Compressive behaviour of steel fibre reinforced concrete

Compressive behaviour of steel fibre reinforced concrete

- —- A study by Rui D Neves, and Joao C. O. Fernandes : Lisbon, Portugal

An experimental study to investigate the influence of matrix strength, fibre content and diameter on the compressive behaviour of steel fibre reinforced concrete is presented. Two types of matrix and fibres were tested. Concrete compressive strengths of 35 and 60 MPa, 0.38 and 0.55 mm fibre diameter, and 30 mm fibre length, were considered. The volume of fibre in the concrete was varied up to 1.5%. Test results indicated that the addition of fibres to concrete enhances its toughness and strain at peak stress, but can slightly reduce the Young’s modulus.

Simple expressions are proposed to estimate the Young’s modulus and the strain at peak stress, from the compressive strength results, knowing fibre volume, length and diameter. An analytical model to predict the stress–strain relationship for steel fibre concrete in compression is also proposed. The model results are compared with experimental stress–strain curves.

by means of confinement with transverse reinforcement. For structures where ductility is very important, such as seismic-resistant re­inforced concrete structures, the design and detailing of confinement reinforcement is often difficult, requiring more labour and qual­ity control and affecting construction costs. The recognised ability of fibres to improve duc­tility of concrete1 –5may be used to overcome that difficulty. Other potential uses of steel fibre reinforced concrete are the compressive layers of block-and-beam and pre-cast perma­nent formwork floors, or discontinuity regions, where loading paths are complex, such as cor­bels, deep beams and post-tensioning ancho­rage zones.

In this context, it is important for designers to know the compressive behaviour of steel fibre reinforced concrete. The aim of the present work was to develop analytical ex­pressions to estimate the main parameters that characterise the behaviour of steel fibre reinforced concrete in compression.

Experimental programme

To evaluate the influence of steel fibres on the compressive behaviour of concrete, two different mixes and two different fibres were used. Concrete mix proportions are indicated in Table 1. The aggregates were siliceous nat­ural sand and crushed limestone (gradings shown on Figure 1). The steel fibres were hook-ended of length lf = 30 mm, diameter df = 0.55 mm and df = 0.38 mm (Table 2), and they were added to the two mixes in volume contents up to Vf = 1.5%.

The concrete was mixed using a laboratory pan-mixer, with mixing times ranging from 3 to 6 min, and compacted on a vibrating table. The compacting time varied between 40 and 60 s, depending on the concrete workability.

Demoulding occurred 24 h after mixing and then the specimens were kept in a wet cham­ber until required for testing at 42 days.

As matrix and fibre type and content varied, different composites were tested. Their identi­fication is shown in Table 3, which shows the mix designation (A, B), followed by the fibre content (in kg/m3) and then the fibre type (Z, R). Each composite was represented by a set of six cylinders, 150 mm in diameter and 300 mm in height.

Tests were performed in a closed-loop, servo-controlled compression testing machine with a load capacity of 5000 kN and an

Introduction

The development of concrete technology has made it possible to reach, for ordinary produc­tion processes, compressive strengths as high as 100 MPa. However, such an increase in compressive strength is in general associated with brittler behaviour of the concrete. In struc­tural applications brittleness can be prevented

approximate stiffness of 2300 kN/mm.6 Before testing, the two ends of each specimen were made parallel by grinding. The tests were per­formed under displacement control with a plate displacement rate of 0.01 mm/s, which corresponds to the lowest limit of the interval identified in Ref. 7. To measure the deforma­tions of the specimens and the force, a clamp type extensometer (HBM DD1) and a load cell HBM P3MB were used. The methods used to determine Q–e curves from load–displacement data and to calculate the average curve

representing each composite are described in Neves.8

Test results and analysis

The main parameters that characterise the compressive behaviour of concrete are the slope of the ascending branch (Young’s modulus), the compressive strength, the strain at peak stress and the area under the Q–e curve (toughness). These parameters were determined from the respective average curve

for each composite and are presented in Table 4.

Young’s modulus

The test results (Figure 2) illustrate the well-known relation between compressive strength and Young’s modulus, and also show that the presence of fibres causes a slight decrease in Young’s modulus. Similar behaviour has been reported by other authors9,10 and can be explained because fibres parallel to the load direction can act like voids and also due to the eventual additional voids caused by fibre addition.

Compressive strength

The reinforcement provided by fibres can work at both a micro and macro level. At a micro level fibres arrest the development of micro-cracks, leading to higher compressive strengths, whereas at a macro level fibres con­trol crack opening, increasing the energy absorption capacity of the composite. Although the primary purpose of fibre re­inforcement is to improve energy absorption capacity after macrocracking of the matrix has occurred, this reinforcement often works also at a micro level. The ability of the fibre to control microcracking growth depends mainly on the number of fibres, deformability and bond to the matrix.11 A higher number of fibres in the matrix leads to a higher probability of a micro-crack being intercepted by a fibre. If the fibre is

stiff enough and it is well bonded to the matrix, it can prevent the microcrack developing.

On the other hand, fibre addition causes some perturbation of the matrix, which can result in higher voidage.12 Voids can be seen as defects where microcracking starts. In addition to fibre quantity, perturbation also depends on the ability of the matrix to accom­modate fibres, which is an important property of the mortar fraction of the concrete.

Therefore the influence of fibres on the compressive strength may be seen as the bal­ance between microcrack bridging and addi­tional voids caused by fibre addition. In the present study different influences were observed (Figure 3): the R fibres increased the strength of the composites by up to 9% whereas the Z fibres reduced it by up to 20%.

As the compressive strength of the steel fibre reinforced concrete depends not only on the fibre type and volume, but mainly on the mix characteristics and as compression testing is quite simple, it is recommended to evaluate the compressive strength by testing, rather than by analytical expressions.

Strain at peak stress

The strains at peak stress obtained in this work
(Figure 4) indicate an increase of e0 with compressive strength, as already observed by other

Knowing Young’s modulus, compressive strength and strain at peak stress, one can express q as a function of p, using equation (7), thus reducing to 1 the number of parameters to be determined. Assuming differ­ent values for p, and calculating the sum of square errors (SSE) between points of the experimental curve and the points of the analytical one, led to a value of p that minimises the error between both curves. Figures 7 to 10 show the suitability of the proposed expression to model the com­pressive behaviour of plain and steel fibre reinforced concrete with strengths and fibre volumes up to 60 MPa and 1.5%, respectively.

The sets of p values that minimise the error between experimental and theoretical curves of each composite are presented in Figure 11.

The ability of the proposed set of expres­sions to predict the stress–strain curve of steel fibre reinforced concrete, even for concrete with compressive strength up to 80 MPa or fibre volumes up to 2.0% is illustrated in Figure 13. The results presented by Otter and Naaman,24 and Hsu and Hsu,25 are compared with the theoretical behaviour given by the proposed model.

It should be pointed out that the results presented in Refs 24 and 25 were also obtained for concrete reinforced with hook-end steel fibres. However, when comparing the proposed model application with the results from other researchers using different experimental conditions, namely straight fibres or smaller specimens, such as cylinders with 100 mm diameter and 200 mm height, the agreement was not so good. So, in situations in which the fibres are of different shape, dif­ferent specimens or even different coarse aggregate is used, the proposed model needs further calibration.

Hybrid Fiber Reinforced Concrete (Blended Fibers)

Performance of conventional Concrete is enhanced by the addition of Fibers in
concrete. The brittleness in concrete is reduced and the adequate ductility of concrete is ensured by the addition of Fibers in concrete. The main reasons for adding steel Fibers to concrete matrix is to improve the post-cracking response of the concrete i,e., to improve its energy absorption capacity and apparent ductility, and to provide crack resistance and crack control.
Fiber Blends – Concrete can be reinforced blends of steel and/or synthetic and/or cellulose Fibers. The reason for using Fiber blends is to enhance the properties of concrete by combing the benefits that each particular Fiber type can impart.
There is no Fiber type that can encompass all the desired properties of fresh and hardened concrete in terms of, for example, providing load bearing capacity at cracked sections, crack control, spalling resistance at elevated temperatures, improved abrasion, impact and frost resistance. However, appropriate blends of Fibers, with or without, traditional reinforcing bars can lead to synergetic effects, i.e. combinations of different Fiber types can enhance concrete in both its fresh and hardened states.
PROPERTIES OF FIBER BLENDS REINFORCED CONCRETE
Steel/Steel Fiber Blends
Small steel wire Fibers are effective in micro-crack bridging, leading to an increased fractural energy and higher flexural strength. Their use, when blended with larger steel wire Fibers, can dramatically increase the peak load and post-cracking performance of concrete. In other words, by combining steel Fibers that are effective in both micro-cracking and in macro-crack bridging, synergetic effects will increase the fractural energy absorption capacity and toughness of the concrete.
Steel/Micro Synthetic Fiber Blends
Steel Fibers do not contribute significantly to the performance of plastic concrete, because their strength and stiffness differs too much from the properties of concrete at an early age. Micro polypropylene Fibers are better suited to take up stresses in plastic concrete due to their lower elastic modulus. Furthermore, their ability to interfere with the capillary forces by which water bleeds to the surface of concrete reduces the risk of plastic settlement due to water evaporation. Consequently, a blend of large steel Fibers and micro polypropylene Fibers can combine structural reinforcement with plastic crack control. The micro synthetic Fibers in the concrete also increase its resistance to spalling in fire situations.
Synthetic/Synthetic Blends
As previously mentioned, micro synthetic Fibers have been used for many years to effectively control plastic shrinkage cracking as well as plastic settlement cracking in concrete floors and slabs. However, once the concrete has set and begun to gain strength, there are no benefits with respect to crack control. Macro synthetic Fibers are dimensionally much bigger than micro synthetic Fibers and therefore they provide very few benefits to the plastic concrete (although there are some commercially available macro synthetic Fibers that are claimed to perform a similar role to that of micro synthetic Fibers).
The main role of synthetic/synthetic blends is to control plastic cracking (in fresh concrete) and drying shrinkage cracking (in hardened concrete), and to improve post-cracking toughness, subject to the previously mentioned provisos on the long-term properties of macro synthetic Fibers. Micro synthetic Fibers also increase resistance to spalling in fire situations.

Steel/Synthetic Blends
A more recent development in Fiber reinforced cement-based composites concerns the use of hybrid blend of Steel and Synthetic PP Fibers. The concrete toughness can be optimized by using steel Fibers that will affect the cracking process during different stages of loading and micro-Fibers improve composite strength by bridging micro-cracks and, therefore, delaying the coalescence of these cracks into macro-cracks. On the other hand, macro-Fibers are more effective in bridging larger crack openings. Therefore, blend can be considered a multifunctional material that is able to achieve a set of desired performances by the use of different Fiber types; as a second example, toughness and ductility can be provided by high-modulus Fibers, shrinkage cracking control by low-modulus Fibers, conductivity by carbon Fibers

Workability of FRC

Workability
SFRC appears relatively stiff and unworkable compared to conventional
concrete. However, when vibrated, the thixotropic mix flows quite well. Additional water should not be added as it may only improve slump, not real workability, and it will certainly have a negative affect on concrete performance. Any problems with workability should be overcome by the use of plasticisers.

The consistency of steel fibre reinforced concrete can be measured using the inverted slump cone method (ASTM C995). The time taken for a vibrator to fall under its own weight through an inverted slump cone full of uncompacted concrete is measured. Values between 10 and 30 secs are recommended. When the standard slump cone test (ASTM C143) is used, values of 25mm-100mm are recommended, ref ACI Committee 544 (1993).

Installing Slabs on Grade

An area of the floor system that is crucially important is the sub-grade on sub-base. The most important item is proper compaction; many floors settle and have structural cracks. Of course organic material cannot be properly compacted and must never be in the sub-grade. It is a simple fact that the floor system rests on the grade and if the sub-grade settles the floor settles.Forming of concrete floors is reasonably straight forward. One must remember, though,that loose or warped edge forms cause uneven floors. Therefore, the care taken with the edge form setting will be proportional to final flatness of the floor.Placing concrete in hot weather, particularly when the walls and roof are not yet completed,creates some additional quality concerns. Plastic cracking is one of the worse problems that occur. Plastic shrinkage cracks form before the concrete hardens and are caused by hot, dry,and/or windy conditions. The cracks resemble the shrinkage cracks seen in clay soils during very dry weather.Curing can also create lots of problems for concrete floors. Since water evaporates so quickly from the large exposed surface, without proper curing methods a floor is likely to rack, craze and dust. The three most common means of curing are:

1.Wet cure by covering, after finishing, with continuously watered burlap.

2. Wet cure by watering finished slab and covering with plastic or paper.

3. Seal cure with liquid membrane curling compound

India Warehousing Show 2012 – PUNE

Fibers and Fiber reinforced Concrete

Continuing our commitment to educate existing and new customers about our product and its capabilities, we at Kasturi Metal Composites Pvt Ltd takes immense pleasure to invite all our industry experts and esteemed clients customers to visit us at upcoming trade show in Pune. You can visit us at the following events

( STAND No: A50A).

 

India Warehousing & Logistics Show Pune 2012

www.indiawlshow.com/

Finishing Of Duraflex™

Placing Of Duraflex™ Steel Fiber Reinforced Concrete:

Generally, placing of SFRC with no vibration is discouraged because, without compaction, the concrete will be less dense, may have air voids, and may have less bond with any conventional reinforcement.. A false loss of slump will occur when fibers are added and the mix will appear less workable than a non-fiber mix. However, the fiber mix can still be placed with adequate vibration. Additional water will not improve the workability of fiber mixes and should not be used. Excessive addition of water can produce fiber balling. Any problems with workability should be overcome by the use of plasticisers. Batch plant operators and transit truckdrivers must be instructed not to add additional water to the mixture based on its appearance and their experience with conventional concrete

In a very thin wall or beam form, e.g., 4 in. (100 mm) or less, which also contains bars or mesh, placement of the concrete may be difficult, especially with longer fibers. This is similar to the difficulties encountered in placing conventional concrete mixtures with larger aggregate in thin, congested sections. When SFRC mixtures are used in congested areas, a 3/8 in. (9mm) maximum aggregate size should be specified to reduce placing difficulties.

Finishing Of Duraflex™ Steel Fiber Reinforced Concrete:

Steel fiber reinforced concrete can be finished with conventional equipment, but minor refinements in techniques and workmanship are needed.

However Following points need to looked carefully while finishing the FRC

  • Screeding should be carried out using a metal screed bar as timber may tend to catch and drag the fibres.
  • Bull floating will push down the fibres as  well as the aggregates and will lead to a smooth fibre free surface that can be given a steel trowel finish using either a mechanical steel trowel or hand trowel ling techniques.
  • A light broom finish is also possible to achieve a textured finish although going too deep into the surface can catch and pull out fibres.

For flat formed surfaces, normally no special attention is needed. The surface will normally be smooth and will not show fibers when the forms are stripped. If chamfers or rounds have been provided at the edges and in comers, the ends of fibers will not protrude at these points when forms are stripped.

To provide added compaction and bury surface fibers, open slab surfaces should first be struckoff with a vibrating screed. The screed should have slightly rounded edges and preferably should be metal. In areas where a screed is not practical, a jitterbug* or rollerbug can be used for  compaction and to establish rough grade control. Care should be exercised when using a jitterbug or rollerbug not to overworkthe surface, bringing excessive mortars to the surface. Magnesium floats can be used to establish a surface and close up any tears or open areas which are caused by the screed. Wood floats tend to tear the surface and should not be used. Throughout all finishing operations, care must be taken not to overworkthe surface. Overworking will bring excessive fines to the surface and may result in crazing, which normally shows up after the curing period.

If excess bleeding occurs or excessive fines are at the surface, such materials should be screeded off and discarded. After completion of any float work, the surface should be left until it can be worked further without damage. This is usually at about the time of initial set. Where a careful finish is not required for appearance or exact tolerance, no further workis needed after floating.

If a texture is required, a broom or roller can be used prior to initial set. Burlap drags should not be used because they will lift up the fibers and tear up the surface. When additional finishing is needed, the next step should be done with magnesium floats.

Power equipment or hand equipment may be used. When done by hand, the float should be held flat and not on edge. It should be moved with a sawing motion (short, quick, back-and-forth movements) as it is drawn across the surface. The magnesium float can be used to obtain a nearly perfect, flat surface, bury or cover all the fibers, and leave a slight texture. This can be followed by hard steel troweling if a smooth surface is desired. During the first troweling phase of the finishing process, blades should bekept at a low angle and the rotation speed should be slow enough to avoid throwing bits of cement and sand across a slab.

Ride-on trowels increase the quality by providing much flatter floors than with walk-behind machines. This was achieved with the introduction of the nonoverlapping riding trowel with pans. These machines do about 80% of the critical workon a floor, correcting problem areas and making them flatter.

Many finishers panic and run the first blade machine as fast as it can go with the blades tilted at a high angle. There are a wide variety of finishing techniques at this phase—too fast, too slow, blades tilted too high, blades tilted too low, rotor speed too slow, rotor speed too fast, travel speed too slow, and travel speed too fast. The most common mistake, however, is running too fast. The trickto getting the pin holes filled in by running your troweling machine at a speed that doesn’t throw surface material across the floor. To do this your blade pitch must bekept relatively flat with only a slight pitch. The machine pass should be run 90 degrees from the previous pass. Finishers are taught to allow some setting time between passes, but an immediate 90-degree pass in the opposite direction will close most, if not all, of these pin holes. So you should lay it down in one direction and immediately make a pass at 90 degrees. Once that is completed, you can return to normal finishing operations and procedures.

The next transition area of concern happens in the final finishing sequence when finishers use the “burn machine” or final ride-on trowel machine to make the final passes. The biggest problem normally seen here is having only one “lay down machine” or one with dirty blades (from running into too wet concrete) or when the second blade machine throws loose concrete off the blades, causing the burn machine (final ride-on trowel) to run over them on the floor. When this happens, darkspots appear all over the floor, making it looklike a spotted Dalmatian—the overall appearance being less than desired. Using these techniques, some excellent finishes of SFRC have been obtained.

 

Benefits of Duraflex SFRC Floors

To the Owner

-          Less Cost , High Quality, Longer Life of Floors

-          Resistance to micro cracks propogating into macro cracks

-          Provides high impact resistance

-          Excellent surface finish can be achieved

-          Eliminates spalling due to corroding reinforcement

-          Reinforces the edge helping to prevent joint failure

 

To the Consultant

-          Cusomized Design Support

-          Suitable for wide range of applications

-          Ease in designing for complete life span of Concrete Floors

-          Ensures Optimum use of material and Technology.

-          Reduce steel reinforcement requirements

 

To the Contractor

-          Faster construction

-          Reduced labor cost

-          Easy to handle, mix, place and finish variety of Floors surfaces.

-          Environmentally friendly

-          Compatible with all surface finish and coating techniques.

 

DURAFLEXTM Steel Fibers and DUROCRETETM PP Fibers are high strength fibers, with favorable shape, orientation, and strength.  The steel fibers are known to have possessed high tensile strength and ductility. The most significant factor affecting resistance to crack propagation and strength of the fibrous concrete and mortar are

 

  • Shape and bond at fiber matrix interface
  • Volume fraction of fibers
  • Fiber aspect ratio and Orientation of fibers
  • workability and Compaction of Concrete
  • Size of Coarse Aggregate
  • Mixing

Construction Phases

CONSTRUCTION PHASE: 

The utilization of SFRC for a concrete floor is an important step in the construction phase process. When people use SFRC for a concrete slab on ground, there are some important points that they should pay attention and follow, as much as possible. ACI 544.3R ). Choose a specific standard for the use and application: ACI Committee 544 and/or other recommendations from steel fibers manufacturers or specialists, especially those ones regarding steel fibers control and mixing.

  • Concrete mix : The concrete mix design should be checked before the construction starts. The compressive strength, the minimum amount of cement and the aggregate gradation, especially, should be checked. People can refer to the ACI 544.1R-8 for aggregate gradation recommendations.
  • Fibers adding, mixing and control : It is recommended to add steel fibers at the batch plant with the other concrete components. You can add steel fibers on the conveyor with the aggregates, put them over the aggregates in the weight balance or put them in the truck hopper when concrete is loaded in it. The slump of the concrete before fiber addition should be 2 to 3 in. (51 to 76 mm) greater than the final slump desired. Steel fibers should be incorporated with the other components at a rate of 20 to 50 kg per minute. Once all steel fibers are added to the mix, the truck should agitate the drum at high revolution for 4 to 5 minutes or until 75 drum revolutions (dry batch plant especially). A good control of the amount of steel fibers required per cubic meter is essential. The rate of steel fibers can be checked at the batch plant or at the job site using a sieve or a specific automatic device. Also, fresh concrete testing should be done at the batch plant and at the job site (ex: slump, air content, temperature).

Most fiber balling occurs somewhere before the fibers get into the mixture. Once the fibers get into a mixture ballfree, they nearly always stay ball-free. This means that if balls form, it is because fibers were added in such a way that they fell on each other and stacked up (in the mixer, on the belt, on the vanes, etc. The most common causes of wet fiber balls are overmixing and using a mixture with too much coarse aggregate (more than 55 percent of the total combined aggregate by absolute volume  (ACI 544.3R-93) 

Placing

Usually SFRC with a proper water-cement ratio  appears relatively stiff and unworkable, compared to conventional concrete. However, use of vibrators or high-range water-reducing admixtures (HRWR) allows easy placing of such seemingly unworkable concrete. placing of SFRC with no vibration is discouraged because, without compaction, the concrete will be less dense, may have air voids, and may have less bond with any conventional reinforcement. Batch plant operators and transit truck drivers must be instructed not to add additional water to the mixture based on its appearance and their experience with conventional  concrete. Water-cement ratios for fibrous mixtures must be carefully controlled. When SFRC mixtures are used in congested areas, a 3/8 in. (9mm) maximum aggregate size should be specified to reduce placing difficulties

Finishing

Steel fiber reinforced concrete can be finished with  conventional equipment, but minor refinements in techniques and workmanship are needed. To provide added compaction and bury surface

fibers, open slab surfaces should first be struckoff with a vibrating screed. The screed should have slightly rounded edges and preferably should be metal. In areas where a screed is not practical, a jitterbug* or rollerbug can be used for compaction and to establish rough grade control. Care should be exercised when using a jitterbug or rollerbug not to overworkthe surface, bringing excessive mortars to the surface. Magnesium floats can be used to establish a surface and close up any tears or open areas which are caused by the screed. Wood floats tend

to tear the surface and should not be used.Throughout all finishing operations, care must be taken not to overworkthe surface. Overworking will bring excessive fines to the surface and may result in crazing, which normally shows up after the curing period. If excess bleeding occurs or excessive fines are at the surface, such materials should be screeded off and discarded. After completion of any float work, the surface should be left until it can be worked further without damage. This is usually at about the time of initial set. Where a careful finish is not required for appearance or exact tolerance, no further workis needed after floating. If a texture is required, a broom or roller can be used prior to initial set. Burlap drags should not be used because they will lift up the fibers and tear up the surface. When additional finishing is needed, the next step should be done with magnesium floats. Power equipment or hand

equipment may be used. When done by hand, the float should be held flat and not on edge. It should be moved with a sawing motion (short, quick, back-and-forth movements) as it is drawn across the surface. The magnesium float can be used to obtain a nearly perfect, flat surface, bury or cover all the fibers, and leave a slight texture. This can be followed by hard steel troweling if a smooth surface is desired. The trowel must bekept flat or the edge will cause fibers to spring out of the surface. Using these techniques, some excellent finishes of SFRC have been obtained.

Use of superplasticizer with SFRC :It is recommended to use a superplasticizer with SFRC in order to prevent to add to much water to get the necessary flowability of the SFRC. Usually, the superplasticizer will be added at the job site after slump testing. The slump before adding superplasticizer should be around 75 to 125 mm (3” to 5”). After the addition, it should be between 125 mm to 175 mm (not more). For usual steel fibers dosage (15 to 40 kg / cubic meter), a dosage of 0,5 L/m3 to 2,5 L/m3 of a normal  superplasticizer is suggested (tests can be done

  • to adjust the dosage). Usually, for 25 kg/m3 of steel fibers, 1 liter of superplasticizer is enough. In some cases, it could be appropriate to add a part of the superplasticizer at the batch plant. 
  • Other Construction Steps

In order to obtain a good overall performance of a SFRC floor, people should pay attention to the following floor construction steps

  • Pre-construction meetings: To communicate the specific information, related to the use of SFRC during the construction, to all the persons involved in the slab construction.
  • Quality control : As any concrete floor construction project, the control should be assumed and done by an independent laboratory. Also, a good quality control of the ready-mix concrete supplier is essential at the batch plant and at the job site.
  • Surface finishing: The concrete can be finished like a normal concrete but the use of a vibrating screed (manual or mechanical) or of a laser screed is suggested strongly in order to obtain a good finished surface without having any problems related to steel fibers at the surface.

Standards and Recommendations: 

Here are some standards and recommendations that can be useful when facing a SFRC floor project:

  • ACI Committee 302 – ACI 302.1R-96 Guide for concrete floor and slab construction
  • The Concrete Society – Concrete Industrial Ground Floors; A guide to their Design and Construction – TR34 Report, General document and Appendix F (Slab design with Steel fibers)
  • ACI Committee 544 – ACI 544.3R-93 Guide for specifying, proportioning, mixing, placing and finishing steel fiber reinforced concrete
  • ASTM C 1116-91 – Standard specification for fiber reinforced concrete and shotcrete
  • ASTM C 1018-94b – Standard test method for flexural toughness and first crack strength of fiber reinforced concrete (using beam with third-point loading)
  • ASTM A 820-M04 – Standard specification for steel fiber reinforced concrete
  • JSCE – SF4 , Japanese Steel fiber standard

SFRC mix design consideration

The design of SFRC slabs on grade involves four considerations:

(1) flexural stress and strength;

(2) elastic deflections;

(3) foundation stresses and strength; and

(4) curl.

The slab must be thickenough to accommodate the flexural stresses imposed by traffic and other loading. Since traffic-induced stresses are repetitive, a reasonable working stress must be established to insure performance under repeated loading.

In comparison with conventional concrete slabs, a fibrous concrete slab is relatively flexible due to its reduced thickness. The magnitude of anticipated elastic deflections must be assessed, because excessive elastic deflections increase the danger of pumping in the subgrade beneath the slab. 

For SFRC ACI Committee 544 (1993) suggests the following:

• Increase paste content to obtain improved workability, possibly by employing pozzolans such as fly ash, slag or silica fume in addition to, or as a replacement for cement.

• Limit coarse aggregate to 55% of total aggregate

• Keep coarse aggregate size to 19mm maximum

• Keep w/c low. A value as low as 0.35 is quite possible. It should not go above 0.55

Design consideration of industrial flooring

Failure or Causes of deterioration of concrete Floorings

  • Mechanical: Abrasion/ Erosion/ Impact/ Vibration
  • Physical: Temperature/Humidity/ Water/Frost
  • Chemical and Biological: Acids/ Oil/  Grease/ Gas/ Micro-organisms
  • Ground conditions: strength, water table, type of soil, sub grade
  • Type of slab: ground supported or pile supported or suspended slab
  • Traffic and other loading requirements: frequency, duty and free or define traffic
  • Method of construction: in stages/bays, in strips or in large pour
  • Concrete mix design (especially critical w/c ratio):
  • The concrete deliveries must be of consistent quality. Otherwise negative impact on wetting/dry-shake workability/final finish performance (abrasion) and appearance.
  • A concrete slump in the range 75 to 110mm will normally give best results. This will depend on the placing method (manual/mechanical)
  • Do not use concrete where cement has partly been replaced with fly ash. This makes the mix is too sticky for proper dry-shake placing and workability, and will cause blisters during power-floating. Blisters are also caused by too early floatng or with inadequate tools (steel instead of wood or magnesium)
  • Slab thickness and reinforcement requirements: steel fibers or re-bars, other fiber types and combination
  • Jointless slabs or join spacing and positioning: Pinwheel contraction joint to separate columns. Design of joints according to traffic requirements. Less joints mean less cost (need for slab connectors) and less chance of damage and wear.
  • Surface smoothness and flatness: TR34, ACI 117, DIN 15185, ASTM E 1155
  • Durability and special operational conditions

Design Considerations and factors for construction of industrial flooring