Crack measurments

Fracture mechanics allow analysis of critical crack size for unstable crack growth leading to fracture. Structures must be inspected periodically to ensure that actual crack size is smaller than critical crack size, otherwise the structure is sure to fail. Critical crack size is the size of a flaw crack in a structure that causes failure at a particular stress level.

Thus, the critical crack size can vary significantly with the type of material. In an ideally elastic brittle material, crack propagation occurs catastrophically if the crack size is above the critical size. At critical crack size, a subcritical crack growth may occur due to environmental effects, such as stress corrosion, resulting in crack growth and failure.

Fracture mechanics is concerned with the formation and growth of cracks, which cause such fractures in brittle materials. Critical crack size is a function of applied stress. The rate of increase in crack size is small at small crack sizes and increases with crack size until the crack becomes unstable at a critical crack size, which depends on the failure criterion.

In stress corrosion cracking, it is not uncommon for growing cracks to be considered acceptable so long as they are substantially less than a critical crack size and can be repaired at the next system shutdown.

When doing this it is important to establish whether the crack is increasing in size. There are various types:. Crack monitors types. The most popular type is made with two slips of overlapping plastic plates with a red cross on one and a marked grid on the other, see picture above and below.

Once this monitor is fixed in position an initial reading is taken. Then more readings are taken at regular intervals to establish if movement is occurring and if so, at what rate.

By comparing the different rates of movement for cracks at different locations in a building, the cause and location of the weakness can be established. The monitors should be left in place for as long as possible to establish whether the crack is moving or not, and for a minimum of eight weeks. There are other types of monitors which are sometimes used, for instance one type is simply a piece of glass which is fixed across the crack.

If the glass is broken it is a sign that the crack has moved. Other methods of crack measurement. There are other means of measuring crack width movement, these include rulers with various crack widths marked on to allow comparison, another type is visual crack microscope device and finally cracks can be measured by simply fixing metal studs either side of the crack and using a vernier calipers to measure the distance.

A sensible approach is necessary when using microscopes and vernier calipers, the extremely small dimensional changes in crack widths which they can measure could simply occur due to daily temperature changes. If using very fine measuring devices the measurements should probably be taken at the same time of the day or at least check the temperature each time a measurement is taken.

Two parameters are often used by engineers to characterize cracks: crack width, and crack depth. In this article, we will review 3 methods for evaluating crack depth in concrete. A crack is a linear fracture in concrete which extends partly or completely through the member OSIM, In a concrete element, tensile stresses are initially carried by the concrete and reinforcement.

When the tensile stresses in the beam exceeds the tensile capacity, the concrete cracks. After this point the tensile force is transferred completely to the steel reinforcement. Cracks can occur during the concrete construction, placement and curing.

They can also at any time during the service life of the structure. Cracks cab be sign of structural problems, or a result of concrete deterioration. Several issues can result in cracks in concrete, including:. In a concrete element, the crack shrinkage, thermal, and service loads width and distribution is mainly controlled by steel reinforcement. In fiber-reinforced concrete, fibers help control cracking.

Cracks that are caused by internal or external chemical reactions, or a result of accidental loads i. Visual inspection and monitoring is the first step towards understanding the nature of existing cracks, and the underlying causes.

For example, inclined cracks over concrete beams near the supports can be a sign of shear stress, or cracks with a sign of rust can be a result of steel corrosion. Usually, crack widths are used to assess the severity of concrete cracks, whereas crack depth is used to evaluate overall structural integrity of the element. Crack severity on the surface of concrete is normally measured using a crack width ruler crack gauge. Depending on the opening of the cracks on the surface, cracks can be described as tiny as hairline, or severe few millimetres opening.

There are cases where structural engineers are interested in the crack depth measurement. Crack depth is used to evaluate structural integrity, and verify durability performance. Crack depth measurement can help repair contractor in evaluating the repair costs.

Depending on the nature of the project, engineers rely on different intrusive and non-intrusive techniques to estimate the crack depth. Extracting core samples from the defects is considered a popular method among inspectors and engineers. Depending on the nature of the crack, and the location of crack is it located in critically damaged area? One approach is simply extracting cores, and visually examining the corehole and the core for possible causes of the crack.

In another method, dye is injected using pressure into surface cracks. Later, concrete cores will be taken from the area under investigation. The sample is studied under microscope for determining the depth of cracks in concrete. In Impact-Echo test, a stress pulse is generated at the surface of the element.

The pulse spreads into the test object and is reflected by cracks, flaws or interfaces, and boundaries. The surface response caused by the arrival of reflected waves, is monitored using a high precision receiving transducer Malhotra and Carino, Fatigue failure is a significant problem in the structural safety of engineering structures. Human inspection is the most widely used approach for fatigue failure detection, which is time consuming and subjective.