inner side of member. Use of steel fibres makes improvements in flexure, tensile, impact and fatigue strength of concrete. It can be used in various types of structures.
Glass fibre is a newly introduced fibre in making fibre concrete which has very high tensile strength of 1020Mpa to 4080Mpa. These fibres are mainly used in exterior building panels and as architectural precast concrete. Carbon fibre is very good in making shapes and it is less dense than steel. Investigations have shown that use of carbon fibre we can make the concrete very durable. The study on the carbon fibres in concrete is limited and it is mainly used for cladding purpose.
Natural fibres are low cost fibres and they are non-hazardous and renewable. Some of the natural fibres are Bamboo, Jute, Coconut husk, Wood fibre, Flax, Sugarcane, elephant grass, etc. Natural fibres increase toughness and flexural strength and induce good durability in concrete.
These fibres are man-made fibres which are resulting from research and development in the petrochemical and textile industries. There are two different physical fibre forms one is monofilament fibres and second is fibres produced from fibrillated tape. Synthetic Fibre include: acrylic fibre, aramid fibre, carbon fibre, nylon fibre, polyester fibre, polyethylene fibre and polypropylene fibre.
1.4 Carbon Steel Fibre:
The term steel is usually taken as mean that an iron-based alloy containing carbon in amounts less than about 2%. Carbon steels also known as plain carbon steels, ordinary steels, or straight carbon steels. It can be defined as steels that contain only residual amounts of elements other than carbon, except those which is added for deoxidation like silicon and aluminium and those which is added to counteract certain deleterious effects of residual sulphur like manganese and cerium. Silicon and Manganese can be added more than those required to meet the criteria of carbon steel so that the upper limits for these have to be set; usually, 0.60% for silicon and 1.65% for manganese. These are accepted as the limits for carbon steel.
Carbon is the most important chemical element in steel after iron. I we increase the carbon content we can produce higher strength and lower ductility material.
Low-carbon steel fibre, carbon content is less than 0.15%. Low-carbon steel benefit for maximum ductility due to too low carbon content.
Medium carbon steel fibre is having carbon content between 0.250 to 0.6%. So that it have balances ductility and strength.
High-carbon steel can be produced by 0.9’2.5% carbon content. Due to high carbon content it is very strong, but less ductile and used for springs and high-strength wires.
Carbon steel fibre having length ranges from 1.5 to 75 mm. Fibre having aspect ratio from 30 to 100. There are many shapes available for carbon steel fibre which is shown below.
Fig-1.7 Different shapes of fibre
1.5 Features and Benefits:
General advantages of CSF reinforced concrete:
The main advantages of Carbon steel fibre reinforced concrete v/s plain or mesh/bar reinforced concretes are:
Cost savings around 10% to 20% over conventional concrete flooring systems.
Fibre provides reinforcement in all direction whereas reinforcing bar provide in one or two direction.
CSF increases flexural strength of concrete member even for thinner section also.
CSF is good for toughness so that the ability to absorb energy can be increase.
It increases impact resistance of concrete member and so that it overcome joint spalling and tear & wear of member.
CSF increases shear strength of member compare to normal concrete.
Tensile strength is more in concrete member which is having CSF.
To achieve high strength, durable and economical concrete, there are some major areas in which we can use Carbon Steel Fibres-
Overlays: Road pavements, Runways of Airport, Storage Yards, Industrial Floorings and Deck slab of Bridges.
Increases impact resistance and Fatigue also
Provide resistance against wear and tear
We can take longer span
Thickness of pavement can be reduce
For CSFC member no or less need to maintain
We can increase service life of member
Pre-cast Concrete members: Frames and covers of manhole, Underground Pipes, Units which is use for Break-Water, Floor slabs of Building, Vertical Wall, Road Krebs, members which should Impact resistant, Blast Resistant members, slippers of railway track, etc.
Increases impact resistance and Fatigue also
Thickness of member can be reduced so that we can overcome cost of handling and transportation
Save precast material and cost of precast process
We can increase ductility and reduce formation of chipping and cracking
We can construct pre-cast units for different sizes and shapes by using CSF
Hydraulic and Marine Structures: Dams for storage purpose and also for electricity purpose, Spillways for water movement, Canals for water transportation, Sea wall, etc.
Provide resistance to cavitation and impact damage which is occur from wave action of water
Highly suitable for repair work of that members which is in contact of water
Defence and Military Structures: Weapon Storage Structures, Structures which should resist Blast load, Underground channels & tunnels etc.
Increases ductility and toughness which gives good resistance to blast load, impact load.
It gives fire and corrosion resistance and long service life
Shotcreting Applications: Underground and mountainous Tunnel Lining, Structural Domes, Elevated Slope Stabilization, Restoration of Structures, etc.
Having good efficiency and economy
For irregular shapes it is fast and single operation
Increases impact resistance and abrasion also
Special Structures: Foundations of Machine in industries, Currency Strong Rooms in banks or other important building, Earthquake Resistant Structures, etc.
Improve behaviour of Column-Beam Joints in seismic resistance structures
Increases impact resistance and abrasion also
Increases ductility which is good for seismic areas
Chapter-2 Objective, Scope of work, Methodology
2.1 Objectives of study
Concrete having weakness in tension and that can be overcome by the use of reinforcement bars and to some extent by the addition of fibres. But for better performance of concrete member we need multidirectional and closely spaced steel reinforcement but that cannot be practically possible with use of reinforcing bars.
So for solution of this problem and to increase the tensile strength of concrete we can use Fibre reinforcement. Carbon steel fibre having greater ductility as well as tensile strength compare to any other fibre. These fibres prevent the formation of the cracks. These fibres are dispersed and distributed randomly in the concrete during mixing, and Thus improve concrete properties in all directions.
The main reasons for using FRC instead of normal concrete is to improve the post-cracking response of the concrete for improve its energy absorption capacity, ductility, and to provide crack resistance and crack control. Also, it increases structural integrity.
Carbon steel fibre can be used in pavements, runways, ground supported slab, ground supported beams, footing, etc.
If we are using FRC with sufficient strength compare to RCC then it will be very economical, cost saving, time saving, good finished, and also we can reduce thickness of member. And it can also resist can be used in road pavements.
Carbon steel fibre reinforced concrete also increases mechanical properties of concrete such as compressive strength, flexural strength and split tensile strength.
Because of it is very ductile material it can resist impact load, wear and tear of concrete member, so it can be used in industrial floors or industrial machine foundation. And it will be more durable compare to normal concrete.
By conducting different tests on prepared specimens, it is intended to analysis of results. By comparing the test results of carbon steel fibre reinforced concrete with normal concrete, as well as comparison between replacements of reinforcing bar either fully or partially with CSF will be done and then conclusion of it will be given.
Scope of the study
This study is concentrated on the performance of carbon steel fibre concrete.
In this research, we have used two different dimensioned CSF, Fibre Type-1 length-50mm & diameter-1mm and Fibre Type-2 length-25mm & diameter-0.5mm, both having same aspect ratio which is 50.
To carry out the best results of compressive strength, splitting tensile strength and flexural strength of Carbon Steel Fibre Reinforced Concrete with respect to various fibre content and aspect ratios.
To compare all results of Carbon steel fibre reinforced concrete with normal concrete.
To validate that the carbon steel fibres can be used as replacement of steel re-bars either partially or completely.
Methodology of work
Literature study was carried out to know past researches, investigations and previous studies related to this thesis.
Collection of raw Materials
All the required materials were collected and delivered to the laboratory. Which are; Cement, fine aggregate, coarse aggregate, carbon steel fibre etc.
Tests were performed on the ingredients of concrete mix to determine their properties required for mix design.
Concrete mix design was prepared for M-25 grade of concrete.
Preparation of testing specimen
The prepared samples include concrete cubes, cylinders and beams.
Testing of Specimens
Different tests were carried out on the prepared concrete samples which are slump test, compressive strength test, split tensile strength test and flexural strength test.
The data collection is done by tests.
Data Analysis and Evaluation
The test results of the samples were compared with the respective control concrete and the results were presented using tables, pictures and graphs.
Comparison between FRC and RCC
Validate the result of flexural strength of partially or fully replaced carbon steel fibre specimen.
Conclusions and recommendations are finally forwarded based on the results and observations.
Chapter-2 Literature Review
2.1 Literature Survey
Amit Rana,’ Some Studies on Steel Fibre Reinforced Concrete’, International Journal of Modern Engineering Research (IJMER), Vol. 3 Issue. 6, pp-3863-3871, Nov – Dec. 2013 :
In this research, a test was carried out on steel fibre reinforced concrete to check the influence of fibres on flexural strength of concrete. Flexural strength testing on beam was done for finding out the optimum quantity of steel fibres required to achieve the maximum flexural strength for M25 grade concrete. A normal concrete beam and SFRC beam of size (700mmx150mmx150mm) with Steel fibre content of 0.75, 1, 1.25, 1.50, 2, 2.50, 3.0, 4.0, 5.0, and 6.0% was casted. The beams would be tested for their flexural strength at 28 days. Two points loading can be conveniently provided by flexural testing machine. The load shall be applied without shock and increasing continuously until the specimen fails. The maximum load applied to the specimen during the test shall be recorded. From the experimental work it was found that with increase in steel fibre content in concrete there was a tremendous increase in Flexural strength. The flexural strength was increased with increasing the percentage of steel fibre as shown in table.
Table-2.1 Increment in Flexural Strength
Sr.No. Required flexural strength increase % of Steel Fibres Required by Weight of Concrete
1 10% 0.36%
2 20% 0.73%
3 30% 2.58%
4 40% 2.92%
5 50% 3.43%
6 60% 3.98%
7 70% 4.42%
8 80% 4.86%
9 90% 5.12%
10 100% 5.30%
11 110% 5.48%
12 120% 5.61%
Flexural strength can be increase from 10% to 120% compared to normal concrete.
Shende.A.M, Pande.A.M, ‘Comparative Study On Steel Fibre Reinforced Cum Control Concrete Under Flexural And Deflection’, International Journal Of Applied Engineering Research, Dindigul, Vol 1 No 4, 2011 :
The hook tain steel fibres assist in controlling the propagation of micro-cracks, first by improving the overall cracking resistance the matrix and later by bridging across even smaller cracks formed after the application of load on to the member thereby preventing there widening into major cracks. In the this paper, the effect of steel fibre reinforcement with different percentage of fibre which is 0%, 1%, 2% and 3% by volume with aspect ratio 50, 60 and 67 are studied for M-40 grade of concrete. The beam is tested for flexural strength under two point loading flexure testing machine. For flexural strength test, beams of size 100mmx100mmx500mm were cast. From testing results, it is observed that flexural strength from steel fibres is on higher side for 3% fibres as compared to for 0%, 1% and 2% fibres. Result shows that, through utilization of 1% steel fibres flexural strength increases from 13.35 to 23.35%, for 2% steel fibres flexural strength increases from 18.35 to 31.65% and for 3% steel fibres flexural strength increases from 20.80 to 48.35%. The addition of steel fibres has significantly enhanced the performance of SFRC beam in flexural compared to the normal concrete beam. When the specimen is tested for flexural strength test, it was visually observed that the SFRC specimen has grater crack control as demonstrated by reduction in crack widths and crack spacing with compared to plain concrete beam. It improves ductility of SFRC over normal concrete because of addition of steel fibres. And from the results it is also observed that for higher percentage of steel fibre deflection of beam is very less as compare to control beam.
A.M. Shende, A.M. Pande, M. Gulfam Pathan ,’Experimental Study On Steel Fibre Reinforced Concrete For M-40 Grade’, International Journal Of Civil And Structural Engineering, Vol 1 No 4, 2011 :
In present literature, investigation for M-40 grade of concrete to study the compressive strength, flexural strength, Split tensile strength of steel fibre reinforced concrete (SFRC). Testing specimens are containing fibres of 0%, 1%, 2% and 3% volume fraction of hook tain where the steel fibres having aspect ratio 50, 60 and 67 were used. For compressive strength test, cube specimens of size 150 x 150 x 150 mm were cast, for flexural strength test beam specimens of dimension 100x100x500 mm were cast and for Split tensile strength test cylinder specimens of dimension 150 mm diameter and 300 mm length were cast. Superplasticizer (0.6% to 0.8% by weight of cement) was added to the all specimens of SFRC and normal concrete. Result shows that compressive strength, split tensile strength and flexural strength are higher for 3% fibres as compared to 0%, 1% and 2% fibres. The compressive, flexural and split tensile strength increases from 11 to 24%, 12 to 49% and 3 to 41% respectively with addition of steel fibres. All the strength properties for 1%, 2% and 3% steel fibres are observed to be on higher side for aspect ratio of 50 as compared to those for aspect ratio 60 and 67. During the test, it was observed that when specimen is tested for split tensile strength and flexural strength the normal concrete specimen has broken into two pieces while the SFRC specimen retained the geometric integrity.
Shende.A.M, Pande.A.M , ‘Experimental Study And Prediction Of Tensile Strength For Steel Fibre Reinforced Concrete’, International Refereed Journal of Engineering and Science (IRJES), Vol 1 Issue 1, September 2012 :
The tensile strength of steel fibre reinforced concrete (SFRC) containing fibres of 0%, 1%, 2% and 3% volume fraction of Hook tain. Steel fibres of 50, 60 and 67 aspect ratio are used. For split tensile strength test, cylinder specimens of size 150 mm diameter and 300 mm length were casted. SFRC and control concrete specimens were tested under compression testing machine. Result shows that, Tensile strength are on higher side for 3% fibres as compared to that produced from 0%, 1% and 2% fibres. Through utilization of 1% steel fibres tensile strength increases from 9 to 15%, for 2% steel fibres Tensile strength increases from 14 to 19 %, for 3% steel fibres Tensile strength increases from 16 to 29 %. All the strength properties for 1%, 2% and 3% steel fibres are observed to be on higher side for aspect ratio of 50 as compared to those for aspect ratio 60 and 67. During the test, it was observed that when specimen is tested for split tensile strength, the normal concrete specimen has broken into two pieces while the SFRC specimen retained the geometric integrity.
D.B.Mohite, S.B.Shinde , ‘Experimental Investigation On Effect Of Different Shaped Steel Fibres On Compressive Strength Of High Strength Concrete’, International Journal of Emerging Technology and Advanced Engineering, ISO 9001:2008 Certified Journal, Volume 3 Issue 1, January 2013 :
Compressive strength is investigated using concrete grade M70 for steel fibre with hooked, flat and wave shaped. The hooked, flat, waving steel fibres having tensile strength 825Mpa, modulus of elasticity 200Gpa and aspect ratio 50 were used. The fibre volume fraction is 0.5, 1, 1.5, 2, 2.5, 3, 3.5, and 4% by weight of cement. The strength was determined by carrying out cube compressive test on 150mmx150mmx150mm size cubes, using UTM. Result shows that the compressive strength increases with increase in fibre content. The maximum increase in compressive strength at 4.0% of fibre content is 14.54% in case of waving shape fibre, 15.72% in case of flat shape fibre and 10.74% in case of hook fibre at 28 days.
Prashanth M.H, Indrani Gogoi, Anil kumar and Ravi Kumar C.M, ‘Experimental Studies On Use Of Helix Fibers In Concrete’, NITK Research Bulletin, Vol 21 No 2, Dec 2012 :
This is an experimental investigation of the effect of addition of helix fibres (High tensile carbon steel wire with electroplated zinc coating) in plain concrete to evaluate the compressive strength, tensile strength and the modulus of elasticity when compared to normal concrete. Concrete Mix of M25 grade was used. The compressive strength and the split tensile strength values for different dosages of helix fibres i.e. 0 (Normal concrete), 10, 15 and 25 kg/m3 were plotted. For compressive strength test cubes of dimension 150mm x 150mm x 150mm and for split tensile strength test cylindrical specimens of dimension 150mm diameter and 300mm height, were cast. It is observed that, with addition of dosages of 10, 15 and 25 kg/m3 of helix fibres to concrete the compressive strength was increased about 14%, 25% and 48% respectively compared to normal concrete, split tensile strength was increased about 10%, 20% and 30% respectively compared to normal concrete and initial tangent modulus was increased about 7%, 23% and 38% respectively. From all testing results, it is observed that the addition of 25kg/m3 of helix fibres in concrete the compressive strength, tensile strength, modulus-of-elasticity, and ductility was increased about 30% to 40%.
Mr. Nikhil A. Gadge, Prof. S. S. Vidhale, ‘Mix Design of Fibre Reinforced Concrete (FRC) Using Slag and Steel Fibres’, International Journal of Modern Engineering Research (IJMER), Vol. 3 Issue. 6, Nov – Dec. 2013 :
This is an experimental investigation focused on the compressive strength performance of the blended concrete containing different percentage of slag and steel fibre as a partial replacement of OPC. In this experimental work, the cement is replaced accordingly with the percentage of 10%, 20%, 30%, and 40% by weight of slag and 0.5%, 1%, 1.5%, 2% by weight of steel fibre. The strength performance of slag blended fibre reinforced concrete is compared with the performance of plain concrete for M20, M30 and M40 grade. The result shows that the optimum dosage for slag is 20% and steel fibre is 1.5%. It is observed that, the percentage of increase in compressive strength for M20, M30 & M40 grade for partial replacement of cement by GGBS (20%) + addition of steel fibred (1.5%) are nearly same for M20, M30 & 2.4% for M40 respectively. The percentage of increase in flexural strength for M20, M30& M40 grade for partial replacement of cement by GGBS (20%) + addition of steel fibred (1.5%) are 20.37%, 25.37% & 56.39% respectively and the percentage of increase in split tensile strength for M20, M30 & M40 grade for partial replacement of cement by GGBS (20%) + addition of steel fibred (1.5%) are 60.19%, 58.73% & 66.27% respectively for 28 days of curing. From the results, it was concluded that the rate of gain of compressive strength of GGBS concrete is slow in the initial Stage i.e. up to 14 days & as the curing period increases strength also increases.
Badorul Hisham Bin Abu Baker, Ayad Amjad Abdul Razzak, James Hassado Haido, ‘Tensile Behaviour Of Steel Fibre Concrete’, IOSR-JMCE, Vol 6 Issue 4, May-June 2013 :
In present paper the test program covered three types of steel fibre and fibre volume percentage 0.5% and 1%. Plain concrete cylinders and steel fibrous reinforced concrete cylinders of dimension 300mm length x 150mm diameter were casted. Crimped steel fibre having aspect ratio-50, one end paddled steel fibre having aspect ratio-16 & 20.5, two ends paddled steel fibre having aspect ratio-34.For spilt tensile strength cylindrical specimens were tested on UTM. Steel fibrous specimens with fibre content of 0.5% and 1% shows increasing of spilt tensile strength about 1.4375 and 1.5761 times respectively. Crimped steel fibre of aspect ratio 50 gives maximum split tensile strength.
Table-2.2 Specimen Tensile Strength
Type of Concrete Aspect Ratio Fibre volume ratio % Tensile Strength
Plain concrete – – 2.54
SFRC : Crimped Fibre 50 0.5 3.2
One end Paddled 16 0.5 2.92
One end Paddled 20.5 0.5 2.9
Two end Paddled 34 0.5 2.74
Steel fibres are widely used as a FRC all over the world. Lot of research work had been done on steel fibre reinforced concrete. Compressive strength of concrete with addition of steel fibre and partial replacement of cement by ground granulated blast furnace slag is nearly same as normal concrete. Hence the addition of steel fibre within FRC is more helpful for the flexural strength than the compressive strength. Compressive strength, flexural strength and split tensile strength was increased with increasing in percentage of steel fibre content.
Chapter-4 Material and Experimental Setup
Concrete is a composite material, main ingredients of concrete are- cement, aggregates, sand and water. Concrete composed of coarse material which is aggregates is a hard matrix of material and binding material cement that fills the space between the aggregate particles and glues them together.
Because of the chemical reaction (called hydration) between cement and water, the resulting material is like a stone structure.
The main concrete mix materials are discussed in this chapter. In this chapter, various test methodology of fresh concrete and harden concrete are discussed as per relevant IS codes.
Initial properties of every ingredient of concrete mix should be required for mix design. Properties in concrete mix like setting time of cement, sieve analysis and specific gravity of the aggregate, water absorption and fineness modulus are calculated by appropriate testing as per IS codes.
The most common type of cement which is generally used in all kind of construction of concrete is Portland cement. Cement is a binding material for concrete and mortar as well as plaster. Which type of cement is suitable for particular work is depend on the overall requirements of concrete, such as strength, durability, etc. OPC-53 grade cement is generally used in construction. This OPC-53 grade cement is easily available in market so use of it is world-wide. When Portland cement is mixed with water, a series of chemical reactions occur between water and cement which is responsible for hardening of concrete. Reactions between water and cement are termed as hydration.
For checking fineness of cement we need the surface area. The accepted fineness of cement is between 350m2/kg and 500m2/kg for OPC-53 grade cement. The typical cement content which is generally obtained is 350 kg/m3 to 450 kg/m3. If we use cement content more than 500 kg/m3 it increase the shrinkage cracks and may be dangerous. If we are using other fine filler (such as fly ash, pozzolana, etc.) then it may be suitable to use less than 350 kg/m3 cement.
There are two types of aggregates used in concrete: Fine aggregates and coarse aggregates. Sand is termed as fine aggregate and natural gravel and crushed stone are termed as coarse aggregate. In concrete, 60 to 75 % volume is of aggregate from the total volume of concrete. So, the properties of aggregate should be satisfactory. And strength of aggregate play important role, like in high ‘strength concrete. But in most of applications the strength of concrete and mix design are may be independent from strength of aggregates. If we add aggregate to the concrete mix we can greatly increase the durability. There are two kind of coarse aggregate available in market based on their origin one is natural aggregates and other one is non-natural aggregates. Aggregates provide concrete with good stability and wear resistance to the member.
For maintain quality of concrete, hard and strong aggregates should be used, it should not contain any undesirable impurities, and it should be chemically non-reactive. For achieving good strength we cannot use soft and porous rock. To avoid segregation in fresh mix amount of fines should be low. To determine particle-size distribution for aggregate grading was done. From grading of aggregate we can obtain the maximum size of it because size of aggregate affects the amount of aggregate. And also affect water-cement ratio, workability and durability of concrete.
Coarse aggregates: Particle size greater than 4.75mm, but generally used aggregate size is in between 9.5mm to 37.5mm in diameter.
Fine aggregates: Fine aggregate is basically sands which are originate from the excavated land or the marine environment. Generally used Fine aggregates are natural sand and crushed stone. Particle size of fine aggregate is less than 9.5mm.
Rough-textured aggregates, angular shaped aggregate require more amount of water to produce workable concrete. Whereas smooth and rounded shaped aggregate require less amount of water. Flat shaped aggregates are not good for bonding so that it must be avoided or limited about 15% by weight of the total aggregate for good quality concrete.
An important ingredient for making concrete is Water. Because it is the main reason for chemical reaction with cement and produce cement gel. Therefore the quantity and quality of water is playing an important role because it affects directly the strength. From Indian standards we can say that, if water is fit to drink it can be used for concrete. For concrete of better quality, water should satisfy all criteria of drinking water.
4.2.4 Carbon Steel Fibre
The term steel is usually taken as mean that an iron-based alloy containing carbon in amounts less than about 2%. Carbon steels also known as plain carbon steels, ordinary steels, or straight carbon steels. It can be defined as steels that contain only residual amounts of elements other than carbon, except those which is added for de-oxidation like silicon and aluminium and those which is added to counteract certain deleterious effects of residual sulphur like manganese and cerium. Silicon and Manganese can be added more than those required to meet the criteria of carbon steel so that the upper limits for these have to be set; usually, 0.60% for silicon and 1.65% for manganese. These are accepted as the limits for carbon steel.
Carbon Steel Fibre is a cold drawn steel wire fibre with Electroplated Zinc coating.
Base Material : High Carbon Steel Wire
Fibre type : Corrugated, Hooked-end
Length (mm) : 25-60
Diameter (mm) : 0.50-1.00
Tensile Strength : 1000-1400 N/mm2
Chemical Composition: C-0.05%
In this research, experimental work was done on two different fibre sizes. Specifications of those fibres are as follow:
Fibre Type 1:
Length ‘ 50 mm
Diameter ‘ 1 mm
Aspect ratio (l/d) ‘ 50
Tensile Strength ‘ 1400 N/mm2
Fibre Type 2:
Length ’25 mm
Diameter ‘ 0.5 mm
Aspect ratio (l/d) ‘ 50
Tensile Strength ‘ 1400 N/mm2
Fig-4.5 Carbon Steel fibre Type-1 & Type-2
4.3 PROPERTIES / TESTS OF FRESH MIX CONCRETE (FRC)
Concrete when it is in plastic or fresh state, is a freshly mixed material which can be moulded in to any shape and any size. The concrete ingredients such as cement, water, fine aggregate and coarse aggregate mainly control the properties of relatively fresh state concrete as well as the control over the hardened state of concrete. The concrete properties in the fresh state should have some requirements like good quality of concrete mix, the water to cement ratio, the mix proportion of all ingredients etc.
4.3.1 Workability of FRC:
The first property of fresh concrete came on mind is workability of it. Adequate workability should be there for any type of construction. But if we increase w/c ratio to increase workability then strength of concrete will be decreases simultaneously. So we can add admixtures to increase workability without disturbing strength of concrete.
Generally we conduct slump test for measure workability of fresh concrete. The test methodology is prescribed as below.
Slump Cone Test:
The most commonly we conduct slump cone test for measure workability of fresh concrete. This test extensively used to assess the workability of the fresh concrete mix. Also the further water requirements for concrete can be carried out by doing the test.
The slump measuring apparatus consists of a mould which is made from metal, in the shape of a cone both ended open with two handles provided on both opposite sides, a steel tamping rod of having 16 mm diameter and 60 cm height. The frustum cone have dimension of 100 mm top diameter, 200 mm bottom diameter and 300 mm height as shown in figure.
Before performing the test, the internal surface of the mould should be cleaned and free from moisture, dirt and any hardened concrete. The mould should be placed on a horizontal, smooth, rigid and non-absorbent surface.
Fig-4.6 Slump of concrete
Then, it is filled in four similar compacted layers which have thickness of each layer being approximately one fourth of the height of the mould. Each layer should be tamped at least 25 times evenly by the steel tamping rod which has the rounded end. When mould is fully filled with concrete, the top surface of concrete is levelled and finished using the trowel. The mould is then removed by pulling it gradually at upward direction with care and free from any jerk. The difference in level between the height of the mould and the highest point of the slumped concrete is measured. The subsidence measured is known as the slump of concrete.
There are three type of slump according to the profile (shape) of slumped concrete. This is as below:
1. Collapse Slump
2. Shear Slump
3. True Slump
Fig-4.7 Types of slump of concrete
When the concrete collapses completely it is called collapse slump as shown in figure. If the slump was collapsed then we can say that the mix is too wet. It shows the high workability mix.
When the top portion of the concrete was slip on side or shears off or one-half of the cone shaped concrete slides down an inclined plane then it is known as shear slump as shown in figure.
If concrete slump type shear or collapse obtains after performed test, it would be repeated.
If the concrete is simply slumped under gravitational force then it is said to be a true slump. This is the only slump which is accepted for various tests of concrete.
4.4. PROPERTIES OF HARDENED CONCRETE (FRC)
4.4.1. Compressive Strength
Concrete is very strong in compression but its contribution to tensile resistance is negligible. The compressive strength is measured by loading moulded concrete specimens in uniaxial compression till the ultimate failure.
Generally, cube specimen of size 15cm X 15cm X 15cm and cylindrical specimen of 15cm diameter and 30 cm length are used for the test. Cubes are mostly used for compression strength testing because in cube test the loading on the cast side can be avoided. But in cylinders we cannot avoid it. Cylinders behave like the actual performance in a structure because in real compression member the loading direction is the casting direction. But which specimen is taken for testing is depends on standards of different countries.
Fig-4.8 Direction of loading with respect to casting direction
Compressive strength of concrete depends on a number of factors that is w/c ratio, content of cement, aggregate type and quality, curing, age, ambient conditions, and specimen geometry.
Various geometry of specimen will affect the compressive strength of concrete. The major factor which affects compressive strength is Height to maximum lateral dimension ratio (h/d). It is expected that the compressive strength of cube is greater than cylinder because central region of cylinder is unaffected during axial loading.
Fig-4.9 Effect of uniaxial loading on cylinder and cube
Before performing the test, the internal surface of the mould should be cleaned and free from moisture, dirt and any hardened concrete. A tamping rod of steel bar having bullet point ended with dia-16 mm and length-60 cm should be used for compaction. In the test procedure, after cleaning of mould the oil should be apply at the inner side of the mould so that the cube specimen can be easily came out during demoulding after 24 hours. Generally, the mould is filled by fresh concrete mix in 3 layers of having each layer size 5 cm approximately. The concrete distribution should be evenly throughout the mould by mechanical vibrator or hand tamping. Each layer of the concrete should be compacted more than 35 strokes per layer using appropriate sized tamping rod. Then the top surface should be well levelled, smooth and finished by using trowel. When the test specimens are removed from the moulds then it would keep in the water for curing until the test is carried out.
At the time of testing, the specimen would be wiping out for remove excess water from the surface. At time of testing, the bearing surface of compression testing machine should be clean. The specimen should place in the testing machine such that the load shall be applied to the opposite surface of the cube cast. The test specimen should be aligning so that it rests centrally on the base plate. After placing of cube on machine, rotate the upper portion by hand till that it touches the top surface of the test specimen. The application of load increases gradually free from shock and at the continuous rate till the failure of specimen. The compressive strength can be calculated by the following formula.
Compressive strength (MPa)=(Failure Load)/(C/S Area of Cube Speccimen)
4.4.2 Tensile Strength
Normally, Concrete is not designed to resist direct tension; we can obtain tensile strength of concrete to estimate the load under which crack will develop. For maintaining the continuity of concrete structure we should give importance to the crack which is developed in concrete member due to any reason. And in many cases for the prevention of reinforcement from corrosion we should minimize the developing cracks. Generally, when diagonal tension developed because of shear stress then cracks was developed. But in the most of cases cracks was occur because of shrinkage and temperature.
The tensile strength of concrete helps to understand the behaviour of reinforced concrete also in many cases when, we have not taken the tensile strength into account for actual design of concrete member. Unreinforced concrete structures, like highway pavements and runways, dams are designed with keep in mind that the member should have some flexural strength also, which is a type of strength in tension. The tensile strength of concrete is only 1/10 to 1/7 compare to the compressive strength of concrete.
For strength in tension we have three tests: test for direct tension, flexure strength test and split tensile test (indirect tension).
Direct tension test:
The direct tension test is the most accurate for the tensile strength in concrete. In direct tension test, application of a pure tension force is very difficult without any eccentricity. And it is also very difficult to gripping the ends of the specimen to the test equipment. As shown in Figure, crushing of concrete can be occurring at the grips even when the specimen will not actually fail in tension.
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This post is not about assessment.
A few weeks ago, before every other word on the internet became Wikileaks*, there was a lot of buzz about this piece in The Chronicle by someone who writes students’ essays for them, for money.
I’d like to think, gentle reader, that you, sitting alone at your computer with a cookie arrested halfway through its trajectory to your mouth, are reeling at this astonishing news: students buying their way through a degree? Say it ain’t so! But if you’re reading this blog at all, you probably already know about essay mills, so finish your cookie and let’s move on, because essay mills make me sad, and they probably make you sad, too.
Anyway, I got to reading the (numerous) comments on the Chronicle article and ended up at another essay-mill confessional. And this one absolutely stopped me in my tracks:
I doubt many experts spent most of a decade writing between one and five term papers a day on virtually every subject. I know something they don’t know; I know why students don’t understand thesis statements, argumentative writing, or proper citations.
It’s because students have never read term papers.
It’s so obvious, in hindsight: students never see enough essays to be able to abstract the rules of what makes a good one. I mean, think about the essay as a form — often the form — of undergraduate assessment: we’re basically asking students to build a working aircraft without ever having seen one. We give students some classes about Newtonian mechanics, show them a few force diagrams, then say “right, build me something that will fly me to France for lunch.” The students duly experiment, with most making emergency landings in fields, while we rally ’round, saying helpful things like “but you didn’t put the right kind of fuel in” and “why haven’t you finished building this wing?” Typical student responses include “there are different types of fuel?” and “but you didn’t cover that part of the wing in class.”**
No wonder some students find it easier to have a quiet word with their local aircraft retailer. And I’m not saying this to excuse the essay-mill companies, whom I deplore for the simple, selfish reason that they are devaluing university degrees, diminishing my own efforts and those of my students. I mean, I don’t think you will ever convince me that their aww-shucks-we’re-just-providing-exemplars-what-do-you-mean-students-are-handing-this-work-in-as-their-own schtick is anything other than a thin veneer of bullshit designed to stave off the lawyers. But I also think that if hacking the system is as easy as paying a few dollars here and a few dollars there to someone who will effectively learn for you, then, well, maybe the system isn’t very good. Simon Bostock has some nice thoughts here on why this problem won’t go away until universities wise up.
But as I say, this post isn’t really about assessment; it’s about learning. Quite a lot of our knowledge is rules-based, like knowing “I before E except after C”, and “don’t talk to Roger until he’s had his first coffee of the day”; we rely a lot on these rules of thumb to help us make sense of the world. Students’ whole lives are about learning rules: how to write an essay; how to format a references list; how to make sure the electricity in your flat doesn’t get cut off. Very, very broadly (it’s possible that this dirty shorthand explanation is going to upset some people), there are two ways of acquiring these rules: learning the abstract principles, and learning by experiencing concrete examples for oneself.
Guess which category most university education falls into.
None of this really cohered for me until I watched a colleague from a different department teaching a group of new students the Harvard style of academic referencing. While not the most stimulating topic, this is nevertheless pretty relevant, because it underpins much of students’ written work during their degree.
Here’s one way of teaching Harvard referencing:
* surname followed by initials
* year of publication
* Title of article
* Title of journal (italics), its volume (italics), page numbers.
These abstract rules work well as a recipe for writing out your own reference list, but they’re not that great if you’re actually trying to internalise the rules. They’re pre-digested; there’s no work left to do there, so the bits of information slide over us, and each other. There’s no friction. Also, there are a lot of pieces of information there: six(ish) basic components, but many more if you also include the order in which they must be assembled, and details like which bits get italicised and which don’t. That’s probably too many.
Here’s a different way of teaching referencing:
Aardvark, J.R. (1980). Ants, and how to eat them. Journal of Orycteropodidae Studies, 80, 11-17.
Barker, R. (1982). Rum babas, and what to do if you’ve got them. Reading: Goodnight From Him.
Halley, W. (1955) Rock Around The Clock. New York: Decca.
Izzard, E. (1998) Cake or Death? Gateaunomics, 10, 195-196.
Lemur, R.-T. (2010) Strepsirrhinoplasty. Antananarivo: Raft Press.
Leonard, E. (1996). Out of Sight. New York: Harper.
Shorty, G. (in press). Okay, so they got me. Los Angeles: Cadillac.
* What are the rules by which this reference list is organised? Name as many as you can.
Here, to understand the rules, we have to do a little work. But it’s sort of fun; working out the rules is a barely-game. And the thing about abstracting the rules for yourself in this way is that the process is messy, tracking its muddy footprints all over your memory. Which is exactly what you want.
Here’s a half-baked thought: you can’t teach abstract principles nearly as well as people can teach themselves using concrete examples.
Science as a university subject relies on practicals as well as theory, but we still spend a lot of time telling students what the rules are, rather than letting them abstract those rules for themselves. For starters, I think this is a very paternalistic*** way of treating people who are supposed to be adults. But also, I’m pretty sure it constitutes poor practice, since putting in a little mental effort is rewarded in the long-term by better retention and understanding. You thought your teachers were sadists, giving you worked example after worked example? Well, maybe they were — but my point is, they were actually helping you out, too.
But university is not high school. And the thing about being an ‘expert’ (and if you’re lecturing to university students, then you are, by many people’s definition, an expert — even if no-one fully understands what it is exactly that you are an expert in, other than that you “do something with computers”) … the thing about being an expert of any kind is that it’s so, so tempting to provide helpful short-cuts, like well-meaning parents who hand down sensible advice about life to their children. We’ve all been given that advice, and I’m pretty sure that we all learned more profoundly from the consequences of ignoring it than we ever would have if we’d listened in the first place. The trick that education often misses is that abstract rules are easy to ignore until we understand their relevance, by which time we’re usually pretty deep in our own personal concrete example. Or deep in something, anyway.
I recently spent some time with a friend who is trying to learn about organic chemistry but finding some aspects of it hard. I enjoyed chemistry at school (um, 20 years ago), so we sat down together for an hour to try and work through the IUPAC scheme for naming chemicals. Now, you can try to learn all the rules for naming molecules in organic chemistry, but there are lots – they go on for two or three pages of my friend’s textbook. That’s a lot of abstraction, and we know that concreteness helps us learn (stodgy academic explanation; human-readable explanation). So instead, we looked at some specific examples of structural formulae, along with their names, and tried to abstract the rules of naming based on the information we had. And you know what, it worked pretty well. In fact, naming in organic chemistry is basically a language and visualisation problem, not a chemistry one, so I learned the rules quicker than my friend did, because language and visualisation are more my bag than they are his. But I’ve yet to meet someone for whom the exemplar approach flat-out doesn’t work.
Of course, when I talk about abstracting the principles from a set of concrete examples, what I’m really talking about is pattern recognition. Pattern recognition will be one of the essential 21st-century skills. It’s not about finding information anymore — now it’s about finding the right information, rejecting the irrelevant stuff, and knowing how we might go about telling the difference. Hat-tip for that link goes to Lee Skallerup, who suggests
Get students to analyze the writing (and the comments) to see what kinds of patterns emerge, what they can see if they take the time to look.
If we want to prepare students for the 21st-century workplace, we should be teaching pattern recognition, using exemplars and letting students figure out the rules for themselves; those are the skills they are going to need when they go out into the world. It shouldn’t takes much effort to shoehorn this sort of activity into the classroom, or to get students to understand the basic process — we abstract rules from concrete examples all the time (take this discussion of what differentiates men’s and women’s shoulder bags, for instance). As @Dave_Ferguson points out in that post,
The effort to make the tacit knowledge more explicit encourages reflection and revision … Concrete examples help people work their way toward more general principles.
And here, try this on: “assessment should fundamentally be about building learners’ capacity to make informed judgements about their work” (@cathfenn, via @hypergogue). I couldn’t agree more with this: success as a teacher or learning facilitator is watching the learner walk away, not needing you anymore (and ideally, exceeding you). But to be able to get to the point of critiquing their own work, the learner must be able to move from tacit or implicit understanding of the rule to being able to describe it explicitly. And I’m struggling to think of a case in which having concrete examples would not make it easier for learners to explicate the rules in this way.
Once I started thinking about this abstract/concreteness issue, I started seeing it everywhere. For example, on my third-year module, I set students a piece of coursework that is opt-in, and pass/fail: in exchange for demonstrating that they’ve done some of the groundwork, successful students receive some additional information that will help them think about the topic. Quite often, students fail to grasp what is required of them (a failure of pattern recognition that may well originate in my explanation of the task), so I ask them to resubmit. Recently, a student emailed me work that wasn’t up to scratch, so I suggested she try again and resubmit, which she duly did. But when I got her work back a second time, it was still missing the point. So I thought about it for a bit, and then I sent her the additional material anyway. And you know what? I got a very nice, very grateful response, saying that she now realised exactly why her original submissions hadn’t been right. Three simple points of triangulation (two “wrong” answers and one “right” one) constituted enough information to start abstracting some rules.
Really, the more I think about it, the more I think that using concrete examples and letting students abstract the rules for themselves is really just another variation on show, don’t tell. Which is honestly the best advice for learning design — or communication of any kind — that I know. And hey, maybe if we want to assess learning in ways that are less easily hackable, we should engineer a system of assessment that requires students to show us, as well as telling us, what they’ve learned. Let’s have assessments that test (a) implicit knowledge of the rules, (b) explicit knowledge of the rules, (c) awareness of situations in which the rules may not apply, and (d) the learner’s awareness of their own progression in terms of grasping (a) through (c), because there’s nothing like getting students all metacognitively tooled-up.
Okay, so maybe this post was a little bit about assessment.
(PS — If you’d like to read more about abstraction and concreteness in learning design, you may find this short paper interesting.)
* Here I’m inserting a reference to current events, so you know I didn’t just record this sometime last year.
** Just so we’re clear, this is a metaphor; I’m fairly sure this is not how Aeronautical Engineering students are assessed. At least, I hope not.
*** I know it’s not the done thing to laugh at your own jokes, but recently I had occasion to tweet But paternalism is good for you. Here, swallow this. You have to seize these opportunities where you find them.