Concrete Construction I Chapter 19

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems © 2008 Pearson Education, Upper Saddle River, NJ All Rights Reserved. Using steel to strengthen concrete Reinforced concrete Reinforcing bars used to accommodate tensile stresses Prestressed concrete High strength steel wires used to introduce permanent initial compression in a concrete member Pre-tensioning (precast, prestressed concrete) Steel strands placed in tension inside formwork  Cast and cured  Steel strands cut, placing concrete in compression, creating camber in beam Post tensioning (cast-in-place) Steel strands encased in sleeves (called tendons) is placed in form  Concrete placed and cured  Steel is tensioned, fixed into position with wedges and cut, producing compression in concrete

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems © 2008 Pearson Education, Upper Saddle River, NJ All Rights Reserved. Three stages in making a precast, prestressed concrete beam

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems © 2008 Pearson Education, Upper Saddle River, NJ All Rights Reserved. Versatility of reinforced concrete Used in many applications Buildings, bridges, tunnels, retaining walls Relatively inexpensive Locally available Fire resistant Compatibility of two structural materials Concrete Steel

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems © 2008 Pearson Education, Upper Saddle River, NJ All Rights Reserved. Concrete used for almost all buildings on Saudi Arabia university campus

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems © 2008 Pearson Education, Upper Saddle River, NJ All Rights Reserved. Concrete used for almost all buildings on Saudi Arabia university campus

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems © 2008 Pearson Education, Upper Saddle River, NJ All Rights Reserved. Cost of concrete 25% - 30% concrete Placing concrete Finishing concrete 20% - 25% steel reinforcement Placing rebar Making rebar cages, etc. 50% - 55% formwork Percent even higher in highly complex shapes and forms

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems © 2008 Pearson Education, Upper Saddle River, NJ All Rights Reserved. Concrete formwork Molds used to give shape to concrete as it cures May be as simple as 2 x lumber and plywood Form plywood has smooth or textured surface as desired Can withstand repeated wetting Requires a range of fasteners, clamps and nails Sheet metal, angles, channels used to form and support concrete Steel & aluminum alloys Glass-fiber reinforced plastic - molded to shape. Forms must be treated with release agent before placing concrete Lubricates formwork, facilitates release Allows for reuse Formwork design is responsibility of contractor

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems © 2008 Pearson Education, Upper Saddle River, NJ All Rights Reserved. Dimensional lumber used for ground-supported slab

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems © 2008 Pearson Education, Upper Saddle River, NJ All Rights Reserved. Shoring & reshoring Adjustable temporary shoring is used to support formwork for elevated concrete structures. Shoring is removed when formwork is stripped In multi-story construction, one shored floor (with formwork) and two reshored floors are required before concrete can be placed on the next floor.

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems © 2008 Pearson Education, Upper Saddle River, NJ All Rights Reserved. Formwork and shores for elevated concrete slab

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems © 2008 Pearson Education, Upper Saddle River, NJ All Rights Reserved. Reinforced concrete shell roof

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems © 2008 Pearson Education, Upper Saddle River, NJ All Rights Reserved. Lumber shores support formwork and reinforced concrete floor

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems © 2008 Pearson Education, Upper Saddle River, NJ All Rights Reserved. Removal of formwork Formwork is removed as soon as possible Early stripping time speeds construction Patching and repair bonds best early in curing process Premature removal Can be hazardous Affects quality of finish Length of stripping time depends on Ambient temperature Strength of concrete Type of component Vertical forms can be removed first and are then wrapped for moist curing Horizontal forms require concrete is self-supporting (70% of design strength). Specified by project engineer.

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems © 2008 Pearson Education, Upper Saddle River, NJ All Rights Reserved. Principles of reinforcing concrete Proper reinforcement requires an adequate amount of steel placed in the correct location Beams Locate reinforcing bars where beam is placed in tension Bottom of beam in simply supported beam Varies in other a conditions Locate stirrups near the ends of beams to resist shear forces Hanger bars restrain stirrups from moving

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems © 2008 Pearson Education, Upper Saddle River, NJ All Rights Reserved. Reinforcement cage for simply supported beam

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems © 2008 Pearson Education, Upper Saddle River, NJ All Rights Reserved. Commonly used stirrup shapes

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems © 2008 Pearson Education, Upper Saddle River, NJ All Rights Reserved. Section of singly reinforced concrete beam: tension steel in two layers

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems © 2008 Pearson Education, Upper Saddle River, NJ All Rights Reserved. Reinforcement in column beam frame

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems © 2008 Pearson Education, Upper Saddle River, NJ All Rights Reserved. Lap splicing reinforcement bars

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems © 2008 Pearson Education, Upper Saddle River, NJ All Rights Reserved. Bond failure of bar: inadequate embedment length

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems © 2008 Pearson Education, Upper Saddle River, NJ All Rights Reserved. Reinforcement cage for simply supported beam

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems © 2008 Pearson Education, Upper Saddle River, NJ All Rights Reserved. Concrete cover An adequate layer of concrete is required to protect reinforcement from corrosion Measured from exposed face of concrete to outer edge of stirrup Bar supports are used to elevate reinforcement above ground or formwork in beams and slabs

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems © 2008 Pearson Education, Upper Saddle River, NJ All Rights Reserved. Reinforcement and formwork for columns Reinforcement used primarily to resist compressive forces Reinforcement located at column periphery To resist bending forces Contain concrete within cage in the case of crushing failure Column reinforcing cage consists of Longitudinal bars (between 1% & 8% of gross area of column) Ties prevent bars from buckling provide shear resistance resulting from bending Formwork Rectangular or square columns: wood or steel Round columns: Steel plate (multiple uses) or waterproof fiberboard (one time use)

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems © 2008 Pearson Education, Upper Saddle River, NJ All Rights Reserved. Tie arrangements in rectangular or square column

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems © 2008 Pearson Education, Upper Saddle River, NJ All Rights Reserved. Column reinforcement using continuous spiral

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems © 2008 Pearson Education, Upper Saddle River, NJ All Rights Reserved. Steel and fiber formwork for round columns

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems © 2008 Pearson Education, Upper Saddle River, NJ All Rights Reserved. Wall formwork: plywood and dimension lumber fastened with removable clamps and form ties

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems © 2008 Pearson Education, Upper Saddle River, NJ All Rights Reserved. Reinforced concrete wall at foundation. Wall is poured after footing

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems © 2008 Pearson Education, Upper Saddle River, NJ All Rights Reserved. Workers erect second layer of wall formwork after reinforcement has been placed and tied off

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems © 2008 Pearson Education, Upper Saddle River, NJ All Rights Reserved. Types of concrete slabs

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems © 2008 Pearson Education, Upper Saddle River, NJ All Rights Reserved. Ground-supported isolated concrete slab Slab is isolated from foundation with joint Used on stable undisturbed soil Vapor retarder placed below slab Prevents passage of moisture & water vapor Provides slip membrane between slab and subbase Aggravates differential shrinkage Subbase Granular material (generally sand) between ground and vapor retarder Levels uneven ground Provides capillary break, resisting movement of subsoil water Generally 4 inches deep

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems © 2008 Pearson Education, Upper Saddle River, NJ All Rights Reserved. Ground-supported isolated concrete slab, cont’d Thickness & strength - typical 4 inch minimum concrete slab 3,000 psi residential, 4,000 psi light commercial Reinforcement Generally not required for structural purposes Place as near the top of slab as possible Used to reduce size of shrinkage cracks Fiber reinforcement can be substituted for steel

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems © 2008 Pearson Education, Upper Saddle River, NJ All Rights Reserved. Ground-supported isolated concrete slab, cont’d Joints Control joints accommodate shrinkage, reduce random cracking Extend about 1/4 of depth of slab Typically 1/8 inch wide, sawcut or tooled Isolation joints Isolates slab from structural components full depth of slab Construction joint (cold joint) Non-movement joint Used where concrete cannot be placed in one operation Shear key  prevents differential movement  Assures aggregate interlock

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems © 2008 Pearson Education, Upper Saddle River, NJ All Rights Reserved. Section through control joint

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems © 2008 Pearson Education, Upper Saddle River, NJ All Rights Reserved. Isolation joints & control joints in interior isolated slab

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems © 2008 Pearson Education, Upper Saddle River, NJ All Rights Reserved. Section: isolated slab at load-bearing wall

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems © 2008 Pearson Education, Upper Saddle River, NJ All Rights Reserved. Keyed construction joint in slab