Residual Strength of the Frost-Damaged Reinforced Concrete Beams

The most severe types of destruction mechanisms are those causing internal cracking and thereby loss of cohesion of the concrete, i.e. internal expansive attacks. The internal frost damage belongs to this category of destructive mechanisms. The frost attack causes a random system of cracks in the heart of the concrete together with cracks parallel to the surface of the concrete. In many cases there are also extended cracks parallel to joints and edges of the concrete, or emanating from corners. The amount of damage can vary from place to place in the same structure. Therefore, in most cases, series of data from many different places are required. Only in special cases data taken from one place can be used for the entire structure.

Internal frost damage appears as loss in compressive and tensile strength, loss in E-modulus, and loss in the bond between the concrete and the reinforcement. Reductions in tensile and bond strength can be as high as 70%, or more. The effect on compressive strength is often limited to about 30% for normal grade concrete. The effect on E-modulus can be extremely high.

Internal damage will affect the moment and shear capacity of slabs and beams, and the compression capacity of columns. It might seriously affect the structural capacity of pre-stressed concrete by significantly lowering the E-modulus of the concrete. It also changes the moment and force distribution in the structure by changing the stiffness in parts of the structure.

In order to study the structural effects of the internal frost-damage large reinforced concrete beams were subjected to frost attack. The frost in combination with high degree of saturation induced internal cracks in concrete. The internal cracks reduced the strength of the reinforced beams and in some cases also changed the designed failure mode of the beams. For instance, beams which in an undamaged condition would fail due yielding of the reinforcement failed as a result of compression fracture of concrete caused by the loss of strength due to internal frost-damage.

Results of this investigation show that internal frost damage, besides causing loss of strength, also causes reduction of the stiffness and extensive visible cracking. Furthermore, this investigation shows that the remaining load bearing capacity of the beams is remarkably high despite the big concrete destruction.