Concrete Works

Bond breaker for joint sealant

Joint sealant should be designed and constructed to allow free extension and compression during the opening and closure of joints. In case joint sealants are attached to the joint filler so that movement is prohibited, they can hardly perform their intended functions to seal the joints against water and debris entry. Polyethylene tape is commonly used as bond breaker tape.

To facilitate free movement, it can be achieved by adding bond breaker tape in-between the joint sealant and joint filler. Primers may be applied to the sides of joints to provide a good bond between them.

Bonding performance to concrete: Epoxy-coated bars vs galvanized bars

The bonding of galvanized bars to concrete is lower in early age owing to hydrogen release when zinc reacts with calcium hydroxide in concrete and the presence of hydrogen tend to reduce the bond strength between galvanized bars and concrete. However, bonding will increase with time until the full bond strength of ungalvanized bars is attained. 

For epoxy-coated bars, there is a 20% decrease in bond strength for bars placed at the bottom of concrete sections while for bars placed on the top there is no major difference in bond compared with uncoated bars.

Coating on concrete – complete impermeability to moisture?

In designing protective coating on concrete structures, stoppage of water ingress through the coating is normally required. Since chloride ions often diffuse into concrete in solution and cause deterioration of concrete structures, the prevention of water transmission into the coating certainly helps to protect the concrete structure. However, if water gets behind the coating from some means and becomes trapped, its effect may not be desirable. Firstly, vapour pressure would be developed behind the surface treatment and this leads to the loss of adhesion and the eventual peeling off of the coating. Moreover, the water creates a suitable environment for mould growth on concrete surface.

In fact, the surface treatment should be so selected that it is impermeable to liquid water but it is permeable to water vapour. This “breathing” function enhances the concrete to lose moisture through evaporation and reject the uptake of water during wet periods.

Crack width limitation (Less than 0.5mm) equal to control reinforcement corrosion?

In many standards and code of practice of many countries, the allowable size of crack width is normally limited to less than 0.5mm for reinforced concrete structure to enhance the durability of concrete. The limitation of crack width can serve the aesthetic reason on one hand and to achieve durability requirement by avoiding possible corrosion of steel reinforcement on the other hand. Regarding the latter objective, site surveys and experimental evidence do not seem to be in favor of the proposition. There was no correlation between surface crack width and durability of r/f concrete structure. In practice, most corrosion problems to the r/f instead of surface cracks perpendicular to the reinforcement.