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

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kgCe

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t)

Bond

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ount

ofw

ater

durin

gde

sorp

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W/C

,(kg

wat

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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

The drying environmental condition

The drying environmental condition

Concrete coating

Estimation of drying time

The Swedish Concrete Association

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