fiber reinforced concrete (FRC) 5.02

Question 1

name natural fiber examples:

Answer 1

-hemp
-wood
-cellulose

Question 2

name organic synthetics examples:

Answer 2

-aramid
-carbon
-poly acrylonitrile
-polyethylene
-polypropylene
-poly vinyl alcohol (PVA)

Question 3

name inorganic fiber examples

Answer 3

-glass
-steel
-ceramic

Question 4

what are the effects on the ductility on the structural behavior?

Answer 4

-increase the bearing capacity under the static load
*redistribution of forces
*stress redistribution
*no concrete spalling
-increase structural safety (early indication of failure)
-high loading capacity under impact loading
*security of structures under high stresses
*reduction in repair cost

Question 5

advantages of FRC in the durability of structures:

Answer 5

• Restriction of crack widths due to stresses and external loads
  *limitation of the penetration of liquids and gasses
  *“self-healing effect” fine cracks
• Minimize the damage of structures by means of corrosion processes
  *Positive effect on the bearing capacity, serviceability and the appearance

Question 6

effects of fibers in crack distributions:

Answer 6

• Prevention of crack propagation
• Spreading in many, very fine, “harmless” cracks
  -under tension test: increase the tensile strength and/or the ductility
  -under compression: improving the cracking behavior and energy absorption

Question 7

FRC composition :

Answer 7

Water-cement value
- w/c: 0.40 to 0.50
- relatively high cement content (workability !)
- usage of superplasticizer
- if necessary: replacement of approx. 30 % of cement by fly ash

Fiber content
- Steel fibers:
general: 0.5 to 2.5 Vol.-%
in situ: 0.25 to 1.0 Vol.-%
- Glass fibers : 1.5 to 4.0 Vol.-%
- Polymer fibers : 0.1 to 2.0 Vol.-%

Question 8

how does grain-size in concrete affect fiber distribution?

Answer 8

• Aggregates which are larger than the average fibre spacing, lead to uneven fibre distribution
• Maximum grain-size should not exceed one-third of fibre length (in steel fibre concrete)
• In case of glass fibre reinforced concrete/mortar: maximum grain-size is 1 to 2 mm
• For HPFRC < 1 mm

Question 9

mixing of FRC

Answer 9

Prevent “balling of fibers” i.e. perfect dispersion of the added fibers
- Gluing with a water-soluble glue into bundles
- Separation of the components before addition to the mixture by sieving
- Influence parameters: l/d, fiber content, fiber diameter, grain size, water content

“Gentle” mixing of glass fibers
- short mixing
- rubber-coated mixing tools
- usage of small, round aggregates
- surface treatment of fibers

Question 10

relation between Fiber and self compacting lightweight concrete (SCLC)

Answer 10

-low amount of fibers: no increase in ductility
-large amount of fibers: blocking, poor deaeriation, not self compacting
-optimum fiber content: high ductility, self compacting

Question 11

what are the percentage of flexural fatigue strength at the flexural strength of static short-term:

Answer 11

• unreinforced concrete 50 - 60 %
• steel fibre reinforced concrete 90 - 95 %
• polypropylene fibre reinforced concrete ca. 70 %

Question 12

creep and shrinkage

Answer 12

• Literature on creep and shrinkage of fibre reinforced concrete are contradictory
• Steel fibres hinder shrinkage deformation and micro cracks due to stresses caused by shrinkage
  = reduction of shrinkage
• Creep deformations in concrete are just little affected by steel fibres
  
  • the stiffening effect of the fibres and the influence
    of the present of high-content of compaction pores
    will equilibrate each other
• polymeric fibres are used specially to limit the crack formation in the setting process of concrete

Question 13

durability of steel fibers

Answer 13

• steel fibres corrodes when they are exposed to moisture and lying near the surface of concrete
  
  • rust stain will occur on the surface
  • there is no concrete spalling because of the small
    diameter of the steel fibres
• steel fibre reinforced concrete samples, which were stored in see water, show after 10 years an increase in
  strength

Question 14

durability of polymer fibers

Answer 14

over the long-term behavior of polymer fibres there are no negative experiences

Question 15

durability of glass fibers

Answer 15

• E-Glass fibres are inapplicable because of their poor alkali resistance
• even by AR glass fibres, lower strength and ultimate strain after many years are expected

Question 16

advantages of fiber shotcrete

Answer 16

• pronounced improvement of ductility
• up to 30 % higher bending strength
• high strength after cracking
• improving the impact strength
• improving the adhesion strength
• reduction of early shrinkage
• increase the fire resistance
• minimizing of crack widths in fibre reinforced concrete hinders the transport processes (durability, serviceability)

Question 17

application and advantages of FRC

Answer 17

-Dynamically loaded structures
* structures in earthquake zones
* explosive structures
* elements with excess load
* impact protection structures
* structures under hydroabrasion because of
cavitations or wear by coarse sediments
* coastal fortifications
* piles
* machine foundations

Question 18

other applications of FRC

Answer 18

• Local strengthening particularly in highly stressed areas of steel or prestressed concrete components.
• Secondary barrier to water-polluting substances. The fibres decrease the crack widths and thus limits
  the penetrations of such unfavorable materials into the concrete.
• As steel fibre reinforced shotcrete in mining and tunneling.
• Thin-walled elements (no sufficient protection of steel rebars by concrete layer possible!).
• As repair material (mortar), for example: soil, dams, bridges, facades (also as spray mortar with glass
  or polymer fibres), structural elements with high fire resistance (steel fibres).