A primer on grout consolidation
The quantity of new reinforced masonry construction projects has increased steadily in the United States during the past several years, led primarily by commercial projects (big-box retail, hotels and low-rise residential) and school construction. Thus, ensuring that you are well informed on the latest requirements, specifications and materials for grouting masonry walls could serve you well. First, let’s clarify what we’re talking about.
Essentially, there are two forms of reinforced masonry construction (RMC): low-lift (simultaneous masonry construction and grouting), and high-lift (grouting after masonry construction). In either form, the majority of corefill grout is used with concrete masonry units, while only a small percentage is used with clay brick construction. With the low-lift method, grout is placed as the courses are laid. The grout then can be consolidated with a “pudding stick” or a mechanical vibrator. In the high-lift method, the masonry is built to the story height, or its full height (not exceeding 12.67 feet), after which grout is poured from a hopper or pumped by mechanical means. The grout then is consolidated with a low-force vibrator. When grouting between wythes, the vibrator should be placed in the grout at points spaced 12 to 16 inches apart.
The tasks seem simple enough, so why does there seem to be so much confusion on this issue?
Why all the confusion?
For starters, Jeff Leonard, vice president of bulk materials for The QUIKRETE Companies, said that because of the nature of masonry grout (with a 10- to 11-inch slump) some contractors think that vibration is not necessary. “This is not the case, however, especially if there is congested rebar within a wall,” he said.
Further, according to some grout suppliers and other industry experts, the code changes within national and local regulations can lead to a great deal of uncertainty. (More changes are, in fact, planned for the national codes, including updates for self-consolidating grout.) Additionally, the introduction of self-consolidating grout (SCG) has led some construction professionals to believe that vibration no longer was necessary, as if all grouts provided SCG’s self-consolidating qualities. It is important to note, however, that SCGs generally only reduce the need for vibration as opposed to eliminating it entirely.
“There is a lot of confusion about grout consolidation in the industry and ACI 530 tells us that anything over a 12-inch grout lift should be consolidated and reconsolidated. This however, does not always happen,” said Nick Blohowiak, a regional manager for Minnesota-based SPEC MIX. “ACI also allows for this consolidation and reconsolidation to be modified, and even potentially eliminated, if an appropriate demonstration panel is built and then tested. It is up to the architect/engineer and inspector to evaluate and enforce these codes on their projects, and all projects are different, requiring different methods to achieve success.”
Further, Blohowiak noted that consolidation and reconsolidation become more critical as the lift heights increase and the amount of congestion increases in the masonry cores. And he said that if the construction is performed in hot weather, the grout has a higher propensity to rapidly loose moisture into the units as it is poured into the cavities, which leads to bridging and voids.
“A proper preconstruction evaluation of the grouting process, including analyzing the reinforcement in the cores, temperatures expected during construction, and proposed lift heights should be done as a precursor to all projects,” he said. “If there is any question on how the grout will perform for the chosen method, a demonstration panel should be constructed and filled, and deconstructive testing should be done to verify the success of the chosen method of grouting.”
National building codes not withstanding, Blohowiak also stressed that local codes and contractor preferences for pouring methods also play vital roles in adding to the confusion and misconceptions for grouting and consolidating. But according to him, “There is no wrong way to grout in terms of lift height selection or placement method; the key to success is ensuring that quality materials go into the wall and that the cores are completely full without voids. This may require consolidation and reconsolidation if standard masonry grout is selected.”
The general consensus within the industry is that the grouting process is the most critical facet of RMC, and consolidation is the most crucial aspect of grouting. The goals are to steer clear of voids in the grout space, to ensure proper curing, and to avoid excessive debris in the grout during placement. To accomplish this, your contractors must be fully aware of the materials and equipment required for consolidating grouted masonry cells.
Basically, there are two types of grout (fine and coarse), both of which are specified under ASTM C 476. Generally, the grout chosen for a particular job is based on the grout space dimensions, with fine grout recommended where large amounts of reinforcing steel are installed or where conduits or piping are laid.
From there, options boil down to personal preference and availability of materials when choosing a grout supplier.
According to Blohowiak, the SPEC MIX system allows the contractor to control the mixing of grout onsite with speed that rivals ready-mixed supplied grouts. This system is gaining in popularity because of its level of quality control, speed of mixing and versatility of the silo systems, he says. “It no longer matters where the project is because bulk bags can be delivered and stockpiled ahead of the grouting schedule, and as little as a pail or as much as 40-plus yards of grout can be efficiently mixed out of one SPEC MIX silo per day,” Blohowiak said.
Leonard said there are two other advantages to using pre-blended grouts (including QUIKRETE’s Core-Fill Grout) with a silo system. First, he said, the mix designs that are produced and sent to the job have prior testing and back up. “If a highly fluid grout – 11-inch slump let’s say – is placed in a cylinder and cured, it will probably test low due to the high water-cement ratio,” he said. “Grout tested according to ASTM C 1019, sometimes referred to as the ‘pin-wheel test,’ allows for the highly fluid grout to bleed excess bleed-water into the block, therefore giving a resulting test that reflects the representative strength of grout placed in the wall.”
Second, Leonard said, a pre-blended product gives total control to the contractor as to the timing of the grout installation. “When receiving grout via a ready-mix truck, there has to be proper coordination. The truck can come too early or late and grout dispensed in the truck is not as quick of a discharge as other pours. If there is a holdup then demurrage charges can come into play if the wait is excessive,” he said.
Further, Leonard advises that grouts should be kept free of contamination. With field mixing, he said, the concern is the contaminates from contact with the earth as the material is dumped on site. In the winter, contaminates, such as salt, should be avoided. “Any contaminate can be detrimental to the ultimate strength of the finished product. Contaminates such as free salts show up in the form of efflorescence and rust shows up as a stain,” he said.
According to the Brick Industry Association, grout in contact with masonry solidifies more rapidly than that in the center of the grout space. It is, therefore, important to consolidate the grout immediately after pouring to completely fill all voids. The best procedure is to have two people performing the operation jointly – one to pour the grout and the other to consolidate it. Normally, a mechanical vibrator is employed for this purpose.
Concrete vibrators are the standard tools used to consolidate grout on masonry jobs. A multiplicity of manufacturers offer concrete vibrators and an assortment of attachments designed for multiple applications. A vibrator is used to ensure that the placed grout is even and free of air bubbles so that the material will maintain its strength characteristics. How the vibrators work is by spinning an off-centered weight as much as 10,000 times per minute, forcing any air pockets in the mixture to disperse. Manufacturers note that the head of the vibrator should always be lowered into the pour slowly and pulled out gradually to avoid the formation of an air pocket. The amount of time required to consolidate the grout varies depending on the mix, but it does not usually take more than 30 seconds to settle the grout sufficiently in each section.
Steve Payne, general manager for Denver Concrete Vibrator, which manufactures flexible shaft, pneumatic and high-cycle concrete vibrators, as well as external form vibrators, said that the common mistake made on grouting jobs is under-vibration. He also noted that it is the changes in the code and the inspectors’ interpretation of those codes that often lead to some confusion with these projects. To avoid these problems, he suggested the following guidelines:
The consolidation technique in general should be five to 15 seconds at the bottom of the cell or lift, followed by pulling the vibrator up at a rate of about three inches per second. If the grout is placed in lifts, he said, it is necessary to penetrate the previous lift with the vibrator and then pull the vibrator up from there to effectively “knit” the lifts together. “Vibrator operators get a ‘feel’ for the job and adjust for grout consistency, et cetera, as they move along to accomplish the task properly,” Payne said. “The pitch of the vibrator may actually change when the consolidation is complete, and the operator will see the surface appearance change and take on a sheen,” he continued.
Payne added that if the radius of effect of the vibrator is too small for the cell, multiple insertions are required to ensure overlap of the fields of action and the elimination of voids where no vibration has taken place. The rule of thumb, he says, is that the field of action (or radius of effect) is about eight times the diameter of the vibrator head.
As Payne noted so well, proper internal vibration increases the bond between the grout and the rebar in RMC. It also decreases the possibility of cold joints, honeycombing, segregation and excessive air, while adding strength to the finished product. The key to ensuring that all of your projects are grouted properly is keeping abreast of the changing standards and regulations – and making sure your contractors also are aware.
Sourced from the September 2007 issue of Masonry magazine.