special concretes [part 01] - aalto · ´ fibre-reinforced and ferro concretes ... − it was...
TRANSCRIPT
Lecture outcomes´ Fast drying concretes
o Understanding the basics of fast dryingconcrete and the factors influence the dryingprocess
o Be able to estimate the drying time forconcrete slabs (floors) before covering them
´ Fibre-reinforced and Ferro concreteso Be familiar with basics and history of FRC and
Ferroconcreteo Listing the type of fibers used in the FRC and
recognizing the properties of FRC and Ferroconcrete
SPECIALCONCRETES[PART 01]
Fahim Al-Neshawy & Esko Sistonen06.11.2015
FAST DRYING
CONCRETES
Drying of concrete• In normal concrete, the amount of water is
180 – 200 dm³/m³− Major part for the workability of the
concrete− Chemically reacted water is 25% of the
cement amount when the hydration degreeis 100%
− The rest of the water amount is physicallybonded into the concrete pores
− If concrete moisture content is higher thanthe ambient moisture content, concretedrys and vice versa
Drying of concreteWstruc = w0 – α.0.25.C – We
− Wstruc is the moisture of the concrete structure [kg/m³]
− W0 is water content of the concrete mix [kg/m³]
− C is the amount of cement [kg/m³]− α is the degree of hydration− We is the physically bonded water in the
equilibrium state [kg/m³]
Bond
edam
ount
ofw
ater
durin
gab
sorp
tion
W/C
,(kg
wat
er/
kgCe
men
t)
Bond
edam
ount
ofw
ater
durin
gde
sorp
tion
W/C
,(kg
wat
er/
kgCe
men
t)
Equilibrium moisture content of concrete having different water-cementratios and hydration degrees.
α = 0,50 when w/c = 0,30,α = 0,60 when w/c = 0,40, andα =0,80 when w/c = 0,50...0,80.(Nilsson 1977, Fagerlund 1980).
Equilibrium moisture content
Construction process
Traditionalprocess
Speedingup theprocess
1. Foundation2. Framework
(skeleton)3. External work
(façade etc.)4. Internal work
(screeding andcoating etc.)
Factors influencing concrete drying
1. Concrete quality requirements2. The structural solution3. The drying environmental condition
− temperature of air− moisture content of air− velocity and direction of air flow
Concrete quality requirements
Factors which affect the time needed for concreteto dry to required moisture levels include:
− Type and amount of cement− Max. aggregate size− Water cement ration− Air content
Concrete quality requirements
− Type and amount of cemento Cement types I, I/II and IIIo Cement content: 178 to 400 kg/m³o Class F fly asho Silica fume (5 – 10% of cement decreases
drying time by 2 and 4 weeks respectively)− Max. aggregate size
o Using of larger aggregate size decreasethe drying time
Concrete quality requirements
− Water/cement ratioo For W/C ratio of 0.50 to 0.70:à the drying time to reach 90% RH is anywherefrom 3 to 9 months, under suitable dryingconditions.
o For W/C ratio of 0.38 – 0.5à typically take 2 to 3 months to reach 90% RHunder suitable drying conditions.
− Air content of concreteo Air entrainment (4 – 6 %)o Substantial air entrainment (8 – 10%)
Structural solution• When the height of the structure is 100 mm and
it can dry to both direction about half of thestructural humidity exits during 3 to 12 months,depending on the density of the structure
• The time to dry quadruples when the thicknessof the structure doubled!
• The time to dry quadruples when the structurecan dry to only one direction
dry to only one direction
Structural solution
The drying environmental condition
Estimation of drying time
The Swedish Concrete Association
Estimation of drying time• Example:
− it was estimated that the time between curing a concrete slab andthe installation of a floor covering would be 3 months
− concrete slab - 100mm− the HVAC turned on, effectively allowing for a 2 month drying
period− maximum relative humidity of 85% at the equivalent depth, and
drying would be one-sided.− The water/cement ratio was to be 0.4− The construction was going to take place during the rainy season.− the drying climate will be 18°C
• From Table 1, the standard time is 50 days• From Table 2, the correction factor for thickness is 0.4• From Table 3, the correction factor for one-sided drying is 2.0• From Table 4, the correction factor for temperature and humidity is 0.9• From Table 5, the correction factor for a rainy season is 1.4 (not shown
in table)• The total time is determined by the following calculation: 50 x 0.4 x 2 x
0.9 x 1.4 = 50 days, which is acceptable compared to the 2 monthsavailable.
The Swedish Concrete Association
FIBRE-
REINFORCED
CONCRETE
Fiber reinforced concretesFiber reinforced concrete (FRC) = compositematerial in which:
− fibers can be distributed randomly or inorganized manner
− fiber length is commonly 10…50 mm− cement-based matrix− Fibers can be in form of steel fiber, glass
fiber, natural fiber , synthetic fiber.
Ferroconcrete (ferrocement) = a thin concretestructure reinforced by a mesh of thin bars (thinlyspaced steel bars having small diameters)
FRC - Historical Perspective• Egyptians used straw to reinforce mud bricks, but there
is evidence that asbestos fiber was used to reinforceclay posts about 5000 years ago.
• In the 1950s, the concept of composite materials cameinto picture.
• In the 1970s, Steel , Glass and synthetic fibers havebeen used to improve the properties of concrete
• In the 1990s - micromechanics, hybrid systems, woodbased fiber systems manufacturing
• 2000+ Structural applications, Code integration, andNew products.
Areas of application of FRC materials• Thin sheets• Shingles• Roof tiles• Pipes• Prefabricated shapes• Panels• Shotcrete• Curtain walls• Slabs on grade• Precast elements• Composite decks• Impact resisting structures
Types of fibers• Fibers include steel fibers, glass fibers, synthetic
fibers and natural fibers – each of which lend varyingproperties to the concrete.
• In addition, the character of fiber-reinforced concretechanges with varying concretes, fiber materials,geometries, distribution, orientation, and densities.
• Aspect ratio (L/d) is calculated by dividing fiber length(L) by its diameter (d).
• Fibers with a non-circular cross section use anequivalent diameter for the calculation of aspect ratio.
Types of Fibers: Steel Fibers
Steel fibers
´ Aspect ratios of [L/d] 30 to 250.
´ Diameters vary from 0.25 mm to 0.75 mm.
´ High structural strength.´ Reduced crack widths and control the
crack widths tightly, thus improvingdurability.
´ Improve impact and abrasion resistance.´ Used in precast and structural
applications, highway and airportpavements, refractory and canal linings,industrial flooring, bridge decks, etc.
Types of fibers: Glass Fibers
Glass fibers´ High tensile strength, 1020 to 4080 N/mm2
´ Generally, fibers of length 25mm are used.
´ Improvement in impact strength.
´ Increased flexural strength, ductility and
resistance to thermal shock.
´ Used in formwork, swimming pools, ducts
and roofs, sewer lining etc.
Nylon Fibers
PolypropyleneFibers
Synthetic Fibers
• Man- made fibers from petrochemicaland textile industries.
• Cheap, abundantly available.• High chemical resistance.• High melting point.• Low modulus of elasticity.• It’s types are acrylic, aramid, carbon,
nylon, polyester, polyethylene,polypropylene, etc.
• Applications in cladding panels andshotcrete.
Natural FibersCoir
(kookoskuitu)
Hay
• Obtained at low cost and low levelof energy using local manpowerand technology.
• Jute, coir and bamboo areexamples.
• They may undergo organic decay.
• Low modulus of elasticity, highimpact strength.
Benefits of FRC• Main role of fibers is to bridge the cracks that
develop in concrete and increase the ductilityof concrete elements.
• Improvement on Post-Cracking behavior ofconcrete
• Imparts more resistance to Impact load
• controls plastic shrinkage cracking and dryingshrinkage cracking
• Lowers the permeability of concrete matrixand thus reduce the bleeding of water
Toughening mechanism• Toughness is ability of a material to absorb energy
and plastically deform without fracturing.
• It can also be defined as resistance to fracture of amaterial when stressed.
Factors affecting the Properties of FRC
• Volume of fibers
• Aspect ratio of fiber
• Orientation of fiber
• Relative fiber matrix stiffness
Volume of fiber
• Low volume fraction (less than 1%)− Used in slab and pavement that have large
exposed surface leading to high shrinkagecracking
• Moderate volume fraction(between 1 and 2 %)− Used in Construction method such as
Shortcrete & in Structures which requiresimproved capacity against delamination,spalling & fatigue
• High volume fraction(greater than 2%)− Used in making high performance fiber
reinforced composites (HPFRC)
Source: P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties,and Materials, Third Edition, Fourth Reprint 2011
Aspect Ratio of fiber• It is defined as ratio of length of fiber to it’s diameter
(L/d).• Increase in the aspect ratio up to 75,there is increase
in relative strength and toughness.• Beyond 75 of aspect ratio there is decrease in aspect
ratio and toughness.
Orientation of fibers• Aligned in the direction of load• Aligned in the direction perpendicular to load• Randomly distribution of fibers
It is observed that fibers aligned parallel to applied loadoffered more tensile strength and toughness than randomlydistributed or perpendicular fibers.
Relative fiber matrix• Modulus of elasticity of matrix must be less
than of fibers for efficient stress transfer.• Low modulus of fibers imparts more energy
absorption while high modulus fibers impartsstrength and stiffness.
• Low modulus fibers e.g. Nylons andPolypropylene fibers
• High modulus fibers e.g. Steel, Glass, andCarbon fibers
Mix composition of FRC• Steel fiber concretes
− Water : cement : aggregates− 0.4…0.6 : 1 : 1.5…2− cement content 350…600 kg/m3
• Glass fiber concretes− Water : cement : aggregates− 0.3…0.4 : 1 : 0.4…0.5− cement content 1100…1350 kg/m3
Comparison of Mix Proportion betweenPlain Concrete and Fiber ReinforcedConcrete
Material Plain concrete Fiber reinforcedconcrete
Cement 446 519Water (W/C=0.45) 201 234Fine aggregate 854 761Coarse aggregate 682 608Fibers (2% by volume) -- 157
The 14-days flexural strength, 8 Mpa, of the fiber reinforcedwas about 20% higher than that of plain concrete.
Effect of fibre aspect ratio on the workabilityof concrete
Workability versus fibre content for matrices withdifferent maximum aggregate sizes
Fiber Reinforced Concrete Normal Reinforcedconcrete
• High Durability • Lower Durability• Protect steel from
corrosion• Steel potential to
corrosion• Lighter materials • Heavier material• More expensive • Economical• With the same volume,
the strength is greater• With the same volume,
the strength is less
• Less workability • High workability ascompared to FRC.
Disadvantages of FRC
• Greater reduction of workability.
• High cost of materials.
• Generally fibers do not increase the flexural
strength of concrete, and so cannot replace
moment resisting or structural steel
reinforcement.
FERROCONCRETE
Ferroconcrete (ferrocement)• Reinforcement mesh having small cross-sections• large Asteel/Aconcrete-ratio• Reinforcement mesh in several layers• Used also in complex shell structures
Ferroconcrete (ferrocement)
Typical cross section of ferrocement
Materials used in Ferro cement
• Cement mortar mix− OPC and fine aggregate matrix is used− sand occupies 60 to 75% of the volume of the mortar− Plasticizers and air entraining admixtures are used− Sand: cement ratio (by mass) 1.5 to 2.5− Water: cement ratio (by mass) 0.35 to 0.60
• Ferroconcrete− Water : cement : aggregates− 0.4 : 1 : 2…3− cement content 500…700 kg/m3
Materials used in Ferro cement
• Skeleton steel− Forms the skeleton of the structure− 3 to 8 mm steel rods are used− Used in the form of tied reinforcement or welded
wire fabric− Used to impart structural strength in case of
boats, barges etc− Reinforcement should be free from dust, rust and
other impurities
Materials used in Ferro cement
• Steel mesh reinforcement or Fibre-reinforced polymeric meshes− Consists of galvanized steel wires of diameter 0.5
to 1.5 mm, spaced at 6 to 20mm centre to centre− Available as woven/interlocking mesh and welded
mesh− Welded wire mesh has hexagonal or rectangular
openings− Expanded-metal lath is also used− Made from carbon, glass etc.
Materials used in Ferro cement
Commonly used reinforcing mesh
Properties of ferroconcrete
• Very durable, cheap and versatile material.
• Low w/c ratio produces an impermeable
structures
• Less shrinkage, and low weight.
• High tensile strength and stiffness
• Better impact and punching shear resistance
• Undergo large deformations before cracking or
high deflections
Behavior of ferroconcrete in tension
RC in tension Ferroconcrete in tension
Applications of ferroconcretes• boats, ships• floating docks, buoys, barges• grain silos, containers, roof structures• large span hangars• façade units, pipes, gutters• roof tiles
References1. PAIKALLAVALUTEKNIIKKA OSA 8. 1995, ” Nopeasti kuivuvat betonit”
RTT Rakennustuoteteollisuus ry, Lahden kirjapaino ja Sanomalehti Oy
2. ACI 544.1R-96: State-of-the-Art Report on Fiber Reinforced ConcreteReported by ACI Committee 544
3. Ferrocement Structures:https://law.resource.org/pub/bd/bnbc.2012/gov.bd.bnbc.2012.06.12.pdf