Precast Retrofitting

Precast Concrete

Domain	Explanation
Topics	Materials & production Frames, components & connections Skeletal frame design Composite precast floor design Joints & connections Structural integrity Handling, transportation & erection
Concrete properties	Inherent plasticity Load resistance Durability Mobility: Handling: points, stress concentrations Transportation Erection
Precast concrete	Definition: Cast one place & erect at another Advantage: assemblage & transport Why use?: Aesthetics Structural: dynamics, seismics Economic: uses, labours Combinations of above Aim: achieve best overall results of above Advantages: Balanced work: foundation do, & beam cast simultaneously Better quality control Closed work environment Phased deliveries Programmed stage-by-stage Uses prestressing, joints & new materials Costs: Material more expensive: high strength Less labour, weather-dependent Higher speed, more erection ease, balance out How: Entire systems approach Close co-operation required Basic relationships established Matching of capabilities Teamwork: Quality: conceive à design à production à erection à completion Better: earlier establishment Criteria: accuracy, finishes, standardization, buildability, modular concept Compromise: circumstances & experiences

Materials	Moulds, selection, compaction, curing, finishes Precast materials: Types: OPC, RHPC, sulphate-resisting, high alumina High strength finish, durability Reinforcement: welded fabric, spacers, more fire-resistance & corrosion-resistant Aggregates: natural & synthetic Water Prestressing: structural shrinkage, handling Additives Mixes: special circumstances Admixtures: compatible, must be uniform, homogeneous & well-compacted; no deterioration, correct & safe use; no deleterious effects Production: Staff, downtimes Materials, mixes Equipment Tolerances Temporary bracing & shims
Moulds	Shape concrete immediately & after set Types: Steel: common Concrete: tolerances, cast into panels to best fit Timber: more versatile Aluminum: roof tile Plastics: complex profiling Plaster: architectural Others: rubbers, plastics Should be: Tight Rigid Finish Selection: Physical: size, shape, variables, fixings, tolerances, finishing Commercial: economy, reusability Design: No & complexity of units Production lengths Variable in shapes & fixings Quantity of projecting reinforcements Size & mass Easy adjustments
Production involves	Mix proportioning Mix: mechanical, uniform, homogeneous, consistent, self-compacting concrete Casting: continuous in layers, uniform, homogeneous, well-compacted Curing: accelerated using covered, steam curing to trap heat Form stripping & finishing Storage: Elements separated & prevented from distortions by proper supports No sagging & twisting Proportional to strength & green state
Repair & corrections	Appearance defects: checks, repairs, the earlier the better Structural defects: Approach designers to consult Manufacturer guarantee By experts Cracks, reinforcement deterioration & cavities
Specific processes	Precast wet-cast: Compaction: table, clamp-on, poker vibration Visual concrete Piles, tunnels, drains, covers Vertical pipes: Hollow: inside & outside moulds Handling points & stresses Vibro-press production: Low W/C ratio: earth-dry or moist High vibration & pressures to cure & compact Spun concrete: Use centrifugal force to compact concrete against mould Hydraulically-pressed production: For elements without much reinforcements: roofing, tiles, kerbs, porous tiles More compaction using vibration & pressures to press water out Extruded & strip form production: Hollow-core slab Stairs Refuse chute Quality of finished element: Requires for colour shades Requirements for type of facing Requirements for cracks Finishing: Self Colour Acid-etched Grit-blasted Flame spalled Tolled Ribbed: split Rolled & tamped Retarded: low set to expose aggregates
Design overview	Lateral stability provided by: Fixed columns Moment-resisting connections Shear walls, cores fixed & braces anchored Façade tubes Load carrying system basic forms: Cantilever systems Unbraced frame systems: self-resisting or MRFs Braced frame systems: lowered deflections, not buckling (sway) Load-bearing façade & façade tubes Comparison of systems with general considerations: Viewed as a whole in its 3-D shape & influences Normally two mutually-perpendicular directions Structural, economic reasons for layout, both horizontal & vertical Selection of components: Precast elements: standard & non-standard Non-standard: specific projects, façade etc. Standard: cast with existing moulds à columns, beams, slabs Assembly: One-storey & low-rise structures: simple construction Frames with moment connections: normally double-T of H configurations Shear walls & cores: walls up to 100% of lateral loads; simple connections, but problems of water leakage Modular planning & layout: Highly systematic & discipline Basis for modular planning: select a suitable planning grid with equal distance between grid-lines Regular shapes Short span: more columns, foundations & higher overall costs Long span: heavier & more expensive elements
Connections	Connection details: Slab-beam connections: must consider the effects of volume changes, transfer of horizontal forces from slab (expansion forces) to the beam; mesh R placed across beam to minimize cracking Beam-column connections: labeled as BC1 to BC7; as boundary conditions Column-base connections: labeled as CB1 to CB5; with non-shrink grout & double nut system; for assumed fixed connections Column-column connections: labeled as CC1 to CC5; ties, anchor bolts, threaded reinforcing bars, inserts; erection & alignment Joints as interface with combinations of: Axial: extension Moment: bending Torsion: twisting rotation Shear: distortion Force transfer between joints & members: Straight-edged sliding joints: utilize shear friction action Castellated jagged-edged joints: utilize shear wedging effect Rough surfaces & interfaces: shear wedging by irregular castellated joints Bedding joints: Ensure grout strength = precast strength Joint strength depends on (modulus, Poisson’s ratio, interface grouting) Stress trajectory with deformation characteristics Grout stronger than precast: grout cracks Precast stronger than grout: precast cracks Bearing joints: Dry: bearing stress < 0.4f_cu Dry packed (with grout): bearing stress < 0.4f_cu Bedded (with levelling packs): bearing stress < 0.5f_cu Elastomeric: rubber pads bearing stress < 0.5f_cu Extended (with bars & grouted): bearing stress < 0.4f_cu Steel (steel or steel-sandwich pack): bearing stress < 0.8f_cu Components: Non-isolated: in events of failures, loads on 1 element are shared with surrounding elements à important for robustness Isolated: no sharing at failure Tension joints: Lap the reinforcements: with lateral bars to reduce concrete splitting & improve integrity at overload & bond failure Threaded couplers: with screws; for good alignment or tolerance Grouted couplers Bolting of the splices Pinned joints: Transfer axial + shear Moment-resisting joints: Transfer axial + shear + moment Locations of joints in a frame: Hollow core slab to beam Double-T floor slab to beam Beam to column Column to column Column to foundation Wall: to column & to beam Rebars placement: Rebars run perpendicular to cracks Fully anchored & mobilized to increase non-isolation Reduce lateral splitting Reduce concrete spalling Reduce compression crushing Links & stirrups concentrated at high shear Floor-slab joints: Steel inserts: bearing stress zones need to be reinforced by steel bearing plates Corbelling: Equilibrium of forces Tie inside corbel into beam or slab General requirements: connection need not be hidden; supports only 1-2 beams Proprietary beam-column joint: boxes cast into elements & joined Beam-column joints: steel inserts & corbelling Column-column joints: Using splices with the following principles: Splices at hinges Splices at zero M: no twisting Splices at staggered levels: no weak horizontal planes formed Splices: simple to construct Splices: to provide immediate stability to columns Splices at levels of columns that equilibrate their self-weight: interior columns spliced more than exterior columns Bolted plates Grouted sleeve splices Upper column normally propped for temporary supports Foundation joints: Pocket foundations: Axial force transfer mechanisms: skin friction & end bearing à conceptualise reinforcements required Adv.: lower costs Disadv: supporting props needed Base-plate foundations: Strength of base plate connections: plate thickness (against bearing) & bolt tensile capacity (against bending) Joints for robustness: against situations of partial collapse leading to overall progressive collapse Capacity to limit damage: accident, explosion, overloading Ronan point Causes: no inter-element ties to mobilize non-isolated load sharing Prevention: use non-isolation measures & ties under right directions & locations Methods: welded connections, anchors, dowel action, ties Tied solution: catenary action Greatest direct stress transfers to adjacent supporting substructures close to failed locations
Design of skeletal frames	Ability to model: Qualitative: kinetics & kinematic effects of members, joints & subsystems Standard element for precast skeletal frames: Roof & floor slabs: direction, magnitude & orientation à 4 types of precast floors (hollow core slabs, double tee slabs, suttering slabs & beam/block floor systems) Staircases Roof beams: tampered precast beams Floor beams: parallel precast beams Columns: normally rectangular or circular cross-sections Walls: range from load-bearing panels to lightweight or composite panels as internal partitions Typical connection details: connections Advantages: High degree of flexibility Large spans & open spaces Basis for design: Handling stress Compatibility Anchorage at supports Joints for movements Stability Durability Shear friction theory: Large shears across concentrated areas Prevention: reinforcement across potential failure surfaces Strut-and-Tie model: Aim: establish flow of forces; check stress levels; determine reinforcement areas Consists of: compression struts; tension ties & joints/nodal zones Assumes: deformation capacity is not exceeded at any point before assumed set of forces is reached System capacity: lower bound on concrete strength, if no element is loaded beyond its capacity Nodal zones: at least 3 forces must meet at a node Crushing strength of a strut: concrete strength, cracking direction, tension strains Design of drapped ends: Correct stress distribution Flow of forces in diaphragm using strut-and-tie models: To check stress levels & reinforcement areas Must have continuity in reinforcement in toppings Compression concentration in the corners between beams Horizontal shears: through activation of composite action
Handling, Transportation & Erection	Aim: max. efficiency of entire cycle from loading of precast elements at plant à transport à erection at site Logistics Handling: Handling points by pre-fabricated holes & steel inserts Stress concentrations Lifting arrangements: proportional to type of precast element Beams: allow transverse lifting Columns: uniform R à aim for uniform lifting stresses induced Cladding to design for support by edge beams or steel inserts Transport: Method of loading: Type of transport: shape, mass & volume of precast Number of elements per load Ideal position on vehicle Overall loaded stability Proper bracing & wedging w.r.t. safety requirements Transport planning: Contracting with contractor’s general programs to coordinate Ensure erection crew & equipment supplied at rate to avoid delays Just-In-Time Coordination between erection crew (sufficient) & equipment (assigned for stability): less delays Delivery: Delivery sequence: flexible to allow full loadings if necessary; control of precast unit position; advance notice of shipment; assure prompt unloading Documentation: purposes to identify precast elements (production no, series, date, mass, ref. to final position in actual structure); should appear on dispatch notes by manufacturer Organization w.r.t.: loading position, route, timing, speed, product protection, site access, insurance, authorization Note & detect damage Acceptance: adequate storage space; arrival & removal of precast in orderly manner Erection: Erection tolerance: 10mm or 0.1% per storey Depends of quality of workmanship of erector Erector: access, no obstruction, access to window & door areas, no repetition Props by: bracings & pocket supports Preparation: structure access; clearance; window access; benchmark lines Tasks: storage à raising à adjusting à connecting à caulking à consistent checking before, during & after à check damage protection against winds, rains, snow & vandalism Sequence in drawings: drawings for every stages Data records: as-built; erection drawings; access routes; topographical survey; others: date & final erections Required: personnel, equipment, documents, precast units, fixing After erections: erection checks; repairs by manufacturers; protection by lowering staining & damage; security: safety nets Final connections: in accordance with design Repairs by manufacturers Storage by contractor: protection of precast units Security
Review	Topics: Introduction Assessment: methods; site practice; test results interpretation; acceptance criteria; diagnosis à prognosis Structural appraisal: appraisal process; visual inspection; design exam Repairs: objectives (restore profile, function, appearance) Repair materials: reinforcements (steel, bars, plastics, FRP); bonding aids; coating: gunite; mortar, epoxy Retrofitting: structural considerations; repair process (remove à repair à bond à cover); crack repair; common repairs (slabs, beams, columns, defective column joints) Precast: Materials & production Compaction techniques Moulds Curing: accelerated curing De-moulding Storage Theory: shear friction theory; strut-tie theory Frames: Lateral stability Lateral force transmission 2-D frames: sub-frames Selection of components: standard types; beams – dapped ends; connection details Assembly: design basis (recommendations, handling stresses, compatibility: connections, anchorage at supports, stability, durability)

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