Ch. 22 and 23 Review Questions

List various types of reinforced-concrete slabs as described in this text, and the essential differences between them.

Ground-supported slabs (also called slabs-on-ground or slabs-on-grade) bear directly on compacted ground (grade) with organic topsoil removed. Elevated slabs rest on and are part of the structural frame of the building. They are referred to as framed slabs or suspended slabs. Elevated concrete slabs are used at the second and higher floors of the building. However, where the soil conditions are unfavorable (or if the building has a basement), they are also used at the first (ground) floor. Isolated concrete slabs are those that are separated from the building's foundation with an isolation joint. Stiffened concrete slab is generally designed to function as a slab-and-foundation combination for wood light-frame (or light-gauge steel-frame) buildings and is particularly well suited for such buildings on expansive soils.

Prestressing Strand

- 6 wires around a center wire (twisted) - Grease - Plastic sheathing

Two-Way Joist Floor (Waffle Slab)

A beamless waffle slab is a type of flat slab structure used in building construction that does not require traditional beams to support the load of the building. Instead, it utilizes a grid of closely spaced ribs and a two-way reinforced concrete slab system to support the load of the building. The waffle pattern provides the slab with increased rigidity and strength, while reducing the amount of concrete used in construction. The grid of ribs also creates a network of channels that can be used for electrical, plumbing, or mechanical systems, which can be more easily installed and maintained than in traditional flat slab systems. In a beamless waffle s

Construction Joint / Cold Joint

A construction joint is a nonmovement joint in a large concrete slab. This is used when the concrete cannot be placed in one continuous operation. The time interval between two concrete placements may be a few hours or several days. A construction joint is detailed as if the component is monolithic across the joint. In other words, the reinforcement goes through the joint undisturbed. However, if the interval between the concrete placements is long, the surface of contact is cleaned with a stiff wire brush before the next batch is placed. This improves the bond between the two placements, enhancing the monolithicity between them.

Flat Plate

A flat plate consists of a solid slab supported directly on columns. A flat plate is similar to a two-way banded slab, except that the beam bands in both directions are concealed within the thickness of the slab. Therefore, the spans that can be achieved economically with a flat-plate floor are smaller than those obtained from one-way or two-way joist floors. Flat-plate slabs are suitable for occupancies with relatively light live loads, such as hotels, apartments, and hospitals.

Flat Slab

A flat slab is similar to a flat plate, but it has column heads, referred to as drop panels. The primary purpose of drop panels is to provide greater shear resistance at the columns, where the shear maximizes.

Explain the difference between one-way slab and two-way slab actions.

A one way slab is supported by beams on two opposite side in one direction Two way slabs is supported on all four sides in both directions

Explain the difference between total precast and mixed precast construction.

A total precast building is constructed of all precast concrete members in which all structural components—columns, load-bearing walls, and floor and roof slabs—are of precast concrete. No steel beams, columns, etc. Mixed precast construction is far more common than total precast. It combines the benefits of both precast and conventional construction. In mixed precast, precast concrete is used only in floor and roof slabs. Using precast concrete floor/roof slabs yields significant savings because a large percentage of the cost of materials and formwork in a concrete structure is embedded in floor and roof slabs

Explain what concrete cover is and the functions it serves.

Concrete cover is the area measured from the exposed concrete surface to the nearest edge of reinforcement. To protect steel reinforcement from corrosion, a minimum amount of concrete cover is required.

Control Joint

Control joints accommodate the shrinkage of concrete. Their purpose is to provide weakness in the slab at predetermined locations to force the slab to crack there. In the absence of control joints, the slab will crack in a random, haphazard pattern. Control joints are generally provided by sawing the slab at intervals to a depth of 0.25 times the thickness of the slab. The width of a saw-cut joint is approximately 1/8".

Double-tee units

Double-tee units are used where the spans are large and cannot be provided economically with site-cast concrete construction or hollow-core slabs. They are commonly used for hotel and bank lobbies. As with hollow-core slabs, a topping of concrete and WWR may be used on double-tees for structural integration and leveling.

Describe the difference between formwork and shores.

Formwork refers to the temporary structures used to create the shape and size of the poured concrete. Formwork can be made of a variety of materials, such as wood, steel, or plastic, and is typically assembled on site. The formwork provides a mold for the concrete to be poured into, allowing it to take on the desired shape and size. Shores are temporary support structures used to hold up the formwork and the weight of the wet concrete while it is still in its plastic state and setting. Shores are typically made of steel or wood and are used to support the formwork until the concrete has hardened and can support its own weight.

Describe the difference between formwork and flying forms.

Formwork refers to the temporary structures used to create the shape and size of the poured concrete. The formwork provides a mold for the concrete to be poured into, allowing it to take on the desired shape and size. Flying forms are a type of formwork system that allows for the rapid construction of large concrete structures. Flying forms are designed to be lifted or "flown" from one level to the next as the concrete is poured and set, allowing for the rapid construction of the structure. Flying forms can be made of steel, aluminum, or other lightweight materials and are typically assembled off-site and transported to the construction site. They are often used in towers, bridges, and high-rise buildings.

Hollow-core units

Hollow-core slabs are precast, prestressed concrete slabs that contain voids in their central region. The voids reduce the dead load of the slab by 40% to 50% compared with a site-cast concrete slab for the same span. Hollow-core slabs are produced in thicknesses of 4 in., 6 in., 8 in., 10 in., 12 in., and 14 in., with the 8 in.-thick slab being most commonly used. The width of the slabs is generally 4 ft. Narrow-width slabs are produced in the plant by cutting standard 4-ft slabs along their length to fit a space that is not a multiple of 4 ft. In addition to 4-ft-wide slabs, some plants make slabs that are 3 ft 4 in. and 8 ft wide.

What is the function of stirrups? Sketch commonly used open-loop and closed-loop stirrups.

Stirrups are used as a shear reinforcement The stirrups help to transfer the load from the concrete to the steel reinforcement, which has a higher tensile strength and can better resist the shear forces. Specifically, when a shear force is applied to a reinforced concrete beam, the beam tends to bend and crack diagonally. The stirrups are placed in a way that they encircle the longitudinal bars of the beam and hold the concrete together in compression, preventing the cracks from widening and the concrete from failing.

Describe the differences between pretensioning and posttensioning of concrete. Is there any difference in the type of steel used in each case?

In pre-tensioning, steel strands or cables are tensioned before pouring the concrete, and the concrete is cast around them. Once the concrete has hardened, the tension is released, which puts the concrete in compression and the steel in tension. This process increases the strength of the concrete and reduces its susceptibility to cracking, which makes it ideal for use in long-span structures like bridges, parking garages, and high-rise buildings. (Structures that use precast) Benefits of pre-tensioning include: - Cost as members are repeated (Cost at Scale) - Less concern over protecting reinforcement (Cover) - Improved Durability Drawbacks of pre-tensioning include: - Requires large stressing equipment - Only able to pretension once (Irreversible) - Cracking at the point of release In post-tensioning, steel strands or cables are inserted into the hardened concrete via sleeves and then tensioned. The tension is maintained by anchoring the cables at the ends of the structure with a wedge. Post-tensioning allows for greater design flexibility and can be used to strengthen existing concrete structures. It can also reduce the size of structural members, resulting in cost savings. Benefits of post-tensioning include: - Only way to build certain structures - Sleeves take almost any shape - Stressing can be performed in the field which allows for necessary adjustments - Improved Durability Drawbacks of post-tensioning include: - Tensioning in the field can be hazardous to safety. - Requires special equipment - Design complications - Strand protection is more challenging

Isolation Joint

Isolation joints in a concrete slab extend the entire thickness of the slab. They are typically 1/2" wide and are provided to ensure that the slab is isolated from the building's structural components so that their movement (creep, foundation settlement, etc.) is not transferred to the slab. The joint space is generally filled with asphalt-saturated fiberboard and covered with a sealant.

Concrete reinforcing bars come in limited lengths. Explain how we ensure their continuity in a long structural element, such as a column in a high-rise building or a long, continuous beam.

Lap splicing allows reinforcement to extend continuously in long beams, large slabs, and multistory columns. Because concrete works as glue, the stress in a bar is transferred to concrete and from there to the lapped adjacent bar.

Inverted-tee beams

Precast inverted-tee beams are generally used as supporting members for hollow-core slabs or double-tee units. Like hollow-core slabs and double-tee units, they are prestressed. They are commonly used in total precast construction. They are also commonly used where the minimum span is 60'.

Describe the difference between reinforcing steel and prestressing steel.

Reinforcing steel, also known as rebar, is typically used in reinforced concrete structures to resist tensile forces. Concrete is strong in compression but weak in tension, and the addition of reinforcing steel helps to offset this weakness. Rebar is typically installed in a concrete structure before it is poured, and it is then encased in the hardened concrete. When the concrete is subjected to tensile forces, the rebar helps to resist those forces and maintain the structural integrity of the concrete. Prestressing steel, on the other hand, is used to create prestressed concrete, which can resist both tensile and compressive forces. In the prestressing process, steel tendons or cables are tensioned before the concrete is poured. When the concrete is cast and hardened, the tension in the steel is transferred to the concrete, creating a compressive force in the concrete. This compressive force helps to offset the tensile forces that the concrete is likely to experience during use.

Describe the difference between shoring and reshoring.

Shoring refers to the temporary support system that is installed to hold up the weight of the concrete slab or beam during construction. It is typically used when the slab or beam is first cast and is still in a plastic state, meaning it has not yet hardened. Reshoring refers to the temporary support system that is installed after the initial shoring is removed. It is used to provide additional support for the concrete slab or beam while it is still in the early stages of hardening. This is necessary because the concrete is still gaining strength and may not be able to support its own weight or any additional loads. Reshoring is typically installed within 1-2 days after the initial shoring is removed and is left in place until the concrete has reached the required strength.

Solid planks

Solid precast concrete planks are typically used when a structure requires a greater amount of load-carrying capacity than what can be achieved with hollow core planks. This is because solid precast concrete planks have a higher weight per square foot, which provides greater structural stability and strength. Additionally, solid precast concrete planks have a better fire rating and sound insulation than hollow core planks.

Using sketches and notes, illustrate the two most commonly used reinforced-concrete floor systems.

The two most commonly used reinforced-concrete floor systems are: One-way slab system: In this system, the slab is designed to span in one direction between beams or walls, and the load is transferred primarily in one direction. One-way slab systems are often used in smaller buildings with shorter spans, where the loads are relatively light and the spans are relatively short. Two-way slab system: In this system, the slab is designed to span in two directions, and the load is transferred in two directions. Two-way slab systems are often used in larger buildings with longer spans, where the loads are heavier and the spans are longer. The two-way slab system can be further divided into two types: flat plate systems and flat slab systems. Flat plate systems have no beams or girders, and the slab is directly supported by columns or walls. Flat slab systems may have drop panels around columns or thicker slab sections to provide additional strength and stiffness. Both one-way and two-way reinforced-concrete floor systems have advantages and disadvantages, and the choice of system depends on several factors, including the building's size, shape, and function, as well as the load requirements, architectural design, and construction budget. If Length/Width > 2 it behaves as a one way slab If Length/Width < 2 it behaves as a two way slab