Over time we get technical questions about shotcrete, as some of these come in we will post them here for your convenience. 


Q: Is it required or necessary to use a bonding agent on a new shotcrete vertical wall that has been shotcreted in the last 7-10 days before shooting another 1/2 inch cementitious liner on the shotcrete?


A: No it is not necessary to use a bonding agent, a bonding agent can become a bond breaker, all that needs to be done is to roughen the surface and get it to SSD with water.


Q: We have a current house project with a 12" shotcrete basement wall. Our last day of shooting was a very long day for many reasons (unplanned), and at about 8:30pm we stopped the work. The final layer of shotcrete that day covered the second layer of reinforcing. It is my understanding that there remains 2" to 2 1/2" of concrete yet to be placed to reach the designed wall thickness of 12". There are two curtains of reinforcing.


Looking at ACI 506 8.5.8 Multiple Layers, additional layers may be applied on another day; for various unforeseen reasons? There are special methods to be followed, and, we think this method applies to our project. However, the structural engineer has sent a Draft fix requiring chip/sandblast excess shotcrete to the centerline of nearest vertical reinforcing, install #3 epoxy dowels at 16" ocew, form and cast in place the remaining section with a low water/cement ratio; half height each pour. The wall is approx 14'-0" high. The engineers concerned about the future loads on the wall from above. We agreed to core samples after the last shotcrete layer. We have concerns about the Engineers repair methods.


In ACI 506 8.5.9 there is mention of placing two reinforced layered walls on different days.


Q: Is ACI 506 correct about Multiple Layers? Are there any other documents to support the method to assure our client of the best job?


A: The ACI is correct the next layer will adhere fine; the existing shotcrete need’s to be rough and clean of laitance. It will need to be water blasted and the surface brought to (SSD) moist, I recommend no bonding agents at this joint, I have done tests and the non bonding agent joints works great, the end result is the added layer can be added with no compromise to the wall, I have done many tests to show this. I would tell the engineer that he can core it when complete, if you prepare the surface you will have no problems.


Q: Our pool contractor shotcreted our pool on Friday, August 13. We live in Louisiana. Right after they began to spray the shotcrete, their machine broke. My husband asked that they fix the machine and spray the shotcrete instead of dumping the mixture into the bottom of the pool. The owner of the pool company agreed. The cement truck driver then became concerned that the pool workers were adding gallons of water into the mixture. He thought it was about 100 gallons of water. I am trying to get the actual records from the cement company.


Is it normal to add water to the truck? If so, how much water is added per yard?


It took about 2 hours and 20 minutes for them to complete spraying from the first truck. How concerned should we be? I have read that the shotcrete has a ninety minute window.


A: Water added to ready-mixed concrete trucks at the job site is not unusual. The amount of water that can be added to the mixture depends on the number of yards of concrete in the truck and how much is allowable to maintain the designed water-to-cementitious ratio. 100 gallons (378.5 liters), if that is what was added, is signigicantly higher than what is typically added fo a full load (10 cubic yards or 7.65 cubic meters) and especially a partial load. 100 gallons would equate to 833 lbs (377.8 KG) of total water added. For a load of ten cubic yards (7.6 cubic meters) of concrete, this would equate to 83.3 lbs (37.8 liters) of water per cubic yard (cubic meter). For example an 800 lb (362.9 KG) cementitiuos factor per cubic yard with a designed w/cm of 0.45, the actual w/cm would now be 0.55. For a 5 cubic yard load this would equate to 166.6 lbs. (75.6 KG) of water added per cubic yard, which would bring the w/cm to 0.65. You can see the trend. It is well known that the shrinkage potential increases with increasing quantities of total water in the mix; Therefore, the potential for cracking would be increased, not to mention the potential negative impact on compressive strength and durability.


Based on Ms. Vicknair's comments (i.e. the quantity of water added along with the amount of time that it took to place the material) suggests that significant slump loss was experience, which required retempering. Ms. Vicknair could contact a registered professional engineer in her area to investigate her concerns. The use of a hydration control admixture can extend the 90-minute window. The 2 hours and 20 minutes placement time does not include the haul time, which would bring the age of the concrete even closer to the initial setting time of the concrete mixture if a hydration control admixture was not used..


Q: We are the engineers for seismic retrofitting for a building that is 158 long ,46 feet wide. It consists of a basement 9 feet height, first and second floor 11 + - feet height. Summarizing, it has the basement and the two stories of 11 + - feet height.


The exterior end walls are Hallow clay tile (HCT) that are unable of taking the seismic lateral forces. This building is located in San Pedro, California (South Los Angeles).


A shotcrete wall along the (46 feet ,narrow side of bldg), 8 inches thick which extends from the basement to the roof has been designed to take the lateral loads . Dowels connect to the existing (HCT).


The existing concrete footing has been widened to accommodate a new footing of the new shotcrete wall.


For logistics (schedule, costs) reasons the contractor is proposing to use the shotcrete on the wall for concreting the new footing.


Q: Is this acceptable?


A: I would advise not using the same mix at a 2’’ slump for the footing, you should see if the shotcrete mix will make the 4000 psi design at a 4’’ slump then your contractor can place and vibrate the footing for proper consolidation and shotcrete the wall after the footing pours, monolithic would be fine.


Q: Would you be able to tell me the risks involved in cutting out the bottom of a gunite pool in order to dig it deeper and then re-steel and re-gunite the pool. Do you know how that would impact the structural integrity of the pool shell? Will that method ultimately result in leaks or cracks down the road in time?                           


Would it be wiser to dig out the pool and start from scratch?


A: You can cut out any part of the pool and make it deeper, you will need to add Epoxy rebar dowels and chip the saw cut to a clean 45 degree joint at a ¼ amplitude. Bring surface to SSD saturated surface dry (wet joint) use no bonding agent You will be fine, then plaster.\


Shotcrete Technical Questions

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