Effect of Steel Fiber on Concrete's Compressive Strength
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The general goal of this research is to investigate whether steel fiber has a significant 'positive' or 'negative' influence on concrete compressive strength, as well as the optimal steel fiber ratio that delivers best result. Manually, cement, fine aggregates, coarse aggregates, steel fibers, and water were mixed together properly. A slump test was carried on the mixed concrete. After determining the workability, the mixed concrete was poured into cubes dimension 150 mm × 150 mm × 150 mm and left for 24 hours. After 24 hours, the samples were removed from the mold and placed in a water tank to cure for 7 to 28 days. The cube was tested for compressive and flexural strength in a universal testing machine after the samples had cured for the required 7 - 28 days. This study focuses on how to obtain high strength concrete using with steel fiber in the Conventional mix ratio to enhance concrete strength. Concrete reinforcement using steel fibers alters the characteristics of the concrete, allowing it to withstand fracture and hence improve its mechanical qualities. This study reports on an experimental study that reveals the effect of steel fiber on concrete compressive strength and the optimal steel fiber ratio that produces the best results. Steel fiber reinforcing improved the compressive strength of concrete. The average compressive strength of normal M25 concrete with 0% steel fibers and curing ages of 7 and 28 days was determined to be 22.97 N/mm 2 and 25.78 N/mm 2 , respectively. The steel fibers are then added in various concentrations, such as 1%, 2%, and 3%, with aspect ratios of 70. The compressive strength of concrete with 1%, 2%, and 3% steel fiber with an aspect ratio of 70 was examined at 7 days and found to be 23.96, 24.80, and 26.14 N/mm 2 correspondingly.
1. Introduction
Conventional concrete, or normal strength concrete, is a structural material that consists of a mixture of cement, fine aggregate, coarse aggregate, and water. On some occasions, admixtures are used to improve specific properties or characteristics [1] . In general, concrete is very good in compression stress and durability. Unfortunately, concrete is brittle and can't tolerate tensile stress more than 10% of its compression strength [2] . To solve this weakness, many ideas have been proposed, such as steel reinforcement to improve the overall flexural ability of concrete and fiber reinforcement to improve the ductility of concrete and crack control [3] . Fiber reinforcement has made significant improvements in recent years. There are various types of fiber reinforcements, such as natural fiber, glass fiber, synthetic fiber, and steel fiber, which are the one we are using [4] . Steel fiber is a small, discrete carbon or stainless steel used for structural enhancement purposes. Steel fiber reinforced concrete (SFRC) is a mixture of conventional concrete and steel fiber (cement, fine aggregate, coarse aggregate, water, and small discrete discontinuous steel fibers) [5] . This study is focused on the improvement of using steel fiber in conventional concrete in its compressive and tensile strength. During this experiment, different tests will be carried out, such as a sieve analysis test for identifying aggregate size. The Slump test is used to determine the work-ability of concrete. Finally, test the cubic samples for compression. This study is focused on finding the optimum volume friction of steel fiber that results in the highest percentage of improvement in compression strength.
2. Literature Review
The current rush of migration from rural to urban regions in quest of economic benefits has prompted the construction of infrastructure utilizing concrete at an unparalleled rate since the creation of concrete, owing mostly to its great compressive strength. However, the brittleness of concrete has long been it fundamental disadvantage, resulting in poor tensile strength [6] .
The necessity to ensure a structure's quality forces designers to think about different loading scenarios more and more. When a structure must survive impact loads, it is critical to understand the behavior of concrete that has been subjected to this sort of loading, as well as the properties of a suitable material capable of withstanding short-term dynamic stresses [7] . Some birds use tiny sticks and grasses as fibrous filaments to mix with clay and build their nests. Therefore, the use of fibrous elements to increase the mechanical characteristics of contentious materials was not invented by man, but rather adopted [8] .
2.1. Research Gap Analyze
Many studies have been conducted to investigate the use of steel fiber reinforcement to improve concrete ductility and crack control. [9] researched just and compared percentages of steel fiber of 1 percent, 2 percent, and 3 percent to find the optimal volume fraction with the greatest compression and flexural strength. Also, further studied their studies with percentages of 1%, 2%, 3%, 4%, and 5% with aspect ratios of 50, 60, and 67 to identify the best volume fraction with the maximum compression and flexural strength [10] . These studies have typically focused on the highest volume friction and the finest aspect ratio of reformed steel fiber. They have not conducted tests on the effect of straight steel fiber on concrete compressive strength. Therefore, to address this gap, this study investigates the effect of straight steel fiber with different aspect ratios and volume fractions on concrete compressive strength.
2.2. Bond Behavior of Steel Fiber
As previously stated, the use of fibers to improve the properties of brittle materials is a fairly old practice. Steel fiber was introduced as a new type of fiber in the early s. The earliest type of that fiber was straight fiber. The friction between the concrete and the fiber determines the fiber's bond. As a result, a rectangular piece with a larger aspect ratio performed better form of strength [11] . Steel fibers play an important role in the propagation of micro cracks. There are two scenarios in which fibers may fail, the first is fiber fracture, and the second is fiber pull-out from the concrete. Because it is more ductile and functions as an energy absorbable, the second scenario is preferred to put it another way, hooked- end fiber and crimped-fiber should bend and yield greatly in order for the fiber to be drawn out. As a result, this procedure will consume a significant quantity of energy.
3. Methodology
Cement, in a broad sense, refers to all binding elements, however in a more specific meaning, it refers to the binding materials used in building and civil engineering projects. It is a dry powder that is used to adhere structural elements together when water is added, and plays an important role to the hardness, strength, and durability of concrete, specially takes a role of keeping structures together. Nevertheless, Ordinary Portland Cement is used to complete all of the activities necessary throughout this experiment.
In general, aggregate accounts for 70 - 80 percent of the total volume of concrete, serves as the main body. Aggregate is one of the most fundamental and necessary components of concrete that gives concrete its body, decreases shrinkage, and has an economic impact. The size of the aggregate used in a concrete is influenced by the structure, whether plain cement concrete or reinforced cement concrete is applied. In construction, the most typical aggregate sizes are 20 mm, 40 mm, and 75 mm. RCC structures employ 20 mm of aggregate, 40 mm of aggregate in PCC structures or mass concreting, and 75 mm or larger aggregate in retaining wall construction [1] . The aggregates used in this study are as follows:
1) Fine aggregate or natural sand smaller than 4.75 mm.
2) Coarse aggregate or gravel is 20 mm.
3.1. Experimental Procedures
Concrete and steel fiber (cement, fine aggregate, coarse aggregate, water, and small discrete discontinuous steel fibers). Steel fibers are classified according to their length, diameter, configuration, tensile strength, and other criteria such as aspect ratio (Parashar & Parashar, Utility of wastage material as steel fibre in concrete mix m-20, ) Steel fibers with lengths of 35 mm, diameters of 0.5 mm, and aspect ratios of 70 were used in this experiment.
Experimental procedure manually, cement, fine aggregates, coarse aggregates, steel fibers, and water were mixed together properly. A slump test was carried on the mixed concrete. After determining the work-ability, the mixed concrete was poured into cubes dimension 150 mm × 150 mm × 150 mm and left for 24 hours. After 24 hours, the samples were removed from the mold and placed in a water tank to cure for 7 to 28 days. The cube will be tested for compressive and flexural strength in a universal testing machine after the samples had been cured for the required 7- 28 days.
3.2. Sieve Analysis
A sieve analysis is a technique for determining the particle size distribution of a granular material with macroscopic particle sizes. When analyzing materials, it's crucial to do a sieve analysis because particle size distribution can impact a variety of attributes, including concrete strength, mixture solubility, and so on. In this study a coarse aggregate has merely been sieved. On each sieve, the retained mass was measured. Then, for each sieve, a retained percentage was calculated to determine the percentage passed. This task was managed to complete using material that pass slump test d sieve 25 and had been retained by sieve 20 opening size as shown in Figure 1.
Slump cone test, is to verify a concrete mix's consistency or work-ability. The
Figure 1. Particle size distribution curve.
consistency of concrete is determined by a concrete slump test, which assesses how well it holds together. The result may be seen by viewing the form of a finely compacted concrete cone after the mold has been removed. In this study slump test results shows that the height is 7 cm out of 30 cm which is true slump.
3.3. Compressive Test
Compression Strength concrete reinforcement using steel fibers changes the property of the concrete, allowing the concrete to endure fracture and enhance the mechanical properties of the concrete. The use of steel fibers as reinforcement increased the compressive strength of concrete. The average compressive strength of standard M25 concrete with 0% steel fibers and 7- and 28-day's age of curing was determined to be 22.97 and 25.78 N/mm2 as shown in the charts below. The steel fibers are then added in different percentages, such as 1%, 2%, and 3%, with aspect ratios of 70. The average of 7 days age of curing the compressive strength of concrete with 1%, 2%, and 3% of steel fiber and an aspect ratio of 70 was studied to be 23.96, 24.80, and 26.14 N/mm2 respectively. Whereas the average of 28 days age of curing the compressive strength of concrete with 1%, 2%, and 3% of steel fiber and an aspect ratio of 70 was studied d to be 27.76, 30.59 and 28.50 N/mm2 as shown Figure 2, Figure 3.
Figure 2. Effect of steel fiber on the average compressive strength of concrete at 7 days.
Figure 3. Effect of steel fiber on the average compressive strength of concrete at 28 days.
4. Conclusion
The reinforcement of concrete using steel fibers changes the compressive property of concrete. Based upon the results of the experiment, the following conclusions are drawn: The use of steel fiber as reinforcement has indeed positive impact on concrete's compressive strength. The use of steel fibers as reinforcement improved the compressive strength of concrete. The greatest compressive strength of 30.59 MPa was reached by the 2% steel fibre volume fraction reinforcement at 28 days age, which was 18.66% greater than the control sample at 7 days of age the highest compressive strength of 26.14.
by George Garber
Thin, short strands of steel fiber are being specified more and more as reinforcement in concrete floors. Sometimes, these fibers are used on their own, and sometimes they are used in conjunction with conventional reinforcing steel. They appear in ground-supported slabs and in composite steel deck slabs.
In ground-supported slabs they are used to control cracks, to allow greater joint spacing, and to justify thinner slabs'though the last goal is controversial because it involves properties of fiber-reinforced concrete that experts disagree on. In composite steel deck-slabs, fibers can replace traditional wire mesh to control shrinkage cracks.
Structural engineers are still figuring out how best to design floors with steel fibers. American Concrete Institute (ACI) 360R-10, Guide to Design of Slabs-on-ground, offers guidance on their use in ground-supported floors. Steel Deck Institute (SDI) C-, Standard for Composite Steel Floor Deck'Slabs, gives basic rules for using them in composite steel decks. However, neither document represents the last word on the subject, so the research continues. Meanwhile, specifiers need to think about how to define this material in contract documents.
Whenever people learn a job will include steel fibers, the first question is always some variant of 'how much?' It seems everyone wants to know the fiber dosage, which is usually stated as the mass added to each unit volume of concrete. Typical units are kilograms per cubic meter (kg/m3) or pounds per cubic yard (lb/cy).
Dosage matters, of course, but it is just a start, because not all fibers are alike. If other key details are not specified, the result is concrete that contains the specified mass of fibers, but does not fulfill the designer's intentions.
Steel fibers in specifications
Since steel fibers can be considered a kind of reinforcement, it is tempting to stick them in MasterFormat Division 03 20 00'Concrete Reinforcing, with rebar and wire mesh. However, fibers are better handled in Division 03 30 00'Cast-in-place Reinforcing or Division 03 24 00'Fibrous Reinforcing. If fibers are put in their own section, it should be referred to in Division 03 30 00'Cast-in-place Concrete as this is where the concrete contractor and ready-mix supplier will look. If the specifications include a special section for the concrete floor, then that would be a good place for the steel fibers.
Every steel-fiber specification should incorporate, by reference, ASTM A820, Standard Specification for Steel Fibers for Fiber-reinforced Concrete. This document lays down rules for strength, bendability, dimensional tolerances, and testing that apply to all kinds of steel fibers commonly used in concrete floors. Fibers must have an average tensile strength of at least 345 MPa (50,000 psi). They must be flexible enough to be bent 90 degrees around a 3-mm (1/8-in.) rod without breaking. They cannot vary from specified length or diameter by more than 10 percent. (This does not need to be put in the project specifications, because ASTM A820 does the work for you.)
ASTM C, Standard Specification for Fiber-reinforced Concrete, can also be incorporated into specifications. This standard regulates how fibers are added to the concrete mix.
Citing ASTM A820 and ASTM C is never enough, however, as these standards explicitly leave important decisions to the designer. A complete specification covers all these points:
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Fiber dosage
The amount of fiber is usually specified by mass of fibers per unit volume of concrete'this is measured in kg/m3 or lb/cy. An alternative is to specify the fiber volume as a percentage of the concrete volume. This makes a lot of sense, especially during the design stage. A volume percentage is easier to visualize, and it stays the same across all measurement systems. However, the workers who actually put the fibers in the concrete have no way to batch by volume. They can only batch by mass, so any volume-based specification will have to be converted along the way. Figure 1 shows equivalents for some specified dosages.
Fiber dosages generally range from 12 to 42 kg/m3 (20 to 70 lb/cy). Dosages below that range are occasionally specified when fibers are used to replace light-gauge wire mesh. Dosages above that range are rare.
Setting fiber dosage is not an exact science, but ACI and SDI offer guidelines. According to ACI's Guide to Design of Slabs-on-ground, the fiber dosage in ground-supported slabs should never be less than 20 kg/m3 (33 lb/cy). When the purpose of the fibers is to allow a wider joint spacing, this guide recommends at least 36 kg/m3 (60 lb/cy). SDI's Standard for Composite Steel Floor Deck'Slabs, has a short, simple recommendation for steel fibers in composite steel deck-slabs: use at least 15 kg/m3 (25 lb/cy). In the end, though, the decision is up to the floor designer, who may rely on experience or on recommendations from one of the steel-fiber manufacturers.
Types
ASTM A820 divides steel fibers into five types, based on how they are made:
Only Types I, II, and V are currently being used in concrete floors.
Predictably, fiber manufacturers disagree on which type works best. From the user's point of view, the main issue is certain properties may not be available in all types. For example, in the current market the only fibers being made with hooked ends are Type I.
When discussing fiber type, one must watch out for confusion between ASTM A820 and ASTM C. ASTM A820, which deals only with steel fibers, divides them into the five types listed above. In contrast, ASTM C, which deals with all kinds of fibers, divides fiber-reinforced concretes into four types, depending on what kind of fibers they contain. In ASTM C, concrete with steel fibers is called Type I. Types II, III, and IV contain glass, plastic, and cellulose, respectively.
Thanks to the two different classifications, one can end up with a Type I concrete mix that contains, say, Type II steel fibers. It is important to remember the classification in ASTM A820 covers fibers, while the classification in ASTM C covers concrete mixes.
Fiber length
The steel fibers used in concrete floors range in length from 25 to 65 mm (1 to 2 1/2 in.).
While it is usually agreed length matters, there is no consensus as to which length is best. It depends on what the fibers are expected to do. Engineers who rely on fibers' ability to limit the widening of cracks after they have formed'a property called residual strength, ductility, or flexural toughness'tend to prefer longer fibers. Those who rely on fibers' ability to prevent visible cracks tend to prefer shorter ones, because they result in higher fiber counts and less distance between fibers. Concrete workers also like shorter fibers, which are less likely to tangle and stick up above the floor surface.
There are limits in both directions, however. The upper limit seems to be close to 65 mm, and any longer will clump to form balls. Even fibers in the 50 to 65-mm (2 to 2.5 in.) range can tangle, and to prevent that problem are sometimes sold in collated form'stuck together with a weak glue that dissolves as the concrete is mixed. The lower limit has not been well-established, but fibers less than 25 mm long are seldom used in concrete floors today. Researchers are working with even shorter fibers, though, so floor designs that rely on lengths below 25 mm may be seen eventually.
If a design is based on fibers of a particular length, the specification should require that length. Length is specified as a single target value (not a maximum or minimum), with an implied tolerance, according to ASTM A820, of ±10 percent.
Effective diameter or aspect ratio
For a fiber with a circular cross-section, the effective diameter is that of the circular section. For a fiber with a cross-section of any other shape, the effective diameter is that of a circle equal in area to the actual section.
For fibers of Types I to IV, the effective diameter is specified as a single target number, with an implied tolerance of ±10 percent. Type II fibers, which are rectangular in section, can be specified by width and thickness instead of effective diameter. Type V fibers are supposed to be specified differently. Since the manufacturing process for Type V results in substantial variation in effective diameter, ASTM A820 suggests specifying a range with upper and lower limits, not a target. However, this rule is not universally followed. Some makers quote a single effective diameter for their Type V fibers.
People sometimes talk about a fiber's aspect ratio instead of, or in addition to, its effective diameter. Aspect ratio is length divided by effective diameter. Since any two of those properties determine the third; all three do not need to be specified. When aspect ratio is specified, be aware ASTM A820 allows the measured value to vary by ±15 percent from the specified target.
In today's marketplace, effective diameters range from 0.58 to 1.14 mm (20 to 40 mils). As with length, choosing the diameter involves trade-offs. Thicker fibers are less likely to tangle while thinner result in higher fiber counts.
Fiber count
Fiber count'the number of fibers per pound or kilogram'is an important factor in the effectiveness of steel fibers as concrete reinforcement. Higher counts result in less distance between fibers, and that generally means better performance. A floor design based on a particular fiber count may not work as well with a lower count, even if the mass of fibers stays the same.
Though fiber count is never directly specified, it is determined by two properties that are specified: length and effective diameter (or length and aspect ratio, depending on preference). Since shorter fibers are usually thinner, too, reducing the length dramatically raises the fiber count. In this figure, both piles have the same mass. The fibers on the right are 50 mm (2 in.) long and have an effective diameter of 1.14 mm (0.04 in.). The fibers on the left are 25 mm (1 in.) long and have an effective diameter of 0.58 mm (0.02 in.). The shorter fibers outnumber the longer fibers, almost eight to one.
Fiber count can be determined from the following equations:
In metric units:
c = 1/[(7.9 x 10-6)L?(d/2)2]
Where c = fiber count per kilogram
L = length of fiber in millimeters
d = effective diameter of fiber in millimeters
In U.S. customary units:
c = 1/[(0.29L?(d/2)2]
Where c = fiber count per pound
L = length of fiber in inches
d = effective diameter of fiber in inches
Fiber counts range from about to 20,000 per kilogram ( to per pound).
Deformations
The earliest steel fibers were smooth, straight pins, and ASTM A820 still recognizes that shape as an option. In practice, though, the fibers used today are all deformed so concrete can grip them better. Deformations take one of three forms: continuous, hooked ends, and flat ends.
A continuously deformed fiber has waves or bumps running down its whole length, much like ordinary steel rebar. A hooked-end fiber has a bend'or multiple bends'at each end. A flat-end fiber has its ends squashed flat, somewhat like a kayaker's double-ended paddle.
Conclusion
While dosage, length, effective diameter, and deformation are the essential features every steel-fiber specification should cover, a few other details are worth considering.
Consider requiring fibers be delivered in containers marked to show the mass. Some specifiers go further and demand containers that include the exact quantity going in each cubic meter or cubic yard of concrete. If the specified dosage is 20 kg/m3 (33 lb/cy), each box or bag would have to contain exactly 20 kg (33 lb). This simplifies batching and reduces the risk of error. Some suppliers may have trouble packaging fibers in anything other than standard quantities.
Fibers should be stored under cover, protected from rain and snow. Left outdoors, boxes can disintegrate and fibers can rust.
Last, it is a good idea to insist all concrete tests, including those needed for approval of the mix design, be made after the addition of fibers. That may seem like common sense, but it will not always happen without a reminder.
Of course, it takes more than the right specification to make a successful steel-fiber-reinforced floor'it also takes a smart designer and a careful contractor. Nevertheless, a full, accurate specification is an essential part of the job when the floor is expected to fulfill the designer's intentions.
George Garber is the author of Design and Construction of Concrete Floors, Concrete Flatwork, and Paving with Pervious Concrete. Based in Lexington, Kentucky, he consults on the design, construction, and repair of concrete floors. Garber can be reached by at .