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2013FallShotcreteEMag

shotcrete: fly ash and mid- to high-range water the water. After calibrating the batch plant, I can reducer/plasticizers. determine how much water is acceptable in the We have found producing a shotcrete mixture material. With that information, I can give the with fly ash is currently unfeasible. There are three operator a maximum water flow rate, so that as ways to enable running a fly ash mixture, but they long as he stays under that number, I know the are either not available or very expensive. One w/c will be adequate. way would be to have a supplier blend it for you, Similar to the water, a little too much admix- but I have not been able to find a supplier in the ture can be highly detrimental to the concrete. northeast that will do that for us. The next way Our batch plant was originally designed to dilute would be to run two different powder hoppers on admixtures into the water and then pump the the batch machine, one for cement and one for fly water into the mixture. We found it difficult to ash. The third way would be to self-blend it before keep up with constantly mixing the proper placing the material in the hopper. As a result, we amount of admixture in mix water because of do not currently have the option of using fly ash our continuous batch methods. We also found in our mixture. Since we do not have fly ash, we the standard admixture pumps were not as con- just use a straight cement mixture. sistent as we would have hoped. Most likely, The specific water reducer/plasticizer consid- volumetric batch plant suppliers offer highly ered for a shotcrete mixture needs to be re - accurate pumps, but we ended up custom-fitting searched. Sometimes a technical data sheet will our machine with swimming pool chemical have all the information, but other times, you pumps. These pumps have variable frequency may need to get in contact with the manufacturer. drives, allowing us to adjust them based on our Some water reducers/plasticizers require several needs, and they are incredibly accurate. I have minutes of mix time, but with a volumetric batch found that monitoring them is unnecessary plant, the mix time is under 30 seconds, and that because once they are set, the accuracy doesn’t is generous. With some of the plasticizers that change. However, I will check—and sometimes take time to react in the mixture, the mixture will recalibrate—the flow if there are substantial look thick with a low slump at the pump. How- temperature changes during the day. ever, as the mixture travels through the hose, the water reducer will start to activate, and by the calibrating the Batch Plant time it gets to the nozzle, the slump will be We require calibrating the batch plant at the undesirably higher. Make sure all of the admix- beginning of each job. Our typical job lasts at tures can start working quickly with very little least 3 days. Perhaps the machine could be mixing time; otherwise, you will just be wasting trusted to maintain all the adjustments and set- Fig. 1: Flow control money and potentially hindering the final shot tings throughout the last job and during trans- flow meter material. As long as all the admixtures in the port between jobs. However, recalibration at mixture can handle a short mixing time, the the beginning of the job will also give you an dosage amounts can typically be kept about the opportunity to re-check everything, and you same as a ready-mix design. may find a new issue with the machine or that Accurate control of water and admixture is a valve or gate got moved and does not seat very important with a volumetric batch plant. itself the same way. The manufacturer provides Adding a few gallons to a ready-mix truck to get a list of steps for calibrating the machines. Our the mixture where you want it may be okay if method could be a little different than other the required water-cement ratio (w/c) is not users’ methods, so I will not go into depth on exceeded, but always be careful not to go over, the entire process, but will highlight some of or you will potentially ruin the entire batch in the important things. The manufacturer should the truck and render the concrete unusable. supply an Excel spreadsheet that will allow you Onsite batching has some advantages over ready- to take measurements from the machine, plug mixed concrete regarding the control of water in them into the sheet, and then tell you how to the concrete. First of all, the volumetric machine calibrate for proper supply of sand, stone, should be equipped with a flow meter that has a cement, water, and admixtures based on your flow control unit installed at a location that is mixture design. Volumetric batch plants have a easily accessible by the operator (refer to Fig. 1). tendency to be a little inconsistent at times if An extra gallon (4 L) in a 7 yd3 (5 m3) truck won’t mistakes are made or assumptions are taken in do much to the mixture, but if your water flow the calibration process. I always diligently is high by 1 gal. (4 L) per minute, that is adding follow the manufacturer’s guidelines for cali- an extra 3 gal./yd3 (15 L/m3) of material based brating the machine and have had great results. on a 20 yd3/h (15 m3/h) material rate. Having a Setting the conveyor belt speed is an important good flow meter with easily accessible adjust- step because it will set the pace of your job. Too ment allows accurate and consistent control of slow and the job will take too long; too fast and Shotcrete • Fall 2013 19


2013FallShotcreteEMag
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