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SEMINAR REPORT-2013

TILT-UP CONSTRUCTION

1. INTRODUCTION “Tilt-Up” is the technique of site-casting concrete walls or elements, normally on a horizontal surface and then tilting them vertically into place.The term “Tilt-Up” was coined in the late 1940’s to describe a method for constructing concrete walls rapidly and economically without the formwork necessary for poured-in-place walls. 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 sufficient strength, they are lifted (tilted) with a crane and set on prepared foundations to form the exterior walls. Although the method is most often called “TiltUp,” it is also called “tilt-wall,” or in specifications and technical papers, “site-cast precast concrete walls.” However, “Tilt-Up” is the preferred and generally accepted term. “Precast concrete” is a generic term meaning the fabrication of concrete building components at a location other than their final position. Tilt-Up, while falling under the category “precast concrete,” refers exclusively to site-cast wall elements. The term “Tilt-Up building” refers to any type of building that employs the Tilt-Up technique for constructing the walls. The Tilt-Up wall panels typically weigh 60 tons or more, averaging only 6 to 8 in. thick. In Tilt-Up construction, formwork is only required for openings and the perimeter therefore, very little formwork material is needed. When the wall panels have attained sufficient strength, usually a week to 10 days, a mobile crane is brought to the job site to lift and set them on prepared foundations. The erected panels are temporarily braced and typically connected to one another. The roof structure is then attached to the walls, the braces are removed, joints are caulked, and the wall finishes are applied to complete the building shell. It is a fast, simple, and economical method of construction, which has been used extensively for one-story buildings and has most recently been adapted successfully to multi-story structures. Today, walls of up to four stories in height are being cast and tilted into position. Currently there have been several instances of wall panels as high as six stories being cast and erected as a unit by the tilt-up method of construction. The economy of tilt-up lies in its simplicity of construction. The critical factors in this

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method of construction are handled in the pre-construction planning stage. Skill in laying out panel erection sequences and designing safe lifting elements which fully utilize crane time will provide for the fast and safe Tilt-up, tilt slab, or tilt wall is a type of building and a construction technique using reinforced concrete. Descriptions of tilt-up construction abound throughout history and some experts believe that tilt-up construction originated more than 2,000 years ago when Roman architects discovered the ease of casting a slab of concrete onto a graded ground surface and tilting it into position. It was in the early 1900’s, when concrete was rapidly becoming the most popular building material, that engineer Robert Aiken introduced the construction method now known as tilt-up construction. Aiken was designing and building reinforced retaining walls at Camp Logan Rifle Range in Illinois. Instead of using the traditional method of cast-in-place concrete walls, Aiken poured the walls on a casting slab flat on the ground and tilted the panels into position. Aiken then used steel rods to anchor the walls into the concrete footing until the entire structure and roof diaphragm was in place. Aiken soon realized that this building method would be advantageous if implemented correctly and used it in several buildings throughout Illinois. The first complete tilt-up building was a concrete factory near Zion City, Illinois. Although tilt-up construction had been known for a long time, it was not until the late 1940’s that it really started gaining popularity with the introduction of the mobile crane. The mobile crane facilitated the panel erection process and allowed for larger panel construction. Ready-mix concrete also started to become available, allowing tilt-up construction to become more efficient and cost effective. These innovations could not have come at a better time. After World War II, business in the United States was booming, subsequently increasing the demand for commercial and industrial structures. Because tilt-up construction allowed contractors to offer high quality projects at a cost effective price and with a reduced construction schedule, the process quickly became increasingly popular. Since that time, tilt-up construction has undergone many more innovations and refinements, and has now developed into a building method commonly used by many concrete contractors and general contractors in the construction industry.

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2. BENEFITS  Flexible finishes and architectural expression Tilt-up panels can be plain and smooth or visually rich. Grooves, texture and colour can be employed creatively by the architect, while plain-finished or painted panels are used for a more subtle effect.  Financial gain Tilt-up is highly competitive with traditional construction for a wide range of buildings types. The inherent fire resistance and security of tilt-up may also result in lower insurance premiums for owners and occupiers. Tilt-up walls can be insulated economically to give the required U values, from a normal building to cold stores. Sandwich tilt-up panels incorporate insulation and minimise follow-on trades. This combination of concrete and insulation builds into thermal mass which can reduce temperature fluctuations and provide a durable internal and external finish.  Robust, easily-sealed surfaces Tilt-up is sealed easily, making it ideal for controlled environments. The low number of joints in a tilt-up building means exceptional air tightness is achievable.  Significant sound and noise reduction The mass of the concrete walls absorbs the sound rather than letting it through as can occur with lighter forms of construction. Noise can be isolated within a building and for airborne sound, tilt-up walls can provide a sound reduction index of at least 52dB compared with about 20 – 30dB for lightweight cladding.  Fire resistance Tilt-up panels can be designed easily for up to four hours resistance and are particularly cost-effective as fire separation/compartment walls.

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 Security Tilt-up panels are frequently used for security walls and prisons because they are virtually impossible to penetrate.  Fast programme to completion A key benefit for using tilt-up is the fast programme to complete a project. After the floor slab is placed, the typical elapsed time from starting to form panels until the building shell is completed may be only four to five weeks. Materials for wall panels are procured easily with minimal lead times, allowing a fast start to this process, which progresses while any products with a longer lead-time are being fabricated.  Health and safety With a tilt-up building, much of the work is on the ground; there is no vertical formwork, no scaffolding, and since the floor slab is poured first, workers have a safer working surface.  Maintenance and durability The wider panels minimise the number of joints and length of sealant, thus reducing maintenance costs. Visual concrete (fairfaced, textured, profiled, tooled and exposed aggregate finishes) and cast-applied facings (inlaid stone, brick etc) require little attention, and modern paints have long life spans. Concrete surfaces are resistant to mechanical damage, and are easily washed down.  Heat Insulation Concrete mass in tilt-up panels not only slows down heat transfer, it also stores heat during the daylight hours and releases it during the cooler night hours. This translates in the need of less costly and more efficient heating and cooling equipment. Tilt-up panels also provide for an excellent enclosure for passive solar designs, because of the concrete’s heat storage properties.

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3. TILT-UP VS. PRECAST CONCRETE BUILDINGS While they may seem similar, there are significant differences. The TiltUp panels are almost always load-bearing and do not require a separate structural frame to resist service loads as architectural precast does. Full-height structural precast panels don’t require a separate structural frame, but are usually limited in width due to transportation requirements. The precaster will frequently say he can build it faster, because he can work in inclement weather and begin casting panels even before the site work has begun. Here are some factors to consider: • How far is the building site from the nearest qualified precaster for transportation costs? • What is the widest, tallest and heaviest panel that can be transported? • Will the precaster use architectural precast, which requires a separate structural system to resist service loads or heavier structural precast? • Due to travel limitations on width, there are approximately three times as many precast panels to erect and brace and three times as many joints to fill. • Will the narrower precast panels accommodate the anticipated opening widths and architectural pattern?

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4. TILT-UP VS. MASONRY BUILDINGS Masonry, when compared with Tilt-Up, is generally more costly and takes longer to build. However, in areas where Tilt-Up expertise is unavailable or where masonry is inexpensive due to a plentiful labor source, you will see many masonry buildings. However, enterprising contractors in these areas are introducing Tilt-Up and often winning jobs that were first designed as masonry buildings. Faced with deciding whether to use Tilt-Up or masonry, the answer you get depend upon who you ask. A masonry contractor will tell you he can build it faster and for less cost, and a Tilt-Up contractor will tell you his method is the better choice. Here are some factors: • For a building of less than about 6,000 sq. ft., the choice is probably masonry, since crane time is uneconomical for a small number of panels. And the limited floor space may not be enough for casting panels. • For a large building in an area of the country where concrete blocks or bricks and mason labor are inexpensive, it could be a toss-up, assuming Tilt-Up expertise and cranes are available; if they are not, masonry wins by default. On a large project an experienced masonry crew can work fast and very efficiently. • If a building has a high clear height inside, say over 24 ft., Tilt-Up is more economical since the thickness of the wall required need only be increased incrementally, whereas masonry jumps in whole block units, such as to 12 in. from 8 in. • For fire resistance, an 8 in. concrete block wall with all cells grouted is rated 4-hours, equivalent to a 6.” concrete wall. If the block wall is not solid grouted, it is rated twohours, which is equivalent to a 5 in. concrete wall • A concrete wall is generally denser and less porous than a masonry wall. With masonry there is the problem of sealing and painting so water does not enter through the joints, or through the blocks’ natural porosity. A Tilt-Up building is faster to erect than a masonry building, given a relatively large building with repetitive panels.

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5. TILT-UP VS. METAL BUILDINGS

Metal buildings fill a market niche for low cost enclosures; however, when they are designed to provide a similar level of performance comparable to a finished Tilt-Up building, they can cost as much, and still lack the durability, fire resistance, low maintenance, and other desirable features of a Tilt-Up building. Recognizing the advantages of a concrete wall, some metal building contractors are using Tilt-Up panels attached to the steel frame. The prefabricated, pre-engineered metal building industry has aggressively marketed their product. They sell the concept of economy through standardized manufactured components. You can almost pick your building from a catalog. Metal buildings have their place for low cost shelter; however, comparing features such as durability, maintenance, fire resistance, insulation, security and others discussed above, there is little comparison. Additionally, a metal building fitted for office use, with drop ceilings, finished walls, insulation, and other features, can cost as much as a Tilt-Up building. The metal building industry has recognized the advantages of Tilt-Up by offering exterior walls of Tilt-Up concrete as an option rather than metal siding. Many other countries have active and growing Tilt-Up markets, including Canada, Australia, New Zealand, South Africa, South America and Mexico.

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6. THE TILT-UP TECHNIQUE-STEP BY STEP

Fig: 1 Tilt-up Technique

 Onsite prefabrication of tilt up panels with face down on floor slab. 

After attaining full strength the panels are tilted up using a crane.

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The tilt up panels is placed over the prepared suitable foundations.

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Joining panels and attachment of roof structure is then done.

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7. CONSTRUCTION STEPS 

Foundation Construction

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Floor Slab Construction

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Panel Layout &Forming

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Panel Structural Reinforcing

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Wall Panel Casting

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Erection of Tilt-Up Panels

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Joining Panels &Structural Steel Frames

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Deck Diaphragm

7.1 FOUNDATION CONSTRUCTION The foundation system for a Tilt-Up building requires a few extra considerations not encountered in a masonry, steel frame, or wood frame building. The foundation system for a typical Tilt-Up building consists of interior footing pads to support columns, interior continuous footings to support bearing walls, and the perimeter footings or pads to support the wall panels. Wall panels are supported on spread or continuous footings, See Figure 2.Spread footings are placed under joints between panels, so that each footing pad supports one half of each adjacent panel and the panel spans between footing pads. Excavation for continuous footings is done economically with a trencher, and little reinforcing is required. Whether a pad or continuous footing is used, the top is held down about 1 in. below the bottom of the panels and this space is filled with grout after the panels are up to provide full bearing on to the footing. For setting the panels at the correct elevation, high density plastic shins or grout bearing pads are constructed on top of the footings under the end of each panel. Customarily just one pad is used under the joint between two abutting panels. These pads are about 1. In. high, set precisely at the correct elevation (bottom of the panel), and are of a length sufficient to safely support the weight of the panel until the grout is placed under the remainder of the panel.

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Fig: 2 Panel Foundations Following is the sequence most often used for constructing the foundations and floor slab: 1. Prepare the sub grade for placing the floor slab. This includes possible importing of fill material, compaction, installation of underground electrical and plumbing, and excavation of the interior footing pads and interior bearing wall footings. 2. Place concrete for the interior footings. This includes setting the anchor bolts using a steel plate template furnished by the structural steel fabricator. 3. Place, finish, and cure the floor slab. 4. Excavate and place exterior foundations. Sometimes this is done before the slab is poured or at the same time. 5. Construct the leveling pads on the perimeter footings in preparation for setting the wall panels. (At this point the wall panels are formed and cast.) Steps 4 and 5 can also be done after placement of panel concrete, so concrete trucks can back close to head of panel. 6. Place grout under the wall panels once they are up and braced for continuous bearing onto the footings. 7. Place the closure strip between the exterior walls and the floor slab. This should be done after the roof is in place but before the wall braces are removed. 8. Grout the base plates for the interior columns once they are up, then place concrete in the blockouts. 10

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9. Check for excessively wide cracks or joints. Route, fill, and patch as required. 7.2 FLOOR SLAB CONSTRUCTION The floor slab of a Tilt-Up building is especially important since it not only functions as the floor surface for the life of the building, but also as a casting base for the panels and a platform supporting construction equipment. Service loads during the life of a manufacturing or warehouse building rarely exceeds 500 lbs. /sq. ft. and usually does not exceed 100 lbs. /sq. ft., particularly in office buildings. Construction loads are the heaviest loads applied to a Tilt-Up building's floor. These loads include the weight of concrete trucks and the crane when lifting panels from the floor slab. In contrast to service loads, construction loads can exceed 40,000 lbs. from the rear axles of a ready-mix truck. Also important is the area over which these point loads are applied. For example, a 5,000 lb. single wheel load from a forklift truck, using hard rubber tires, may be supported on an area only8 in. wide by 2 in. long. In this case the bearing area is only 16 square inches and the pressure under the hard rubber tire is over 300 psi, which explains why hard rubber tire forklifts are so hard on joints in a floor. In contrast, the unit pressure applied by a ready mix concrete truck, or a pneumatic tire forklift, will not exceed the tire inflation pressure, which may average 75 psi. The pressure under crane tires can be as much as 150 psi. Various steps in designing a floor slab include: 1. Determine what loads will be on the floor during its life, or at least the foreseeable future. This may include steel storage racks, forklift trucks, palletized storage, stacked goods, or other types of loads but not crane loads (addressed later). 2. Determine the required thickness of the floor slab. 3. Specify the concrete, including aggregate size, cement content, water-cement ratio, 28-day compressive strength, slump, and any admixtures. 4. Decide whether to include a granular base, and if a vapor barrier or retarder is required. 5. Decide whether or not to reinforce the slab either with welded wire fabric (also referred to as “mesh”) or reinforcing bars. 6. Select joint spacing. 7. Select type of joints to be used, as well as dowel sizes and spacing. 11

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8. Specify testing (quality control) requirements. 7.3 PANEL LAYOUT & FORMING One critical task a Tilt-Up contractor performs during the planning stage is panel layout and sequencing. Variable factors affecting the layout process include the following: • Crane types and capacities available (including their costs) • The decision to lift from inside, outside or both • The ability to stack cast and where to use stacking as an advantage • If and how many temporary casting beds are required Numerous design options are available for tilt-up construction varying from project to project. In the design of a tilt-up structure, the panels themselves should always be designed to withstand the in-place loads, lifting loads, and whatever features that the wall will have. Architectural features such as color, finishes, exposed aggregate or rustications should also be incorporated in the design. The designer should also take into account the lifting inserts layout; usually it is done after all other aspects of the design are complete. The following criteria should be considered when designing a tiltup panel:

Fig: 3 Window formed in panel 12

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 Loads A Tilt-Up panel is subjected to forces from three directions: 

Vertical (downward) forces from loads imposed by roof or floor framing, and from the weight of the panel itself, tend to cause the panel to buckle like a column, or be overstressed in compression. The thinner the panel, the more likely the tendency to buckle.

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Lateral (sidewise) forces, from wind or seismic, tend to bend the panel. To resist this bending, the panel spans like a flat slab between points of support, which are usually provided by the floor and roof diaphragm. Alternatively, the panel is designed to span horizontally between pilasters, but this is rarely done.

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The third force acts in the plane of the panel—parallel to it—and tends to cause the panel to shear, or slide on its foundation. In this case the panel acts as a shear wall.

Resistance to all three forces is provided by the thickness of the panel and its reinforcing. The forces that nearly always control the design, and determine the panel thickness and reinforcing, are the lateral forces—wind and seismic.  Panel with Openings Panels with openings are becoming more and more popular. These panels can be analyzed by finite element methods, but this is a very expensive and time consuming process. An approximate analysis generally provides results that are sufficiently accurate for most designs. Local codes should be consulted to meet requirements. Where openings occur, the lateral and axial loads, including self-weight, on the entire panel must be carried by the continuous vertical legs on each side of the openings. If there is more than one opening, vertical legs should be able to carry all loads. Often, it is only necessary to increase the loads acting on the legs by the ratio of the total panel width to the leg width. Some designers may assume point loads are exerted by door and window frames. When the remaining panel width besides the opening is significantly large, the width of the leg carrying the additional loads should be restricted to approximately 12 times the panel thickness in design calculations. The use of a greater width may fail to detect the danger of buckling. For very wide openings, it is 13

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recommended to design the panel legs or segments as beam columns extending the full height of the panel. In some cases design loads may be substantial. Items to consider in the design are:

segments and use of closed ties.

-of-plane forces -plane shear and frame action Since the panel reinforcement around the openings, and determined by analysis, is often considerable, added crack control reinforcement may not be necessary.  Lifting Stress As the tilt-up panel is picked up for erection, it is immediately subjected to bending that causes both compressive and tensile stresses which must be resisted by the panel. Manufacturers of inserts locate the picking points so that the overhanging portions of the panel will reduce the bending moments between pickup points, therefore reducing the compressive and tensile stresses in the concrete. The flexural stresses can be determined by treating the panel as a beam supported by tension loads on the inserts and the ground reaction. The gross concrete section of the panel is used as lifting analysis. The load applied along the beam is the normal component of the panel selfweight. The tension and shear loads on insets vary as geometry in the rigging changes. Maximum stress usually occurs at 0 degrees and between 30 and 50 degrees for two and four row lifts. In the case that the allowable stresses are exceeded, additional reinforcement, higher strength concrete, strongbacks, or another lifting arrangement should be considered.

7.4 WALL PANEL CASTING Good planning is a key part of all successful construction work, but it is especially critical in tilt-up construction if all the cost, schedule, and safety benefits are to be realized. “Success of each construction sequence depends on the success of the

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preceding construction event.” Successful construction of tilt-up panels requires careful organization and planning. It is recommended to check site access and the jobsite conditions as an early step of construction planning. It is common for permits to be required by different entities depending on the location of the jobsite. For example, special permits are required for schools and churches since these are usually built in residential areas where tonnage restrictions, noise abatement, and dust control regulations could exist. The contractor and erection sub-contractor, if any, should determine site access and the building access for the crane. Problems with uneven terrain or obstruction from other construction trades should be noted and solved beforehand. Additionally, a suitable location for crane assembly and rigging should be determined. Some local governments will not allow this activity on public streets. In the early stages of construction planning it is important to be aware of overhead wire problems. Panel drawings can usually be found on the architect’s /engineer’s plans. Complete panel drawings are essential. Plans should include panel identification, dimensions, physical characteristics, reinforcing, location and identification of embedded items, insulation, finishes and textures, and rigging and bracing information. To insure an efficient construction procedure, consideration must be given to casting location of panels and erection sequence. The contractor should develop a panel casting layout, which can include temporary casting slabs or the floor slab, which will ensure the most efficient casting and lifting of the panels. Panels must be properly located to have efficient construction access and to minimize crane movement. Prior to panel erection, it is important to watch for special bracing conditions, particularly at corners and other interruptions of a straight building line. Cross-bracing will be required at the corners, where braces will be required to pass either over or under the braces of the previously erected panel. Avoid “fill in” panels where possible. It is recommended for panels to be erected in a consecutive way were possible. 7.4.1 Casting Surface The concrete floor slab is usually the most convenient place for casting tilt-up panels. It is recommended for the floor slab to be placed as soon as possible as it

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used as the “work table” for all tilt-up panel forming, casting, bracing, and erecting. If the floor slab is not available or there is simply not enough space available for casting the panels, panels can be “stack-cast” one on top of another or in a temporary casting platform or casting bed on an accessible location outside of the building. The floor slab must be level and smoothly troweled for a high quality finish. A smooth surface is important to prevent mechanical bonding and is necessary for clean cleavage when lifting. A properly constructed floor slab should have a compacted subgrade with adequate strength and thickness to carry the loads of material trucks and heavy mobile cranes that may be needed in the erecting process. Floor slab jointing should be located between panels whenever is possible. If the floor contains openings for pipes or other utilities, a ¾-inch skin coat over sand fill can be used to close up the openings temporarily.

Fig: 4 Casting Slab and Tilt-Up Panels

7.4.2 Edge Forms One of the advantages of tilt-up construction is the savings in form material and labour. Panel edge forms, which are a relatively easy form to erect, should be securely bolted or weighted down to the casting surface. Top and bottom edge forms 16

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need to be braced and squared. A recommended method for securing panel edge forms, is to make L-shape forms on the site or use prefabricated steel angles, then anchor them to the floor slab by drilling 3/8-inch holes through the forms (prefabricated steel angles should already have these holes) into the casting slab, and anchored with double headed form nails wedged into the hole in the concrete. Using pieces of plastic wedged between the nail and the concrete also facilitates the stripping of the forms when removing the nails from the casting bed. Holes can be patched with epoxy at completion of the work. In constructing panel forms it is important to avoid variations in form measurements and insure a good straight line in the panel sides and from panel to panel. It is recommended to use a chalk line for laying out the forms, and spraying a sealer over the chalk so weather will not wash the lines away.

Figure.5: Tilt-Up Panel Forms Slight variations in form measurements will result in plumpness variations and will affect joint dimensions. Windows and door openings in panels are usually not a problem to form. The common method used by contractors of including windows or doors in a tilt-up 17

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panel is to cast directly against the steel sash or frame. All opening forms should be held down on the casting slab with either weight or by the method described above. Exposed surfaces of sash or frame should be given a coat of bond-breaking compound to prevent concrete adhesion. 7.4.3 Bond Breakers A good bondbreaking agent is also essential in tilt-up construction to obtain an easy lift from the floor slab. There are three basic bondbreaker types. 

Synthetic petroleum, hydro-carbon, resin solutions

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Solutions of waxes with metallic soaps

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Solutions of organic esters and silicones It is important to be aware of some of the effects each bondbreaker

might have on the final appearance of the panels. For example, some bondbreakers leave residue, especially when applied excessively to the casting surface. Residues affect the coloration of the panels and can prevent adhesion of paint, adhesive, sealers, or any other treatments. If the possible effects of the bondbreaker on the panels are unknown, it is recommended to consult the manufacturer before using it. It is very important to test the bondbreaker agent with the curing agent for compatibility, usually before casting the slab. The bondbreaker coat should be applied prior to placement of the reinforcing steel. Care should be taken to ensure that the bondbreaker is not sprayed over dirt, sawdust, dew, frost, or surface water. An easy method for testing separation is to sprinkle water on surface were the bondbreaker was applied. If the water beads as on a newly waxed floor, the panels will separate. If the water is immediately soaked into the floor, the panels need re-spraying. Bondbreakers that have been heavily exposed to rain or to weather for longer than three days should be checked. Re-spraying might be necessary. Bondbreakers not applied correctly will result on panel surfaces that stick. Not being able to separate the panels can result in damaged and cracked panels, pull out of lifting inserts, or exceed crane load capacity creating schedule delays and panel re-construction costs.

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7.5 PANEL STRUCTURAL REINFORCEMENT, INSERTS & EMBEDMENTS Having the reinforcement in the right location, in the right sizes and the right quantities, is essential for the structural integrity of the panel during its service life and for stresses that can occur during lifting. In addition to the reinforcement, a number of other items must be cast into the panel. These include lifting inserts, wall brace inserts, anchor bolts, beam connections, weld plates, and other items necessary for connections to the panels. Embedments consist of beam pockets, support angles, plates for attachment of structural components, and other items that are an integral part of the panels after they are erected. (Note the term “embedments” is used to indicate items embedded into the concrete. The term “embedments” is also used and means the same, but the former is more often used.)

Fig: 6 Panel Reinforcement Around openings, reinforcement bars are added to strengthen the corners and edges. In the locations were wire fabric is the principal reinforcement, dowel bars should be provided where panels and columns are tied together. To provide for clear cover, mats of reinforcement should be lowered onto plastic chairs. Chairs made of steel wire should have plastic tips. 19

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Fig: 7 Panel Reinforcement 7.6 INSULATION There are a number of options for insulating tilt-up panels but the most economical method is to cast normal weight concrete directly over rigid, low density insulation which is laid in the form. Panels constructed this manner have to be cast face up with lifting inserts located on the outside surface. Another method is to install insulation panels or apply insulation directly to the inside wall surface after placing the wall panel in a vertical position. A wall panel of sandwich construction is preferred for many buildings. Sandwich panels consist of two wythes of reinforced concrete bonded together to a core of rigid, low density insulation. Sandwich panels can be load bearing or non-load bearing. It is common for sandwich panels to consist of one layer of structural concrete and the other of lighter-weight architectural concrete. The concrete layers help provide thermal mass. That is, the concrete is able to store significant amounts of thermal energy and delay heat transfer through the building walls. According to the Fundamentals Handbook of the American Society of Heating, Refrigerating and Air Conditioning Engineers, Inc. (ASHRAE), this delay leads to three important results: 

Slower response time tends to moderate indoor temperature fluctuations under outdoor temperature swings.

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In hot and cold climates, energy consumption is significantly reduced over that for a similarly sized low mass building.

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Because the mass is adjacent to the interior of the building, energy demand can be moved off-peak periods because energy storage is controlled through correct sizing of the mass and interaction with the HVAC system. The first step in sandwich panels is to pour and level the exterior layer of

concrete. Then the insulation panels or sheets are arranged on top of the unhardened concrete according to the individual panel drawings. The insulation panels can be prefabricated and pre-drilled by the manufacturer. Connectors are inserted through the insulation sheets. Most connectors are made from a fibre composite material. In the past, it was common to use steel to hold the panel together, but testing has proved that fiber composite connectors are better suited for sandwich wall construction because steel conducts heat energy at a rate over 50 times greater than that of fiber composite ties.

Fig: 8 Panel Insulation 7.6.1 Thermomass Fiber Composite Connectors The connectors hold the wall together by developing a keying action within the concrete wythes using specially designed notches. Connectors provided by Thermomass (See Figure 9) have been tested to have pull-out capacities in concrete

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exceeding 5,000 lbs., shear capacities as great as 2,500 lbs. each, and a tensile strength up to 140,000 psi.

Fig: 9 ‘Thermomass’ Fiber Composite Connectors 7.7 CONCRETE PLACEMENT AND CURING The concrete is placed the same way as the floor slab. Care must be taken to prevent rock pockets along the bottom of the edge forms and to prevent damage to the bondbreaking material. Concrete must be properly consolidated using an appropriate vibrator. Avoid any contact between the vibrator head and the casting slab; this will affect the bondbreaking surface resulting in lifting problems. Additionally, it can damage the appearance of the exterior surface. 7.7.1 Layout, Alignment, Leveling, Bracing and Connecting Prior to erection the panels should be labeled for identification, laid out on the exterior foundations, and establish the exterior wall line. A commonly use method of alignment requires the contractor to mark the limits of each panel and drill ¾-inch holes into the foundation approximately 5-inches deep. Then install #5 dowels, two on each side of the panel. This process is helpful when placing the panels as it reduces the effort in keeping the panels aligned. The day before erection of the panels, install shims using a level to avoid placing the panel in a tilted position and more importantly setting the panels at the wrong elevation. A simple and commonly used method is to set up the shims to a desired or specified elevation below finish floor or 22

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grade. After panels have been erected, it is important to backfill and compact the perimeter strip. The perimeter strip is a 3ft to 5ft wide and 4ft to 6ft deep strip at the edge of the finish floor. After backfilling, grout is used to fill the voids between the panels and the finish floor. Bracing should be done prior to erection of the panel. Bracing done after erection and while the crane is holding the panel in position is a costly procedure as it requires the use of additional equipment such as lifts or temporary scaffolding. Additionally, it places personnel in a hazardous position. Figure 10 above illustrate how complex the bracing layout can be on a given project. The braces should be adjustable and positively anchored to imbedded inserts of sufficient strength to resist both horizontal and vertical forces acting on the panel. The angle between the brace and the finish floor should be between 45 degrees and 60 degrees, all connecting bolts must be checked to ensure tightness and prevent movement of the braces.

Fig: 10 Complex Bracing

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7.8 ERECTION OF TILT-UP PANELS Erection of the panels is a critical phase during tilt-up construction. It is imperative that the designer, contractor, and usually an erection sub-contractor plan and check all rigging and tilting procedures carefully to ensure that the tilt-up phase is done safely and efficiently. Not planning for erection and not taking the necessary safety precautions can be fatal. The correct crane should be selected prior to construction and based on the panel design. The general rule for sizing the crane is that the crane capacity should be a minimum of two or three times that of the heaviest panel including the weight of all the rigging gear. In addition to the general rule for sizing the crane, the American Concrete Institute recommends the following three questions to be answered before determining the crane size: 1. How far must the crane reach to lift the panel? 2. How far will the crane have to travel with the panel? 3. How far will the crane have to reach to set the panel? After selecting the correct crane, alternative crane positions should be considered to make the erection phase as efficient as possible. When the wall panels are to be erected before construction of the structural frame or roof diaphragm, it is recommended that the crane operates on the floor slab. If this is the case, panels can be formed and casted beside each other in rows or “stack-casted” to allow a substantial amount of panels to be erected while minimizing crane movement. Before rigging the contractor should provide clean working area and the panel should be cleaned of all debris and loose tools. 7.8.1 Rigging An even number of lifting points is recommended unless panels with special shapes require different rigging procedures like some sort of automatic equalizing system is being used. Panel erection should be done in a continuous and smooth motion to avoid overloading. It is important to make sure that the bottom section of the panel is not dragged on the casting bed or ground during erection, as this can damage the panel. 24

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Fig: 11 Panel Erection It is important to use wedges and pry bars to help release the panel. Panels stuck to the casting bed can double the load on the pick-up inserts causing possible withdrawal. As lifting starts and cables start to get tensioned, they will inevitably try to rotate. A rope can be used to prevent undesired rotation and to keep the panel aligned. Ropes should also be used to prevent braces from getting trapped by the rigging when the panel is being set in its final position. Braces that get trapped can damage the floor slab or cause the insert to snap, permanently damaging the panel. 7.8.2 Placement If it is a closure panel, measurements should be taken prior to lifting the panel to make sure it will fit correctly. ` A theodolite can be used to “fine tune” the final panel plumbness by making sure the panel is in a 90 degree angle with the floor. All pipe braces are designed to have threated adjusting units; this can be used to position the panel as plumb as possible even after the braces have been attached to the floor. 25

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Fig: 12 Panel Placements

7.8.3 Temporary Bracing Once the panels are erected in place, tilt-up panels must be temporarily braced against wind loads and other lateral forces until all final structural connections are completed. The most commonly used method for bracing tilt-up panels is the use of telescoping pipe braces. 7.9 JOINING PANELS Panels are joined end to end with only a narrow joint between them, which is caulked for water tightness. The precision to which panels are set is such that there is rarely more than 1/8in. variation in thickness along the length of the joint. When a wall must be fire resistive, there are joint fillers available that give two-, three-, and four-hour fire wall ratings. Choice of the joint filler is important since it must remain relatively flexible to accommodate the expansion and contraction of the wall and adhere properly to the concrete to insure its water tightness. Structural connections 26

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between panels are usually made by welded chord bar splices, or splicing the steel ledger, at the roof line. This type of connection affects a continuous tie along the length of a wall for resistance to lateral forces imposed by the roof and floor diaphragms. In areas of low seismic risk the structural ties between panels are sometimes omitted. In the earlier days of Tilt-Up the panels were joined by poured-in-place concrete pilasters or “stitch joints.” In this method, which is rarely used now since it is more costly and often resulted in cracking, a one-foot gap is left between panels and the horizontal wall reinforcing projects into this gap. Concrete is then placed to make the closure.

Fig: 13 Common panel joint with backer and rod partially placed

7.10 ROOF DIAPHRAGMS A horizontal diaphragm is used to transfer lateral (wind or seismic) forces to bracing elements—usually the end walls of a building. They are considered either flexible or rigid; the former being plywood or metal decking, and the latter being concrete. A diaphragm can be thought of as a plate girder on its side, spanning between vertical shear resisting elements (usually shear walls), with the web of the girder (sheathing, plywood or decking) resisting shear and the flanges (chords) resisting tension and compression.After erecting all panels and completing the roof structure, it is recommended to install backer rod, spray water-proof foam and then seal and caulk all wall panel joints. If the panels are not to be painted, a cleaning agent and a sealant can be used to provide protection.

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8. FINISHING AND ARCHITECTURAL TREATMENTS FOR TILT-UP PANELS A wide range of colored coarse aggregates and sands, several possible colors of cement, and many other architectural available applications offer an owner and designer an array of possible colors and textures which can be applied to tilt-up panels. In the United States there are four or five commonly used methods for obtaining various finishes on tilt-up panels. These methods include exposed aggregate that is usually obtained by sandblasting, the seeding method, the sand bed method, and finishes obtained by using a textured liner. Decorative patterns and even stone-face can also be added to obtain the desire design. 8.1 EXPOSED AGGREGATE FINISHES  Sandblasting The main classifications for sandblast finishes are light blast, medium exposed aggregate, and to fully exposed aggregate. This can be obtained by varying from fine to coarse sand and applying various pressures. It is recommended to sandblast the face-down of a panel because a higher concentration of aggregate, as gravity tends to move aggregate towards the bottom. Additionally, the face-down surface does not have any lifting or bracing inserts which are very difficult to patch in order to match the surrounding surface. The most commonly used finishes are light to medium exposed aggregate. The reason for this is that concrete in the panels has to reach sufficient strength to avoid creating stress and bending cracks when tilting the panels. Most contractors allow the concrete to reach at least 3000 psi before tilting, while normal sandblasting should be done before the concrete reaches 1500 psi to 2000 psi, heavy sandblasting is required making this a more costly operation.  Seeding Method To create the desired exposed aggregate finish, the aggregate can be either seeded over the fresh concrete or hand placed on a wet mortar. The method chosen typically depends on the aggregate size. To accomplish this, a structural concrete mix can be poured in the forms and consolidated to a level of 1/8-inch to 7/16inch below the desired finish which allows for the volume of the selected aggregate to 28

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be seeded. Once the structural concrete has been screeded to the proper level and consolidated, the aggregate is seeded carefully by shovel or by hand to completely cover the concrete layer.  Sand Bed Method The sand bed method is recommended when the aggregate selected for exposure is 1-inch or more in diameter. This includes large field stone materials. The two most common methods of utilizing the sand bed method to expose aggregate are: (1) Place the aggregate on the casting bed and sprinkle sand between the aggregates (2) Spread a layer of fine sand over the bottom of the form and then imbed the aggregate. 8.2 TEXTURED LINE A variety of patterns may be produced in large tilt-up panels by using plastic materials as form liners. Common liners used are vacuum-formed thermoplastics, fiberglass-reinforced plastics, or plastics formed into shape by heat and pressure. Some advantages of using this method is that liners can eliminate the extra care of preparing the casting bed so that joints, cracks, block-outs, etc will not be reflected in the panel. Liners also provide a wide range of textures and appearances. Finishes obtained with liners include simulated sandblast, ribbed finishes, boarded textures, bush-hammered finished, and many more. The use of liners in tilt-up construction is usually limited to larger projects where higher utilization of liners can be obtained. Liners are expensive and usually require a minimum of three to four uses to be economically comparable with other finishes. An important aspect to consider is that plastic does not bond to concrete, so parting agents are frequently not needed.

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Fig: 14 Textured Cedar Ridge High School, Austin, Texas

Fig: 15 Cedar Creek High School, Austin, Texas 30

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Fig: 16 Liner at Charlie Rouse High School, Austin, Texas

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9. CONCLUSIONS Tilt-up construction has been around for a long time, but it was not until the late 1940’s that it really started gaining popularity with the introduction of the mobile crane. Since then, tilt-up construction has undergone many innovations and refinements, and has now developed into a building method commonly used by many concrete contractors in the construction industry. In the design of a tilt-up structure the panels should always be designed to withstand the in-place loads, live loads, wind loads, and most importantly, lifting loads. The designer should take into account the lifting inserts layout and all architectural features incorporated into the final design of the structure. Good planning is the key part to all successful construction work, but it is especially critical in tilt-up construction if all the cost, schedule, and safety benefits are to be realized. It is important for the contractor to develop a panel casting layout to ensure the most efficient casting. The floor slab can be used efficiently as the “work table” for panel forming, casting, bracing, and erecting. The lifting or erection of the panels is a critical phase during tilt-up construction. It is imperative that the designer, contractor and usually an erection sub-contractor plan and check all rigging and tilting procedures carefully to ensure panel erection is done safely and efficiently. The correct crane should be selected based on the panel design. A wide range of colored coarse aggregates and sands, several possible colors of cement, etc available offer an array of possible colors, textures, and finishes that can be applied to tilt-up panels. In

conclusion,

tilt-up

construction

basically

involves

job-site

prefabrication of concrete building members under controlled and relatively economic conditions. Tilt-up construction is generally identified with industrial and commercial building, but has been increasingly been used in other types of buildings such as parking lots, residences, shopping centres, and churches. It is important to recognize the opportunities and advantages tilt-up construction has to offer. Even though tilt-up is not a new idea, it has been proven to be one of the most efficient and cost effective construction methods known to date.

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