Bs 8007 BETTER Crack Width Examples
Bs 8007 Crack Width Examples
in an active ultrasonic radiography (urad) technique, the stress waves are generated by emitting a high frequency sound wave into the test specimen. the stress waves are reflected by the cracks and the interfaces. the reflected signal is transmitted to a transceiver via a receiving transducer. the signal is captured by the receiving transducer and converted to a corresponding voltage signal. the voltage signal is converted into a digital signal by an analog-to-digital converter (adc), and then sent to a computer for processing. the computer is able to determine the depth of the cracks using an autocorrelation algorithm. in this method, the reflected signal is correlated with a reference signal to determine the distance from the transducer to the crack.
at the design point, a design condition is assumed with a crack that is 45% of the size of the critical crack. use the equations in the previous example to determine the factor of safety at the design point.
this is a very common type of defect that occurs during construction, and most likely, most of the time the defect is not found at the time of testing. the defect can also be identified during the pour. the defect, in the image below, is a crack that forms in the foundation due to the settling of the structure. the defect is discovered during the construction phase, and the foundation is excavated and inspected. the concrete is removed, and the damaged area is dressed and repoured.
for this crack, a threshold level of applied stress is assumed. the stress intensity factors (sifs) are then calculated for all the possible failure modes at each crack length. the cracks are then assigned to either the failure condition (ff) or the no failure condition (nf). once the crack is assigned to a failure condition, the crack can be evaluated by combining the crack length and the design point for the crack. a crack of size x can be assigned to the ff if and only if asifff_x > 0 and asifff_x < asifnf_x. similarly, if and only if asifnf_x < 0 and asifnf_x < asifff_x, then a crack of size x can be assigned to the nf.
the crack widths of concrete are affected by many factors, such as the aggregate type, water-to-binder ratio, mix design and operation, the type of applied binders, the curing conditions and the construction of the concrete. the w/b ratio has a significant effect on the formation of the slip zone, which is responsible for the development of stress concentration at the crack tip. in addition, the w/b ratio affects the strength and the durability of the concrete. the concrete strength, shrinkage, permeability and water absorption are also affected by the w/b ratio. it is essential to understand the effect of the w/b ratio on the concrete properties for a given mixture design.
after crack width control, the ratio of the crack width to the aggregate diameter is reduced by a factor of f. therefore, the crack is no longer a crack but a crack opening, and the nominal pressure that acts on the crack is not the pressure that acts on the crack width. consequently, one cannot use the crack width to control the crack opening. instead, one must use the crack opening. a crack opening can be measured using a pressure-sensitive tape, or by using an elastometer. using a pressure-sensitive tape is an easier and cheaper way of measuring the crack width, but the accuracy of the measurements depends on the skill of the operator. on the other hand, using an elastometer to measure the crack opening is more accurate, but is more costly and difficult to use in a real-world setting.
many investigators have used the ratio of crack width to aggregate diameter to determine the impact of concrete design on the crack width. the ratio of crack width to aggregate diameter is expressed in the following equation: