Time-Dependent Cracks Width
Although all cracks that appeared during the application of the load or in the period of observing of 360 days, have been registered, the cracks width have been measured only in the middle third of the beams, i.e. in the part of the beams with constant bending moment.
As it was predicted, the cracks in all beams from group 3 (Fig. 9) appeared between the level of permanent and service load. At the reinforced concrete beams (C30/37), denoted as A31 and A32, 4 and 6 cracks appeared from the precracking load, respectively. At the steel fibre reinforced concrete beams (C30/37 FL 1.5/1.5), denoted as B31 and B32, 2 cracks appeared in each beam, while at the beams from C30/37 FL 2.5/2.0, denoted as C31 and C32, 2 and 1 crack appeared. After removal of the variable load “q”, the cracks width decreased and increased thereafter up to the moment of appearance of new cracks. The mentioned new cracks were with smaller crack width and therefore they decrease the average value of all crack widths. This can be also noticed from the figures in the first 20 to 100 days. In total, at the beams A31 and A32, 11 and 7 cracks developed, at beams B31 and B32, 6 and 3 cracks developed, while at beams C31 and C32, 4 and 8 cracks developed, respectively. After forming of all cracks there is only increase in their width.
From the Fig. 9, can be noticed that the instantaneous crack widths at the moment of precracking and after precracking for C30/37 FL 1.5/1.5 are bigger than C30/37, while for C30/37 FL 2.5/2.0 are significantly smaller. However, total crack width of C30/37 and C30/37 FL 1.5/1.5 are almost the same, while the one of C30/37 FL 2.5/2.0 is reduced for 31.8 %.
On the other hand, the long term crack widths of C30/37 and C30/37 FL 2.5/2.0 are similar, while the one of C30/37 FL 1.5/1.5 is decreased for 27.1 %. The long term crack widths of C30/37 FL 2.5/2.0 are of the same order of magnitude as C30/37 due to the fact that the instantaneous deflection of C30/37 FL 2.5/2.0 is much smaller and the cracks due to loading opened up later.
However, having in mind the complexity of the process of cracking, the randomly oriented fibres and the low level of stress from the permanent load, which is about 20 % from the concrete strength, can be concluded that steel fibres decrease the crack widths.
The beams from group 4 were first loaded with permanent load “g”. At this load level there was no cracks appearance. Afterwards, the beams were loaded additionally with variable load “q”. The variable load “q” was kept for 8 h and removed in the next 16 h. This was repeated every day in the period of 360 days. On certain time intervals, before and after application or removal of the variable load, measurements of the cracks width with crack microscope were performed.
In Fig. 10, time-dependent cracks width is presented as an average value of two beams. Dashed lines represent the cracks width at the level of permanent load “g”, while the full lines represent the cracks width at the level of service load “g + q”.
From Fig. 10, much clearer situation than for the group 3 can be noticed. This is due to the higher load level in the time intervals for 8 h each day for group 4.
Fig. 10 Time-dependent cracks width for the beams from group 4
Instantaneous cracks width at the level of service load for concrete type C30/37 FL 1.5/1.5 are 17.6 % smaller than C30/37, while for C30/37 FL 2.5/2.0 are 58.4 % smaller than C30/37.
Total cracks width at the level of service load, when compared to C30/37, were decreased for 29.2 and 53.1 % for C30/37 FL 1.5/1.5 and C30/37 FL 2.5/2.0, respectively. At the level of permanent load similar decrease was obtained, 28.1 and 53.4 % for C30/37 FL 1.5/1.5 and C30/37 FL 2.5/2.0.
Long term cracks width at the level of service load, when compared to C30/37, were decreased for 40.3 and 47.9 % for C30/37 FL 1.5/1.5 and C30/37 FL 2.5/2.0, respectively. At the level of permanent load similar decrease was obtained, 42.0 % for C30/37 FL 1.5/1.5 and 46.9 % for C30/37 FL 2.5/2.0.