When concrete is in need of repair, the hardened repair material properties must be considered. Compressive strength, Young’s modulus, modulus of elasticity (e value), Poisson’s ratio and tensile stress resistance must all be taken into account. The moduli differences between the mother concrete and the repair material should not be vast, but it is preferable that the hardened repair material is more durable and slightly stronger than that of the mother concrete at the time of repair.
Each side of the bond line should be concrete of the same modulus. If one side is stronger than the other, you are likely to have concrete failure rather than bond line failure. The stronger side will cause stress to be transferred to the weaker side and cause what is similar to a point load.
In many cases, the degraded concrete may not be repairable and should be demolished and new concrete installed. In some cases, good strength fit-for-purpose resurfacing of weak concrete floors can be successfully achieved, but it is recommended that specialist advice first be sought.
Repairing floors with a bonded topping or screed
If the screed is excessively strong and the bond-line rigid, curl stresses in the cured topping will be transferred to the bond-line and a lower modulus bonder may be required to improve the peel adhesion resistance of the bond medium. If the bond-line is too rigid and high strength, the new topping in service could cause de-bonding and a ripping away from the mother concrete which is of lower strength. Suitable joints should be cut in the new topping in order to provide curl stress relief.
Our sincere thanks and appreciation to David Samson for the information contained in this article.
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