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Bonding new concrete to plastic and hardened concrete is an important task that is needed in many construction works. The formation of a good bond between new and existing concrete is essential when you plan to resume the concreting of an incomplete construction member, and for other repair works.

Bonding generation involves roughening the concrete surface, pouring new concrete, and finally curing the poured concrete. The roughening process is vital for the success of bond formation.

Use sandblasting, acid washes, or bush hammering for a fully hardened concrete surface. However, for newly placed concrete or plastic concrete, wire brushing can roughen the surface easily.

Bonding new concrete to old concrete
Fig. 1: Bonding new concrete to old concrete

How to Bond New Concrete to Hardened Concrete?

  1. Roughen and clean unprepared hardened concrete surface using sandblasting, bush hammering, acid washing, or any other suitable roughening means.
  2. Use a bonding agent for the hardened concrete surface after roughening and clearing. The bonding adhesive ensures a good adherence between existing and fresh concrete.
  3. The bonding agent will not perform its function properly if the dust layers of cement, fine aggregate, oil, or grease are on the surface of the concrete. Hence, the concrete surface should be free from dust and dirt.
  4. During repair work, use milling or grinding to reach hard concrete and then apply a bonding agent.
  5. Sand-cement and water-cement grouts are economical and practical adhesive agents that can be used for bonding hardened and fresh concrete.
  6. The sand-cement grout consists of 1 cement: 1 sand: ½ water. However, the water-cement grout consists of 1 bag of cement and 6-7 gallons of water.
  7. Cure the poured concrete properly; otherwise, the bonding strength would be lost. Employ wet burlap, wet sand, plastic sheets, curing paper, cloths, curing compounds, or combination thereof.
  8. If the modulus of elasticity, coefficient of thermal expansion, and shrinkage strains of existing concrete are different from that of fresh concrete, then thermal movement due to temperature variations can generate ample stresses to break the bond during the curing process.
  9. Do not employ curing compounds because they may interfere with bonding between concrete surfaces.
  10. Use a hammer or some other dull object to tap the poured concrete gently on the repaired area after 24 hours to check the bonding condition. Listen to the tapping sound. If the sound is hollow, then the bonding is not right, and cracks may develop in the future. But, if the sound is a dull thud, then the bond is good.
  11. For plastic concrete or recently set concrete, wire brushing can roughen the surface of the concrete. Alternatively, employ retarder on the surface of the concrete to postpone setting time of concrete.

FAQs on Bonding new concrete to plastic and hardened concrete

?How to bond new concrete to plastic concrete?

For plastic concrete or recently set concrete, wire brushing can rough the surface of the concrete. Alternatively, employ retarder on the surface of the concrete to postpone setting time of concrete.

?Is a concrete bonding agent necessary to bond new concrete to hardened concrete?

The bonding adhesive ensures a good adherence between existing and fresh concrete.

?How do you bond new concrete to existing concrete?

Roughen and clean hardened concrete surface, apply bonding agent on the roughened concrete surface, and pour new concrete, and finally cure poured concrete using suitable curing regime.

?How to roughly test the bond between new and old concrete?

Use a hammer or some other dull object to tap the poured concrete gently on the repaired area after 24 hours to check the bonding condition.
Listen to the tapping sound. If the sound is hollow, then the bonding is not right, and cracking may develop in the future. But, if the sound is a dull thud, then the bond is good.

Madeh Izat Hamakareem

Madeh Izat Hamakareem

EDITOR
Madeh is a Structural Engineer who works as Assistant Lecturer in Koya University. He is the author, editor and partner at theconstructor.org.

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