Serviceability limit states including cracking are of increasing importance and often dominate the design of reinforced concrete structures. Furthermore, actual crack widths and their shape play an important role in the assessment of service life of existing reinforced concrete structures. Crack width is inherently a random variable of considerable scatter due to randomness of material properties, geometry of the structure, loading and model uncertainty in crack width estimates. The state-of-the-art concepts for serviceability verifications were recently presented in fib Model Code 2010 that is jointly with Eurocode EN 1992-1-1 considered as key background materials in this study. To assess the sufficiency of code requirements and design procedures, crack widths of water retaining structures are investigated in detail using probabilistic methods of structural reliability. The current codes seem to be well calibrated to reach a target reliability index of 1.5 in the serviceability limit states. The two variables dominating structural reliability are uncertainty in crack width model and concrete cover. Numerous topics need to be further investigated including revision of crack width limits, improvements of mechanical models, quantification of model uncertainty, methodology for load combinations, treatment of spatial variability for large structures, and optimisation of target reliabilities.
Crack Width Limit For Water Retaining Structures Designl
For crack width less than 0.2mm, it is assumed that the mechanism of autogenous healing will take place in which the crack will automatically seal up and this would not cause the problem of leakage and reinforcement corrosion in water retaining structure.
Advertisements(adsbygoogle = window.adsbygoogle []).push();When the cracks are in inactive state where no movement takes places, autogenous healing occurs in the presence of water. However, when there is a continuous flow of water through these cracks, autogenous healing would not take place because the flow removes the lime. One of the mechanisms of autogenous healing is that calcium hydroxide (generated from the hydration of tricalcium silicate and dicalcium silicate) in concrete cement reacts with carbon dioxide in the atmosphere, resulting in the formation of calcium carbonate crystals. Gradually these crystals accumulate and grow in these tiny cracks and form bonding so that the cracks are sealed. Since the first documented discovery of autogenous healing by the French Academy of Science in 1836, there have been numerous previous proofs that cracks are sealed up naturally by autogenous healing. Because of its self-sealing property, designers normally limit crack width to 0.2mm for water retaining structures.
Structural loads, structural analysis and structural design are simply explained with the worked example for easiness of understanding. Element designs with notes and discussions have added to get comprehensive knowledge. Also, construction materials, shoring system design, water retaining structures, crack width calculations, etc. have discussed in addition to other aspects.
In water retaining structures, the crack width will be limited to 0.2mm to avoid movement of the water. Actually, water moves through the crack having a width of 0.2mm; however, self-healing will seal these cracks.
Sine we discuss the cracks in the basement wall, firstly, it is required to know whether the wall is in contact with water or not. In general, there are concrete walls, we could assume it is a basement wall retaining the earth.
Incase outer face of water tank retaining wall is covered with soil and inner face with small amount of water for example 200mm. Shall we use the allowable crack width value for water retaining structures ? I read it's 0.1mm for water retaining and 0.3mm for soil.
Whatever your water level is in the tank and soil outside the tank the limits for the crack width will remain as per the code, as mentioned above these are conservative to say but should be followed, the workmanship is not that great locally, so its better to be conservative.
At the Mix StageHow to Verify Adverse Conditions in Underground Water Retaining Structures (Worked Example)The use of an angular aggregate shape instead of a rounded shape would help to achieve better strain capacity by reduction of active cement content.The use of low aggregate modulus would help to achieve a higher fctm/Ecm ratio and yield a higher strain capacity.
An aggregate type with low αc is preferable.
The use of binder additions such as fly ash, and GGBS would help to reduce the effect of cement binder on cracking. However, this should be subject to limiting binder content.The use of water reducers and superplasticizers would also help to reduce cement content and check durability requirements.
At the Construction Stage, the following measures could be adopted to reduce the risk of cracking
When AS 3600-2009 is selected as the design code, reinforcement is added to keep the steel stress within the limits specified in sections 8.6.1 or 8.6.2. The exception is when the option to "Check Capacity of User Reinforcement without Designing Additional Program Reinforcement" is selected. In that case, the program will not add additional reinforcement, but a design failure would result if the crack control requirements were not met. Since there are no direct equations for crack width in AS 3600-2009, calculated crack widths are calculated using the method in the paper cited in your question when that design code is selected.
Accomplishing adequate research on crack characters are primary before optimizing life-cycle performance of concrete structures or even predicting corrosion initiation. Rasmussen et al. [7] proposed an approach for outlining crack development in beams involving description of the system of cracks and estimation of reinforcement stresses. Rasmussen defined the width of crack as the sum of slip from both sides adjacent to the considered crack, and deduced an analytical approach for the prediction of crack width from summation of steel strains. To accurately describe the geometrical characteristics of shrinkage cracks, Zhu et al. [8] observed the whole process of shrinkage cracking in concrete and analyzed cracks statistically. The experimental results showed the depth-to-width ratio and cracks tip angle were approximately 44.3 and 2.6, respectively. Laterza et al. [9] proposed an analytical model for describing the stress state provoked by a confining rectangular hoop, also taking into account the effects of additional external strengthenings. The model is capable of describing the confinement state within the section core at any increment of axial strain, and the constitutive principle of cracking is well explored. Naotunna et al. [10] applied an axial tensile load on specimens, and the generated cracks were sealed by using epoxy. Then, the cracked specimens were cut and the crack width propagation along the concrete cover depth observed. They found that specimens with ribbed bars behave in a way more related to the no-slip theory, while specimens with smooth bars behave in a manner more related to the bond-slip theory. Tung et al. [11] carried out an analysis of crack development and shear transfer mechanisms in beams with a low amount of shear reinforcement. The investigation was analyzed using a group of appropriate equilibrium conditions, geometric conditions, and constitutive relationships. A simplified model for the shear resistance of rectangular beams was obtained and the validated analysis can be used to investigate the relative contributions of different shear resisting mechanisms during the shear crack development. Cheng et al. [12] presented a 2D diffusion-mechanical model for concrete cover cracking caused by non-uniform corrosion of reinforcement. The model showed that rebar diameter has limited influence on the cracking patterns, but smaller diameters will result in longer crack initiation, extended through time, and an increase in surface crack width. Yang et al. [13] developed a numerical method to predict concrete crack width for corrosion-affected concrete structures. A cohesive crack model for concrete was implemented in the numerical formulation to simulate crack initiation and propagation and the surface crack width was obtained as a function of service time.
The standards of each nation have different control ranges and requirements in crevice of concrete building. The bond-slip and no-slip theory are employed to calculate crack width by the British Standard Institution (BS) [14] and Eurocode (EN) [15]. The maximum crack width is specified on the basis of the different working conditions and purposes of structures. For example, for buildings with aesthetic needs, the maximum crack width is 0.1 mm; for buildings in good conditions which are used indoors, the maximum crack width can be 0.4 mm. Due to the discrete characteristics of concrete, the actual crack widths cannot be accurately calculated. Therefore, the American Concrete Institute (ACI) [16] no longer adopts this method for directly calculating the crack width of components. Instead, their method limits the spacing of reinforcement bars to control the crack widths. In essence, the expression of maximum crack width is established according to the no-slip theory. On the other hand, referring to the specifications of ACI, Frosch [17] proposed the expression of maximum crack width (shown in Table 1). Under sufficient statistics and comprehensive theory of bond-slip and no-slip, the Chinese code (GB) [18] decides that the first step is to determine the average crack spacing and crack width under short-term loads. Then, they consider the effects of long-term load. The model is a half-theoretical and half-empirical formula. Table 1 summarizes the calculation formulas for crack width in each norm and Table 2 compares the maximum allowable crack width of buildings in Europe and China. 2ff7e9595c
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