To improve the quality and durability of concrete structures, several measures can be taken. Firstly, reducing the maximum internal temperature of the concrete can help prevent cracking and improve its strength. Additionally, delaying the onset of cooling and controlling the rate at which the concrete cools can also be beneficial. One way to achieve this is by insulating the exposed concrete surface during the first five days of curing. This can be done using 50mm thick thermocol sheets encased with polythene sheet laid over concrete surfaces already covered with hessian cloth and a water sprinkler keeping the hessian wet. It is important to maintain a temperature gradient of no more than 150 C between the core of the concrete and the surfaces.
Another way to improve the quality of concrete is to increase its tensile strength. This can be achieved by reducing the concrete temperature at placement up to 32 0 C. Using low heat of hydration cement or fly ash to replace part of the cement can also help. During the winter, keeping the steel formwork warm by air heating can prevent the concrete from freezing and cracking. Using thermally insulating material as formwork and keeping it for a longer duration can also help.
Lastly, for repairs, sealing and grouting of concrete cracks can help prevent water and other harmful substances from penetrating the concrete structure. It is recommended to use low-grade cement, such as OPC 33 grade, or cement with high C2S content for improved durability.
Types of Design and Detailing Errors in Construction and their Prevention
Following are the different design and detailing errors in construction, their symptoms and prevention methods:
(1) Inadequate structural design
Inadequate structural design can cause concrete to be exposed to greater stress than it can handle or result in an increase in strain beyond its capacity, leading to failure. This failure is usually characterized by spalling or cracking of the concrete. When compressive stress is excessively high due to poor design, the result is usually spalling of the concrete. On the other hand, high torsion or shear stresses can cause spalling or cracking of the concrete. Cracking of the concrete also occurs when it is subjected to high tensile stresses.
To identify inadequate design as the cause of structural damage, a thorough inspection of the structure is necessary, and the locations of the damage should be compared to the types of stresses present in the concrete. In the case of rehabilitation projects, a comprehensive petrographic analysis and strength testing of concrete from elements to be reused are necessary.
To prevent inadequate structural design, it is crucial to thoroughly review all design calculations carefully. Any rehabilitation method that involves the use of existing concrete structural members must also be carefully reviewed to ensure that they are structurally sound.
(2) Poor design details
In structural engineering, it’s important to consider the impact of poor design details on structural members. Even if a design appears to be adequate to meet the requirements, poor design details can result in localized concentrations of high stresses within the structure. These high stresses can lead to the cracking of concrete and create pathways for water or chemicals to penetrate the concrete. As a result, poor design detail may not necessarily result in structural failure, but it can cause deterioration of the concrete over time.
To prevent such problems from arising, it is crucial to conduct a thorough and careful review of plans and specifications for the construction work. By doing so, potential issues can be identified early on and addressed before they cause significant damage. It’s also essential to consider the types of poor design detailing that can affect the structure, as well as their potential effects.
One type of poor design detail is the insufficient thickness of concrete cover over reinforcement. This can lead to corrosion of the reinforcement, resulting in structural damage. Another type of poor design detail is inadequate spacing between reinforcement bars, which can cause concrete cracking and reduce the strength of the structure. Additionally, inadequate detailing of joints and connections between structural members can lead to reduced structural stability, making the structure susceptible to failure during earthquakes or other extreme events.
In summary, careful attention to design details is critical to ensure the longevity and safety of structural members. By reviewing plans and specifications thoroughly and identifying potential issues early on, engineers can prevent problems such as cracking, corrosion, and deterioration of the concrete from occurring. It’s essential to consider the types of poor design detailing that can affect the structure and take the necessary steps to address them before construction begins.
(a) Abrupt changes in section:
When there are sudden changes in the shape or dimensions of a structure’s section, it can create areas of stress concentration that may lead to cracking. This is particularly true when thinner sections are connected to much larger sections or patches in a rigid manner. Similarly, if replacement concrete is not uniform in its plan dimensions, it can also cause stress concentrations and potential cracking. These scenarios highlight the importance of maintaining a consistent and well-planned structure to avoid issues with stress concentration and structural integrity.
(b) Insufficient reinforcement at corners and openings:
Stress concentrations can lead to cracking, particularly in areas with corners and openings. Therefore, it is crucial to take steps to prevent stress concentrations in such areas. One effective strategy is to provide additional reinforcement to mitigate the potential for cracking.
By reinforcing the sections of a structure where stress concentrations are expected, you can help distribute the load and reduce the likelihood of cracking. This is especially important in areas where the structure will experience significant pressure or force.
It’s important to note that prevention is key in avoiding cracking caused by stress concentrations. Once a crack appears, it can quickly spread and lead to larger issues. By taking proactive measures, such as additional reinforcement, you can help ensure the long-term durability and safety of a structure.
(c) Inadequate provision for deflection:
When loads exceed the expected values, there is a risk that certain members or sections will be subjected to stresses beyond their intended design capacities. This can occur particularly in walls or partitions, leading to the development of cracks as a result. It is therefore crucial to be aware of the potential for such deflections to occur and to take appropriate measures to prevent them from happening. By doing so, it may be possible to avoid the need for costly repairs or replacement of damaged structural elements.
(d) Inadequate provision for drainage:
Improper drainage of a structure can lead to the accumulation of water, which can have serious consequences. If water is allowed to pool, it can result in leaks or cause the concrete to become saturated. These leaks can cause damage to the interior of the structure and leave unsightly stains and buildup on the outside. If the concrete becomes saturated and the structure is located in an area that experiences freezing and thawing, the damage can be severe and potentially irreversible. Therefore, it is crucial to pay close attention to the details of draining a structure to prevent these issues from occurring.
(e) Insufficient travel in expansion joints:
When it comes to expansion joints in concrete structures, improper design can lead to spalling of the concrete around the joints. To avoid this issue, it is crucial to consider the full spectrum of temperature differentials that the concrete is likely to encounter when specifying the appropriate expansion joint. It is important to note that there is no one-size-fits-all solution for expansion joints, as the required specifications will vary depending on the specific temperature differentials involved.
To ensure that concrete expansion joints are effective, it is imperative to take into account all possible temperature variations that the concrete may experience. If the expansion joints are not appropriately designed, the concrete around them may spall, which can be both unsightly and structurally problematic. By considering the full range of expected temperature differentials, the appropriate expansion joints can be specified to ensure the longevity and durability of the concrete structure.
The specification of expansion joints for concrete structures requires careful consideration of the temperature differentials that the concrete is likely to encounter. Failure to properly account for these variations can result in spalling of the concrete around the expansion joints, which can have serious consequences. It is important to understand that there is no single expansion joint that will work in every situation, as the specific requirements will vary depending on the temperature differentials involved. By carefully evaluating the full range of potential temperature differentials and selecting the appropriate expansion joints, the durability and longevity of the concrete structure can be ensured.
(f) Incompatibility of materials:
When materials with varying properties, such as different modulus of elasticity or coefficient of thermal expansion, are used adjacent to each other, it can lead to issues such as cracking or spalling. These problems can occur as the structure is loaded or when it is subjected to daily or yearly temperature changes.
The difference in properties between adjacent materials can create stress concentrations at their interface, leading to the development of cracks. Additionally, the materials may expand or contract at different rates in response to temperature changes, causing them to pull apart or push against each other, resulting in spalling.
Such problems are a common issue in construction and engineering, and they can compromise the integrity and safety of the structure. To mitigate these issues, various techniques can be employed, such as the use of intermediate layers or the selection of materials with compatible properties. It is crucial to carefully consider the properties of the materials being used and their compatibility with each other to avoid potential problems in the future.
g) Neglect of creep effect:
When creep is disregarded, it can result in comparable consequences to those that arise from insufficient provisions for deflections in structures. Furthermore, failing to account for creep in prestressed concrete members can lead to excessive loss of prestress, which in turn can cause cracking as the loads are applied.
(h) Rigid joints between precast units:
When utilizing precast elements in design, it is crucial to ensure that there is allowance for movement between adjacent precast elements or between the precast elements and the supporting frame. The absence of provision for such movement can lead to the development of cracks or spalling, which can compromise the integrity and durability of the structure.
To prevent such failures, designers must consider the potential for movement and incorporate appropriate measures to accommodate it. This may include the use of expansion joints or other flexible connections between precast elements or between precast elements and the supporting frame. These measures allow the structure to respond to changes in temperature or other environmental factors that may cause movement and avoid the development of unwanted stresses that can lead to cracking or spalling.
By providing for movement between precast elements, designers can ensure the long-term performance and durability of the structure. Failure to consider this aspect can result in costly repairs or even structural failure, compromising the safety and functionality of the building or infrastructure. Therefore, it is essential to prioritize the incorporation of measures for movement accommodation in any design utilizing precast elements.
(i) Unanticipated shear stresses in piers, columns, or abutments:
When expansion bearing assemblies are neglected and not properly maintained, they can become stuck and immobile. This can lead to the transfer of horizontal loads onto the concrete elements that support the bearings. As a consequence, the concrete may start to crack, and this problem can be exacerbated by the infiltration of water into the concrete.
It is important to note that freezing of expansion bearing assemblies can have serious consequences for the structural integrity of a building. The transfer of horizontal loads to the concrete can lead to cracking, and this can ultimately compromise the safety and stability of the entire structure. Therefore, it is crucial that regular maintenance is carried out to ensure that the bearings remain in good working condition.
Furthermore, the effects of water infiltration into concrete can be far-reaching. Water can cause concrete to deteriorate rapidly, leading to the loss of strength and stiffness in the material. In addition, the presence of water can encourage the growth of bacteria, fungi, and other microorganisms, which can further weaken the concrete and cause it to degrade. Therefore, it is essential that steps are taken to prevent water from entering the concrete, particularly in areas where expansion bearing assemblies are located.
Overall, it is vital that proper maintenance procedures are followed to ensure that expansion bearing assemblies are kept in good condition. Failure to do so can result in significant damage to the concrete elements supporting the bearings, leading to potential safety hazards and costly repairs. Additionally, steps should be taken to prevent water infiltration into the concrete, as this can exacerbate the problem and cause further damage. By prioritizing regular maintenance and taking preventative measures, the structural integrity of buildings can be preserved, ensuring the safety of those who use them.
(j) Inadequate joint spacing in slabs:
Cracking of slabs-on-grade is a common issue that occurs due to various reasons. However, one of the most frequent causes of this problem can be attributed to a specific factor. Identifying this factor and taking measures to address it can significantly reduce the occurrence of slab cracking and improve the overall durability of the structure.
It is important to note that slabs-on-grade refer to concrete slabs that are poured directly on the soil without any additional support or foundation. Due to their direct contact with the ground, these slabs are susceptible to various forms of stress that can cause them to crack. Some of the common causes of cracking in slabs-on-grade include shrinkage, settlement, and frost heave.
However, among these causes, one factor stands out as the most frequent cause of cracking in slabs-on-grade. Identifying and addressing this factor is crucial to prevent slab cracking and improve the long-term performance of the structure. By taking proactive measures to address this issue, builders and engineers can significantly reduce the occurrence of slab cracking and ensure the durability of the building.