Chapter Three. Rear End Plugs and Material Cylinder Packing.
Rear end plugs and material cylinder packing.
This might not be an everyday occurrence but it does happen. The material cylinders of today's hi-volume pumps can have a piston face of 10 or 12” that moves the material thru a very long stroke into an immediate series of transition and reduction thru the valve, then reducing continually and finally to the delivery system only a few feet from the piston face. All this takes place within 6 ft of the big piston...it has to reduce from 10” down to 4.5 on long booms. That's pretty remarkable. You're never going to see another situation where concrete is called upon to do so much in such a short time. Since the 200yph barrier has been broken for quite some time, high velocity concrete gets its speed from the piston speed and from the huge reduction that it goes thru. As it reduces it also accelerates and velocity increases. Lubrication is never more necessary than it is during this time in transition. This is also where you have the maximum power applied to the concrete directly from the hydraulic pistons. Understanding all of this will help you manage a difficult mix that won't make it thru transition and this will help you in keeping everyone in the proximity of the rear end, safe. We all know what ass-end diapers are really for. Not about drips and leaks, about explosions and containing the shrapnel. Because this is where the max power is, the pressure generated by these super pumps is greatest right there. Don;t think that your low volume pump is any less dangerous. Actually, the smaller the piston the more potential pressure that it can generate in the rear end.
The cause of getting “rocked up” in the rear end.
The first thing that comes to mind is the mud itself. Even though we are dealing with a reduction of this magnitude, from 10” to 5 or less in as many feet, Good pump-able concrete will do the reduction easily. If it will NOT, then there is a mix problem that you will have to compensate for. The pump is the best inspector on the job. Always has been. It'll tell you if a mix is lean, if the sands are too course, washed sands, no fines, if the aggregate is too heavy or too much of one size and a shortage of the intermediate aggregates. It will tell you everything you need to know if you know what to look for. Since you will be the ONLY person on the job that can possibly understand what the pump is telling you, its up to you to figure out how to present this data to the person that can make changes in the mix and utilize this information to keep the job moving.
Rocks..lots of rocks
Back “in the day...” when we all used river rock that was round and smooth and like ball bearings we rarely saw this kind of trouble unless it was light-weight or some awful ugly sands that we were pumping. Round rocks pump nice. But, we started running out and conservation prevented us from dredging up what was left. So, came quarry rock that went thru a crusher to get it to our size. Crushed aggregates...The early days were a nightmare. Sands were nowhere close to being able to form a good thick slurry to support and carry these jagged sharp rocks, and the rocks themselves ate thru boom system like a knife thru warm butter. Besides plugging up every stroke, the stuff tore up everything it touched. The aggregate industry wasn't at fault. We ask them... It had to be our pumps. As an industry we saw more development of pumping technology than ever before. It was pretty obvious that they were not going to change, or couldn't, so we had to. We got some nice stuff out of the deal, too. There are those that believe that evolution stops the instant that it is not threatened. Had we not needed machines that would pump cinder blocks, we wouldn't have them. Since there is no real magic inside a 5” pipe on anybody's boom, the fun is all in the concrete valve and transition.
Dealing with crushed aggregates in gate valves or S-tubes, was that crushed aggregate had more surface area that had to be coated with slurry than the round river rock of yesterday. Huh? How so? Take an round shaped rock. Paint it with some color. Split that rock once, split those pieces once. You now have several rocks that are only half coated with the very lubricant/color that they must have, yet they weigh exactly the same as the single round rock did. Weight is how concrete is batched and designed... Crushed aggregates required richer mixes. That was lesson 1.
The weights of the aggregates dry and wet made huge differences. This means that the weight of a batch could/would be different with the various types of rock. Some are more porous than others and able to absorb more of the batch water than others would when they were under the extreme pressure in the ass end of a pump. This is what caused problems when wet mud in the hopper that dry plugged in the valve or rear end. The water is still there, its just been forced into the aggregate by the pressures in the rear end. Much like light-weight that had not been pre-soaked. Rocks DO absorb water like a sponge. The higher the aggregates Specific Gravity (SG) the harder it is to absorb water, but it will when the pressure becomes high enough. Don;t let a nice hard chunk of limestone fool you...it's like a sponge when its in water and pressure is applied.
What to do? Batch-plants must make batch adjustments to allow for free moisture and absorbency of the aggregate. This is often solved by simply keeping a water sprinkler on the stockpile.
Crush aggregates bind against each other whether in a slurry or simply when piled up dry. The multifaceted aggregate lock together and grab each other. In a pile you can see thru the gaps where slurry would be if that were a mix. The bigger the gaps the weaker the mix, so Gradation comes into play. This means there is different sized aggregates to help fill in those gaps and and minimize the dependence on grout to do that. The grout should be a binder, not a filler. Because grout is the weakest part of concrete, we want the majority of the mass to be made of the strongest material, the aggregate. This binding makes for some super strong mixes too,. Much stronger than the river rock mix of 30 years ago. The natural binding was a benefit that nobody saw coming and for a while concrete batch plants started to think that they could reduce their cost on other things...like cement since the real work was being done by the aggregate. That does not work obviously but some may still try. How does this binding effect of crush aggregate ruin our day? The same way it makes the mix stronger. When it binds as it does in the reducers and cannot slip and adjust to the shape of reduction or transition and that “ball bearing” effect is negated we will get some good solid plugs. When we see a mix that has poor gradation, as in the rocks are all the same size, this binding can be almost constant while under pressure because there are no smaller rocks to fill in between the edges of the larger rock to allow for some rolling and transitioning. IF a mix has 1800 lbs of 1” rock per yd its going to be tough unless its a very rich mix and even then it'll be a challenge. How well the cylinders load will give you an idea of how well its going to pump. If the mix cannot “flow” well or its not “plastic” then its a poor mix. In 1800 lbs at least 300 can be replaced by 1/2” aggregate and another 100 with pea-rock. Then, you have a more fluid mix that will reduce and flow with less gaps in the aggregate. This gradation is so critical to mix design and performance that a poorly graded mix can effect the result of a slump test by stacking up and standing as the cone is pulled, appearing to be low slump. Between the gap graded aggregate and too much water the mix was never going to reach its designed strength and nobody could figure out why.
I once knew of an operator that had a simple test for crushed aggregates that were not pumping well....pour some on the ground. Stand on the pile. If you sunk into the pile the mud was OK and not at fault. If you could stand on the pile of poured concrete, that indicated how poorly it was graded and it was not going to pump. I have seen a rock valve or an S-tube so “rocked up” that the valve could not shift to help chop the plug out. That's why you're always “on the button” so the rock jam doesn't end up worse than it has to be. Fortunately these happen in the rear end mostly and can easily be worked out by reversing a stroke and returning to forward. IF its marginal it might take 1 stroke back for every 3 forward. I've pumped 600 yd like this when the mix was very lean and had almost no lubricating slurry. As long as you can react and reverse before its a plug you can manage a rocky mix. Its not fun, but it is getting done...
If its hopeless, it won't go past the rear end and changes must be made or its not going to be pumped. The addition of cement, ash or whatever enhanced lube helps but the real solution is in the gradation and getting a better blend of the larger, medium and small aggregate before getting into the sands. The aggregates when crushed must be of various size in order to fill the gaps that are created naturally in the matrix. This not only helps hold the slurry/mortar to bind the aggregate, it assist in keeping the quality of the cement lubrication as high as possible.
Gimme 20 ! NOT !
A common solution to a mix that's trying to plug. Adding water? I see that even today in some parts of the country they still believe that water fixes everything. That's as backwards as marrying your cousin. In the case of a rocky gap graded mix,. The absolute worst thing to do. If there are too many rocks and the mix is struggling to support and carry the aggregate, why would you want to thin the slurry more and let the aggregate sink or separate in the mix? Water increases the slump, but thins the slurry making what little support there is inadequate to do the work of lubricating and carrying the aggregates. Grout is like paint...thin it too much and it does not cover. Too thick (dry) and it does not spread well. You have to exercise good judgment in these cases and think it thru...is there enough cement to support some additional water? 5.5sk or more? Is there already so much water that the rocks are more wet in appearance than they are coated by slurry? Are the rocks suspended in slurry? Can you see the color of the rock? I have pumped many mixes that were extremely rich or lean, that were very very water sensitive and would react to 1 gallon added to a 9.5 yd load. Beware the water reducing agent that may be in the mix...it makes a drop act like a 5 gallon bucket. So does “super-P”. Know what you;re dealing with before trying to apply a solution. The admixtures change everything. Overdosed super-p mixes can sometimes fix themselves if you can wait out the life of the add-mixture (usually 45 min), if its clear to everyone that it was overdosed.
The grout or slurry should be supporting the rock, not washing around it like a liquid running down the driveway. As the slurry moves it should carry the aggregates with it. Ask yourself what can you do IF you do add too much water or plasticizers to use that load? Remember that water is best added by someone that can afford to pay for the load, not the operator unless he has the specific permission and blessing of the site superintendent. Can the load be blended? Is the next load so marginal that this wet load would not blend with it to be able to be offloaded and used? Think it thru before trying to use water to solve a non water problem. Most often the best way to pump rocky mixes is with a low slump so the slurry can support the excess aggregate. The placing crew may not enjoy it, and you might get some opposition to this, but the alternative is no concrete they often find it within themselves to cooperate and get over it. If you encounter a rocky mix that you can stand on, where the rocks roll down the chute instead of sliding within the slurry, then the best immediate solution is to keep it stiff and use the thick slurry as the buffer against the excess rock binding and plugging. I've seen loads so rocky that they could not be pumped at all, and the solution was to get a load of 6sk grout and blend that with the several loads of rocks to give it some additional slurry to carry or keep the aggregate suspended in the mix. That is not done often anymore, but 30 yrs ago we even climbed up on the RM truck and added bag cement to a load to make it pump-able. You haven't had fun until you've added 10 bags by hand......This was widely accepted since it was not reducing the strength of the concrete like water would. Just not viewed with enthusiasm by the operator or whoever got to climb. Today the plants prefer to take the material back and make the changes themselves so the QC can be documented and their liability minimized. Everything is potential liability these days so avoid placing yourself in the position of buying a load. Make suggestions, explain the why and how. Let someone else make the decisions to do it or not.
What makes a great sand castle? Sand that is packed together so well that it can stand on its own and defy the elements. Good blends of sand in other words. Sands of various textures and grade. To a geologist there is no such thing as sand. Just tiny rocks or the smallest aggregates. Often referred to as “fine” aggregate, sand has the ability to trap and hold water or let it slip thru and take the lubrication with it. Since sand is crystalline in shape its like its cousin, crushed aggregate, it can have spaces between each particle. Because of its irregular shapes it does not fit well together with the next particle in random, so the best that we can do to bind sand without some form of adhesive is to fill the gaps between particles with smaller sand particles. The absence of these smaller particles is often referred to as “gap graded” sands. This means that the sand is all of one size and consistency minus any “filler” size particles. Gap graded sand allows water to pass thru, does not bind well, does not conform well. Its gritty consistency prevents it from filling the smallest voids in between aggregates and this will cause some issues in the level of lubrication in the concrete as far as pumping is concerned. The ability to self lubricate comes with gap graded sands. The best mixes that achieve the best compressive strengths will have 2-3 grades of sands to make the microscopic voids between the larger aggregates as minimal as possible leaving the cement to do bind it all together. Cement is sometimes used in conjunction with fly-ash to make up the difference in the absence of fine grade sand. It takes lots of cement, but in a rich mix is can be difficult to tell the difference. The symptoms of poorly graded sands will include (not limited to) rear end plugging especially in the tube or reducers, or wet slurry right next to what looks like a dry pack. Dry packs of almost pure sand in transitions and reducers. This is where the water has been squeezed out and the slurry lost its ability to self lubricate. If a marginal mix makes it past the rear end reduction it has a chance of being pumped to its destination but it can “bleed” in certain boom configurations so if stopping for any period of time is necessary, its advisable to suck back a couple strokes to relieve the head pressure. Pressure is what forces the water to bleed out of gap graded sands. The solution? Until there can be adjustments from the plant, reduce head pressure by lowering the speed. Speed is pressure and pressure is what squeezes out the water. Just pump it gently and baby the mix.
See if the batch plant can up the fines and reduce the moderate or coarse grade sands. It only takes 100lbs per cu yd to make a difference in many cases. Often the fines or “silt sands” are in short supply because of environmental issues that only allow the plant to store what can be contained, not piled outside because a light breeze can carry the stock pile away. Fine sands or silts are just a step above powder, so they can be difficult to store. I have pumped many special slurry mixes for special applications in pressure grouting or jacking or long distance void-fill that demanded an extremely flow-able, almost liquid slurry. Silt sands were the solution and sometimes had to be special ordered by the batch plant if they could even get them. Most plants do stock pile fine sands of their definition often a cement-sand 'blend' that is general purpose sand for concrete. This is adequate under most conditions but there are situations or mixes that strain the limits of what these medium and coarse sands can do in a pump mix. These cement sand blends often leave the “fine” grade to be filled by the cement or fly-ash. This is why every pump pour mix design should be reviewed by the pump company before going to the job-site. It's easier to change or adjust the mix before trucks are on the road.
The pump is OK...it has to be the mix !
Maybe. What about the gap in the cutting ring? Anyone looked at the underside of the ring where the real wear is? The top edge may be worn and getting chewed up a bit, and look OK, but the bottom side can be missing since it wears at minimum 2 times faster than the top edge. It travels farther so it wears faster. When a tube is adjusted its often brought up to the top edge being snug. This often makes the ring bind and not fit well, because the bottom was looser. Grooves that were worn into the wear plate prevent the ring from sealing and each cycle forces the cutting ring back inside against the spring as the gap between wear plate & ring increases as the Tube travels across the wear plate.
The ring is designed to float as it travels so it does not break. This means that as it touches a spot that’s higher on the wear plate the ring will have a gap in one edge or the other. This is where the fines in the mix will be lost and squirted back into the hopper instead of the concrete valve because its easier. Same result, plugging in the rear end. Problem is,. How do you address this without your attorney being present? Any admission places you in the hot seat and makes you liable for the time and material that may be lost due to pump-ability problems. Its best not to take a pump out that has a bad cutting ring or wear plates because if you;re caught....you;re caught.
Been knocking out those little spots of build-up in the rear end? What about the half-moons in the bottom of the S-tube or Rock? Build-up happens where ever there is/was an air leak. Rock valve discharge ports are good for this where there is an O-ring that gets neglected and leaks a little air. With every stroke there is a little bit of air that's forced thru under great pressure and that is like shot-Crete on the inside of the transition or discharge port. If this is allowed to keep growing it will become tomorrows blockage when a boney rocky mix just cannot make it thru the reduced passage size. Just knock it out. It only takes a minute with bar. Funny thing is that the easier the mix is the more prone to building up in the rear end where the velocity is low. The rougher and rockier the mix the less there is to worry about build-up in the rear end.
Sometimes fresh hard surface welding can be a problem with a marginal mix. Its not so much being a smaller passage as it is being a much rougher passage. A nice hard surfaced elbow on the back end might be hell to pump thru for a few days until the welding has had some yards pass over it to sand it down. That's all that concrete wear is, liquid sand paper. Sanding down the rough welded beads takes a few yards but it turns into nice smooth bumps that deflect the abrasion and extend the life of the part several times over.
Mismatched parts or alignment will do the same thing.; if there is an edge that the concrete can hang up on, it will. A new elbow next to a worn discharge port piece will do this. As the concrete travels it forms a slug that likes to exert force against the pipe wall as it moves. If there is an edge there as small as 1/4” that is enough to disrupt the slug and cause the rocks to hit and catch the edge instead of the slurry being the boundary layer.
Finally, one of the strangest ways to plug up the back end is running worn out mud cups. I've seen cups so loose and worn that they could not hold water in the box for more than a single load. They passed so much water to the mud in the cylinder that a 3” slump in the hopper turned into a 6+ at the discharge. If you're dealing with rocks, this can kill ya. Again, its something that you never want to admit to, because it makes YOU responsible, but it can be entertaining watching an inspector try to figure out how a mix gets wetter inside a pipeline. Having a mud cup damaged on the job is a little different. Generally something had to cause that damage, most often a piece of steel that came down the chute and got in the material cylinder where it cut the cup or got wedged between the cup and the cylinder. This can be a legal mess too, because a good chrome material cylinder is very expensive and the downtime adds up pretty fast as well. A redi-mix company isn't going to come running out there with a check....
If this happens to you, and you know it to have happened on this job, today, get your office involved asap (as with any trouble), document everything that you can, and find the cause if possible and hold onto it. Finding a chunk of steel is your proof that your pump was damaged there and someone else is responsible. Let your office tell you what to say and when to say it as far as whats happening and why. If the problem has been identified then usually the office wants the customer to be aware, but let the office decide.
Hopefully these examples and details will help you in identifying the cause of a rear end plug or the “rocked-up” scenario. The difference in this and other plugging is that here you have more control over the situation since its not able to travel into the system where the potential for a time consuming repair exist. In the rear end it can get ugly but as long as you don;t abuse the power that’s available to you,. You can prevent it from getting out of hand. Identify the problem, make adjustments. Remember that power is not always your friend, and neither is water. It is far better to have these issues in the ass-end than it is to have them pumped well into the boom. The pumps of today are all quite capable of pumping almost anything into the boom, so its the operator that has to be pro-active in what he allows to fall thru the grate. Look at it, know what it's supposed to be and what that mix should look like. Learn what the slumps are and ways to judge what that is by the way the mud slides in the chute. And most of all, know the limitations of the pump. Keep it tight, and keep it clean. If you do these things as part of your job you probably won;t be faced with these issues very often. Its an ongoing education. Learn what to do so you won't have to do it...
By Lee Horton Published by ConcretePumping.com All rights reserved.