Contraction cracks

Contraction stresses develop when slab/environmental temperature decreases. Contraction tensile stresses should be defused otherwise, contraction cracks will develop. To avoid contraction stresses, adequately spaced contraction joints are provided. 

Early thermal contraction cracking or thermal stress cracking... 

Early thermal contraction cracking, also known as thermal stress cracking or curling, is caused by excess temperature gradients (deferential) or moisture content between the top and bottom of a slab, attributed to rapid cooling when ambient temperature drops and, in the case of thick (mass) concrete elements, to excess heat generation during hydration of the cement. 

The early thermal contraction cracks are linear and formed in the slab s corners or edges. The cracks may be deep and of variable length depending on the size of the concrete element. The typical time of appearance is after 1 day to 2-4 weeks. 

Early thermal contraction cracking - occurs within a few days of casting. 

Fig. 9.14 A contraction crack in the fillet of a small giinmetal sand casting.

In other cases contraction cracks arise during the period when the metal is solid but still at a temperature where the mechanical strength is low (Fig. 9.13). 

Contraction cracks and hot tears result from the hindered contraction of the casting this gives rise to complex internal stresses. 

Contraction cracks Tearing of the metal under (i) Use collapsible moulds Avoid abrupt changes... 

Defects in casting, 166 et seq. blowholes, 166 cold shuts, 174 contraction cracks, 174 crossjoint, 178 flash, 178 hot tears, 174 misruns, 185... 

Fig. 9.13 A contraction crack formed in art aluminium alloy die casting during cooling in the mould after solidification was complete.

Il jSL Uld be noted that contraction cracks often have an external wi(i ,.ii,iiii e very similar to a shrinkage cavity (see Section 9.6). ruify can lie dill ercntiated from the latter because there is no cavity... 

It should be noted that contraction cracks often have an external appearance very similar to a shrinkage cavity see Section 9.6). They can be differentiated from the latter because there is no cavity or porous area beneath the surface. Furthermore, contraction cracks arc normally located in positions clearly related to the geometry of the casting and to the restriction of free contraction by parts of the mould. A good example of this is shown in Fig. 9.14, where cracking has occurred in the fillet region between two parts of the casting. 

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