In construction, there always comes a time when there is need to bond old hardened concrete (substrate) with fresh concrete topping/overlay. This post aims to explain how to bond old and fresh concrete successfully, and also review the strength of interfacial bond between old and new concrete based on already carried out experimental works.
Bonding is very important for adequate performance of finished concrete when fresh concrete topping is used to overlay an existing hardened concrete. This construction feature is usually found during bridge deck construction, concrete pavement, precast filigree slab, pile caps (in some cases) etc. The truth is that adequate bonding is not always guaranteed between the two layers, unless adequate precautions are taken.
For adequate bonding, it is very important to prepare the surface of the substrate adequately. The preparation of the surface usually involves roughening the surface, and removal of all dirt, oil, grease, loosened or unbonded portions of the existing concrete. By implication, the surface of the substrate should be hard, firm, clean, and free from loosened particles. This can be achieved by the use of chipping hammers, wire brushing the surface etc. After this is done, the exposed concrete surface can be cleaned by using pressurised clean water, air, etc. The man hours involved depends on the area of the surface, location, and the ease of cleaning (e.g reinforcement interference).
|Fig 1: Precast Filigran Slab|
After surface preparation, there is usually need to apply bonding agent on the surface of the existing concrete in order to facilitate the bonding. Epoxy based bonding agents are very popular for such operations. It is recommended that bonding agent is applied prior to casting the fresh concrete. In essence, the procedure should be ‘wet-to-wet’ as the bonding agent should not be allowed to dry before the fresh concrete topping is placed.
|Fig 2: Hardened Concrete With Bonding Agent Ready for Topping/Overlay|
In a research carried out by Vandhiyan and Kathiravan (2017), the compressive strength of monolithic and bonded concrete was compared using 150mm x 150mm cube specimens at 28 days. With epoxy based bonding agent, the compressive strength of the bonded concrete was about 5% less than the monolithic strength, while without bonding agent, the compressive strength was about 28% less than the monolithic compressive strength.
Research has also shown that the moisture condition of the substrate affects the shear bond strength of bonded concrete. Shin and Wan (2010) investigated the interfacial bond strength of old and new concrete considering saturated surface dry (SSD) and air dry conditions. Saturated surface dry is condition that can be described as the concrete containing moisture that is equal to its potential absorption, without the surface being wet or damp. At water/cement ratio of 0.45 (for the topping concrete), the shear bond strength at the interface was about 44% greater when the substrate was at SSD condition than when it was air dry. At water/cement ratio of 0.6 for the topping layer, an increase in shear bond strength was recorded, but there was a reduction in the compressive strength of the concrete.
So the recommendation in this article is that when casting a topping layer of fresh concrete on old concrete, adhere to the following guidelines;
(1) Prepare the surface properly
(2) Make sure that the substrate is at saturated surface dry condition
(3) Use a bonding agent and follow the manufacturer’s technical recommendation properly.
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Vandhiyan R., Kathiravan M. (2017): Effect Of Bonding Chemical On Bond Strength Between Old And New Concrete. SSRG International Journal of Civil Engineering- (ICRTCETM-2017) – Special Issue – April 2017 ISSN : 2348 – 8352 pp 129-134
H-C. Shin, Z. Wan (2010): Interfacial shear bond strength between old and new concrete. Fracture Mechanics of Concrete and Concrete Structures – Assessment, Durability, Monitoring and Retrofitting of Concrete Structures- B. H. Oh, et al. (eds) ⓒ 2010 Korea Concrete Institute, Seoul, ISBN 978-89-5708-181-5 pp 1195 – 1200