This article discusses the various factors that contribute to the development of cracks in different parts of reinforced buildings. Such cracks can cause the building to fail in terms of serviceability design, making it important to identify and mitigate them. To this end, the article explores some of the principal techniques that can be used to prevent or reduce the occurrence of cracks in reinforced buildings. These techniques are described in detail to help readers understand how they work and how they can be applied effectively in building design and construction. Overall, the article provides valuable insights into the prevention and mitigation of cracks in reinforced buildings, which can help improve the safety and durability of such structures over time.
Measures to Mitigate Cracks in Reinforced Concrete Structures
When planning the layout of restraining members in a structure, it is important to consider factors such as the expected loads and potential areas of stress. By strategically placing these members, it is possible to distribute the forces evenly and increase the overall stability of the structure.
Structural separation is another key consideration, as it can help to reduce the risk of damage in the event of an earthquake or other natural disaster. This involves dividing the structure into separate sections, each of which is designed to withstand the forces it is likely to experience.
When it comes to the actual construction process, there are several important factors to consider. Closure strips, joints, and favorable pouring sequences can all play a role in ensuring that the finished structure is as strong and stable as possible.
In terms of released connections, it is important to carefully consider the various types of releases that may be required. This includes wall/slab release, slab-column release, and wall joints, among others.
Finally, improving the layout of mild reinforcement and tendons can also help to increase the overall strength of the structure. By carefully considering the placement and distribution of these elements, it is possible to create a structure that is not only safe and stable, but also built to last for many years to come.
Planning the Layout of Restraining Members
The effective prevention of restraint-cracks during building construction can be achieved by careful planning of the positions of columns and walls. By placing an equal number of walls with the same length, the tendency for cracks to develop can be reduced. This is because such arrangements allow the slab to move freely in the direction of the planned point of zero movement. A visual representation of this concept can be seen in Figure-1. Therefore, careful consideration of the placement of columns and walls during architectural planning can greatly improve the structural integrity and longevity of a building.
Fig.1: Favorable Arrangement of Restraining Cracks
In certain scenarios, the placement of walls and overall layout of a structure can impede the natural movement of those walls. This can lead to the development of cracks, as depicted in Figure-2. When walls are unable to shift and adjust as needed, any pressure or movement that occurs within the structure can cause stress to build up in certain areas, ultimately resulting in cracks. It’s important to consider the overall design and layout of a building to ensure that walls have the necessary freedom to move and shift as needed, reducing the likelihood of cracking and other structural issues.
Fig.2: Unfavorable Arrangement of Restraining Walls
Structural Separation to Mitigate Cracks in Reinforced Concrete Structures
Slabs with irregular geometry are prone to cracking, and Figure-3 provides an example of such a slab. The figure illustrates a structural separation between a larger post-tensioned rectangular slab and a smaller square slab. The width of this structural separation typically falls within the range of 13 to 26 mm.
It is important to note that the function of a structural separation is limited in time and usually lasts for only two or three months. In contrast, an expansion joint’s purpose is to make room for temperature-induced movements and must continue to serve this function throughout the structure’s lifespan.
Therefore, while both structural separations and expansion joints provide essential functions in preventing cracking and damage in slabs, their respective roles and lifespans differ significantly. It is crucial to understand these differences to ensure proper installation and maintenance of these features in any construction project.
Fig.3: Separation between Large and Small Slabs that Create Irregularities
Closure Strips, Joints and Favorable Pouring Sequences
A closure strip is a temporary space that separates two regions of a slab that are constructed and post-tensioned differently, allowing them to experience shortening independently. The width of the closure strip is determined based on the distance needed to install a stressing jack between the two slabs and is typically between 76-91 cm. Non-shrink concrete is used to fill and consolidate the space between the two regions of the slab, usually after one to two months.
The duration for which the closure strip remains open is determined by the extent of shortening required before joining the two slabs. Reinforcement extending into the closure strip from each side of the concrete slab provides continuity between the two portions. The amount of steel reinforcement embedded in the closure strip is calculated based on the bending moment and shear forces at the closure strip location, considering the entire slab as a continuum.
It is recommended that the stressing ends of tendons stopped in the closure strip be cut, sealed, and grouted for corrosion protection purposes. Based on experience, the closure concrete is poured after considering a shortening of approximately 6.35 mm on each side of the closure strip.
Fig.4: Closure Strip Width Between Two Regions of a SlabConstruction joints
Construction joints are used in concrete slabs to provide separation for a short period of time between two regions of the slab. The purpose of this is to control cracking. These joints are typically introduced between two concrete placements, and their positions are specified in advance.
One additional benefit of construction joints is that they can help to divide large slabs into smaller sections, making it easier to manage the construction process. This can be particularly useful when working on larger projects where there is a lot of concrete to be poured. By breaking the slab up into smaller sections, it becomes easier to pour, level, and finish the concrete.
Overall, construction joints are an important part of the concrete slab construction process. They help to ensure that the slab remains structurally sound over time by controlling cracking, and they make it easier to manage the construction process when working with larger slabs.
Fig.5: Construction Joint; (A) Without Stressing, (B) With Intermediate Stressing
The purpose and function of construction joints depicted in Figure-5 differ from that of cold joints. While construction joints are specifically located as per the designer’s instructions to manage and control the formation of cracks in concrete structures, cold joints result from a pause in concrete batching and the subsequent delay of three to seven days before the next concrete placement.
In post-tensioned slabs, intermediate stressing is employed for long tendons due to substantial stress loss. Closure strips are implemented in post-tensioned slabs to enhance their performance, and guidelines exist regarding their placement. For instance, when the slab length is shorter than 76 meters and supporting walls are located favorably, closure strips or structural separation are not necessary. If the slab length ranges between 76 and 114 meters, one centrally located closure strip is suggested. On the other hand, for slab lengths exceeding 114 meters, structural separation is recommended.
Released Connections: Wall/ Slab Release, Slab-Column Release, Wall Joints
The passage discusses the use of released connections in construction to prevent cracks in buildings. Released joints are designed to allow for limited movement of the slab in relation to its support, but they must be properly constructed and placed in order to be effective. If the supporting structural members are not well-positioned, or if closure strips and construction separation are not adequately applied, then released connections can be particularly useful in preventing structural separation and related problems.
There are three types of released connections that can be used in construction. The first is the wall/slab release, which allows for movement between the wall and the slab. The second is the slab-column release, which permits movement between the slab and the supporting column. The third type is wall joints, which are used to connect walls and allow for some degree of movement between them. By using these types of released connections appropriately, it is possible to reduce the risk of cracks and other structural issues in buildings.
Wall / Slab Release
Several types of joints are available for construction purposes, and all of them utilize slip-resistant materials to prevent slipping. The joint release with ties is the most effective type, but its use is limited because walls are usually necessary to transfer shear force and gravity loads at the wall-slab interface. In Figure-6, different types of wall-slab release are demonstrated for exterior walls and terminating slabs. However, with some modifications, all of these types can be employed for intermediate slabs and interior walls.
Fig.6: Wall-Slab Release Types
Slab-Column Release
The given context describes the possibility of designing columns that can resist lateral forces without experiencing any signs of damage. These columns may also be designed to allow for relative displacements at the joint between the column and the slab. The figure provided as an illustration shows the use of hinged construction at the base ends of the column, as well as the detailed design of the joints. By using this approach, it is possible to create a column that is able to withstand lateral forces while also allowing for necessary movements and displacement without causing any damage.
Fig.7: Hinged Construction at Base of End Columns; (A) Elevation View, (B) Joint Detailing
Wall Joints
Vertical joints are a type of joint that is located between adjacent walls. These joints are designed to accommodate the displacement of slabs and beams that are supported by the walls. The purpose of these joints is to prevent the development of cracks in the beams, slabs, and walls that they support.
The effectiveness of vertical joints in mitigating cracks is significant. They are particularly useful in preventing the development of cracks in beams, slabs, and supported walls. By allowing for the displacement of slabs and beams, these joints help to ensure that the load on the walls is evenly distributed, reducing the risk of cracking.
The importance of vertical joints is illustrated in Figure-8, which shows a plan of a rectangular slab supported by interior columns and perimeter walls. The vertical joints in this illustration are critical to ensuring that the slab and beams are properly supported and that cracking does not occur. Without these joints, the slab and beams would be more susceptible to cracking, which could compromise the integrity of the entire structure.
Fig.8: Wall Joints; (A) Plan Showing All Joints and Closure Strips, (B) Plan Showing Arrangements of Different Wall-Slab Joints
Addition or Improved Layout of Mild Reinforcement
The previous sections have outlined various measures for crack mitigation. However, in certain cases, it may still be necessary to install additional reinforcement to address potential crack development. This is particularly relevant in situations where release joints cannot be provided due to shear transfer requirements specified in the design.
One example of this is at the slab and its supporting walls, as illustrated in Figure-9. In such cases, the installation of mild reinforcement at potential distress positions can help to prevent or mitigate cracking. This additional reinforcement can provide the necessary support to the structure, reducing the likelihood of cracks developing due to stress or strain.
While proper release joints are often the preferred method for crack mitigation, there are situations where this may not be feasible. In such cases, installing extra reinforcement can be an effective alternative. This approach can help to ensure the structural integrity of the building, minimizing the risk of damage or collapse due to cracking.
Fig.9: Crack Mitigating Reinforcement next to Shear Walls; (A) Interior Shear Wall. (B) Exterior Shear WallIt
The effectiveness of placing reinforcement in slabs parallel to shear walls has been demonstrated in Figure 10. The reinforcement is placed over a distance of nearly 3 meters vertically to the wall. This placement has been found to be substantially effective in improving the stability of the structure.
The reinforcement ratio used in this method is 0.0015 multiplied by the slab’s cross-sectional area over one third of the transverse span. The spacing between bars is 1.5 times the thickness of the slab, and they are installed alternately at the top and bottom.
Overall, this method has been found to be an effective way to improve the stability of structures. By placing reinforcement in this manner, the slabs parallel to shear walls are better able to withstand the forces acting upon them. This can help to ensure the overall stability and safety of the structure, which is of utmost importance in construction and engineering.
Fig.10: Reinforcements at the Corner of the Slab
Addition or Improved Layout of Tendons
To minimize anticipated losses in certain areas, it is recommended to install tendons that can apply additional compression. In order to achieve this, overlapping and dead-ending tendons are arranged in Figure-11 and Figure-12. It is crucial to pay close attention to the layout detailing of the strands around openings and discontinuities, as this can greatly impact the effectiveness of the tendons in these areas. Proper placement and arrangement of tendons can help to optimize the overall performance and durability of the structure.
Fig.11: Tendon Arrangement for Mitigating Cracks in Mid Spans
Fig.12: Tendon Arrangement to Compensate Restraining Effects of Transverse Wall
Fig.13: Tendon Arrangement at an Interior Opening