Do you take your best shot from up on top or at stages along the way to the top? In other words, do you use high-lift or low-lift grouting techniques?
"In its simplest form, high-lift grouting is typically done with a grout or concrete pump, as high as 24 feet in one pour, and using large pours of grout," notes Stan Harwell, Eastern regional manager for Spec Mix, Mendota Heights, Minn. The Virginia-based Harwell continues, "There is even a movement today to increase that height to 42 feet. Low-lift grouting, on the other hand, is done in lifts up to about 4 feet, and sometimes every two or three courses of block."
According to Harwell, "There are two schools of thought. The contractors that like to pump think that they save money by waiting and having one crew do all the grouting at one time. They'll lay up as much wall as they can and then throw the bricklayers on another wall while the grouting team comes in and pumps the first one.
"The guys that do low-lift grouting swear that they're faster because they don't have to stop production. They'll have two guys just following behind the bricklayers as they lay their wall. As the wall goes up, laborers will work behind and, using 5 gallon buckets, dip grout out of the tub and pour it in or they'll use a Grout Hog."
MCAA member Bill Dentinger of Bill Dentinger, Inc., a mason contractor in St. Paul, Minn., voices this opinion: "Personally I like high-lift grouting for production reasons. As long as the core is large enough an 8- or 12-inch block core I like to grout in 8-foot lifts. With 12-inch block, you can go higher if you need to. For us, it works best if we can put in a cleanout and don't have to worry about the other side of the wall being finished."
However, he adds, "We do a lot of institutional work, jail cells and such. When you're in a partition wall in a cell, for example, low-lift grouting is really the only way to do it. The walls are too difficult to get the grout pump or hopper to a lot of times."
So the choice of high or low can be situational sometimes one is better than the other because of the type of construction, the type of block or the location.
The Masonry Institute of Washington (MIW), for example, says that high-lift grouting is usually preferred over low-lift grouting because the contractor gains efficiency and, because the inspector can be certain that all of the steel (rebar) is properly in place.
Whichever method is used, some "universals" are followed. Proper preparation of the grout space is very important. With high-lift, all mortar droppings and debris should be removed through the cleanout openings or by other approved methods. In high- or low-lift grouting, mortar that protrudes more than about 3/8 inch into the grout space should be removed so that grout will not bridge at these location causing incomplete filling of cores. When high-lift grouting is used, protrusions can be knocked off and removed through the cleanout openings; with low-lift, they should be removed as soon as the course is completed to make reaching them easier.
In high-lift, cleanouts and wall ties must be built into the wall and the collar joint must be at least 3 inches wide unless it contains no horizontal steel, in which case it may be 2 inches wide. The reason for such great width is to prevent the grout from hanging up and leaving voids in the wall.
Vertical grout dams should be built into collar joints to control the horizontal grout flow. These dams should not be more than 25 inches apart. Without these dams the grout might segregate, or the inspector might find himself watching a continuous grout pour that takes all night to complete.
Cleanouts are usually created by omitting every other brick in the bottom course of one tier. Mortar droppings and other debris are flushed out daily. A good way to keep them from sticking to the foundation is to sprinkle some sand into the collar joint after each cleaning.
Quantity and quality
MIW suggests that after the wall has cured for at least three to five days, depending on the weather, and after the inspector has assured that the steel is properly placed and that the collar joint is clean, the cleanouts may be sealed and the wall grouted.
Normally, cleanouts are sealed by laid bricks braced to prevent them from blowing out. A better way, which can be used when the bottom course is not exposed to view, is merely to brace a plank against the bottom course. The contractor saves the expense of laying all those closer bricks and, when the plank is removed, the inspector has a foolproof way to inspect the bottom of the grout pour.
Using the low-lift technique, grouting should proceed in lifts not more than 4 feet before puddling or consolidating. Grouting of any section between dams should be completed in one day with no interruptions over an hour long. Grouting too rapidly might blow out the wall; too long an interruption might cause shrinkage cracks or laitance.
Dentinger, who works in the cold winters of Minnesota, notes that weather can make a difference in the choice of high- or low-lift. "In the winter, the cover of the scaffold will sometimes dictate whether we want to do high- or low-lift grouting. This is because of accessibility, the ability to get our grout in to the work area. When we're grouting in the winter, there is also the problem of keeping the grout warm. If you can't keep the grout warm in a tall-lift situation, it's better to do a low-lift where you can control the heat better. When I'm on a large wall of a big retail store, I would just as soon do high-lift grouting and as long as I can keep my heat up and high-lift is allowed by the architect."
And there is one of the rubs, having authority to do high-lift. "I'll be honest with you," Dentinger admits, "we seem to have trouble getting our architects to go along with high-lift grouting. It's probably the type of work we do: industrial, schools and institutional buildings. As much as 90 percent of the grouting we do is low-lift because the architects won't allow us to do high-lift. I wish they would allow high-lift more often, because it's much more productive."
Another problem is quantity. High-lift grouting requires a lot of grout in each cell since it has to fill from the bottom of the cavity to the top, often 20 or more feet. But even that isn't a lot when you compare it to a truckload. As Dentinger says, "What determines it, as far as we are concerned, is if we can get enough wall ready to have a concrete truck come. We need enough ready to make it practical to use a 5-yard load, which is the minimum they would give us.
"Our company doesn't do any high-lift grouting unless we can get the grout delivered by a concrete truck. If we don't have access to a pumper or have enough volume to use a concrete truck, we will do low-lift grouting," Dentinger says.
Once Dentinger gets the delivery, he has to get the grout to the top of the wall. "We use two different systems. In most cases, with high-lift grouting, we would pump it with a grout pump with a 2-inch line. Or we could use a grout hopper system, like Grout Hog. The concrete would be dumped into the pump, and then pumped into the core or be put into the hopper and dumped in with the forklift."
With high-lift, care must be taken to insure the cleanouts are properly replaced and braced. MIW says, "Face shell plugs or mortared units should be adequately braced to resist the fluid pressure of the grout. In hollow unit masonry, the mortar should be permitted to cure sufficient before grouting to prevent mortar joints from being blown out by pressure of the grout. When filling the cavity space between two wythes of masonry, a specified period of time is recommended to prevent 'blowouts.'"
And one final issue, the "cramped quarters syndrome." As Harwell indicates, "The problem we run into is that the architecture/structural engineer sees this 4-inch square core as a perfect concrete form. He takes two pieces of rebar that have to be lapped as they're installed in the block. So inside this 4- to 6-inch square you have two pieces of rebar sitting side by side, and then you have the little pieces of reinforcing wire, call them 'butterflies,' and that stuff can block up that hole. Add to that the mortar that is squeezed into the cavity mortar snot and that hole is getting smaller and smaller. The engineer still sees it as a perfect concrete form, but it never is."
Low-lift grouting, what MIW defines it as "when the grout is poured as the wall is being built," has many fans. Harwell has found that many engineers call for low-lift, as do architects and owners, for a variety of reasons. He recalls, "Wal-Mart requires all low-lift grouting, and I believe the Army Corps of Engineer requires it as well. They feel the inspectors can keep a better eye on what's happening with low-lift. It's easier to make sure that the cavities are clean and unobstructed when you're low-lifting. And you get better consolidation."
Some considerations when doing low-lift include being sure the collar joint is wide enough to provide proper steel clearance. It must be at least 3/4-inch wide. One tier may be carried up 6 feet before grouting, but the other must be laid up and grouted in lifts not higher than 5 feet or the wall might blow out. And the last grout pour of the day should stop about an inch to an inch and a half below the top of an unfinished wall. This provides a keyway in the cold joint.
Harwell expands on that idea, saying that contractors should be sure that the space provided is always adequate. "Structurally, what we want is to make sure that grout stays down about an inch and a half below the top of the face shell of the block. That will provide a good mechanical connection so that the moment of inertia is carried through that wall a little better. People tend to grout it right to the top of the block and then cut it off. That way, we have a mechanical connection at the same point as the coursing of the block and that's not a good technique."
Consolidate and reconsolidate
Consolidation of the grout is required in low-lift as well as high-lift pours. This is accomplished with a lintel head concrete vibrator and doing it is an art form in itself. As Harwell says, "You have to be careful of the head pressures. They can't be allowed to get too high and you can't vibrate it too much if the wall's green because you can blow out the wall you would blow the mortar joints right out of it. Consolidation is the thing that structural engineers worry about when they specify high-lift grouting."
He continues, "You're supposed to consolidate and then reconsolidate. When grout comes in contact with the block, the block is so porous it absorbs all the excess water. You may start out with a very high water to cement ratio, but after it's in the block and the suction of that block takes that excess water out, you're back down to a water to cement ratio of about .49 or .50. The result is that you get some shrinkage. Cementitious products tend to shrink and that's why you have to do the reconsolidation. Typically it shrinks at that top, not necessarily at the side."
So should you high-lift or low-lift? The right answer is, there is no right answer. Harwell summarizes the dilemma by saying, "The engineering of the wall can determine the best method of grouting. If I have a lot of horizontal bond beams, then I would low-lift, because you can't go any faster than installing your bond beams will allow you. If I am doing a building that has a lot of reinforcing, then I would low-lift simply because I don't have to keep pulling my masons off and sending them to set up on another wall while the grouting team comes in."
He adds, "It never ceases to amaze me that a contractor is getting all this production and then has to stop and go to another wall and wait for the grout crew to come in and grout. If he's low-lift grouting, he can grout right behind the mason. As soon as they get up 4 feet, they just knock it out and keep on going. They can put two laborers with a tub of grout, or a forklift and a laborer and a Grout Hog, and there they go, they never have to stop."
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