"Tilt Up Constructions"
Seminar Report 2007-08 ABSTRACT The paper deals with the salient features of tilt –up construction and the delicate ways by which it can be applied to the present construction scenario. It describes the potential of tilt - up concrete construction technique as a viable alternative to conventional methods of construction under favorite conditions. It also establishes the fact that tilt- up (also called tilt wall) concrete construction is finding its way among its competitors in the fiercely competitive field of construction methods. This highly efficient, practical method of construction makes beautiful buildings possible at costs that rival the most utilitarian industrial building. Tilt-up contractors are innovative, and continually work to improve Tilt-up techniques. Today, there are many contractors are experts in this specialized construction. Design-build construction scheduling further enhances the Tilt-Up concept! The use of computers in the preplanning and layout process provides the opportunity to make innovative improvements before the design is finalized. Dept. of Civil Engineering i Govt. College of Engg., Kannur Seminar Report 2007-08 CONTENTS 1. INTRODUCTION 1 2. EVOLUTION OF TILT-UP 2 3. CONSTRUCTION PROCEDURE 3 4. CONSTRUCTION DETAILS 6 4.1 FOOTING AND FLOOR SLABS 6 4.2 TILT-UP PANELS 8 4.3 REINFORCEMENT 13 4.4 EMBEDS AND INSERTS 15 4.5 CONCRETE PLACEMENT 15 4.6 PANEL ERECTION 16 4.7 PANEL FINISHING 19 4.8 INSULATING THE PANELS 20 5. FIRE RESISTANCE OF TILT UP PANEL 21 6. ADVANTAGES OF TILT-UP 22 7. INTEGRALLY COLOURED TILT-UP CONCRETE 23 8. STEEL AND TILT-UP TOGETHER: USING THE STRENGTHS OF EACH 24 9. PRECAST AND TILT-UP CONCRETE 24 10. SITE CONDITIONS THAT LIMIT TILT-UP 25 11. CONCLUSION 26 REFERENCES 27 Dept. of Civil Engineering ii Govt. College of Engg., Kannur Seminar Report 2007-08 1. INTRODUCTION Tilt up concrete construction, which began in southern California in the late 1950‟s as an economical and fast way to construct concrete walls for ware houses, has become today a multi-billion dollar industry, accounting for over 10,000 buildings annually. It is now used for shopping centers, distribution facilities, ware house, manufacturing plants, office buildings, prisons, schools, churches, and nearly every type of one to four-storey building. According to a survey by the tilt wall concrete association, over 300 million square feet of tilt wall buildings were constructed in 1999. The term tilt wall was coined in the late 1950‟s to describe a method for constructing concrete walls rapidly and economically without the formwork necessary for poured-in-place walls. Simply, it is a two-step process: First, slabs of concrete, which will comprise sections of wall, are cast horizontally on the building floor slab, or separate casting slab. Then, after attaining proper strength, they are lifted (tilted) with a crane and set on prepared foundations to form the exterior walls. When they have attained sufficient strength, usually a week to 10 days, a mobile truck crane is brought to the job site to lift and set them on prepared foundations. The erected panels are temporarily braced, connected, and the joints between them caulked. The roof structure is then constructed and attached to the walls to complete the building shell. Techniques continue to improve as new innovations and research evolve, furthering the advantages and economics of tilt-up, and as information about tilt-up is more widely available, tilt-up is attracting worldwide attention and activity. For architectural attractiveness, the architect has almost unlimited freedom to arrange and assemble the panels, plus a wide choice of surface finishes, to achieve what are often award-winning designs. Construction time for a tilt-up building, from completion of the floor slab to completion of the building shell, is often less than four weeks. Dept. of Civil Engineering 1 Govt. College of Engg., Kannur Seminar Report 2007-08 2. EVOLUTION OF TILT –UP CONSTRUCTION Tilt wall is a simple concept. Its origin may have been 2,000 years ago when some anonymous roman builder discovered it would be easier to cast a slab of concrete on the ground then hoist (tilt) it up into position, than to build two wood forms, place concrete between them, and then remove the forms. Tilt wall, as we know it today, evolved from such early innovations, and undoubtedly during the history of concrete construction has had many applications. However its feasibility for constructing large thin wall panels had to await reinforced concrete, which came into use in the early1900‟s. Credit for one of the first reinforced concrete site-cast walls that was cast horizontally then tilted up, goes, according to the archives of the Portland Cement Association, to an innovative builder, Robert Aiken. In about 1908 he constructed several buildings using lifting jacks or derricks to raise the panels. One of the buildings was a two-storey warehouse at Camp Logan, Illinois, another at Camp Perry, Ohio. He also reportedly constructed a Methodist Church in Zion. In Chicago, in about 1912, a four- storey factory was reportedly constructed using a derrick to raise the concrete walls. But the popularization of tilt wall had to wait for the one essential element necessary to turn it into a fast, economical production method: the mobile truck crane. These became available in the late 1940‟s, at the close of World War II. Another essential for on-site construction was availability of ready-mixed concrete, introduced a few years earlier. With the availability of truck cranes and ready-mixed concrete, a third fortuitous condition was present: a post-WWII pent-up demand for new construction, including warehouses and manufacturing plants. These were ideally suited for tilt wall, with long, nearly windowless walls, and repetitive panels. Some anonymous contractor (or contractors) cautiously tried the method, it worked, and the tilt wall industry was born. Over the years innovative architects, engineers, contractors, and hardware vendors, have contributed numerous refinements in design and construction methods, resulting not only in award winning architectural designs, but improved design and construction methods, making possible very tall panels (forty feet high is not uncommon), Dept. of Civil Engineering 2 Govt. College of Engg., Kannur Seminar Report 2007-08 faster erection time (lifting, setting, and bracing 20-30 panels a day is commonplace, achieved through well-trained crews and innovative ground-release lift attachments), and a wide choice of finishes available for architectural attractiveness (such as patterned form liners used as a casting surface, resulting in a textured exterior surface). Tilt wall construction is most frequently used for one-storey commercial buildings such as warehouses or office buildings, though two, three and four storey office buildings are becoming commonplace. Condominiums and hotels as tall as ten stories have been constructed with tilt wall concrete. However, tilt wall concrete is no longer limited to use in industrial and commercial buildings. In 1993, tilt wall concrete panels were used to construct trickling filter tanks at a wastewater treatment plant. 3. CONSTRUCTION PROCEDURE Brief descriptions of the six steps involved in the construction of tilt wall building are as follows: - 1. First, the concrete floor slab (as shown in figure1.), which will serve as a casting base for the panels, is placed on the prepared sub grade. At least a five- inch thick slab on a well-compacted sub base is essential. The slab is held back several feet around its perimeter to allow space for setting wall panels, which will be later, filled in with concrete. At the same time, the perimeter foundations are constructed, upon which the wall panels will be set. 2. The wall construction process begins with laying out the panels on the floor and constructing the perimeter forms. Forms are typically dimensioning lumber, making the panels 0.1397m, 0.18415m or 0.23495m thick Fig. 1. Wall panel cast on floor. Dept. of Civil Engineering 3 Govt. College of Engg., Kannur Seminar Report 2007-08 3. With the edge forms in place, door and window openings are blocked out and set in place, reinforcing added (typically #4 bars at 0.45m on center horizontally and #5 bars at 0.3m on center vertically, all placed at the panel center for equal bending resistance the greater reinforcing is used vertically since most panels are designed to span vertically from floor to roof), placement of embedded items, such as for attaching the lifting cables, are put into place. 4. Concrete is placed in the forms; finished, cured, and let set for 5 – 10 days while the concrete attains sufficient strength for lifting. 5. On lifting day, cables are attached to inserts cast into the panels and the crane lifts each panel in sequence and sets it on the prepared foundation, proceeding around the building perimeter. Before releasing each panel, temporary braces are installed (at least two per panel) to brace the panel until the roof structure is attached. An experienced rigging crew can erect 30 or more panels in a day usually enough to complete a building enclosure (as shown in figure 2). Fig. 2. Crane lifts the panel. 6. Connections between panels are made (usually by welded splices of steel ledger angles), joints between panels (typically 0.01905meter) are caulked, patching is done if necessary to repair blemishes; concrete is placed in the closure strip between the panels and the floor slab (usually a two foot wide strip); and the bracing is removed when the roof has been permanently connected to the walls as shown in figure 3. Dept. of Civil Engineering 4 Govt. College of Engg., Kannur Seminar Report 2007-08 Fig. 3. Temporary bracing in place. Fig. 4. Roof structure erected. CONSTRUCTION PICTURES Fig. 5. Forming panels on Fig. 6.Panel with Fig. 7.Placing concrete in the floor slab reinforcing and ready for panels concreting Dept. of Civil Engineering 5 Govt. College of Engg., Kannur Seminar Report 2007-08 [ Fig. 8. Lifting a panel Fig. 9Panel bracing prior to Fig.10.Construction roof structure installation progressing 4. CONSTRUCTION DETAILS 4.1 FOOTINGS AND FLOOR SLABS Once planning is complete, construction can begin. Install footings as level as possible. Spread footings are used most often for tilt-up buildings, but pier footings can be considered if conditions warrant their use. Footing locations, heights, and dimensions should be checked and verified for correctness. Crane time is expensive if modifications must be made while a panel is suspended over an incorrect plate or footing. Fig. 11 floor slab Minor mistakes in floor slab (fig.11) construction often can be masked or corrected. A poor floor slab, however, can affect the appearance of the tilt-up panel. The slab should be smooth and hard and construction joints should be positioned where they will have minimum impact on panel. If the joints must fall beneath the panel, clean the joint and fill it with caulk. Floor sealing or hardening compounds must be compatible Dept. of Civil Engineering 6 Govt. College of Engg., Kannur Seminar Report 2007-08 with any chemicals or paints used on the panels or there may be problems when the panels are stripped or painted. Proper sub-grade preparation is essential. 90% compaction is minimum. The floor slab will only be as good as the sub-grade under it. The construction loads (trucks, cranes etc.) require at least 5-inch slab thickness. A thumb rule is adopted for determining the proper spacing and type of joints in the floor slab. Concrete slump does not exceed 4-inches, this helps in controlling shrinkage problems. In concrete tilt wall construction, some regional areas have seen tilt wall panels not attached to the spread footing foundation providing bearing support for the panels themselves. In these buildings, wall panels are vertically supported on foundation pads and shear reinforcement is provided only into the narrow pour-strip of the slab-on-grade. Much of Section 1915.8 specifically indicates that direct foundation connection is required for cast-in-place concrete construction. However: 1997 UBC 1918.104.22.168 exception states: “In tilt-up construction, this connection may be to an adjacent floor slab….” Consider temporary erection loads on footing. TABLE 1: CONVENTIONAL FOOTINGS (UBC TABLE 18-1-C) Number of Floors supported by the foundation Dimension 1 2 3 Below grade 0.3048m 0.4572m 0.6096m Wall thickness 0.1524m 0.2032m 0.254m Footing thickness 0.1524m 0.1778m 0.2032m Footing width 0.3048m 0.381 0.4572m Dept. of Civil Engineering 7 Govt. College of Engg., Kannur Seminar Report 2007-08 I. Where design is not provided, the above Table may be used as a minimum foundation design for stud bearing walls, unless expansive soils are present. (UBC 1806.3.) II. Width, depth and reinforcement of the footings must meet the soils report recommendations or design engineer's calculations if more restrictive than what is noted above. III. In seismic zone 4, continuous footings are required to have min. 1-#4 bar @ top & bottom. IV. In seismic zones 3 & 4, 1- #4 bars are required at top and bottom of footings with stem walls & turned-down footings with slab on-grade. (As per UBC 1806.7.1 & 1806.7.2.) V. When ground slopes more than 1 unit vertical in 10 unit horizontal, stepped footings are required. 4.2 TILT-UP PANELS 4.2.1 FORMING TILT-UP PANELS The standard practice in laying out panels is to snap a chalk line on the floor slab. Wooden, plastic, polystyrene, or metal blocks or strips can be used as forms. They should be non-corrosive and non-staining, firmly Axed in place so they will not move during placement. Woods are the most common material used for side forms. Often the panel depth is designed to fit the depth of standard dimension lumber; so 0.135m and 0.184m thick structural panels are common. The form sides can be supported and secured to the slab by a wood or steel angle support. Any common concrete anchor can be used to attach forms to the slab. Remember that holes in the slab left by anchorages must be repaired. Cant strips should be used at the juncture of the side form and slab. They reduce concrete leakage, and give a neater appearance. A bead of caulk often is added as an additional seal between the cant strips and the slab. The available floor area for casting should be at least 25% greater than the area of the wall panels; otherwise the panels will need to be cast on exterior side. Dept. of Civil Engineering 8 Govt. College of Engg., Kannur Seminar Report 2007-08 Form the door and window openings after framing the panel perimeter. Brace the interior of the openings to prevent bowing or movement. If the opening is closer than 24 inches to a panel edge, a strong back may be needed for additional support during panel lifting and placement. Fig. 12. workers place brackets to outline where the tilt-up panels outside boundaries, windows and door openings will be placed. Apply form release agent and bond breaker to the slab and forms. Compatibility between bond breakers, form release agents, and paints or coatings used on the panels is critical. Check compatibility by consulting with the product suppliers. Typical thickness of wall is 0.1397m and four-hour firewalls are 0.165m thick. Fig. 13.Workers build the tilt-up panel forms out of wood connected to the braces. The roof structure holds the tilt-up panels in place. It acts as a diaphragm to horizontally support the wall at the top, and the curb on the footing supports it laterally at the bottom. The panels are generally not connected together to allow for expansion and Dept. of Civil Engineering 9 Govt. College of Engg., Kannur Seminar Report 2007-08 contraction without cracking. The panels are only positively connected to the roof at their centers near the top. Avoid „L‟ and „T‟ shaped panels because they are difficult to erect and usually requires strong backs. Keep the ratio of height to width of solid panel between openings to less than 4:1. For greater ratios, the code requires additional reinforcing requirements. . When bottoms of panels must be stepped (such as for sloping grades), the step upward should not be more than about one-third width from a panel edge. Since panels are lifted by tilting along their lower edge, this edge should be at least two-thirds of the panel width. 4.2.2 NUMBER OR LOCATION OF OPENINGS There is no limit to the number of openings, but their location can be critical. Openings closer than the minimum from the end of the panel supporting a concentrated roof load can add considerable reinforcing thickened concrete or steel columns. The minimum distance is one-eight the eave height, or two feet, whichever is less. . Keep door and window openings at least 18" from the edges of panels and a similar minimum distance between openings. If less, there can be structural problems. 4.2.3 ANALYZING A PANEL It is conservative and perhaps most common to design multi-storey tilt-up panels as simple spans between supports while accumulating axial loads from the storey above. However the lateral wind and seismic forces transferred from the panels to the floors and roof should be calculated using a continuous span. A simple span assumption will result in an unconservative calculated connection force. The engineer should also make sure not to forget to add the connection force due to thermal bowing restraint at the floor connections. When designing multi-storey panels one must also consider the design of the panel for loads during construction. Typically when a panel is on braces, the span of the panel to the brace point is probably considerably more than the final design span. This condition many times will control the reinforcing design of multi-storey panels. Dept. of Civil Engineering 10 Govt. College of Engg., Kannur Seminar Report 2007-08 4.2.4 SEQUENCE IN WHICH CASTING A PANEL They should be cast with the lifting sequence in mind, so that the top panel on a stack is the first to be placed and so that it is not necessary to lift and reset panels. Consideration should also be given so that a panel still on the floor will not block the setting or bracing of another. Crane reach and travel should be considered as well. The panels should be cast so that the crane can lid them with a minimum of handling, and so that the concrete ready-mix truck can reach them. If possible, it is desirable to cast them with the inside face up so that the connections are visible, lifting inserts are patched to the inside, and so the wind braces can be connected before the panels are lifted. 4.2.5 RECOMMENDED TOLERANCES FOR THE PANELS The American Concrete Institute Committee on Tilt-Up is writing a state-of-the- art paper, which uses the following tolerances: Height and width of panel up to 9 meter - +0.00635m. Each additional 3 meter increment in excess of 9 meter for height or width - + 0.003175m. Thickness of panels - + 0.00635m or – 0.003175m. Size and location of openings cast into the panel - + 0.00635m. Location of embedded connections - + 2.9m. Reinforcing steel cover – 0.00635m. 4.2.6 STACKING OF PANELS Basically, the lower panel is cast with a form that is two panels thick. The first panel is cast and cured for a day, and then the reinforcing and hardware are installed in the next panel above it. The second panel is poured and cured for a day, and then the double thickness form is moved up one thickness to form the next panel. The form is held to the panel for this purpose with various items, including bolt, J-hooks and J-snap-ties Dept. of Civil Engineering 11 Govt. College of Engg., Kannur Seminar Report 2007-08 Panels can be stacked as high as desired, since it is unlikely that the floor will be overloaded. However, it will not be possible to place concrete directly from ready-mix trucks if over about three feet high. Another consideration is the lifting techniques off of a stack of panels. When the lower panel has a large opening the hole can be blocked out in wood or filled with wet sand. A thin layer of concrete, grout or plaster is cast over it and trowel led to match the panel surface. Bond-breaker is then applied to the surface to seal it and prevent bonding of the concrete above it. 4.2.7 SURFACE TREATMENTS It is popular to impart a pattern or texture to the face of the tilt-up panels using reveal strips. Typically these are anchored to the base slab after side forms are erected, but before the reinforcements are placed. Concrete nails or other anchors often are used to fasten the strips. This surface treatment must be carefully planned and executed, but the results are striking. 4.2.8 JOINTS AND CORNERS Corners can be either overlapping or mitered. This must be detailed on the plans. When overlapped, be sure one panel is shorter to allow for the overlap. Joint should be treated for fire ratings. 2, 3 and 4 hour ratings require special sealants. Joint width should be ½ inch up to 24 feet high and ¾ inch for taller panels. Delay welding of connections between panels as long as possible, the longer the delay, the more shrinkage will take place in the panels and the less chance for shrinkage cracks at the connections. Dept. of Civil Engineering 12 Govt. College of Engg., Kannur Seminar Report 2007-08 Fig. 14. Panel joint. 4.2.9 THICKNESS Generally 5 ½ inch thick panels are used. Four hour firewalls are 6 ½ inch thick. . A rule of thumb for required panel thickness is to divide the unsupported height in feet by four, to give the required thickness in inches. For example, an unsupported height (floor slab to roof ledges) of 22 ft. would require approximately a 5.5 inch panel, round it off to the next higher form lumber size when available. 4.3. REINFORCEMENT Standard Grade 40(Fe 415) or 60(Fe 600) bars are used. The use of plastic support chairs instead of steel chairs is recommended to avoid rust on the panel face. The correct chair height to be used to maintain the proper depth of reinforcing. Minimum panel reinforcement ratio is 0.0012 for horizontal bars and 0.002 for vertical bars (UBC-‟94 1914.3.8). Panel reinforcing should be #4 or #5 bars. Additional sizes may be required for special conditions, but not larger than #7 for practical Dept. of Civil Engineering 13 Govt. College of Engg., Kannur Seminar Report 2007-08 placements. Spacing of bars cannot exceed 18 inches, per code, and should not be closer than about 10 inches, so workman can step between the bars. Add two #5 bars at the bottom of panels, with hooked or 90º bends to minimize likely hood of diagonal cracks at the lower corners of panels. Fig. 15. Rebar is placed in the completed forms. 2- #5 bars Where 2 feet min. cannot be obtained, extend as far as possible & bend 90 degree. Pad type footing. Floor line Fig. 16.Panel Reinforcing At Openings Dept. of Civil Engineering 14 Govt. College of Engg., Kannur Seminar Report 2007-08 4.4 EMBEDS AND INSERTS The next step is to install embeds and inserts. Embeds are pre fabricated steel plates with lugs that are cast into the panel to attach it to the footing, other panels, or the roof system, or for attachment of building accessories after the shell is completed. They can be attached to the side forms if they are on the panel edges, or they can be wired to the reinforcing. Inserts provide the attachment points for lifting hardware and braces. They usually are sized by the suppliers, who also furnish engineering drawings showing insert locations. Install inserts according to the manufacturer‟s recommendations. If there is a field changes in panel size, opening location, or other conditions, the insert supplier should be contacted to confirm the location and selection of hardware. 4.5 CONCRETE PLACEMENT Before placing the concrete for tilt-up panels, clean the base slab. Use compressed air to blow away dirt, leaves, and other loose debris. Also remove any standing water on the slab. Fig.17. Workers pour the concrete in to the forms Dept. of Civil Engineering 15 Govt. College of Engg., Kannur Seminar Report 2007-08 Concrete placement methods for tilt-up panels are the same as those for floor slabs. Since the panels are the structural members, make sure the concrete mix meets specifications. Direct chute placement is the most economic method, but pumping and bucket placement also work. Consolidate the concrete to ensure good flow around the reinforcing steel. A trowel finish is suitable for most projects. As with other types of concrete construction, plan for the unexpected. If the weather will be cold, have insulation blankets ready. If it looks like rain, delay the pour or have a suitable covering material available. Provisions should be made to block off a pour if a concrete truck breaks down or gets stuck in traffic. On hot or windy days, be prepared to cure the panels by water misting or by applying a suitable curing compound. Fig. 18.Use of power trowels to make sure the panels are level and smooth. Fig.19. Panels ready for erection after curing and finishing the holes. 4.6 PANEL ERECTION The erection sequence should be determined well in advance, but it is a good idea to review it immediately before panel erection. Also, thoroughly review safety procedures with all tilt-up crewmembers to help prevent accidents. Discuss crane operation, bracing and anchorage details, cable releases, and job communication. As soon as possible after Dept. of Civil Engineering 16 Govt. College of Engg., Kannur Seminar Report 2007-08 the panels are erected dry pack or stiff grout should be placed under the panels (usually a one-inch space) and vibrated so full bearing is achieved. Locate and clean inserts and embeds and attach before lifting the panels. It is much quicker and safer to do this work while the panel is flat rather than doing it on a ladder after the panel is upright. Don‟t remove braces until after the roof and decking are installed. Once the braces are removed, workers can patch holes in the floor and complete other finish work. Before erecting tilt-up concrete panels allow them to gain enough strength to with stand lifting stresses. Erecting panels before achieving the proper strength may lead to damage, costly repairs and even serious accidents. As concrete gain strength more slowly in cold weather and quickly at higher temperature, curing time alone is not an accurate method for determining in place concrete strength. Only reliable testing can produce accurate estimates of in-place concrete strength. The most common testing method makes use of field – cured cylinders. Fig. 20.Mobile crane erecting the panel to place it over the footing. Dept. of Civil Engineering 17 Govt. College of Engg., Kannur Seminar Report 2007-08 4.6.1 SAFETY PRECAUTIONS NECESSARY BEFORE AND DURING LIFTING Provide the crane company with a set of panel lifting details well before the day of lifting so that they can plan necessary rigging. Double-check that the rigging configuration and that the spreader bars and slings are the correct length. Watch for ditches, holes and overhead obstructions that might prevent the crane from moving. Have a safety meeting at the beginning of the erection to ensure that everyone knows what is expected and how to operate safely. Make certain everyone on the rigging crew knows how the lifting hardware is supposed to work. 4.6.2 BRACES Braces should be attached to the panels when they are still on the casting bed if at all possible. It speeds and simplifies the erection when the braces are raised with the panels. The braces should be removed after the roof connections and diaphragm is completed. Braces should be disconnected from the floor first and then carefully disconnected at the top and lowered to the ground. Fig. 21. After placing the panel over the footing braces are connected properly to the floor slab. Dept. of Civil Engineering 18 Govt. College of Engg., Kannur Seminar Report 2007-08 22.214.171.124 NUMBER OF BRACES Minimum two bracings are needed for aligning the panel, but large panels require more. Check with the design charts provided with the erection design for type and quantity of braces. 126.96.36.199 PRECAUTIONS NECESSARY WHEN BRACING TO THE FLOOR Be certain that the floor is of adequate thickness and strength. The inserts should not be near edges or joints, and must be placed in large enough slab sections to withstand uplift forces. 188.8.131.52 BRACING AND SHORING OF LINTEL PANELS The panel bracing and shoring should be designed by a professional engineer, and the shores should be braced against lateral movement separately from the panel. All members should have firm bearing on adequate foundations, and the connections should be fixed before releasing the panel from the crane. 4.7 PANEL FINISHING The finish of a panel is limited only by the creativity of the architect and the abilities of the contractor. Common sand blasted or exposed aggregate finishes can be done immediately after panel erection. Painting, however, must wait until partial curing has taken place and residue from the bond breaker has been removed. Most tilt-up concrete panels have an uneven or splotchy appearance when first stripped. These splotches usually fade after time. Uneven bond breaker application, standing water, slab porosity, and other factors can produce this effect. Sand blasting most of these inconsistencies. Washing also can improve appearance, but most tilt-up panels are eventually painted. Before cleaning and painting panels, caulk the joints and correct significant imperfections. The most frequently used paints are acrylic-based. Textured paints can be used for special effects. Building or striping is a popular technique to produce variety and interesting tilt up buildings. Reveals or recesses cast into the panel often are painted a contrasting or darker colour for accent. Dept. of Civil Engineering 19 Govt. College of Engg., Kannur Seminar Report 2007-08 Many tilt-up contractors are discovering that they can save time and money, in addition to increasing the architectural options they can offer customers by applying an exterior insulation finish systems (EIFS) to the exterior of concrete tilt-up panels. EIFS, sometimes called synthetic stucco, is a generic term for wall systems that cover exterior surfaces, ranging from plywood to concrete. The expanded polystyrene is applied to the surface with an adhesive and then is covered with a thin, polymer modified, cementitious base coat embedding a mesh of glass fibre in the base of reinforcement. The final step is application of an elastomeric topcoat, which is available in various colours and textures. Tilt-up contractors can create interesting architectural projections and decorative moulding by varying the size and shape of the expanded polystyrene. If the goal is to simply add color and texture to the panels the elastomeric topcoat can be applied directly to the panel surface. 4.8 INSULATING THE PANELS The most effective method of insulating Tilt-Up walls, however, is the method known as "sandwich". This method is placing a layer of insulation between a structural concrete layer and an architectural or non-structural concrete layer during the casting of the panel and then tilting this entire construction as a panel. This method is made possible by structurally connecting the two concrete layers through the insulation layer. As soon as the panel is erected to final position, the inside layer becomes structural and load bearing, while the outer concrete layer is suspended from it to allow for temperature changes without cracking. It is critical that these two layers remain independent from each other except for the connection through the insulation. The basic forming and pouring process must be modified slightly to accommodate the sandwich wall system. Higher side forms are needed to accommodate the insulation, and some systems require concrete placement on separate days. Tilt-up sandwich panels with as much as 6 inches of insulation and R- values of 30 can be built Dept. of Civil Engineering 20 Govt. College of Engg., Kannur Seminar Report 2007-08 5. FIRE RESISTANCE OF TILT UP PANEL. The fire resistance of Tilt-Up walls is easily referenced or calculated in the current UBC and IBC codes. Fire resistance is related to an R rating that determines time duration based on the thickness and type of material. The following table is reprinted from the data contained in both the UBC and IBC codes. It shows the relative thickness required for each aggregate type to meet the listed fire resistivity rating. TABLE 2 Fire Resistance of Single-Layer Concrete Walls (UBC/IBC Tables) 1 hour 11/2 hour 2 hour 3 hour 4 hour Aggregate type Minimum equivalent thickness (meters) Siliceous 0.09 0.1092 0.127 0.16 0.1778 Carbonate 0.08 0.1016 0.117 0.14 0.1676 Fig. 22. Steel joist to wall. Dept. of Civil Engineering 21 Govt. College of Engg., Kannur Seminar Report 2007-08 6. ADVANTAGES OF TILT –UP CONSTRUCTION The construction market is highly competitive, and tilt-up is chosen only when its advantages, given the local and circumstances of a project, clearly favor it. And not every building is suitable for tilt-up. To effectively and economically use tilt-up, some basic criteria should be met. The building should be at least 5,000 square feet (the larger the more economical), to allow enough room to cast panels and effective use of crane and rigging crew. There should be extensive wall surface, such that it can be divided into lift able panels and there should not be over about fifty percent openings in the panels. While one- and two-storey buildings are most economical, many construct three- and four- storeys high. When these basic conditions are met, tilt-up offers numerous advantages: 1. ECONOMY:-In areas where tilt-up design and construction expertise is available- particularly a trained crane and rigging crew tilt-up has proven to be more economical than competing construction methods for similar types of buildings. Tilt-up concrete is an economically viable method for building individually designed reinforced concrete structures. The process requires few forms and makes efficient use of modern mechanical equipment. RMC for tilt- up is locally available and special labour skills are not required. Panels are formed and cast on the jobsite, and can be quickly tilted, lifted, set in place, and braced with the aid of high capacity mobile cranes. 2. SPEED OF CONSTRUCTION: - From the time the floor slab is placed, the typical elapsed time from starting to form the panels until the building shell is completed is 4–5 weeks. 3. DURABILITY: -Materials and methods of tilt-up construction ensure durability. Early tilt-up construction, erected in the 1940's, remains in good condition today. The use of plastic support chairs instead of steel chairs is recommended to avoid rusting. 4. FIRE RESISTANCE: - Concrete is an obvious first choice for fire resistance. A 0.165 m thick wall, for example, has a four-hour fire resistive rating. Dept. of Civil Engineering 22 Govt. College of Engg., Kannur Seminar Report 2007-08 5. LOW MAINTANANCE COSTS: - Building maintenance requirements are limited to a new coat of paint every six to eight years. 6. LOWER OPERATING COSTS: - There are a number of ways to form individual panels. 7. EXPANDABILITY: - By planning for the possibility of expansion, panel connections can be designed so the panels can be detached and relocated. 8. SECURITY: -Panel thickness and strength make them virtually impenetrable. Forced entry can only occur through door and window openings. 9. LOW HEATING/COOLING COSTS: - The insulation value for tilt-up concrete often exceeds masonry and wood frame construction. Tilt-up walls can be economically insulated to give higher insulation values. 10. ARCHITECTURAL AESTHETICS: - Patterns or texture can easily be added to the face of Tilt-Up panels. 11. LOWER INSURANCE RATES: - The fire resistance of tilt-up concrete walls results in lower premiums. DISADVANTAGES 1. Lack of design flexibility. 2. Not a factory controlled production to assure quality. 3. It is a weather dependent construction. 4. For erecting highly equipped machines are needed. 7. INTEGRALLY COLOURED TILT -UP CONCRETE While most tilt-up buildings are still finished with paint or other coatings, the wave of innovation in the industry has introduced integrally colored concrete as an alternative for high-quality tilt-up projects. Integrally colored tilt-ups can benefit concrete producers in three ways Colored tilt-ups help increase the overall use of concrete in a community Dept. of Civil Engineering 23 Govt. College of Engg., Kannur Seminar Report 2007-08 By strategically promoting colored concrete tilt-up and developing a reputation for product quality, a ready-mix company can position itself to become the preferred supplier to tilt-up contractors in its community Many ready-mix producers report more profit from value-added services like colors than from the concrete itself 8. STEEL AND TILT –UP TOGETHER: USING THE STRENGTHS OF EACH A large steel office building or retail center may use tilt-up concrete panels for interior fire walls or exterior facades. The Mervyn's department store in the Dallas / Fort Worth, Texas area (fig .25) was constructed in just this manner. Fig. 24. Mervyn's building in the Dallas / Ft. Worth 9. PRECAST AND TILT -UP CONCRETE Over the last decade joint precast and tilt-up parking garage construction is probably the best-known combined application of the two systems. Concrete cast in the position it is to occupy in the finished structure is called cast in place concrete. Concrete cast and cured elsewhere is called precast concrete. Tilt-up concrete is a special type of precast concrete in which the units are tilted up and placed using cranes or other types of lifting devices. Dept. of Civil Engineering 24 Govt. College of Engg., Kannur Seminar Report 2007-08 10. SITE CONDITIONS THAT LIMIT TILT- UPCONSTRUCTION Access by the crane to the job site. Relatively flat terrain to allow the crane operation. Any power lines, ditches, railroad tracks, or other obstructions, which limit crane operation. Other buildings very close to where panels must be placed. Dept. of Civil Engineering 25 Govt. College of Engg., Kannur Seminar Report 2007-08 11. CONCLUSIONS Tilt-up construction is a fast growing construction method. In comparison to other modes of construction, it is economically and practically viable and advantageous. The advantage that it can claim is manifold, and in comparison, its drawbacks are mere and insignificant. The tilt–up construction technique is widely used and accepted in developed nations such as America. Owing to its versatility, it can be said with confidence that this technique can be applied to its full potential even in developing nations such as India. Dept. of Civil Engineering 26 Govt. College of Engg., Kannur Seminar Report 2007-08 REFERENCES 1. ACI 551-92, „Tilt-Up Concrete Structures’, American concrete institute, Detriot. 2. Brooks.H.E, „The Tilt-Up Design and Construction Manual’, second edition, 1990, H.B.A publications, Newport Beach. 3. Edward sauter.J, ‘Special Formwork for Tilt-Up’, Concrete international, October 2002. 4. Edward sauter.J, „Today’s Tilt-Up – Commingling Structural Design and Greater Aesthetics ’, Concrete international, April 2002. 5. James. R. Batty, ‘A Convenient Marriage: Tilt-Up And Precast’, Concrete international, May 2004. 6. Nick Paris and Michael Chusid, „Integrally Coloured Tilt-Up Concrete’, Concrete international, August 2000. Dept. of Civil Engineering 27 Govt. College of Engg., Kannur