Crack control

Khludnev A.M. (1995a) On contact problem for a plate having a crack. Control and Cybernetics 24 (3), 349-361. 

Telega J.J., Lewinski T. (1994) Mathematical aspects of modelling the macroscopic behaviour of cross-ply laminates with intralaminar cracks. Control and Cybernetics 23 (3), 773-792. 

This relies on the integrity of the concrete construction itself with reference to design (crack control, joint spacing and detailing, concrete mix) and skilled workmanship and supervision. 

To improve the interfacial bonding between the PP fiber and a cementitious matrix, commercially available PP fibers with different geometries were fluori-nated and the effect evaluated with regard to crack control, impact resistance (AC1 Committee 544 Report), and water absorption (ASTM C-948).M,34a In all cases, as seen in Table 16.13, fluorination improved the induced properties. 

Sample No. Crack control (%) Impact resistance (blows to cause failure) Water absorption (%)... 

B. A. Graville, The Principles of Cold Cracking Control in Welds, (1975), Dominion Bridge Company, Montreal, Canada. 

Root knot and other pest nematodes cause poorly colored, deformed potatoes with rotted areas under the skin, surface blemishes, and surface cracks. Control pest nematodes by applying a chitin or parasitic nematodes to the soil. Prevent problems by planting nematode-resistant cultivars such as Heart , Jasper , Jeweir. Kandee, Nemagold , and Nugget . 

Nuclear Reactor with a Hole in the Head On March 6,2002, personnel repairing one of the five cracked control rod drive mechanism (CRDM) nozzles at Davis-Besse Nuclear Plant, Oak Harbor, Ohio, discovered extensive damage to the reactor vessel head. The reactor vessel head is a dome-shaped structure made from carbon steel housing the reactor core. The reactor vessel head is placed such that it can be removed when the reactor is shut down to allow spent nuclear fuel to be replaced with fresh fuel. The CRDM nozzles connect motors mounted on a platform above the reactor vessel head to control rods inside the reactor vessel. Reactor operators withdraw control rods from the reactor core to start the operation of the plant and insert the control rods to shut down the operation of the reactor. 

Zhang Hua, Liu Zhao and Wang Libo. Discussion on temperature crack control of concrete box girder. 

TABLE 8.2. Stress-Corrosion Cracking Control Measures [22] ... 

Improving the serviceability conditions such as flexural crack control, reducing deflections at service load level and delaying the onset of yielding of the steel reinforcement... 

At the front face of stem, using 5 12 in. in both longitudinal and horizontal direction, it meets the crack control requirements of the AASHTO specifications. 

Crack control and resistance against local loads in ferrocement is improved considerably by the addition of fibres and polymers into the matrix com-... 

To improve crack control and tensile strength various kinds of continuous fibre reinforcement are used for road and runway overlays with different fibres polystyrene, nylon, glass or steel. 

Barluenga, G., Hemandez-Olivares, F. (2007) Cracking control of concretes modified with short AR-glassfibers at early age. Experimental results on standard concrete and SCC , Cement and Concrete Research, 37 1624—38. 

Crack control by fibres is also visible in those regions of the load-deflection curves where, after the unloading-reloading cycle, the elasticity of fibres bridging the cracks enables the recovery of the bearing capacity of the specimen before unloading. 

There is another notion derived for fibre-reinforced matrix and related to the post-cracking behaviour. It is the critical value of fibre volume fraction Yf which corresponds to the increase of the bearing capacity. For insufficient reinforcement, when Yf< Yf i the load decreases after cracking of the matrix. The role of fibres is limited to the crack control, but its efficiency is not sufficient for any load increase beyond the level of the first crack in the matrix. The expression for was derived by Hannant (1978) on the basis of several simplifying assumptions for composites reinforced with short dispersed fibres. The numerical examples show that the values calculated in that way are much lower than the results obtained from tests. The notion of Yf i, may be generalized for all kinds of reinforcement and used to estimate their efficiency. 

Toughness FRC as a result of steel fibre crack control was studied by Brandt (1996) who has shown that there was no single measure to characterize the ability of a composite material to absorb energy. All methods proposed by different authors have their shortcomings and the only way is to diligently use a few of them and carefully analyze the results. 

It has been shown on elements reinforced with fibres and subjected to bending how the role of the fibres is decisive for crack control. 

The proposed formulae are sufficiently precise to enable the analysis of different cases of steel fibre reinforced elements, however, their simplified form and assumed hypothesis leave scope for further development and improvement for example, more sophisticated mechanisms may be used, without modifying the principle of the design method, based on the conviction that the best solution corresponds to the maximum area under the load-deflection or stress-strain curve. That maximum reflects the most effective crack control by the fibres. 

The application of dispersed reinforcement and other measures of crack control are considered as an effective way to counterbalance the natural ductility of cement-based materials. This may lead to a high volume of reinforcement, for example SIFCON, where a high strength and very brittle matrix is transformed into a crack resistant and quasi-elastic material (cf. Section 13.5). 

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