Trends in utilization of FRC

2nd EA-CCE Summit, UNECA, Addis Ababa April 19-20, 2017

Organized by i-Capital

By Yoseph Birru (PhD)

Contents • What is Fiber Reinforced Concrete (FRC). • FRC - Historical Perspective • Weakness of conventional concrete • Fibers potentially used in concrete • Factors affecting FRC performance • Advantages of fibers over reinforcing

steel bars • Application of FRC in Structures • Future trends of FRC applications • Features of FRC • Some considerable facts about FRC

What is Fiber Reinforced Concrete (FRC)?

• FRC is a composite material consisting of cement paste, mortar or concrete with closely spaced and uniformly dispersed fibers of steel, polypropylene, nylon, asbestos, coir, glass, carbon … (circular or flat).

Steel fiber

Natural fibers

coir hay

jute bamboo

FRC - Historical Perspective

• BC Horse hair • Egyptians used straw to reinforce mud

bricks • 1900 asbestos fiber was used to reinforce

clay posts • 1920 Griffith, theoretical vs. apparent

strength • 1950 Composite materials • 1960 FRC

FRC - Historical Perspective

• 1970 New initiative for asbestos cement replacement

• 1970 SFRC, GFRC, PPFRC, Shotcrete • 1990 micromechanics, hybrid systems,

wood based fiber systems manufacturing techniques, secondary reinforcement, HSC ductility issues, shrinkage crack control.

• 2000+ Structural applications, Code integration, New products.

Background

• Large practical application of FRC in construction is mostly hampered by insufficient knowledge and subsequently absence of relevant standards, based on performance concepts.

Background

• It should also be admitted that the cost of fiber reinforcement and related technological operations is certainly an obstacle for use of FRC in ordinary structures.

• On the other hand, in successful applications in demanding structures very special requirements are satisfied; probably future developments will go in this direction.

Objectives of the presentation

• In this presentation the main fields of application of FRC composites are examined and future perspectives discussed with the objectives of creating awareness to Ethiopian CI.

Materials evolution

• The menu of materials has expanded so rapidly that be

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advanced

• The menu of materials has expanded so rapidly that designers who left college some 20 years ago can be forgiven for not knowing that half of them exist.

• But not-to-know is, for the designer, to risk disaster. Innovative design, often, means the imaginative exploitation of the properties offered by new or improved materials.

• And for the man in the street, the schoolboy even, not-to-know is to miss one of the great developments of our age: the age of advanced materials.

Weakness of conventional concrete

• The low tensile strength of concrete is being compensated for in several ways, and this has been achieved by the use of reinforcing bars and also by applying prestressing methods.

• Though these methods provide tensile strength to concrete, they do not increase the inherent tensile strength of concrete itself and moreover installation of reinforcements is time consuming business.

Weakness …

• Existence of one phase (i.e., steel or concrete) does not improve the basic strength characteristics of the other phase and consequently the overall performance of the traditional reinforced concrete composite is dictated by the individual performance of the concrete and steel phase separately.

Fibers potentially used in concrete

• Since the early use of asbestos fibers, a wide variety of other fibers have been used with hydraulic cements: conventional fibers such as steel and glass; new fibers such as carbon and low modulus fibers, either man-made (polypropylene, nylon) or natural (cellulose, sisal, jute …).

• These types of fibers vary considerably both in properties, effectiveness and cost.

steel

Polypropelene

Glass

others

Source: Global research Analysis, Volume : 2 | Issue : 2 | Feb 2013

Fiber used in concrete reinforcement in world

Factors affecting FRC performance

• Type of the fibers & Geometry.

• Volume percent of the fiber.

• Aspect ratio (the length is divided by the diameter of the fiber).

• Orientation of the fibers in matrix.

Fiber size

• Round steel fibers have diameters in the range 0.25 to 0.75 mm.

• Flat steel fibers have cross sections ranging from 0.15 to 0.4 mm thickness by 0.25 to 0.9 mm width.

• Crimped and deformed steel fibers are available both in full length or crimped at the ends only.

Fiber size

• It is important to emphasize the role of fiber size on the mechanical behavior of the composite.

• To bridge the large number of microcracks in the composite under load and to avoid large strain localization it is necessary to have a large number of short fibers.

• The uniform distribution of short fibers can increase the strength and ductility of the composite.

Fiber size

• The presence of long fibers significantly reduces the workability of the mix and its volume fraction should be determined with care.

• In some applications, such as slurry-infiltrated-fibered concrete (SIFCON), workability is not a concern therefore a high percentage of long fibers can be used.

• Large numbers of short fibers can bridge the microcracks leading to increase in strength and ductility.

• Small number of long fibers can bridge macrocracks causing a significant increase in the ductility of the system.

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Classifications

• It is convenient to classify the fiber reinforced composites as a function of their fiber volume fraction:

1. Low volume fraction (<1%)

2. Moderate (between 1 and 2 %)

3. High volume fraction (>2%)

Advantages of fibers over reinforcing steel bars

• Disperse fibers offer various advantages or steel bars and wiremesh to reduce shrinkage cracks:

1. The fibers are uniformly distributed in 3D making an efficient load distribution;

2. The fibers are less sensitive to corrosion than the reinforcing steel bars;

3. The fibers can reduce the labor cost of placing the bars and wiremesh.

Advantages …

• Fibers may not be efficient in withstanding the tensile stresses, however, because they tend to be more closely spaced than conventional reinforcing bars, they are better at controlling cracking.

• Thus, conventional reinforcing bars are used to increase the load-bearing capacity of concrete; fibers are more effective for crack control.

Application of FRC in Structures

• A) Highway And Airfield Pavements – Repair of existing pavement – Reduction in pavement thickness – Increase in resistance to impact – Increase in transverse and longitudinal joint spacing – Light weight long span bridge

• B) Hydraulic Structures – Resistance to cavitations or erosion damage – Repair of spilling basin – Thin wall pipes

• C) Fiber Shotcrete (FRS)

– improves many of the mechanical properties of the basic material such as the toughness, impact resistance, shear strength, flexural strength, and ductility factor.

Other Practical Aspects of FRC

• The use of FRC jackets as an external mean to strengthen existing RC columns has emerged in recent years with very promising results.

• Such strengthening technique has proved to be very effective in enhancing their ductility and axial load capacity (Matthys et al., (2005), Benzaid et al., (2008).

• Composites can also be used to strengthen/retrofitting and wrap columns and bridge supports that are partially damaged by earthquakes and other environmental factors.

Wrapping an old structure with glass polymer composite

Wrapping an old structure with glass fiber-reinforced polymer composite

Wrapping columns of a bridge withWrapping columns of a bridge with FRP composites

Strengthening a concrete bridgeStrengthening a concrete bridge with FR polymer bars

Encasement or enlargement of the column cross section (jacketing).

Current Applications of FRC

• It is important to recognize that in general, fiber reinforcement is not a substitute for conventional reinforcement.

• Fibers and steel bars have different roles to play in modern concrete technology, and there are many applications in which both fibers and continuous reinforcing bars should be used together.

Current Applications …

• Recently FRC technology is gaining more and more popularity among constructors for shotcrete work in which steel fibers of various types and shapes were introduced for the application of unstable slopes, landslides, tunnel lining, box girder bridge shells and road embankments.

• Runways • Sleepers • RCC with steel fibers • Roof tiles

Current Applications …

• Components which must withstand locally high loads or deformations, such as blast resistant structures, or precast piles which must be hammered into the ground.

• Overlays of air fields, road pavements, industrial floorings and bridges decks etc.

• Canal and refractory linings

Precast concrete railroad track slabs for high-speed trains. Source: Brite-Euram (2002). • Steel-FRC in combination with

traditional reinforcement to significantly reduce crack width and/ or the required amount of reinforcement leading to durability improvement.

• A reduction of reinforcing bar up to 50% is possible while keeping crack width constant.

Segmental tunnel lining using steel-FRC.

Precast concrete sewer pipes. Reinforcing precast concrete pipes using only steel fiber is economically advantageous for pipe diameters up to 36 in. (900 mm).

Exemplary Successful FRC Applications

• The most remarkable application was building of 18 runways of airports in USA between 1972 and 1980, which demonstrated excellent performance for about two decades towards crack and local damages.

Exemplary …

• The first structural use of steel-FRC was in 1971 for the production of demountable 3250 mm2 by 65 mm-thick panels for a parking garage at London’s Heathrow Airport.

• The concrete contained 3 percent by weight of 0.25 mm-diameter by 25 mm-long cold-drawn steel fibers. At the time of the last reported inspection, after 5 years of use, the slabs showed no signs of cracking (Kumar et al. 2006).

Exemplary …

• Many segments are reinforced with only steel fibers, but reinforcing bar can be used in addition if required to carry large moments, and monofilament polypropylene fibers can be added for fire resistance.

• It is estimated that there are more than 60 completed projects constructed with steel-reinforced segmental linings around the world, comprising some 450 to 480 km of tunnels, with more than 60 km in the USA (Brite-Euram. 2002).

Exemplary …

• Two fires in heavy goods vehicles being carried on trains in the Channel Tunnel linking England and France, one in 1996 and one in 2008.

• Both of these resulted in extensive damage to the HSC tunnel requiring costly and time-consuming repairs.

• The inclusion of polypropylene fibers in the concrete during mixing was considered overcoming this effect; these rapidly melt and provide pressure-relief channels.

Fire damage to the concrete lining of the Channel Tunnel. [Photograph courtesy of Paul Acker]

Opened in 1994, the English Channel Tunnel was an impressive engineering achievement and culminated the old dream of connecting England and France through underwater transportation. Over $15 billion was spent on the project which used state-of-the art construction methods and materials, including 50 MPa high-strength concrete liners. On November 18, 1996 a fire broke out. Approximately 50 m of tunnel lining was damaged and in some areas its thickness was reduced from 40 to 17 cm. The spalling of concrete, caused by the fire, led to local buckling in some sections of the reinforcing grid. It is interesting to note that the post-fire study on the undamaged concrete showed that the actual compressive strength was about 100 MPa. The lining was repaired with FRC.

Application of ultra-high-performance FRC to a slender bridge. With a 120-m span and a 130-cm arch depth the Cheong footbridge in Korea has the world record of slenderness (i.e., of the ratio: depth divided by span length). Even though the engineering applications of ultra-high-performance FRC are still at their infancy, the material has unique properties and has the potential of changing the structural design philosophy (Kumar et al. 2006).

Future trends of FRC applications

• A new generation of FRC materials has been developed, the so-called ‘high performance’ FRC, which exhibit multiple cracking and strain hardening beyond the point of first cracking, with a concomitant increase in energy absorption capacity.

Typical stress–strain curves for conventional and high performance FRC (after ACI

544).

Consumption of fiber

• Even though the market for FRC is still small compared to the overall production of concrete, in North America there has been a yearly growth rate of 20% and Li reports that the world-wide yearly consumption of fibers used in concrete is 300,000 tons (Kumar et al. 2006).

Consumption concrete

• At the current rate of concrete consumption the demand for concrete is expected to rise to about 16 billion tonnes a year by 2050.

• Thereafter, the consumption should start declining depending on how soon and how seriously we pursue the task of introducing into our everyday construction practice the principles of industrial ecology and enhancement of durability of the structures that are being designed and built right now.

Forecast of future population growth and concrete consumption. (From Mehta, P.K., Concr. Int., Vol. 24, No. 7, pp. 23–28, July 2002.)

Features of FRC

• Fibers can be used in concrete for added strength, impact resistance, and/or weight losses are desirable.

• FRC can be used in the construction of prefabricated building panels and tunnels formed of lightweight concrete.

• FRC can result in the outstanding architecture of lasting performance and beauty.

Features …

• To prevent Steel rusts tons of concrete has to be used for cladding – by contrast FRC is less prone to rusting and cracks caused by salts, water or other environmental factors.

• As a result, – more durable and also lowers maintenance and

modernization costs.

– requires far less protective concrete (which in turn reduces transport costs) and use of materials and resources is significantly lowered, as are CO2 emissions.

Features …

• Thinner structures delivering the same tensile strength, it can be used in more flexible and filigree ways, even enabling highly angled shells to be constructed.

• FRC is adaptable in that it can be poured or sprayed. – When sprayed, the surface finish has no pits or

bug-holes.

– If it is poured, it is easily shimmied to remove all pits and bug-holes prior to hardening.

Future potential of FRC

• It is obvious that lack of holistic approach

in meeting our socioeconomic needs is the

primary cause of environmental problems.

• Chokri Cherif, Director of TU Dresden’s Institute for Textile Machines and High-Performance Textile Materials Technology: “The concrete structures of tomorrow will stand out for their refined, light and aesthetic properties”.

Future …

• The German Federal Ministry of Education and Research (BMBF) is likewise convinced that the light construction material has a strong future: In summer 2013, 45 million euros were provided as a grant to the “C3 – Carbon Concrete Composite” consortium, which currently has 119 partners, all dedicated to making construction and maintenance more economic, more efficient and ecologically sustainable.

Some considerable facts about FRC

• Fibers can improve characteristics of concrete

– Increase flexural strength and toughness

– Increase impact resistance

– Reduce shrinkage and cracking

– Improve durability by stabilization of microcracks and decrease in permeability

• With increasing infrastructural demands, utilization of advanced materials for construction such as FRC is not a matter of choice but desire.

… facts

• In FRC crack density is increased, but the crack size is decreased.

• The addition of any type of fibers to plain concrete reduces the workability.

• Although every type of fiber has been tried out in cement and concrete, not all of them can be effectively and economically used.

– Each type of fiber has its characteristic properties and limitations.

… facts

• HSC behaves differently when exposed to fire with higher tendency to spall in an explosive manner, hence Fiber is needed as a complementary material.

• A new approach in design and in the utilization of this material, to account for both increase in performance and economics is therefore, needed.

What about horneros? Professor Alberto Fava of the University of La Plata in Argentina points out that the hornero is a tiny bird native to Argentina, Chile, Bolivia, and other South American countries. The bird had been painstakingly building straw-reinforced clay nests on treetops since before the advent of humans.