Why cracks are the common problem in concrete?5 answersCracks in concrete structures are a common issue due to various factors such as faulty design, construction errors, overloading, and internally induced stresses. These cracks can be classified into structural and non-structural categories, with structural cracks posing safety risks to buildings. The formation of cracks in concrete can lead to reduced load-bearing capacity, compromised tightness, and aesthetic deterioration of structures. Detection of cracks is traditionally done manually, which is time-consuming and impractical for regular monitoring, leading to the development of automatic detection methods using artificial neural networks and machine learning. Despite the significant impact of cracking on construction projects, the exact financial implications are not always clear due to inadequate record-keeping, potentially resulting in hidden costs and underestimation of the true impact.
How does the orientation of cracks affects the compressive strength of concrete?4 answersThe orientation of cracks in concrete can have an impact on its compressive strength. Studies have shown that cracks formed due to frost action or other expansive reactions can lead to further damage and increase moisture transport. The extent of consequential damage is likely influenced by the orientation of the initial cracks. Traditional methods of quantifying crack orientation are time-consuming, but a proposed method using automatic image analysis based on image gradients has been found to be quicker and produce accurate results. Additionally, an experimental study found that cracks had no pronounced effect on the modulus of rupture (MOR) of concrete. However, further research is needed to fully understand the effects of crack orientation on the compressive strength of concrete.
What are the drawbacks of catalytic cracking?5 answersCatalytic cracking, a process used to convert various feedstocks into lighter hydrocarbons, has some drawbacks. One drawback is the potential for fouling when resin or asphaltene constituents interact with catalysts, especially acidic support catalysts, leading to incompatibility on the catalyst surface. Another drawback is the high coking potential of heavy hydrocarbons, which can lead to coke formation in the reforming process. Additionally, catalytic cracking of heavier hydrocarbons can result in the production of sulfur compounds, which can poison and deactivate components of the reforming process and fuel cell stack. These sulfur compounds need to be removed through subsequent sulfur adsorption processes. Overall, these drawbacks highlight the challenges associated with maintaining catalyst performance and preventing the accumulation of unwanted byproducts in catalytic cracking processes.
Why cracking occur in concrete pavement?5 answersCracking in concrete pavement can occur due to various factors. One of the main causes is plastic shrinkage, which happens when there is an imbalance between bleeding and evaporation, leading to negative capillary pressures at the pavement surface. Different environmental conditions, such as ambient temperature, wind speed, and relative humidity, can affect the rate of evaporation and the time of balance, thereby influencing the severity of cracking. Another factor that can contribute to cracking is the design of the pavement, including joint spacing and mix design. Concentrated stresses under the asphalt layer, caused by repetitive slab joint movements or upward curling of the concrete slab base, can lead to reflective cracking. Additionally, defects in the cement-concrete pavement, such as corrosion of steel reinforcement and inadequate compensation seams, can result in loss of pavement structure strength and the formation of cracks. Therefore, a combination of environmental conditions, design factors, and structural defects can contribute to the occurrence of cracking in concrete pavement.
What is the minimum width of a crack in concrete that can be repaired effectively?5 answersThe minimum width of a crack in concrete that can be effectively repaired is approximately 0.013 mm. Cracks smaller than this critical crack width are considered to have a negligible influence on the rate of chloride penetration inwards, while chloride penetration accelerates above this critical crack width. Additionally, a long-term experiment indicated a critical crack width of 0.04 mm, suggesting that crack healing may play a role in the difference between short and long-term experiments.
What is the maximum crack width that can be effectively repaired with self-healing concrete?5 answersThe maximum crack width that can be effectively repaired with self-healing concrete varies depending on the self-healing mechanism used. Autogenous healing mechanisms, such as the use of mineral additives or polymers, are generally limited to healing crack widths of about 100-150 µm. However, autonomous self-healing mechanisms, such as the application of encapsulated polymers or bacteria, can heal cracks of 300 µm or even more than 1 mm. These autonomous mechanisms usually act faster than autogenous healing mechanisms. Therefore, the effectiveness of self-healing concrete in repairing cracks depends on the specific self-healing technique employed, with autonomous mechanisms having a greater potential for repairing wider cracks.