Concrete dams | Discussion and analysis of damage
Above: Figure 6. Repaired shrinkage cracks in the face slab of the Nam Ngum 2 CFRD in Laos
Impact damage of slab elements close to abutments due to rockfall.
Shrinkage and thermal cracks in concrete slabs resulting in accelerated corrosion of reinforcement in face slab.
Typical damage along vertical and horizontal joints in the face slab of the 156m high Zipingpu CFRD in China are shown in Figures 4 and 5. This damage was caused by the ground shaking during the magnitude 8 Wenchuan earthquake of May 12, 2008. The concrete spalling along the vertical joint was the result of large normal stresses and the detailing of the joint (Figure 4). It is interesting to note that the joint damage is all along the joint despite the fact that the thickness of the concrete slab increases linearly from the top to the bottom of the dam. This means that the high compressive stresses in the face slab are almost independent of the slab thickness. This is typical for structural elements subjected to imposed deformations like, e.g., restrained temperature strains. Unlike the overthrust displacement shown in Figure 5, there is no such component visible along the damaged joint in Figure 4. The damage to the horizontal joint with the vertical offset and the overthrust displacement is due to the differential movement of the slab support caused by the deformations in the rockfill, which cannot be prevented by the face slab (Figure 5). The basic structural function of the face slab is to transfer and distribute the water load over a larger area and to compensate for any variations in the stiffness of the rockfill support. Therefore, for the hydrostatic and hydrodynamic loads the face slab will experience some bending moments due to nonuniform rockfill deformations. These bending moments are rather small and will cause cracks as in reinforced concrete structures. In order to ensure watertightness the crack width should be less than 0.1mm. This is a serviceability limit state problem and not a strength problem. In Figure 6 shrinkage cracks in the face slab of the 182m high Nam Ngum 2 CFRD in Laos are shown. All visible cracks were repaired to minimize any leakage through the face slab and to protect the steel reinforcement from corrosion. Steel reinforcement was provided at the top and bottom of the concrete slab. The development of a shrinkage crack can be avoided by using fibre reinforced concrete for the face slab, which increases the tensile strength of concrete.
44 | July 2023 |
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mechanisms of face slabs The observed damage to the face slabs above the reservoir level was the result of high in-plane stresses and was not caused by the water load. These stresses are due to the different deformational behaviours of the rockfill and the concrete slab. The modulus of elasticity of rockfill may be of the order of 100 MPa, whereas that of concrete under seismic action may be 30-40 GPa, which is a factor of 300 to 400 times larger than that of rockfill. Therefore, if the rockfill portion of the dam deforms, it is partly restrained by the face slab and thus the shear stresses are mobilized along the rockfill-face slab interface. The shear stresses are limited by the friction between the face slab and the rockfill, which depends on the normal stresses due to the dead load of the face slab and the water load. The friction forces are relatively small for the face slabs above the reservoir level and increase almost linearly with increasing water depth in the submerged part and therefore the in-plane stresses will be much larger in the submerged part of the face slab than in the part above the reservoir level. The in-plane stresses in the face slab are caused by the deformations of the rockfill support and such stresses are also known as stresses due to imposed deformations, similar to temperature, shrinkage and creep strains. These imposed strains can be released by joints or cracks, Therefore, if an adequate joint spacing and joint width between adjacent concrete slabs is provided, these strains can be reduced significantly. Therefore, if concrete spalling is observed along joints and cracks in face slabs, it can be assumed that the joints are closed and compressive stresses can be transferred from one slab to the other, i.e. the face slab acts almost as a monolithic plate subjected to compressive stresses. In the case of tension, the joints will open.
Usually, the selection of the joint width is based on the assumed static deformations of the body of the rockfill dam, which along the crest provide tension zones near both abutments and compression in the centre of the crest. To minimize in-plane stresses due to such deformations the joint width in the compression zone should be increased. This concept works for static deformations, i.e. dead load and water load, however for seismic action the joints can be closed or open because of the oscillatory nature of the ground motion. Therefore, dynamic compressive stresses will occur even in static tension zones when the joint is closed. These compressive stresses may damage the joints when they reach the values of the compressive strength of concrete, which are of the order of 30 MPa. Cracks may also develop when the biaxial strength of the face slab is exceeded in zones with principal stresses in tension and compression rather than under biaxial compression. Inclined cracks are due to high shear stresses. If cracks develop or a joint is damaged due to high
compressive stresses, then in the case of imposed deformations or seismic action the other parts of the face slab and the other joints will be protected from similar damage. This is also an observation made when tensile cracks are formed in concrete dams due to seismic action. Therefore, in order to protect the face slab from damage, it is recommended that the width of the vertical joints is determined in such a way that they will
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