Updated: Jan 6
Written by: James Lim
When we observe a crack on concrete slab or beam, we can either patch it with something to close it or we can seal it with a high strength resin which provides compressive strength and bond strength. For this, engineers prefer epoxy resin because it has strength and good bonding properties.
Most epoxy resin available in the market caters for dry cracks condition as epoxy resin will not set when it is mixed with water. For wet cracks, either underwater epoxy resin or high strength polyurethane resin shall be selected for crack injection. In this respect, 2 components polyurethane resin is ideal as it will react with the water to form a stiff material with a bonding strength of 1.5 N/mm2. It is important to know that structural polyurethane resin can achieve a compressive strength of 80 MPa and this is equivalent to the strength of an epoxy resin. Upon contact with water, polyurethane resin forms a rigid structure that does not allow air and water to pass through the cracks. When the cracks are sealed and air tight, corrosion of the steel reinforcement can be prevented.
At the job site, it is not difficult to spot a group of injection nipples or injection packers sticking out along the line of crack to illustrate the repair job is completed by visual inspection. Now the question to ask is whether the resin has penetrated into the cracks as intended by the structural engineers who specified the method of repair. Surprising enough, nobody seems to be interested in asking whether the cracks are sealed properly. The sight of seeing rows of injection nipples or packers basically confirmed that the job was done as intended. So it is basically good showmanship because we don’t normally questioned whether the cracks have been properly sealed with injection resin or not.
If you were to conduct a random test to check the resin penetration by core sampling test, you will notice most of the cracks are not filled with resin due to poor injection methodology. As engineers are not trained to recognized whether the specialist applicator has done a good job, the outcomes are often left for imagination. This is because the showmanship of crafting the rows of injection packers were suffice to confirm the job was done to reasonable standard. No further tests or quality control are put in place to ensure the resin actually penetrate the cracks. So real injection or great showmanship?
The easiest way of finding out whether the injection is good is by looking at the way how the holes were drilled. The drilled hole is the only passage for the resin to travel into the crack. As simple as it may seems, drilled holes that are not drilled deep enough to intercept the cracks are likely to result in a road block situation. Road block means injecting resin into a solid wall of concrete because the holes were too shallow. To intercept the crack effectively, a drilling angle of 45 degrees are recommended versus drilling perpendicular to the concrete surface because the cracks are often not straight when you take a cross section view of the cracks.
For method that uses surface packer or injection nipple where only epoxy adhesive is used to bond the nipple to the concrete, the injection resin could not travel far into the cracks because surface packer or nipple are not designed to handle high injection pressure. Low injection pressure limits the penetration depth of the resin and it is likely to result in incomplete filling of the cracks or even no fill at all if the packers/nipples were not aligned with the crack. Repair specialist prefers this method because it is quick and easy to do since drilling is not required. But it is often more of a showmanship to show the structural engineer that injection has been done to seal the cracks.
What can we observe from the site picture taken above?
1. Do you need that many holes for an injection job? What surprises the most is that the contractor was even allowed to drill so many holes in a small area.
2. Inconsistent drilling angle. The bottom row is drilled at an angle and the upper rows were kind of perpendicular to the wall. Potential honeycomb location to inject perhaps.
3. Resin rebound back at the location of the packer and this indicates a road block situation. Therefore, injecting into solid concrete.
4. Injection packers were left on site. Perhaps to show proof of injection by showing the number of packers installed.
With a perfect hole that connects the cracks to a high pressure piston injection pump, the chances are you will not require to drill as many holes as seen in picture above. The one with the most number of packers are the likely outcome of an amateur contractor with good showmanship.
Unless the structural engineer requires to fill and bond the cracks properly, the injection shall be done with diagonal drilled holes staggered at both sides of the crack and then inject it with a high strength resin. With good drilled holes achieved, the required injection pressure should not be more than 1000 psi at the start and it will quickly drop to 300 psi when the resin starts to flow into the cracks.
The concept of injection consists of an understanding of 3 elements below:
1. Connecting the cracks/zone of injection with a drill hole. Mostly like a longer drill bit to hit the spot.
2. Injection pressure. The key is to achieve consistent injection pressure during the process of injection.
3. Injection resin mix ratio and setting time. An ultra-fast setting resin requires a plural pump set up and built-in flushing capability.
As simple as it might seems, real injection requires careful planning of packer position, resin selection and choice of injection pump. Asking the right questions in the process of resin selection and injection methodology will allow the engineers to get a better outcome when it comes to crack injection.