There is considerable controversy within the concrete repair industry regarding the use of bonding agents as a pretreatment method for the repair of deteriorated concrete. This paper examines the use of bonding agents in the concrete repair industry and presents the results of a search of the literature and the test result data from an actual field concrete repair project using a bonding agent.
Bond strength refers to the resistance to the separation of a concrete patch, overlay or structural concrete repair to the underlying substrate concrete. A concrete patch, overlay or structural concrete repair must develop adequate bond strength to the substrate concrete to remain firmly anchored in the repair cavity over the service life of the structure: to efficiently transfer applied loads across the repaired composite structural element, withstand the development of stresses imposed by differential volume changes of the concrete repair material and substrate concrete, such as drying shrinkage, thermal expansion and contraction and creep, and prevent a hazardous condition, the debonding and falling of the repaired concrete.
Bonding agents used in the concrete repair industry are mixtures of substances that are prepared and applied to the substrate concrete to enhance the development of bond strength between the concrete repair material and the substrate concrete.
An evaluation must be undertaken to ascertain the interrelationship with the selected surface preparation technique, the shrinkage rate (potential for cracking) of the selected concrete repair material, the in - service environment of the repaired concrete, the compatibility of the repair material to the substrate concrete and if there is a need for a bonding agent to obtain a required bond strength. Thus, a holistic approach is required to determine the proper concrete repair material to use and the need for a bonding agent.
Bonding Agents Panacea or Snake oil?
Bond strength develops as a result of the absorption, adhesion, diffusion and mechanical interlocking bonding mechanisms between the concrete repair material and the substrate concrete, and is time dependent.1-2
Bonding agents are used to enhance the bond strength between the concrete repair material and the substrate concrete, and can generally be placed into three basic groups: epoxy, latex, and cement based materials.3-4 There are many proprietary bonding agents on the market which consist of some proportions of the above basic groups combined with other materials.
The direct tensile bond test is a widely used testing procedure to evaluate early-age bond strength in the concrete repair industry.
The long term bond strength and integrity of the bond will depend on the dimensional compatibility between the concrete repair material and the substrate concrete, its ability to withstand the loads (stresses) imposed, and the degradation processes associated with the exterior and interior environments.
Various laboratory and field tensile bond strength testing devices are available such as the Germann Instruments Bond Test device and the Proceq DYNA Z15 and the Hilti Tester 4 devices are used for measuring the early age bond strength between the concrete repair material and the substrate concrete.
Adequate tensile bond strength is fundamental in developing monolithic action between the concrete patch or overlay repair and the substrate concrete. According to the International Concrete Repair Institute (ICRI) Guideline No. 03739 adequate tensile bond strength of 250 psi is achievable with available surface preparation and repair techniques in moderate to good quality substrate concrete.5