British Columbia Carpenter Apprenticeship Program

Transcription

1 British Columbia Carpenter Apprenticeship Program Level 2 Line G Competency G-9 Install Specialized Formwork, Pre-cast and Pre-stressed Concrete

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3 SAFETY ADVISORY Please note that it is always the responsibility of any person using these materials to inform him/herself about the Occupational Health and Safety Regulation pertaining to his/her work. The references to WorkSafeBC safety regulations contained within these materials do not / may not reflect the most recent Occupational Health and Safety Regulation (the current Standards and Regulation in BC can be obtained on the following website:

4 Competency G-9 Install Specialized Formwork, Pre-cast and Pre-stressed Concrete Contents Objectives... 2 Learning Task 1: Describe Tilt-up Construction... 3 Learning Task 1 Self-Test Learning Task 2: Describe Pre-cast Concrete Learning Task 2 Self-Test Learning Task 3: Describe Pre-stressed Concrete Learning Task 3 Self-Test Learning Task 4: Seal Joints Learning Task 4 Self-Test Learning Task 5: Describe Slip-form Use and Construction Learning Task 5 Self-Test Learning Task 6: Describe Construction Methods Used for Mass Concrete Learning Task 6 Self-Test BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2 1

5 Competency G-9 Install Specialized Formwork, Pre-cast and Pre-stressed Concrete Innovations in construction technology have provided the construction industry with many methods for constructing concrete buildings. These new methods of construction require specialized formwork. Carpenters must keep up-to-date with changes in technology and safety regulations for these specialized formwork methods. Objectives When you have completed the Learning Tasks in this Competency, you will be able to: describe tilt-up construction and its uses describe formwork procedures for tilt-up construction describe procedures for tilting up panels state the purpose of pre-cast concrete describe methods of forming pre-cast members state the purpose of pre-stressed concrete members describe methods of pre-tensioning pre-stressed members describe methods of post-tensioning pre-stressed members seal joints describe slip-forms describe construction methods used for mass concrete Competencies Written: Install Pre-cast and Pre-stressed Concrete You will be tested on your knowledge of terminology, construction methods and uses of specialized formwork. 2 BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2

6 Competency G-9 Learning Task 1 Learning Task 1 Describe Tilt-up Construction Tilt-up construction is used extensively for single story warehouse and retail buildings in British Columbia. It is the most economical method of constructing large single story buildings. The tilt-up process is very fast, allowing the owner of the building to be receiving rental income sooner than with other methods of construction. Tilt-up Construction Tilt-up construction is a specialized form of pre-cast concrete construction. Concrete walls are pre-cast on site in a horizontal position. They are then lifted into place using a crane. The crane is attached to the face of the panel and tilts the panel into a vertical position. This type of construction is best suited for one-story buildings, but it may be used in multistory structures. Tilt-up Drawings The employer must ensure that a registered professional engineer for the project certifies a comprehensive set of drawings and specifications. These drawings and specifications must include: all erection and bracing procedures all lifting insert types, installation details and locations required concrete strength before lifting can begin the design wind pressure used for the design the maximum wind gust speed that can be supported by the temporary bracing (if the wind exceeds this speed, the site must be cleared of workers) requirements for steel braces needed during lifting method of rigging and handling every panel any site specific considerations the weight of each panel BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2 3

7 Learning Task 1 Competency G-9 Panel Lifting and Setting Tilt-up panel lifting and bracing must be done under the direct supervision of a qualified person. The following are some of the important considerations: panel lifting must not begin until the specified concrete strength is reached bond breaker must be used between the tilt-up panel and the previously placed concrete wedges and pry bars should be used to help release the panel when lifting position crane so that there are no blind lifts (a blind lift is where the crane operator cannot see the panel being lifted) during panel lifting, only workers that are involved in the bracing of the panel are permitted in the danger area of the down side of the panel all bracing must be in place before the rigging can be slackened additional bracing and cross bracing must be installed as soon as the rigging is released and all bracing must be installed for all erected panels at the end of the day panel lifting must not be done if the wind gusts to over 55km/h (35mph) all workers must leave the job if the wind gusts exceed 100km/h (60mph) The engineer that certified the erection procedure must inspect the site and issue an inspection certificate prior to the start of tilt-up panel lifting. Temporary bracing may not be removed until inspected by the building design engineer and he/she has issued a written statement that the bracing is no longer needed. Figures 1, 2, 3 and 4 show the basic procedures used in tilt-up construction. WALL IN FINAL WALL IN FINAL POSITION WALLS ARE CAST ON THE FLOOR SLAB FOOTINGS ON UNDISTURBED SOIL WELL COMPACTED GRANULAR FILL Figure 1. Typical cross section of a tilt-up wall and slab 4 BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2

8 Competency G-9 Learning Task 1 The footings and the main floor slab are constructed first. The slab construction is critical: it must have a very smooth surface it must be very strong the sub grade must be well-compacted The tilt up panels are cast and allowed to cure to the required strength specified by the engineer. A mobile crane is used to lift the panels into place. The lifting of the panel is very carefully controlled (Figure 2). CRANE LIFTS WALL INTO FINAL POSITION Figure 2. Lifting a wall panel The crane must hold the panel in place until the steel braces are bolted into place (Figure 3). THE CRANE HOLDS THE WALL WHILE THE BRACING IS SET TEMPORARY BRACING IS BOLTED IN PLACE Figure 3. Braces installed After the braces are installed, the crane can release the panel and pick up another. BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2 5

9 Learning Task 1 Competency G-9 The panels are bolted together with a steel angle iron. This angle is used as a ledger to support the roof trusses. The trusses are set and the infill concrete is placed after all of the panels have been erected (Figure 4). TEMPORARY BRACING IS REMOVED INFILL CONCRETE PLACED Figure 4. Completed building The joints between the tilt-up panels are finished with a backer rod and caulking. Uses of Tilt-up Construction Tilt-up construction was used by Thomas Edison to build housing over a hundred years ago. Today many types of buildings are built using tilt-up construction: warehouse buildings office buildings civic and institutional buildings residential buildings retail buildings The following are statistics of actual tilt-up building projects: Warehouse Building The erection of the panels for a large warehouse building having 522,000 square feet floor area took only 15 days. There were 288 panels in total all 12'6" wide and 38' 42' high. Retail Building A 520,000 square foot retail building was built using large panels. The panel size varied from 14' 18' wide and 42' 46' high. These large panels were very heavy, weighing between tons each. 6 BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2

10 Competency G-9 Learning Task 1 The panels were lifted with a mobile crane, some had to be walked into place. Walking the panel requires the crane to move while carrying the panel. During the walking procedure, the crane s outriggers are kept slightly above the slab. The floor slab must be very strong to allow a moving crane to roll across it without cracking. Office Building A four-storey office building that included 250,000 square feet of finished area was built using tilt-up construction square feet of 12" thick insulated walls were part of the structure. Procedures for Tilt-up Construction The formwork for the wall panels used in tilt-up structures is similar to the formwork for regular concrete slabs. The main difference between regular slab construction and tilt-up wall panel construction is that the tilt-up wall panel is lifted from a horizontal position into a vertical position. The rigging for the lifting is connected to inserts that are cast into the concrete wall panel. The correct inserts must be located exactly as shown on the engineer s drawings. Failure to follow the drawings may result in the inserts pulling out and the panel falling. Footings The footings for the tilt-up wall panels are cast on undisturbed soil at an elevation that will put the bottom of the tilt-up wall panel below the elevation of the floor slab by at least 12" as shown in Figure 5. INFILL CONCRETE GUIDE PINS GROUT PAD Figure 5. Section view showing the footing location Reinforcing bars in an L shape are cast into the wall panel. These bars are used to connect the infill concrete to the wall panel. BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2 7

11 Learning Task 1 Competency G-9 The crane sets the wall panels directly on the footings. The in and out position of the base of the wall panel is guided by rebar dowels cast into the footing, as shown in Figure 1. The guide pins need to be in a perfectly straight line and are aligned using survey equipment. Grout pads are set on each footing to allow the elevation to be exactly controlled. Two pads of grout are placed on each footing. Using an angle grinder, minute amounts are ground off, while checking the elevation with a surveyor s level, until the exact elevation is obtained. The tolerance for the finished grout pads is very small, plus or minus " or 0.5 mm. Figure 6 shows two wall panels resting on the grout pads on the footing. GUIDE PINS 3/4" GAP GROUT PAD AREA THAT WAS GROUND TO ELEVATION Figure 6. Grout pads on footings After the panels are in their final location, the footing is covered with a mound of fresh concrete to keep the panels from moving during the backfilling of the infill area. Floor Slab Preparation The floor slabs must be designed to withstand extra loads imposed by the concrete wall panels and the equipment used to lift them. The surface must be very smooth, very flat and free of blemishes. The wall panels are rigidly held in place at the base by reinforcing steel that is cast into the floor slab and infill concrete. The floor slab is kept back from the perimeter of the building by approximately 4 to 6 feet to allow working room around the footings supporting the tilt-up wall panels. 8 BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2

12 Competency G-9 Learning Task 1 The curing of the floor slab must be carefully controlled to maximize the quality of the finished surface. WARNING! If there are blockouts in the floor slab, they should be covered to keep rainwater from softening the sub grade under the slab. The weight of the crane will crack even the strongest slab if the sub grade is soft. Tilt-up Wall Panels The wall panels are cast directly onto the floor slab. The shape of each panel is detailed on the structural drawings. Edge forms are built, as shown in Figure 7, to create the edge of the panels. Figure 7. Wooden edge forms The chamfer strip shown in drawing A and D is used to provide a finished edge at the joint of the panels. The panels are cast so that the bottom of the panel is the outside of the building. Any imperfection in the floor slab will show up on the outside of the building. Adding form liner to the bottom of the wall panels will texture the surface of the outside of the building. Many different textures have been tried over the years but for most buildings, a plain concrete surface is used. BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2 9

13 Learning Task 1 Competency G-9 Window and Door Openings Window and door bucks are made and fastened to the slab to blockout the openings. The bucks must be braced square and fastened to the slab to keep them from moving during the placement of concrete. Stacking Panels Panels can be built on top of another to conserve floor space. If stacking panels are to be built, only stack full panels without window or door openings. Release Agents Release agents are used to allow the tilt-up wall panel to be separated from the floor slab. Specific chemicals have been designed for tilt-up construction, and must be used as directed by the manufacturer. The floor slab must be clean of oil and other materials that may stain the finished surface of the wall panel. Clean the area of the floor slab prior to positioning the forms. Lifting Inserts The shop drawing for each panel will show the type of insert (Figure 8) and the location of the insert for each lifting point. Usually there are six lifting points but there may be more depending upon the shape and size of the panel. Figure 8. Lifting inserts Pay careful attention to the details of how the insert is to be placed in relation to the reinforcing steel. Extra bars are usually required at the lifting insert location. Bracing Inserts Metal wall braces are attached to the wall panels to support them. The wall end of the brace is bolted to an insert that is cast into the concrete wall panel. Locate the wall brace inserts as shown on the panel shop drawing. 10 BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2

14 Competency G-9 Learning Task 1 Strongbacks are added to provide support for delicate details around windows and doors in the wall panels. The strongbacks are bolted to inserts in the wall panel. Reinforcing Install the reinforcing as shown on the structural drawings. The reinforcing is used to support the lifting inserts and strengthen the wall during the lifting process. Take care to position the bars correctly around the lifting inserts. Concrete Placement and Finishing Concrete Placement The concrete used for the tilt-up panels is high strength concrete. An accelerator is used in the mix to speed the strength gain and allow the lifting of the panels to begin soon after the placement of the concrete. Vibrate the concrete well, paying particular attention to the lifting inserts and the corners of the formwork. Finishing The bottom side of the tilt-up panel is the finished side. The top side of the panel is the inside of the building. The inside of the building is usually covered with insulation and interior wall finish, unless insulated panels are built. The topside of the wall panel is finished as a regular slab would be finished. Decorative form liners can be used to create designs on the surfaces of exterior panels. The use of liners is usually restricted to wall panels that are manufactured in a pre-casting plant. Curing Immediately after finishing the surface of the wall panels, the curing agent should be applied. If multiple layers of wall panels are being cast on top of one another, the curing agent must be compatible with the release agent that is being used. The site engineer is responsible for deciding when the concrete strength of the wall panels is adequate for the lifting to begin. The engineer will often use a rebound hammer to determine the actual strength of the concrete for each panel. A rebound hammer is a spring-loaded device that bounces a dowel of steel off of the surface of the concrete being tested. The amount that the hammer rebounds indicates the strength of the concrete. The stronger the concrete, the more the hammer rebounds. BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2 11

15 Learning Task 1 Competency G-9 Procedures for Tilting Up Panels A qualified person must supervise the entire lifting process. The qualified person does not have to be an engineer but they must be experienced in the lifting of tilt-up panels and capable of working with the engineering principals of tilt-up construction. Lifting of the panels must follow the detailed instructions set out in the erection drawings prepared by the registered professional engineer. Lifting Hardware Each component of the lifting hardware must be listed in the erection drawings for the tilt-up project. The use of alternate components is only allowed if approved by the engineer. Figure 9. Lifting brackets and plates for tilt-up panels Wall Braces Wall braces (Figure 10) extend from the supporting slab to near the top of erected panels. They are secured at both ends with bolts. The wall braces are supported near their midpoint by knee braces. The wall braces should be attached to the floor slab with bolts and inserts. Drilled inserts are most often used. A safety factor of 4 is always used with drilled anchors. 12 BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2

16 Competency G-9 Learning Task 1 Figure 10. Bracing wall panels The position of the attachment to the slab should be such that the brace will end up square to the wall as shown in Figure 11. Figure 11. Wall braces laced together BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2 13

17 Learning Task 1 Competency G-9 Lacing The wall braces are connected together with lacing, shown in Figure 11. The lacing is continuous from one end of the building to the other and is attached to the braces with brackets like the one shown in Figure 12. Adjustment The braces are adjusted for length using the screw jacks at the base (Figure 13). All locking pins and shackles should be wired in place to prevent accidental dislodgement. Figure 12. Lacing bracket Figure 13. Adjustable base Openings Strongbacks are added to panels with openings in them to support the narrow pieces of the panels during the lifting process. Prior to lifting, the strongbacks are bolted to inserts cast into the wall panels. Figure 14 shows strongbacks at the sides of a door opening. The strongbacks will be removed after the final bracing is in place. 14 BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2

18 Competency G-9 Learning Task 1 Rigging The rigging for the lifting of the panels is detailed in the erection drawings. Most rigging systems use a large spreader bar that is equipped with pulleys that allow the sling lengths to self-adjust as the panel is tilted up from the floor slab. Figure 15 shows a typical rigging system for a small panel. The wall braces are attached to the panel before it is lifted. Rigging systems for larger panels use six and eight attachments to the wall panel. All systems use pulleys to allow each leg of the rigging to automatically support an equal amount of the load. The connections of the rigging to the panel can often be disconnected from the panel from ground level. This eliminates the need for a worker to climb a ladder and manually unbolt the connection. Pulling a rope attached to the release mechanism releases the attachment. Figure 14. Strongbacks Figure 15. Lifting hardware BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2 15

19 Learning Task 1 Competency G-9 Safety Meeting Before the lifting begins, the supervisor should hold a safety meeting to define the responsibilities of each person in the lifting crew. Every person on the lifting crew must attend the meeting. At the meeting the supervisor should: 1. Instruct all workers to never stand: under a panel that is being lifted between the crane and the panel on the blind side of the panel while the crane is transporting it 2. Identify the rigging foreman, and ensure that the foreman and the crane operator know all the hand signals they will be using. 3. Instruct all workers on the lifting crew that the only person who should signal the crane operator is the rigging foreman. 4. Provide the crane operator with the erection schedule showing the weight of each panel to be lifted. 5. Demonstrate the correct procedures used to rig the panels. 6. Demonstrate the bracing process including the types of bolts and tools to use. 7. Instruct workers to never use their hands to reach under a panel to adjust a shim or bearing plate. 8. Prepare a checklist that describes all of the safety precautions and procedures used for that specific job and have each worker on the lifting crew sign the checklist. Lifting Panels Make sure that concrete has reached the design lifting strength before beginning the lifting process. The stresses caused by the lifting process are much greater than the stresses of supporting the building s weight. 1. Continually check the wind speed and stop lifting if the wind gusts to over 55km/h (35 mph). If the wind is gusting to over 100km/h (60 mph), evacuate the job site because the wind speed may be reaching the point that even the wall panels that are fully braced could blow down. 2. While lifting panels, clear all workers away from the lifting area. 16 BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2

20 Competency G-9 Learning Task 1 3. Have the crane s outriggers fully extended and blocked. 4. Inspect the rigging system before connecting the slings to the lifting inserts. The rigging must be properly aligned and free of any twists. The rigging configuration must match that shown on the erection drawings. 5. Use wooden wedges and pry bars to assist in breaking the bond while the crane pulls the panel with just enough force to lift it without jerking. 6. Position the panel and install braces. Do not use bent or damaged braces. 7. Never release the crane if the bracing does not appear adequate. Install knee braces and lacing (lateral bracing) as required by the erection drawings. 8. At the end of each day, check each brace attachment. Keep a log of the inspections of the panel bracing. Place concrete at the base of all erected panels each day to stabilize the bottom of the panels. Installing the Roof The roofing system forms the permanent bracing of the panels at the top. A steel ledger, used to support the roof trusses, is bolted to the top of the wall panel as shown in Figure 16. CONTINUOUS STEEL ANGLE IRON BOLTED AT 600mm O.C. TO ALL WALL PANELS Figure 16. Roof truss attachment to the tilt-up wall panels The roof construction forms a huge diaphragm (horizontal shear surface) that braces all of the walls together. The tops of the walls are finished with a cap flashing, the flashing continues onto the roof deck. BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2 17

21 Learning Task 1 Competency G-9 Vertical Wall Joints The joints between the wall panels are normally caulked water tight as shown in the plan view of a wall panel connection (Figure 17). A rubber insert can be used in place of the backer rod and caulking. The rubber insert comes in rolls and is simply pressed into the space between the wall panels. EXTERIOR BACKER ROD AND CAULKING Figure 17. Finishing the vertical panel joints Pilaster Forms Pilasters are used in multi-storey buildings to support the vertical loads of overhead beams. The pilasters are formed using pilaster clamps shown in Figure 18. Figure 18. Pilaster clamps Now complete Learning Task 1 Self-Test. 18 BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2

22 Competency G-9 Learning Task 1 Learning Task 1 Self-Test 1. Explain the tilt-up construction process. Draw a sketch showing the basic steps in the construction process. 2. Why is the floor slab required to be very strong? 3. Why should the floor slab be very flat? 4. List four types of buildings that are built using tilt-up construction. 5. Where is infill concrete placed? 6. Describe the formwork that is used for the tilt-up wall panels. 7. How are newly erected tilt-up panels usually held in position? 8. What is the purpose of the grout pads? 9. Describe how the base of the tilt-up wall panels is held in place. 10. List two differences between a normal floor slab and a floor slab for a tilt-up building. 11. Which surface of the finished wall will have the lifting inserts exposed? 12. What is a rebound hammer? 13. Sketch the configuration of the rigging used to lift tilt-up wall panels. 14. Describe what is covered in the pre-lift safety meeting. 15. Describe the lifting of a wall panel. 16. When are pilasters used to join wall panels? 17. What holds the wall panels permanently in place at the top of the panel? 18. What are the wall braces connected to on the wall panels? BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2 19

23 Learning Task 2 Competency G-9 Learning Task 2 Describe Pre-cast Concrete Construction with pre-cast members requires careful planning so that components are installed correctly and without damage. Building with pre-cast concrete units is like stacking blocks of wood, one unit rests upon the other. Like a pile of wooden blocks, the concrete units will fall down if not properly connected and braced. Moving and Placing Moving pre-cast members, which may be quite delicate, requires careful rigging and lifting by ground workers and crane operators. Cradles, strong-backs, and nylon slings are often used. The slings are connected to inserts or lifting loops cast into the units. The lifting ring or loop is cut off once the pre-cast components are set in place. Permanent fastening of components is made by bolts, grout or welding. Temporary Bracing The temporary bracing of pre-cast components must be completely installed before the crane releases the tension from the hoisting lines. All temporary bracing should be inspected prior to leaving the job each day to ensure that it is still securely attached and supporting the members adequately. Temporary bracing must be designed to withstand the wind pressures from winds in excess of 100km/h. Order of Assembly One advantage of pre-cast buildings is that they can be erected very quickly. The engineer prepares detailed erection drawings, which described the erection procedures. The engineer must approve any changes to the erection procedure prior to implementing the changes. The assembly sequence for structures with pre-cast units varies with the size and availability of lifting equipment. Once the materials are ready, the crane should be continuously on site throughout assembly of the building. Columns are sometimes placed before rough floor slabs are cast, but usually footings and floor slabs are in place before columns are erected. 20 BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2

24 Competency G-9 Learning Task 2 Girders that are supported by the columns are installed next. Smaller beams that intersect with the girders are installed against them. All of these elements are connected and braced while the crane holds them in place. The temporary connections may be made with bolts or welds and the bracing is usually done with guy wires. After girders and beams are in place, wall panels may be set up then pre-cast floor and roof slabs may be installed. Interior concrete stairs, dividers, mezzanines, ramps or other items are lifted into place before roof or floor assemblies block crane access. Connection Methods The most common connection between pre-cast units is made with welding. Steel plates are cast into the members and then these plates are welded together after the member is in place. Pre-cast members are designed to rest on top of one another with gravity holding them in place as well as the welded connection. The shear walls provide permanent lateral bracing. The shear walls may be pre-cast or cast-in-place. Guy wires provide temporary lateral bracing. Columns The columns have steel anchor plates cast into them and are connected to their supports with bolts. The bolted connection allows the elevation of the column to be adjusted during installation. Anchor bolts are cast into the supports for the column. During installation, a nut is placed on each anchor bolt and adjusted to the approximate height for the column. The gap under the base of the column is filled with expansive grout after the erection is complete (Figure 1). ANCHOR PLATE The column is set and braced. The height of the column can be adjusted by turning the nuts under the anchor plates. When the height is set, the top nut is tightened and the grout placed. Figure 1. GROUT BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2 21

25 Learning Task 2 Competency G-9 Girders, Beams and Joists The girders, or beams are welded to the columns. Steel welding plates are cast into both members and welded together. An alternative method is shown in Figure 2. In this case, an additional steel angle is used to make the connection. This allows the members to be positioned slightly apart with the steel angle making up for the space. STEEL ANGLE WELDED TO STEEL INSERTS STEEL WELDING PLATES CAST INTO CONCRETE MEMBERS Figure 2. Beam to column connection Dowels are often used to align the members but welding the steel plates together makes the final connection. The horizontal members are always supported directly on a corbel or ledge in the column or girder that supports them The ends of single and double tee joists (Figure 3) are supported on ledges formed into the sides of girders or beams. They are held together by welding steel plates to anchors cast into the joists and by a topping slab. SINGLE TEE DOUBLE TEE Figure 3. Single and double tee joists 22 BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2

26 Competency G-9 Learning Task 2 Slabs Cored slab panels (Figure 4) are often used to build floors and roof structures. These thin pre-cast slabs can support heavy loads and span long distances. CORED SLAB Figure 4. Pre-cast cored slab The slabs are placed on high-density plastic bearing strips that level out the support for the slab. Once in place, a topping slab is cast, which ties the slab panels together. The reinforcing in the topping slab holds the panels in place (Figure 5). STEEL DOWELS EXTENDED INTO REINFORCED CONCRETE TOPPING SLAB HOLLOW CORE SLAB HIGH DENSITY PLASTIC BEARING STRIP STIRRUPS EXTEND INTO REINFORCED CORBEL CONCRETE TOPPING SLAB PRE-CAST CONCRETE BEAM Figure 5. Installing cored slabs BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2 23

27 Learning Task 2 Competency G-9 Curtain Walls Curtain wall panels carry no loads other than their own weight. They are used to close in the outside of the building envelope. These wall panels can be used with a pre-cast structural frame or with a cast-in-place frame. The installation is done by hanging them onto the structural frame with gravity connections and then welding or bolting them in place. If a bolted connection is used, inserts are cast into the panel to hold the bolt. Finishing Between Pre-cast Units Grouting usually finishes the connection between structural members. The grout provides a continuous bearing surface for vertical connections and stiffens horizontal connections. Installing backer rod and caulking finishes exterior pre-cast units. The backer rod is inserted before the caulking, to support the caulking and assist in sealing the joint. Roof and Floor Concreting Single or double tee joists, or cored slabs form the structural components of the roof or floor slab. A topping slab is added to provide roofs and floors with a level and smooth finished surface. The topping slab is between 30 mm and 150 mm thick and usually contains wire mesh reinforcement. For roof slabs, the slab may be made with low density insulating concrete. 24 BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2

28 Competency G-9 Learning Task 2 Methods of Forming Pre-cast Members Pre-cast concrete members are cast in the controlled environment of a pre-casting plant. The advantages of using pre-cast concrete are many: higher concrete strengths are easily obtained efficient use of formwork quality of the finished product is easily controlled complex shapes are possible The pre-cast plant uses a steam curing system, called autoclaving, which allows rapid strength gain in the concrete. The quick curing of the concrete allows the forms to be recycled rapidly. The speed of production of pre-cast concrete members keeps the cost per unit down. The formwork for pre-casting concrete members is usually made from steel. Steel is used because of its strength and durability. Pre-casting forms are reused thousands of times. They must be strong to resist being crane lifted and shaken with external vibrators. Pre-cast Units Many commercial buildings are built using pre-cast concrete units. These members are usually built in pre-cast plants where high production formwork and steam curing (autoclaving) allow for very efficient manufacture of the units. CORED SLAB SINGLE TEE DOUBLE TEE INVERTED TEE BEAM Figure 6. Pre-cast concrete units BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2 25

29 Learning Task 2 Competency G-9 Attachment Details for Pre-cast Units The assembly and attachment procedure for the pre-cast units must be carefully designed to ensure a safe and efficient installation. The hollow core slab shown in Figure 7 is attached to the cast-in-place concrete wall by casting dowels into the reinforced concrete topping slab. The plastic bearing strip is used. to shim the slab units to the correct elevation, it will also make up for slight imperfections in the corbel. STEEL DOWELS EXTENDED INTO REINFORCED CONCRETE TOPPING SLAB HOLLOW CORE SLAB HIGH DENSITY PLASTIC BEARING STRIP STIRRUPS EXTEND INTO REINFORCED CORBEL CONCRETE TOPPING SLAB PRE-CAST CONCRETE BEAM Figure 7. STEEL ANGLE WELDED TO STEEL INSERTS STEEL WELDING PLATES CAST INTO CONCRETE MEMBERS Figure 8. Welding pre-cast concrete units together Pre-cast members can also be attached to each other and to cast-in-place concrete by welding a steel angle iron between two welding plates that are cast in the concrete. 26 BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2

30 Competency G-9 Learning Task 2 Stairs The formwork for pre-cast stairs is usually built upside down with the formed surfaces providing the finish to the treads and the risers (Figure 9). The reinforcing steel for a set of pre-cast stairs is installed during the placement of the concrete. The unit rise and unit run are not variable. The landings have to be set to the correct height to ensure that the top unit of rise is the same as all of the others in the flights. Figure 9. Steel stair form with reinforcing Vaults Vaults or chambers are used for making electrical or mechanical connections. The vault is usually buried outside the building. A typical vault is rectangular in shape and has access holes for pipes and conduits. The construction of pre-cast vaults is usually done in sections of various sizes. The lifting equipment that is used to deliver and set the components usually controls the size of the sections. Figure 10 shows two typical configurations. THREE PIECE TWO PIECE Figure 10. Pre-cast concrete vaults BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2 27

31 Learning Task 2 Competency G-9 The various components are joined with cement mortar or caulking compounds. The base for the heavy concrete components must be well-compacted gravel. The connection between components is often made with a fitted joint. Figure 11 shows a typical joint for the two-piece style of vault. The access holes for the conduit and piping are formed as knock out holes. Figure 11 shows the weakened section of the wall of the vault. KNOCK OUT MATCHED JOINTS Figure 11. Knock out holes and joints Now complete Learning Task 2 Self-Test. 28 BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2

32 Competency G-9 Learning Task 2 Learning Task 2 Self-Test 1. How are cored slabs connected to the beams that support them? 2. What type of grout is used under pre-cast columns? 3. How are steel angles used to connect pre-cast members? 4. How are topping slabs reinforced? 5. How are pre-cast columns set on their bases? 6. Describe the order of assembly of pre-cast building components. 7. What is a curtain wall? 8. What are vaults used for? 9. How are pre-cast stairs formed? BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2 29

33 Learning Task 3 Competency G-9 Learning Task 3 Describe Pre-stressed Concrete Pre-stressed concrete members are used when heavy loads must be carried or when long spans are desired. Pre-stressing Concrete Pre-stressing the concrete, means applying stress to the concrete member before any load has been applied to it. Pre-stressing does two things; it provides up-lift from the pre-stressing tendon to counter the gravitational loads applied to the member, and it removes any tendency for the reinforcing to stretch as the loads are applied to the member. Figure 1. Stresses and anchor points in a pre-stressed beam As the pre-stressing tendon in Figure 1 is tensioned, or stretched, into a straight line, it forces the concrete member upward. As pre-stressing forces are applied to the concrete members the centre of the member will often lift upward away from the formwork. Loads Gravitational loads are: the weight of the materials, the weight of the occupants of the building, and the snow and rain loads. Pre-stressing can also be used to resist other loads such as wind loads, water pressure and earthquake loads. Stretch Concrete beams reinforced with traditional steel reinforcing, consisting of deformed steel bars, will sag over time. When the concrete member is put into service and is loaded with the designed live load (vehicle traffic if the member is supporting a highway overpass or snow if the member is a roof beam) it will sag as the reinforcement stretches slightly. 30 BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2

34 Competency G-9 Learning Task 3 The actual steel reinforcement will stretch slightly, and the bond between the steel and the concrete will tend to slip as well. During the pre-stressing process, the stretch of the reinforcement is eliminated, and the pre-stressing tendons are continuous with no overlaps to slip. Tension Concrete can resist ten times more stress in compression than in tension. Concrete beams are reinforced with steel to resist the tensile forces. Traditional steel reinforcement is placed into the forms and the concrete bonds to the deformations in the steel. The bond of the concrete to the steel transfers the tension loads on the concrete to the steel. LOAD LOAD LOAD LOAD PLAIN CONCRETE Figure 2. Plain concrete As a load is applied to the un-reinforced concrete beam, shown in Figure 2, the beam will deflect and will soon fail and fall to the ground. LOAD LOAD LOAD LOAD Figure 3. Reinforced concrete REINFORCED CONCRETE As a load is applied to the reinforced concrete beam, shown in Figure 3, the beam will deflect but will be able to support the designed live load. The prestressing tendon placed in the concrete beam shown in Figure 4 is not yet tensioned. BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2 31

35 Learning Task 3 Competency G-9 TENDON ANCHORS PRE-STRESSING TENDONS IN PLACE BUT NOT TENSIONED Figure 4. Pre-stressed concrete The tensioning jacks pull the tendon out of the concrete member. The base of the jack pushes against the tendon-anchors. The tension in the pre-stressing tendon pulls against the anchor at the opposite end of the beam compressing the beam. TENDON ANCHORS PRE-STRESSING TENDONS TENSIONED Figure 5. The effect of pre-stressing a concrete beam The demonstration of pre-stressing shown in Figure 5 is an exaggeration, but the principal is accurate. The concrete resists the tension in the pre-stressing tendons. This resistance results in the concrete being compressed. The compression forces the concrete beam to bow upward. During the pre-stressing of the tendons, the tendons are fully stretched so when the load is applied the beam will not deflect as much as a beam that was reinforced with traditional reinforcing steel. Pre-stressed concrete beams and slabs can be reduced in cross sectional size, and can span further than beams and slabs made using traditional reinforcing. Pre-tensioning vs. Post-tensioning Pre-tensioning Pre-stressed concrete is pre-tensioned if the tension is applied to the pre-stressing tendons before the concrete is placed and cured. Post-tensioning Pre-stressed concrete is post-tensioned if the tension is applied to the pre-stressing tendons after the concrete is placed and cured. 32 BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2

36 Competency G-9 Learning Task 3 Methods of Pre-tensioning Pre-stressed Members In pre-tensioned concrete members, the tensioning of the pre-stressing tendons is done before the concrete is placed in the formwork. Once the concrete has cured to the design strength, the tendons are released from the abutments and the tensile force is transferred from the abutments to the concrete. Tensions in the pre-stressing tendons are great. The abutments that support the tensioning jacks must be solidly built to support the forces until the concrete has gained enough strength to carry the load (Figure 6). Figure 6. Tie-downs for pre-tensioned concrete The formwork for the pre-cast member is built between the tensioning abutments. The pre-stressing tendons are draped into the formwork. Hanging the tendons in a curve promotes the uplift of the concrete member when the tension is released onto the concrete. The shape of the curve of the pre-stressing tendons is carefully dimensioned on the shop drawings for each concrete beam. The pre-stressing tendons are held down during the tensioning process. The tiedown locations are also described in the shop drawings. Tensioning Process Hydraulic jacks are used to pull the pre-stressing tendons. The jacks pull the tendon against the concrete abutments on the casting bed. The tension in the tendon pulls against the opposite abutment, as show in Figure 7. DEAD END HYDRAULIC JACK Figure 7. Tension from the tendon is supported by the abutments BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2 33

37 Learning Task 3 Competency G-9 The tensioning abutments must be very strong to resist the stresses of the tensioning. The tensioning is done in small increments because the hydraulic jack can only pull the tendon a short distance. This requires a wedge-like device to hold the tendon against the abutment so that the tension is not released when the jack is let go. There could be hundreds of tendons to tension and they are tensioned in groups or individually. After the tensioning process is complete, the concrete is placed into the forms. The concrete is steam cured until the design strength is reached then the tiedowns and tendons can be released. The tie-downs must be released first to allow the beam to be free to move when the tendons are released. Upon release of the tendons, the concrete member will shrink in length slightly and the bottom will lift off of the casting bed. Methods Of Post-tensioning Pre-stressed Members In post-tensioning, tendons are tensioned after the concrete is cast and cured to the design strength. The tendons for post-tensioning differ from those for pretensioning. They are encased in a sleeve that allows the tendon to be pulled through the concrete. Safety The high stresses used in pre-stressing concrete create a significant hazard. During the stressing of the tendons there is the possibility of a blowout. Blowouts happen if the concrete is not strong enough to resist the stressing forces, or if the prestressing tendon was improperly positioned in the concrete. Pre-stressed concrete construction is designed by an engineer. The engineering drawings and stressing schedules must be followed very carefully. The following are a few precautions to be taken when working around pre-stressing operations. The tendons must be protected against damage during all phases of the construction process. A slight knick in a tendon can cause it to fail. Workers not involved with the tensioning must be kept clear of the danger area. Signs and audible signals should be used to warn workers of the danger areas. During pre-stressing, guards must be positioned to protect workers from flying materials if the tendon was to break. All blowouts that occur must be reported to the structural design engineer and investigated. A log of all blowouts must be kept for future reference. Handle pre-stressing tendons carefully their strength capability makes them stiff and likely to spring loose and injure those handling them. 34 BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2

38 Competency G-9 Learning Task 3 Renovations Drilling a hole in a pre-stressed slab, beam, or girder is not a simple operation. If a prestressing tendon is damaged it may break releasing over 100,000 lbs of stress. This release of stress is like a bomb exploding and the broken pieces will fly apart. WARNING! Before drilling, cutting or fastening to a concrete slab, determine if it is a pre-stressed slab. If a pre-stressed slab needs to be worked on, obtain written procedures from a registered professional engineer before continuing. Flat Plate Slabs Flat plate slabs are thin slabs that are supported by columns only, there are no beams, girders or slab bands used. These slabs are post-tensioned to increase the span of the slabs between columns. Figure 8 shows the plan view of the layout of post-tensioning cables for a typical slab. A TYPICAL 12 24" COLUMN POST-TENSIONING TENDONS B C Figure 8. Post-tensioning tendons in flat plate slabs BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2 35

39 Learning Task 3 Competency G-9 The post-tensioning tendons used in flat plate slabs, take the place of the beams that would be in a slab that was supported by beams. The six tendons at gridline B, shown in Figure 8, would equate to a deep beam, the four along gridline 2 would be a shallower beam and so forth with the tendons along gridlines 1 and 3 equal to the smallest beam. Slabs of this type are often used in apartment construction where the soffit of the slab is the finished ceiling of the apartment. Not having beams in the way is a great advantage. Traditional Reinforcement Normal deformed bars are used between the post-tensioning tendons. The spacing will be very similar to the spacing used if the slab was spanning from beam to beam. Reinforcing Bolsters Reinforcing bolsters are used to position the tendons and keep them the correct distance from the slab form. The post-tensioning tendons are positioned as shown in Figure 9. POST-TENSIONING TENDON Figure 9. Elevation of the position of post-tensioning tendons Bolsters come in different heights to allow the tendon s vertical position to be set accurately. The bolsters are stapled down to the slab forms at the positions shown on the drawings. BOLSTER Figure 10. A typical bolster 36 BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2

40 Competency G-9 Learning Task 3 The width of the bolster will depend upon how many tendons are to be supported, various widths are available up to 48" wide (Figure 10). Post-tensioning Tendons The post-tensioning tendons, used for a flat plate slabs, come to the job site pre-cut to specific lengths. They are coiled and tagged to allow them to be easily identified. The tendons transfer the load of the tensioning to the concrete with a dead end at one end and an anchorage at the other end. Both the dead end and the anchorage are shown in Figure 11. DEAD END WEDGE-LIKE CLAMPS PLASTIC SLEEVE ANCHORAGE Figure 11. Dead end and anchorage A plastic sleeve covers the tendon preventing the concrete from bonding to it, and thus allowing the tendon to be pulled through the concrete. Between the plastic sleeve and the tendon there is a layer of grease to help the tendon slide. Tensioning Tendons are heavily stressed with special hydraulic jacks called stressing rams. They grip the ends of the tendons as they pull. After the required tension has been reached, the tendons are released and the wedge-like cones transfer the stress to the anchorage. When the tendons are stressed, the concrete is lifted slightly. Forms must be designed so that they do not interfere with movement of the concrete member as it is being post-tensioned. BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2 37

41 Learning Task 3 Competency G-9 Beams, Girders and Slab Bands Pre-stressed beams, girders and slab bands are post-tensioned in the same manner as flat-plate slabs. An engineer designs the entire process and a very specific stressing schedule is followed. Post-tensioning Pre-cast Members Pre-cast members that are to be post-tensioned are built with conduits in them to allow the post-tensioning tendons to be installed after they are set in their final position. The tendons are installed and anchored at one end to a permanent abutment. The tendons do not have a plastic sleeve because the conduit allows them to slide past the concrete. Tensioning jacks are used to stress each tendon, this lifts the precast members from their supports. The conduits are filled with expansive grout to complete the bond between the tendon and the pre-cast members. Now complete Learning Task 3 Self-Test. 38 BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2

42 Competency G-9 Learning Task 3 Learning Task 3 Self-Test 1. Explain the process of pre-stressing concrete beams. 2. How does pre-stressing a concrete beam affect the strength of the beam? 3. Explain the difference between pre-tensioning and post-tensioning. 4. During the tensioning process what forces are applied to the concrete beam? 5. What type of stress is reinforcing steel used to resist? 6. What is the maximum compressive force that a concrete sample can resist if it could resist a tensile force of 500lbs? 7. Give two reasons why concrete reinforced with traditional reinforcing will deflect under load. 8. What is the purpose of the tie-downs? 9. Draw a sketch showing the forces acting on the tensioning abutments after the tension has been applied to the tendons. 10. How are the tendons held against the tension force while the concrete cures? 11. What is used to tension a pre-stressing tendon? 12. Where is the design information found for pre-tensioned concrete beams? 13. Who is responsible for the design of the pre-stressing procedures? 14. What is the purpose of the plastic sleeve on a pre-stressing tendon? 15. What is the purpose of the grout when post-tensioning pre-cast members? 16. List five safety precautions used when working around a pre-stressing operation. 17. What is used to hold the stressed tendon in place? 18. Explain how post-tensioning supports a flat plate slab. 19. Explain the following terms: a. Pre-stressing b. Pre-tensioning c. Post-tensioning d. Pre-cast 20. Explain how post-tensioned pre-cast members are used. BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2 39

43 Learning Task 4 Competency G-9 Learning Task 4 Seal Joints There is a caulking compound for every purpose. They come in two sizes of tubes and are used with a caulking gun. Caulking compounds can also be purchased in bulk form and loaded into the chamber of special guns. Figure 1. Caulking guns WARNING! Some caulking compounds are hazardous to your health, so protection such as goggles, gloves and respirators must be worn when using them. Caulking Compounds Many caulking compounds are available for caulking and sealing concrete. Some have a water base (latex caulking) and air-dry to a hardened state; others use a solvent as a base (butyl rubber caulking) and dry very slowly. Other caulking compounds use a two part system consisting of a base and a catalyst. Once mixed, the chemicals harden and cure by chemical reaction and can be used underwater or in other areas where they will not be exposed to the air. 40 BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2

44 Competency G-9 Learning Task 4 Types of Caulking Compounds Acrylic Base Acrylic-base caulking cures by evaporation of the solvent, and it must be applied in air temperatures between 4 and 27 C. It is used to seal wood, glass, concrete, ceramic tile and aluminum. It can be applied over a damp surface, but it is not suitable for continuous immersion in water. The finished caulking can be painted. This is a low-odor, nonflammable compound, which can be cleaned up with water while the caulking is wet or cleaned with solvents after it has cured. Polysulfide Base (Rubber Base) Polysulfide caulking is used to seal pre-cast tilt-up panels, pre-cast facings and curtain walls. It is considered to be a general building caulking. Polysulfide is a rubber-based sealant that comes as a one or two-part, chemically curing caulking. All surfaces must be thoroughly clean and dust-free. A special primer is used to prepare the surfaces for the caulking. The caulking should not be applied to wet or frosty surfaces. Safety Precautions for using Polysulfide caulking: 1. Avoid prolonged or repeated contact with the skin, breathing the vapors or ingestion. Use adequate ventilation or air-supplied respirators during applications, and wear gas-tight goggles and gloves. 2. Obtain immediate medical attention if the material is ingested. If the primer or caulking comes in contact with the eyes or skin, wash it off immediately with plenty of water. 3. Wash hands and face before eating. 4. Keep this sealant away from heat, sparks, and open flames; it contains flammable and volatile solvents. 5. Keep out of the reach of children. BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2 41

45 Learning Task 4 Competency G-9 Butyl Base Formulated from a blend of butyl rubbers, this caulking provides a soft flexible seal with minimum shrinkage. It is used for sealing the gap between the exterior finish and window and door frames. This caulking is available in many colours and can be painted. Mineral spirits are used to clean tools and caulking gun smears. Polyurethane Base This one-component joint sealant with a polyurethane base is designed to resist moisture and withstand movement, shear and deflection. A high-performance, moisture-curing sealant, it is used where considerable movement is expected. It is not recommended for horizontal joints in floors and decks, or for joints that will be submerged in water for prolonged periods of time. It should not be used with polystyrene. Immediately remove all excess sealant and smears adjacent to the joints as work progresses. Use solvents, recommended by the manufacture, for clean-up. Safety Precautions for the use of polyurethane caulking: 1. Do not use in confined areas without proper ventilation. Avoid inhaling vapors. Contains ignitable solvents. Keep away from an open flame or high heat. 2. If inhaled over a prolonged period, move to fresh air and avoid further overexposure. No special treatment is required. 3. Wash hands and face before eating. 4. Guard against ingestion and contact with skin and eyes. 5. If this sealant comes in contact with your skin, wash with a hand cleaner, then with soap and water. If it gets in your eyes, flush them immediately with running water for 15 minutes and get medical attention. If ingested, call a physician immediately. 42 BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2

46 Competency G-9 Learning Task 4 Silicone-base Silicone-base caulking is a one-part, silicone-rubber sealant. It works well with nonporous materials such as glass, aluminum and ceramic tile. Silicone base caulking cannot be painted. Surfaces should be clean and dry before applying the caulking. It needs atmospheric moisture for curing. Prior to applying caulking, clean surfaces with vinegar. Masking the sides of the bead before applying will simplify clean up. Remove the tape immediately after tooling. Safety precautions for the use of silicone caulking: 1. Avoid repeated or prolonged contact with the skin. 2. If uncured sealant comes in contact with the eyes, immediately flush them with plenty of water. 3. Use goggles when working overhead. Industrial Caulking Buildings constructed from pre-cast components, such as slabs-to-wall, and tilt-up panels, will require many litres of high quality caulking to be placed. The caulking specified for each job is chosen to meet the needs of the building. Industrial buildings that will house and/or use chemicals will require a caulking that is resistant to those chemicals. Tilt-up buildings need a caulking that is waterproof and highly flexible (Figure 2). They also should be UV protected and have a long lifespan. Because of their special properties, these caulks tend to be expensive. Figure 2. Caulking between tilt-up panels BC CARPENTER APPRENTICESHIP PROGRAM LEVEL 2 43