Theoretically, what is the main stresses created on new concrete cast over old one, that might effect its bond?
Bonding coats are available which can be used to connect old and new concrete. In addition to above shear connectors are also useful in connecting old and new concrete.
Dear Hussam,
Very good question. There isn't a simple answer because when you bond new on old concrete, this will effectively lead to non-monolithic behaviour (as a result of materials with differing stiffness, thermal properties, shrinkage, etc.).
The level of the stresses that result from the bonding (interface bond strength) may be affected by parameters such as moisture condition, age and condition of the old concrete, state of bonding surface (roughness), etc.
The nature of the stresses you have will depend on the elements being bonded together. The key strength to watch for is the shear bond strength.
In fact modelling of interface is very difficult but the same can be done by any FEM Simulation technique to ascertain the behaviour.
Shrinkage and expansion are the worst cases and can be avoided by proper choice of materials. Also the new concrete should be kept cool with small streams of lowing water to avoid high temperature in the new concrete as it cures and exotherms.
Maturity of the old concrete is always a concern. If it has lost strength and become subject to crumble then it should be repaired or removed before new concrete is cast.
One example is a warehouse where old concrete makes dust from wear of fork lift trucks. One remedy is to seal the old concrete with epoxy for concrete and as it cures to cast on top another 5 centimeters of high strength crush resistant concrete filled with tiny beads of stainless steel several sizes from which surface shine was removed in advance..
What do you mean by the old one!!? Is it a well-known architectural monument or a particular building class?
Since they are two different concrete casted at different time, the bond between them is their weakest part, but more is when the strength of the new concrete is not up to that of the old one, for any pressure I think it will be safe.
Assuming that you are referring to pavement over pavement, the primary forces involved are lateral shear in two directions. If the pavement is predominantly uni-directional (e.g. roadway), then a roughening of the surface in combination with scoring the existing pavement perpendicularly to the travel direction, followed by a thorough cleaning (oil-free), should be sufficient. Make sure that the depth of the scoring is no deeper than 15%-20% of the existing slab, and that the score widths are a minimum of 1.25x the largest diameter in the new aggregate.
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