SPECIAL CONCRETES [PART 01]

Transcription

1 SPECIAL CONCRETES [PART 01] Fahim Al-Neshawy & Esko Sistonen Lecture outcomes Fast drying concretes o o Understanding the basics of fast drying concrete and the factors influence the drying process Be able to estimate the drying time for concrete slabs (floors) before covering them Fibre-reinforced and Ferro concretes o o Be familiar with basics and history of FRC and Ferroconcrete Listing the type of fibers used in the FRC and recognizing the properties of FRC and Ferro concrete

2 FAST DRYING CONCRETES Drying of concrete In normal concrete, the amount of water is dm³/m³ Major part for the workability of the concrete Chemically reacted water is 25% of the cement amount when the hydration degree is 100% The rest of the water amount is physically bonded into the concrete pores If concrete moisture content is higher than the ambient moisture content, concrete drys and vice versa

3 Drying of concrete W struc = w 0 α.0.25.c W e W struc is the moisture of the concrete structure [kg/m³] W 0 is water content of the concrete mix [kg/m³] C is the amount of cement [kg/m³] α is the degree of hydration W e is the physically bonded water in the equilibrium state [kg/m³] Equilibrium moisture content Bonded amount of water during absorption W/C, (kg water / kg Cement) Bonded amount of water during desorption W/C, (kg water / kg Cement) Equilibrium moisture content of concrete having different water-cement ratios and hydration degrees. α = 0,50 when w/c = 0,30, α = 0,60 when w/c = 0,40, and α =0,80 when w/c = 0,50...0,80. (Nilsson 1977, Fagerlund 1980).

4 Construction process Traditional process Speeding up the process 1. Foundation 2. Framework (skeleton) 3. External work (façade etc.) 4. Internal work (screeding and coating etc.) Factors influencing concrete drying 1. Concrete quality requirements 2. The structural solution 3. The drying environmental condition temperature of air moisture content of air velocity and direction of air flow

5 Concrete quality requirements Factors which affect the time needed for concrete to 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 cement o Cement types I, I/II and III o Cement content: 178 to 400 kg/m³ o Class F fly ash o Silica fume (5 10% of cement decreases drying time by 2 and 4 weeks respectively) Max. aggregate size o Using of larger aggregate size decrease the drying time

6 Concrete quality requirements Water/cement ratio o For W/C ratio of 0.50 to 0.70: à the drying time to reach 90% RH is anywhere from 3 to 9 months, under suitable drying conditions. o For W/C ratio of à typically take 2 to 3 months to reach 90% RH under suitable drying conditions. Air content of concrete o 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 the structural humidity exits during 3 to 12 months, depending on the density of the structure

7 Structural solution The time to dry quadruples when the thickness of the structure doubled! The time to dry quadruples when the structure can dry to only one direction dry to only one direction The drying environmental condition

8 The drying environmental condition The drying environmental condition

9 Concrete coating Estimation of drying time The Swedish Concrete Association

10 Estimation of drying time The Swedish Concrete Association Estimation of drying time Example: it was estimated that the time between curing a concrete slab and the 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 months available. The Swedish Concrete Association

11 FIBRE- REINFORCED CONCRETE Fiber reinforced concretes Fiber reinforced concrete (FRC) = composite material in which: fibers can be distributed randomly or in organized manner fiber length is commonly mm cement-based matrix Fibers can be in form of steel fiber, glass fiber, natural fiber, synthetic fiber. Ferroconcrete (ferrocement) = a thin concrete structure reinforced by a mesh of thin bars (thinly spaced steel bars having small diameters)

12 FRC - Historical Perspective Egyptians used straw to reinforce mud bricks, but there is evidence that asbestos fiber was used to reinforce clay posts about 5000 years ago. In the 1950s, the concept of composite materials came into picture. In the 1970s, Steel, Glass and synthetic fibers have been used to improve the properties of concrete In the 1990s - micromechanics, hybrid systems, wood based fiber systems manufacturing Structural applications, Code integration, and New 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

13 Types of fibers Fibers include steel fibers, glass fibers, synthetic fibers and natural fibers each of which lend varying properties to the concrete. In addition, the character of fiber-reinforced concrete changes 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 an equivalent diameter for the calculation of aspect ratio. Types of 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 improving durability. Improve impact and abrasion resistance. Used in precast and structural applications, highway and airport pavements, refractory and canal linings, industrial flooring, bridge decks, etc. Steel fibers

14 Types of fibers: Glass Fibers High tensile strength, 1020 to 4080 N/mm 2 Generally, fibers of length 25mm are used. Glass fibers Improvement in impact strength. Increased flexural strength, ductility and resistance to thermal shock. Used in formwork, swimming pools, ducts and roofs, sewer lining etc.

15 Synthetic Fibers Man- made fibers from petrochemical and 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 and shotcrete. Polypropylene Fibers Nylon Fibers Natural Fibers Obtained at low cost and low level of energy using local manpower and technology. Jute, coir and bamboo are examples. They may undergo organic decay. Coir (kookoskuitu) Hay Low modulus of elasticity, high impact strength.

16 Benefits of FRC Main role of fibers is to bridge the cracks that develop in concrete and increase the ductility of concrete elements. Improvement on Post-Cracking behavior of concrete Imparts more resistance to Impact load controls plastic shrinkage cracking and drying shrinkage cracking Lowers the permeability of concrete matrix and thus reduce the bleeding of water

17 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 a material when stressed.

18 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 shrinkage cracking Moderate volume fraction(between 1 and 2 %) Used in Construction method such as Shortcrete & in Structures which requires improved capacity against delamination, spalling & fatigue High volume fraction(greater than 2%) Used in making high performance fiber reinforced composites (HPFRC)

19 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.

20 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 load offered more tensile strength and toughness than randomly distributed 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 imparts strength and stiffness. Low modulus fibers e.g. Nylons and Polypropylene fibers High modulus fibers e.g. Steel, Glass, and Carbon fibers

21 Mix composition of FRC Steel fiber concretes Water : cement : aggregates : 1 : cement content kg/m 3 Glass fiber concretes Water : cement : aggregates : 1 : cement content kg/m 3 Comparison of Mix Proportion between Plain Concrete and Fiber Reinforced Concrete Material Plain concrete Fiber reinforced concrete Cement Water (W/C=0.45) Fine aggregate Coarse aggregate Fibers (2% by volume) The 14-days flexural strength, 8 Mpa, of the fiber reinforced was about 20% higher than that of plain concrete.

22 Effect of fibre aspect ratio on the workability of concrete Workability versus fibre content for matrices with different maximum aggregate sizes

23 Fiber Reinforced Concrete Normal Reinforced concrete High Durability Lower Durability Protect steel from Steel potential to corrosion 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 as compared to FRC.

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25 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

26 Ferroconcrete (ferrocement) Reinforcement mesh having small cross-sections large A steel /A concrete -ratio Reinforcement mesh in several layers Used also in complex shell structures Ferroconcrete (ferrocement) Typical cross section of ferrocement

27 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 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

28 Materials used in Ferro cement Steel mesh reinforcement or Fibrereinforced 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

29 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

30 Applications of ferroconcretes boats, ships floating docks, buoys, barges grain silos, containers, roof structures large span hangars façade units, pipes, gutters roof tiles References 1. PAIKALLAVALUTEKNIIKKA OSA , Nopeasti kuivuvat betonit RTT Rakennustuoteteollisuus ry, Lahden kirjapaino ja Sanomalehti Oy 2. ACI 544.1R-96: State-of-the-Art Report on Fiber Reinforced Concrete Reported by ACI Committee Ferrocement Structures: