towards special inspectors, it's important to note that much of the material presented here will be considered outside the normal scope of the inspector's work. This information is presented here in an effort to offer a broad informational overview of precast concrete construction. So, let's get started. The agenda over the next few meetings will cover code requirements, the types of precast concrete structures, types of precast concrete components, a general overview of precast concrete manufacturing, the documents required for effective inspections, condition assessment of precast concrete components, and the erection of precast concrete structures. We will talk under the erection portion about the preparation of cast-in-place concrete foundations, various bearing conditions, and the connection of these precast concrete components to their foundations or other elements. We'll begin by looking at the code requirements specific to precast concrete construction. Section 17 of the International Building Code 2015 establishes the requirements for special inspections and testing. Section 17.04.2 requires that the owner or the registered design professional in responsible charge, acting as the owner's agent, they must hire the approved special inspection agency. Section 17.04.2.1 of that section requires that in order for an inspection agency to be considered approved, it must provide written evidence of its experience or training. The intent of this program is to provide focused training for inspectors intending to qualify as approved special inspectors of precast concrete structures. We understand that others in our industry will get a great use out of this program as well. Where load-bearing members are manufactured at a fabrication shop, those members also are required to have special inspection under Section 17.04.2.5. However, under this section, it goes on to clarify that special inspectors or special inspections aren't required at a place of fabrication so long as that fabricator uses fabrication and quality control procedures that have been approved by and periodically inspected by the building official or the fabricator's procedures and quality control manuals have been reviewed and approved and they are periodically audited by an approved agency. In most cases, PCI certified plants are considered to satisfy the second scenario. However, the ultimate decision to accept this resides with the building official. Once the project moves from plant to job site, the International Building Code 2015, Table 17.05.3 requires periodic inspection for the erection of precast concrete members. Where ACI 318, Chapter 26.8 is the reference standard. It's worth pointing out that the reference to Section 26.8 of ACI 318 seems to be questionable. That section in the reference is reference to embedded materials or embedments. In past versions of IBC, the reference was to Chapter 16 of ACI, which was an entire chapter devoted to precast concrete. With the 2014 reorganization of ACI 318, precast topics are now addressed throughout the standard. Perhaps what ICC intended was to reference ACI Section 26.9, which is Additional Requirements for Precast Concrete. In addition to the general special inspection requirements for precast concrete, there are several topics that have been flagged out specifically by IBC that the special inspector should be aware of. Table 17.05.3.1 refers to the special inspector to AWS D1.4 for requirements unique to the welding of reinforcing bars. Also requirements are anchors embedded into concrete that are subject to periodic special inspection. Also included is post-installed anchors required either continuous or periodic special inspections depending on the manner in which they are used. You can see that also in the table under Item 4. In designated seismic systems as defined in the construction documents, they are required to be examined by a special inspector for structures that are assigned to seismic design categories C, D, E, and F. As we begin our discussion, let's take a look at the various types of structures that are commonly associated with precast concrete. Parking structures are arguably the building type most commonly associated with precast concrete construction. Long-spanning floor members and speedy construction allow for inviting and open parking spaces that can be quickly brought online. Precast concrete school construction have gained prominence since the turn of the century through the frequent use of exterior precast concrete wall panels or total precast concrete structures. Precast aesthetic flexibility along with energy efficiency afforded through the use of insulated wall panels make it an excellent choice for this type of construction. While precast concrete cladding panels are a common component of many office buildings, the use of total precast concrete structures has also gained prominence. The efficiency, cost-saving, and construction schedule benefits of modular precast concrete cells have led to their widespread use in correctional facilities throughout the country. These modules are often equipped with furniture, lighting fixtures, plumbing fixtures, insulated walls, and painted surfaces so that they can be nearly ready for occupancy once they arrive at the job site. The benefits of insulated precast concrete walls make them a frequent choice for storage and distribution centers. Precast concrete's flexibility, resistance to fire, and its structural robustness make it an ideal material for heavy industrial applications. Precast concrete wall panels, beams, columns, and floor slabs have been used to construct many multi-unit residential structures across our country. It is not unusual to combine several of these building functions within one precast concrete structure – parking, retail, office, etc. – in the same structure. The flexibility of framing that precast makes it an ideal choice for these structures. Most of the major athletic and entertainment venues in the country make use of precast concrete stadia or seating components. Total precast concrete framing of these types of structures is frequently used. We'll stop there with the building types, but there are, of course, numerous other types of construction that incorporate precast concrete. Next to consider are the various types of precast concrete components commonly used in construction. Considered by many to be the workhorse of the industry, double Ts are commonly used to create floor and roof spaces. Their efficient cross-section allows for spans which, in turn, support the creation of open spaces within structures, such as parking garages or industrial buildings. In these photographs, you can see the double Ts on the left after they've been produced and are stacked in the yard, ready for delivery. And on the right, you can see a double T being swung into place on a multilevel parking garage. Countless variations of precast concrete beams have been created to offer support to floor and roof members. The photos here shown are of inverted T beams, first in the production facility on the left and later in its final position supporting floor double Ts. Columns are used to carry loads from the superstructure down to the building's foundation. Because these loads can be very substantial, the use of high-strength concrete in these components is not unusual. We see that a column on the left, marked number one, holding up an inverted T, which is holding up floor Ts. And then, in the middle, you see a tall column that's holding up spandrels, which are also holding up double Ts, or maybe at the end of a non-load-bearing wall. And in the yard, on the right, we see columns that have been produced, ready for delivery. Precast concrete spandrels typically serve a number of functions on a structure. They can serve as a beam. They're used to support floor and roof members. As an exterior element of the building, they can provide aesthetic features through the use of colored concrete and enhanced finishes. The height of the spandrels can be established as to provide vehicle protection or a barrier or pedestrian fall protection. On the left, again, we see spandrels in their final position holding double Ts. And on the right, we can see them before they're being loaded, after the columns have been stood, and the spandrels that go from column to column. Precast walls can be used in many ways throughout a structure. Thickness is usually dependent on the structural requirements of the application. By introducing a layer of insulation within their thickness, wall panels can be used to create a thermally efficient enclosed space. Width, height are often dictated by shipping and handling limitations. So, here you can see plain wall panels, thin brick wall panels, lots of different looks, diversal use of precast wall panels. Light wall panels are largely unique to parking garages. By introducing a high percentage of open space and providing support for floor members on both of its faces, these wall panels eliminate the need for dual load-bearing members, while allowing abundant light to pass through a garage's interior column lines. There are numerous types of light wall systems throughout the precast concrete industry, typically driven by manufacturer's standards. As you can see in the two pictures, the amount of light that's allowed to go in there without having to block off by using a solid wall. Shear walls and frames are used as the central resisting elements for lateral loads created by wind, earthquakes, or earth pressure. These members are vital to the overall stability of the building and merit thorough inspection during installation. The left photo, two solid wall panels in the center are shear walls carrying inverted T-beams inside the building. The center photo is a shear wall being erected. Note the dowels being lowered into the grout-filled sleeves at the bottom of the panel. There's also a rectangular pocket in the center that is set to receive an inverted T-beam. The right photo is a K-frame, which provides lateral support while also providing some openness. Note the frame here also carries an inverted T-beam. These are becoming less prominent as the International Building Code is often interpreted to prohibit their use in moderate to severe seismic applications. Precast concrete stairs are proven as a viable solution for all manner of multilevel buildings. They are of particular benefit during construction since they are installed as part of the precast structure, hence providing an immediate access between floors as opposed to waiting for the metal pan stairs to be installed at the completion of the precast erection. The left photo shows an incomplete stair, meaning no handrails. For illustration, the center photo shows one level of all of the completed stair, and on the right, you would see precast stairs with temporary handrails. Flat slabs are commonly used as floor or roof members in small area applications. We can see the canopy or catwalk along the outside of the building there on the left, and a larger flat slab being used as a roof structure in the picture on the right. Polycore slabs are typically produced as part of a continuous long bed of concrete. After the concrete is cured, the slabs are cut to their designated length and shape. They are frequently used in moderate span length applications and are well suited when the minimum floor system depth is desired. So you can see different types of precast polycore on the left, stacked in the yard ready for delivery, and then in their placement on the picture on the right. For the purpose of this seminar, not much specialized focus has been given to precast cladding, although the basic concepts for inspection of total precast structures will carry over to the inspection of precast concrete cladding systems as well. Stadia units that we mentioned earlier are used to create the seating areas of athletic and entertainment venues. The structural robustness and general mass of precast concrete make it ideal material for withstanding the high loads and vibrations of these structures. The left photo, a triple riser, shown stored in a precast fabrication plant in preparation for final patching and quality control checks. The photo on the right shows a precast concrete riser unit being erected on a steel raker beam. Raker beams are another part of the stadium arena construction. They can provide support for the previously mentioned stadia components. The list of available products could easily go on and on, but we'll stop with this list for more common components. So on the left, we can see precast rakers held up by precast columns with precast beams in between them. And on the right, you have vomitory walls and stadia and precast rakers. In this part of this presentation, we're going to look at the facet of precast concrete construction that makes it unique from other methods of concrete work. Precast fabrication is at a specialized manufacturing plant. The keystone of the precast pre-stressed concrete industry is the Precast Pre-Stressed Concrete Institute and its plant certification program. In general terms, it is broken out into two segments, structural precast and architectural precast. Each segment has its own guidelines for quality control, Manual 116 for structural precast and Manual 117 for architectural precast. When a precast manufacturer participates in PCI's plant certification program, it is required to create and maintain a quality systems manual that defines the operation's custom quality assurance and quality control procedures, all of which must follow the guidelines of either Manual 116 or 117. Plants participating in the certification program are audited twice a year by an independent engineering firm retained by PCI. These audits are unannounced and the subject plant is required to respond in writing to PCI with corrective action on all reported non-conformances. Before looking at the various steps in precast concrete manufacturing, let's first consider the general philosophy. First of all, precast concrete components are cast at a manufacturing facility away from the job site under strict manufacturing assurance. Components are then shipped to the job site and assembled to create the final structure. Fabrication of precast components and construction of job site foundations can occur simultaneously, thus allowing the schedule to be condensed. These components are produced under rigid quality assurance processes in a controlled environment. In its most fundamental definition, precast concrete is steel-reinforced concrete. All precast concrete manufacturing facilities will make use of non-pre-stressed reinforcing steel. Many facilities will also utilize pre-stressed reinforcing steel. These two photographs on the left side will show that the non-pre-stressed reinforcing steel for a wall panel and a close-up of the conventional mild reinforcing bars that are typically solid bar with surface deformations. The deformations serve to enhance the bond between concrete and steel. The photos on the right show the pre-stressed reinforcement in an inverted T and a close-up of the typical pre-stressing strand. Note that these strands are comprised of seven separate high-strength steel wires that are arranged in a spiral configuration. The cylindrical steel sleeves around the strands are chucks. These are used to grab hold of the strand, stretch it, then keep it stretched as the concrete cures. It can be noted that non-metallic types of reinforcement are gaining some prominence in the precast concrete industry and may also be encountered in precast concrete components. Looking first at how mild reinforcing works in precast concrete beams, these beams are designed for various levels of performance. This slide illustrates the mechanics through which the concrete and steel combine to create a structural system. The precast concrete reinforcement is placed in the bottom, then encased in concrete. Once a beam has been made to span between the bearing points, it begins to deflect and build up tension in the bottom surface. If the tension stress exceeds the concrete's tensile strength capacity, then cracking occurs. Once the cracking occurs, it is left solely to the mild reinforcing bars to resist the tension. It's good to note where appearance or durability of specific concern, the designer may elect to detail the mild steel in such a way to keep cracking width to a minimum. This is usually done through the use of closely spaced small bars or wire in lieu of large widely spaced bars. Alternately, the designer may elect to size the component and connect it in such a way to prevent the member from ever exceeding the concrete's tensile strength, thus preventing cracks from ever starting. This approach may require the use of shorter spans or a larger cross section. When prestressing enters the discussion, there are two methods to consider. Pre-tensioning and post-tensioning. Pre-tensioning means the concrete is cast and allowed to cure around the prestressing steel that has already been stressed to a specific tension force. Post-tensioning, on the other hand, the prestressing steel is encased in a conduit or sleeves to allow the tensioning to occur after the concrete has been cast or cured. This approach may utilize single and multiple strands within a given sleeve or conduit. In some circumstances, both methods may be used in a given component. Prestressing strand, in this illustration, is stressed to predetermined force, usually between 25,000 and 35,000 pounds, between abutments. Pre-stressed force will be determined based on the strength or size of the utilized strand. Concrete is then poured around the tension strand to allow for it to cure. This concrete is bonded to the concrete strand. Once the concrete has achieved its adequate strength, the strands are cut. It should be noted that many variables, cure time, temperature, humidity, will influence the amount of camber or the arc of the piece. As such, the designed camber is an approximation and is not to be considered an exact objective. It should be noted that camber of pre-stressed components can increase, particularly when members that have been designed to carry large superimposed loads or those that have been stored for an extended period of time in an unloaded state. The camber occurs as the strand tries to return to its original, un-tensioned length, and the pre-tensioned force is transferred to the concrete. In most flexural components, the strand is located near the bottom of the component so that the internal force causes the member to camber upward. After a pre-stressed beam is erected in its designed position, the load is applied. The compression imposed on the bottom of the beam by the strand begins to reduce. Concurrently, the camber in the beam begins to reduce. Depending on the beam's geometry span, pre-stressing, and concrete strength, a significant amount of load can often be applied without ever causing the bottom of the beam to go into tension. This can be good and bad, as such high pre-stressed force can also cause the bottom of the beam to shorten over time, which may, in turn, shorten the bearing at each end of the beam, which we'll be talking about later in the erection process. In addition to the load applied, the contribution of the pre-stress may eventually be exceeded, which would then allow the bottom of the beam to go into tension, or negative camber. This would further load the tension of the bottom of the beam, will eventually exceed the concrete's tensile strength, and cracking will occur. Unless an inordinate amount of load is applied to a beam, well above that assumed for a properly designed beam, the beam will likely return to its original camber and internal stresses once the load is removed. It should be pointed out that in the case of precast structures utilizing rigid connections and or continuity, that these basic principles may not apply. In those instances, the structural engineer of record and the precast or specialty engineer should be consulted to determine those areas that are critical by the hybrid configuration. Having considered the fabrication process, let's look at what happens once the erection of the structure begins. First, let's review that all of these components are largely independent units, and they're not relying on the behavior of adjacent members. The primary design concerns are positive flexure and steer. Primary reinforcement is located near the bottom of most horizontal components. With primary reinforcement near the bottom of components, its exposure to the elements is reduced and its vulnerability to corrosion lessened. Redundancy is limited as components behave independently. Steer reinforcement configuration is similar to conventional cast-in-place concrete construction. As we said before, let's get started looking at the erection and the on-site inspection where most of the work will occur. In this section, we're going to take a look at that facet of precast concrete construction that makes it unique from the other methods of concrete work. The fabrication in a specialized manufacturing facility. Before conducting inspections, the special inspector should familiarize her or himself with the approved erection drawings and the erection plan developed by the precast concrete erector, precast concrete manufacturer, and or the precast concrete specialty engineer. Instructor's product data should also be reviewed for materials to be used during erection, such as grout, epoxy adhesives, post-installed anchors, and other proprietary items. So again, the precast erection drawings, the erection plan, and any materials needed. The first set of documents we're going to review are the erection drawings. Initially, these drawings are prepared by the precaster or the specialty engineer for the purpose of illustrating the project design team, the intended plan for configuring and erecting the building. Once these drawings are approved and components begin arriving at the job site, the erection drawings are referenced by the erector for information on how to connect the components to each other as well as other structural systems. When we're looking at the erection drawings, we're looking at the general notes, how the embedded items are in the foundation or adjacent frames, any other framing plans, exterior elevations, internal elevations, and sections and details. General notes included in the precast concrete erection drawings, usually found in the upper right-hand corner, provide a summary of the criteria to be used by the specialty structural engineer to design the structure such as applicable codes, design loads, and material properties. This sheet may also include special instructions to the erector about the erection sequence, handling procedure, and connection requirements. The foundation embed plan will locate all weld plates, dowels, anchor bolts, and other connection hardware required to be cast into the building's foundation. This would be the plan used by the general contractor or construction manager or subcontractor to place those embeds, which will be integral in the system. The foundation embed plan will show those embedded items should be identified by a unique part number, which gives a description of the item. The location of the item should be referenced off established gridlines for the project. Drawing user will be further directed to additional information by section and detail call-outs. In this example, the detailed use of the piece is shown. Here in detail 100, expanded sections of details provide thorough description of how components are to be positioned and connected. In this case, the item JMP001 located on the embed plan is now described and located. The user can then look at elevation A, once again see the JMP001 embedded in the foundation. Additional weld information is provided for reference at this time, and the precast concrete wall above is erected. We can see by the flags that those will be field welds, quarter-inch fillet welds, and the design of the fillet weld and where it should go. Framing plans offer downward views of each level of the structure. Included in this view are pertinent dimensions for the building's footprint, identification numbers of each piece for the floor system components, and locations for the floor system connection. In this magnified view of the previous example framing plan, you can see column grid line, line spacing, precast concrete component size, component identification numbers, or piece marks. Note that in this case, piece mark DT, usually for double T, 220, is used for multiple pieces, indicating that the design and geometry of this component is identical, and it also shows the connection locations. As with the foundation embed plan, the framing plans will reference expanded sections and details to show exactly how the connections and component positioning in greater detail. Drawings that show elevation will offer views of the vertical components throughout a precast concrete structure. As with the framing plan, these drawings will include piece marks for vertical components, pertinent dimensions for these members, and reference to expanded sections and details. In summary, the erection drawings convey the intentions of the precast engineer, and they should be used extensively by the erector and the special inspector to make sure that actual construction meets those intentions. In addition to the erection drawings, there should also be an erection plan. The erection plan should lay out the erector's plan for meeting the expectations of the precast engineer. While the responsibility for preparing this document may vary from project to project, input is imperative from both the project's erector and the precast engineer. Engineers may participate, such as the structural engineer of record, the general contractor, or the precast concrete manufacturer. In most instances, the special inspector will not be required to monitor the erector's adherence to the erection plan, however it is important to recognize the significance of this document in the successful construction of a building. We'll take a look at it here, just as the information, so that we can show the special inspector has an expectation of the guidelines the erector will be working under. The erection plan should lay out the erector's plan. These are the items that should be covered under the erection plan. In this example, the precast concrete manufacturer's engineering department, with input from the job's erector, has developed a guide for the project's grouting, erection, and bracing procedures. After its final review, it is acknowledged and signed by a representative of the engineering department and a representative of the erector. Throughout the erection process, this plan should be readily accessible to the erection crew for direction and the inspectors as they perform their work. Again, an example. Detailed instructions are provided for the grouting and load-bearing wall panels and columns. How they should be grouted, when they should be grouted, how they should be inspected. A detailed description in the erection sequence is provided to illustrate the intended progression of the construction process. This would also include the placement of the crane. In these drawings, you can see where braces are to be placed. A number of things can be included in the sequence plan. Here the erection plan offers a section view of a brace structure. At the bottom of the page, instructions are provided to illustrate the minimum accepted amount of welding during erection. It shows in the clouded note that these welds are required only if work is proceeding down the deck ramp. It gives some direction from the engineer to the erector on what should be occurring before the next phase. Two other sources of good information on precast concrete erection are available through PCI. The Erection Safety Manual and the Erector's Manual both offer valuable recommendations and guidelines that can be used by erectors. They form the backbone of the Certified Erector Program. When determining the parameters of which precast components must be set, PCI's Tolerance Manual and Erector Manual can be useful tools. The final set of documents that should be available to the erector inspector on site is any product information necessary for the proper use of proprietary items, such as grout material, post-installed anchors, and chemical adhesives. The need and nature of this information should be pretty self-explanatory, so we won't spend much time in that area. That takes us to the actual inspection part of the precast concrete erection process. As precast concrete components begin to arrive on the site, it is imperative to realize that these members have been poured at an off-site facility, moved to a storage area, placed back on a trailer, and subsequently shipped to the job site. Upon arrival at the job site, these components are then lifted from trailers, hoisted into their position on the structure, oftentimes with the help of a pry bar. Throughout these processes, the components are subjected to unpredictable loads and impacts that may cause damage. In most instances, the damage is minor, but the inspector should be on the lookout for potentially serious damage. In most instances, the damage caused by transportation and handling poses no threat to the member and can be readily repaired. However, it is important to be able to recognize those types of damage that may indicate a serious structural concern. Once erected, they should then be checked for proper placement and stabilization. In all cases, the precast engineer should be consulted to assure proper action is taken. The following slides will illustrate various precast components and types of distress they sometimes sustain. Some of the things that we're going to be looking for are cracks and spalls, placement, and stability. Let's first take a look at columns. As they are often the first components to arrive on the site, columns will be discussed first. Of course, these members are typically required to collect and carry the large gravity loads down to the structure's foundation. In addition, they may be subjected to flexural or bending stresses that can create tension in a portion of the cross-section. Because the demands are high in these components, it is important to assure that their condition is suitable before the load is applied. As these members arrive on the site, the erector and periodically the special inspector should review them for damage following the several types of distress that may be evident in columns prior to erection. As a column is stripped from its form in the plant, in the case of prestressed columns, it is detensioned. It may then bind in the form and place unintended stress on the corbels. Depending on the concrete state of cure, this may result in minor cracking where the corbel meets the main body of the column. On the assumption that there is confidence that the correct amount of corbel reinforcing was properly installed in the column, a hairline crack in this instance is usually deemed non-critical and is either epoxy injected or in the case of very narrow cracks, simply patched over. So those would be the cracks that go from the top near the corbel into the column. During stripping and detensioning or handling of a column, it is not unusual for minor chips and spalls to occur at the corners or edges. These are rarely a structural concern and most frequently are repaired by patch material and a bonding agent. The severity of this condition depends upon the design criteria for the connection's fixity at the base or the top of the column. This condition should be reported to the precast engineer for determination for the appropriate corrective action if we have cracks at either the top or the bottom of the columns as shown in the drawings. If a column is improperly handled or stored, these types of cracks can develop. Pre-stressed columns are less susceptible, although not immune to this type of cracking, as the internal compression pre-stress force will resist tension caused by bending. Depending on the severity of the cracking, these cracks will usually close after the pre-stressed column has been erected, often to the extent for which they can no longer be discerned. For non-pre-stressed columns, these cracks will close somewhat under applied loads, but not usually to the extent that cracks in pre-stressed columns will close. In the case of exterior architectural columns, consideration should be given to the appearance of those cracks. While most cracks can be readily repaired, it is important to recognize when the damage is too significant to salvage the component. In this example, a pre-stressed column was loaded on a defective trailer hauled over 500 miles. During the trip, the column was subjected to repetitive bouncing, which led to cracking and an eventual yielding of the pre-stressed strand, or a loss of bond between the pre-cast concrete and the strand. The end result is irreparable damage and cause for a new column to be made. As always, the pre-cast concrete specialty structural engineer or structural engineer of record should be consulted when determining whether a component can be repaired. As previously mentioned, some cracks will be nearly imperceptible once the column is stood upright. In this photo, you can see a subtle indication of a prior crack, but the implications are very minor, so no corrective measures were taken in addressing this condition. Now let's take a look at beams. Pre-cast concrete beams are often required to support large areas of a structural frame. Beams with this type of heavy demand will typically have a robust design, utilizing a significant pre-stressed or non-pre-stressed reinforcement. Because they are designed to carry such heavy loads, the stresses imparted on them during handling and transport are usually small compared to those they will encounter while in service. As a result, damage resulting from handling and transport of these beams is often limited to minor chips and spalls. As we look at the two photographs, the one on the left shows an L-beam, afferably named, and an inverted T-beam. Each will carry double Ts or other units. You can see that they are in storage, ready for delivery. In the case of heavily pre-stressed beams, hairline cracking may be occasionally observed at the top surface of the beam. This is sometimes caused by heavy pre-stressed force concentrated near the bottom of the beam, coupled with the absence of inter-counteracting applied dead load while the beam is in storage. Once the dead load of the members has been supported by the beam and the beams are in place, these cracks typically become imperceptible and do not pose a structural or durability concern. For more information on occasionally encountered cracks and beams, consider this PCI Journal article published in May to June 1985. Although published nearly 30 years ago, this article offers recommendations that remain valid to this day in the assessment of pre-stressed concrete beams and columns. Similar to columns, wall panels carry gravity loads, applied loads, or their own weight to the building's foundation. They typically bear directly on the foundation or on the wall panel below. Prior to their installation, the erector should inspect the wall panels for handling or transportation damage. The influence of cracks and spalls on wall panels' structural integrity is similar to that described in the columns. Oftentimes this damage will not merit a structural repair, but instead be aesthetically addressed. As always, the precast engineer or engineer of record should be consulted to determine the appropriate cause of action. One of the precast concrete industry's most widely used products, double Ts, have been subject to much study when it comes to assessing and addressing and preventing handling and shipping damage. Their overall slender cross-section can sometimes leave them susceptible to torsional stresses. However, the efficient configuration of the cross-section also allows them to be fabricated in increasingly long dimensions. Their prominent use in all fashion of structures around the country, coupled with significant research efforts by the precast concrete industry, have allowed the precast concrete design community to gain a firm grasp on the double Ts' capabilities. Here we see a double T being pulled from a bed in a manufacturing plant. To gain an appropriate perspective of the capabilities of a double T, consideration should be given to this series of photographs. As part of a PCI research program to determine the amount of shear reinforcement needed in double Ts, this typical double T was positioned 49 inches off the floor of a testing area. At the test's heaviest load, the specimen was subjected to 160 pounds per square foot or four times the current code required load, live load, for parking structures. After this final test, the double T came to rest on the floor without failing. As illustrated by the last slide, double Ts and other pre-stressed simple span concrete components are capable of withstanding significant load and deflection when subjected to normal uniformly applied loads. It is very rare that a pre-stressed concrete beam or double T is compromised due to cracking or damage at the center of the span. Even in instances of strong impacts and considerable damage, pre-stressed components will often retain their integrity. In the case of the one shown here, the bottom of a double T was struck by a forklift, knocking out a large portion of the concrete and exposing the pre-stressing strand. Despite the damage, the member span remained in place. In consideration of the extent of the damage, precautionary shores were placed below the double T and traffic was prohibited on and beneath the double T and the damaged T stem was later repaired with an epoxy resin and carbon fiber fabric. Because of its limited thickness and its cantilever off the double T stems, the top slab or flange may sustain damage during handling or shipping. Most common are cracks caused by tensioning stresses through the transport or cracks caused by accidental contact with other components. Here you can see a roof leveled double T on a parking garage that was repaired 12 years ago by gravity feeding low viscosity epoxy into the crack and then rubbing over the repaired crack with a cementous slurry. In the second photograph, you can see there is no evidence of the crack propagating through the underside of the double T flange nor any sign of moisture passing through the flange. An appropriate fix. It is also not uncommon for excessive loads to be mistakenly placed on double T flanges by other trades during the construction phase. Because of the cause, most damage to double T flanges are readily fixed and will not jeopardize the durability or the integrity of the component. They can usually be formed up, poured, and meet their design. In the case of dapped double T's, that is a double T in which the bearing is not the full height of the T. It is not unusual to see some hairline cracking in the re-entrant corner of the dapped bearing assembly. This may be caused by the internal stresses, pre-stress force within the T stem, or a load applied to the double T. These cracks are not typically a structural concern. Depending on the width of the crack, the precast engineer may elect to patch over or seal these cracks with epoxy to preserve the long-term durability of the component. Unlike minor cracking at the daps, the presence of significant shear cracking in the double T stem should be considered a serious structural concern. As with the concrete beam, shear failure within a double T can take place quickly. In the picture above, the series of cracks, including one major shear crack, has developed near the end of a double T stem. This should be brought to the attention of the precast engineer or the engineer of record immediately. In this instance, it was discovered that the building's rooftop planters were not draining properly. In order to correct the situation, the soil from all the planters were removed and stockpiled in one location. The resulting pile was well over the intended design load for the double T, which caused the observed damage we saw in the slide before. Once discovered, emergency shoring was put in place to secure the damaged component until repairs could be carried out. The PVC resin and carbon fiber fabric were used to restore the double T to its intended strength. For additional information on occasionally encountered double T cracks, consider the PCI Journal article, Fabrication and Shipment Cracks in Pre-Stressed Holocore Slabs and Double T's. Like the similar article published in 1985 for beams and columns, this document still offers noteworthy recommendations on possible causes and corrective measures for cracks and double T's. This now ends Part 1 of our Special Inspectors Training Program. I hope that you were prided with enough information. If you have any questions, feel free to forward those to us. We are happy that you are part of this program and that you have an interest in this training. Thank you very much, and we will talk again next week.

precast concrete construction

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manufacturing

inspection documents

erection

special inspections

parking structures

double Ts

fabrication

inspection process