Free shrinkage has a greater effect on smaller aggregate sizes than on larger aggregate sizes. This results in a linear relationship between free shrinkage and the crack width of concrete. See Figure 6.

Grzybowski and Dorell and Nordberg conducted several ring tests trying to reduce the crack width caused by free shrinkage. Grzybowski used straight, smooth, brass-coated steel fibres, and Dorell and Nordberg used smooth straight fibres with hooked ends. See Figure 7 for a typical cross section. Their results show that the smallest amount of steel fibres resulted in crack width reduction caused by the free shrinkage of smaller aggregate sizes. See Figure 8.

Figure 7: Cross section of steel fibres and concrete (units in millimetres)
 
 

Figure 8: Average crack width versus steel fibre content