اليكم هذاالتقرير اخوتي كتبه بروفيسور ومهندس مشهورين من جامعة لافال في فرنسا بخصوص الخلط والخليط المثالي والراجع من الخلط في عملية الخرسانة المقذوفة:
mixing :
Through mixing of all ingredients , especially any coarse aggregate or accelerator if used , is essential to good quality shot concrete the material tend to cake on mixer blades and inner surface of the shell and the mixer requires frequent cleaning to maintain mixing efficiency
-The mixing period should not be less than 1.5 minutes, unused cement and aggregate mixed material that stands longer than 1 hour should be wasted .
-If an accelerator is used it should be well dispersed in to the shotcrete mix immediately before entering the shooting equipment.
the optimum mix :
Rebound has a greater percentage of coarse sand particles and much smaller cement content than the shot concrete as it leaves the nozzle. the cement content of materials as mixed should, therefore be less than that desired for shot certe in place.
-although increasing the water content decreases the amount of rebound , water content, must be limited as over wetness of the shot concrete causes it to slough from it's initial position on the structure.
-the optimum mix contains a little less water than that which will cause sloughing and just enough cement for the desired water – cement ratio .
-on one large job the optimum mix for sand shot concrete ( as discharged at the nozzle ) was 1 part cement to 4.5 part sand and coarse aggregate by weight 1: 4.5 : 4.5
-this gave proportions in place of 1 : 2 : 3 to 1 : 3 : 8 .
-the water – cement ratio of the fresh shot concrete in the place was about 0.57 for sloping and 0.54 for over hanging surface, these were approximately the maximum ratios that could be use with out causing sloughing .
-we can add the diatomaceous earth equal to 3 percent of the weight of cement was added to make the mix more plastic .
-although the ratio of 1 : 4 : 5 is commonly used as a trial mix , this ratio is sometimes modified to meet strength criteria.
a minimum cement content per cubic yard as discharged from the nozzle is usually specified the proportion of fine coarse aggregate ranges between 0.55 and 0.6 percent
-the proportions are adjusted during early test to provide minimum rebound
rebound :
-because of the velocity at impact , aportion of the mixture bounces from the surface on which is being applied .
this material is know as rebound . when applying dry – mix shot concrete to over hanging surfaces or squaring of corners , the rebound averages about 30% , for vertical surface about 25% , and on nearly level surfaces it is close to 20%.
-the amount of rebound tends to increase with increasing nozzle velocity . within the range of ordinary consistencies , when other factors remain the same , the amount of rebound for sand shot concrete varies inversely with water – cement ( w/ c ) ratio .
-as the percentage of water is increased , the mortar becomes more plastic and sticky and has greater tendency to adhere to the surface.
-unless the accelerator in coarse aggregate shot concrete coarse aggregate will be incorporated in place.
-if the accelerator is working properly , the quantity of the rebound should not exceed that of the sand – type shot concrete
-the amount of coarse aggregate rebound is believed to be related to particle shape to some degree with more angular particles rebounding to a lesser degree .
- Professor Kamran M. Nemati Winter Quarter 2007
- Simon Reny is a Technical Representative for King Packaged
Materials Company. His areas of expertise include applications, mixture designs, rehabilitations and durability, new technologies, and equipment for both dry-and wet-mix methods. He received his degree in civil engineering from Laval University.
الخميس ديسمبر 03, 2009 11:29 pm