Placing and finishing lightweight concrete can be a tricky and risky task. Success requires teamwork from the owner, architect/engineer, general contractor, ready-mix producer, and concrete contractor. Each must understand their responsibility for successful placing and finishing of high-quality lightweight concrete slabs. The entire team must embrace their roles in the planning and execution phases.
Teamwork begins with the building owner, who must understand which final floor finishes are realistically achievable with lightweight concrete and which floor tolerances (FF/FL) are necessary to provide a high-quality building. The owner must apply that knowledge in purchasing a realistic floor finish and employ a testing agency for floor tolerance testing after each floor slab gets placed and finished. Next, the architect/engineer must specify achievable concrete properties such as unit weight, air content, strength of concrete, and floor finish tolerances. This list should consider performance specifications for the lightweight mix design while leaving specific attributes of the mix to the supplier.
The ready-mix supplier should consider all aspects of the project and concrete requirements when designing the mix, take necessary and appropriate actions to properly presoak the lightweight aggregate, and make adjustments based on concrete contractor and testing agency feedback. The concrete contractor must plan for placing and finishing lightweight slabs by choosing the correct pump, slickline, and finishing equipment, as well as use proper finishing techniques, observe proper timing, be aware of weather conditions that promote surface crusting conditions, and communicate and provide feedback to the other team members. With these roles properly executed, the team can ensure successful lightweight concrete placement.
Most lightweight mixes are pumped, which affects the mix design.
James McHugh Construction's, Chicago, current project illustrates how this team approach works. McHugh is the concrete subcontractor for the 353 N. Clark St. building, Chicago, a 50-story structural steel lightweight composite metal deck slab and concrete core building. It features two and a half levels of normal weight supported slab construction in the lower levels of the building with lightweight concrete used in the rest of the work. The lightweight concrete supplier is Prairie Materials, Bridgeview, Ill. The total slab surface area is approximately 1.5 million square feet, with nearly 1.25 million square feet of hard-troweled lightweight concrete slabs. Project specification called for a floor flatness (FF) of 30 with a local minimum of 18. McHugh is placing two floors per week (or 60,000 square feet). The average FF is 36 with a local minimum of 27.
The owner's role
It is important that the owner understands which floor finishes can be attained realistically and specify lightweight concrete placed on metal deck when appropriate. ACI 117 classifies a moderately flat floor surface as one that would be suitable for carpeted, ceramic tile, or vinyl tile in commercial office buildings or an industrial building with low-speed traffic. Per Table R4.8.4 in ACI 117, a moderately flat floor would have an FF of 25, or a maximum gap of ? inch under a 10-foot straightedge. When the owner requires a floor flatness greater than necessary with lightweight concrete, problems can arise. Requiring overly flat floors means costly finishing procedures and added risk to the concrete contractor. These factors also unnecessarily drive up the cost of the project and increase the risk of blisters or delaminated areas of the slab as the finishers try to meet the high FF values. If the owner understands these factors and understands the team can produce a quality floor with a lower FF value, the finishing costs will be lower and the risk of failed floors and problems with the concrete reduced.
Another key to success is for the owner to employ a testing agency to perform the proper tests for the project. The testing agency should perform FF testing per ACI and ASTM standards. Although ACI requires the results be provided to the concrete contractor within 72 hours, a faster turnaround is not only possible but preferred.
The design team, including the architects and engineers, plays a key role in successfully placing and finishing lightweight concrete. The designers must specify the lightweight mix design based solely on performance, not on mix issues. In most cases, the only concrete performance criteria important to the design team are compressive strength and unit weight of the concrete for both the fire rating and overall weight of the structure. Specifying air content, slump, and/or the w/c ratio puts unnecessary restraints on the ready-mix producer who designs mixes to meet the actual performance requirements. When a designer specifies quantities, they actually can exceed the performance requirements. If the ready-mix producer can provide 4000 psi lightweight concrete at a unit weight of 115 ± 3 pounds with an air entrainment of 3% to 4% and a slump of 8 inches, it's unnecessary to specify that the air must be 4% to 8% with a maximum slump of 5 inches, because performance can be achieved at other values.
The ready-mix producer
Although the designer and other team members play a vital role, the ready-mix producer may be the most important part. The producer must develop a mix design that is pumpable, placeable, and finishable, while also meeting the project specifications. Most projects that specify lightweight concrete require the equilibrium weight of the concrete to be in the range of 115 ± 3 pounds per cubic feet (pcf). The equilibrium weight of the concrete is not the wet weight of the concrete, but the weight at which the concrete has cured or dried enough so the relative humidity of the concrete equals the relative humidity of the ambient conditions. According to Gary Hall, quality control manager for Prairie Material, the concrete's batched weight could be anywhere from 4 to 10 pcf heavier than its equilibrium weight. As a result, the mix designer must design a lightweight mix that achieves the required unit weight of the concrete with a lower amount of air entrainment in the concrete. Some air entrainment is needed in lightweight concrete to attain the required unit weights and to make the mix pumpable. However, it is equally important to understand that too much air can cause finishing difficulties, along with delamination and blistering. From experience, it is recommended to keep the air content at 3% and no more than 4% to help ensure success. Air entrainment readings between 6% to 8% can cause great problems in finishing.
Along with the proper air content, the amount of cement used in the mix is significant as well. Hall advises that lightweight mixes be designed with a minimum cementitious content of 564 pounds of cement because lightweight aggregate isn't as strong as normal weight aggregate and requires more paste in the mix to make it pumpable. To make sure mix proportions are correct, Prairie measures the unit weight of the lightweight aggregate daily to be added to each batch by volume, not weight, because the specific gravity of lightweight aggregate fluctuates based on the level of saturation of the aggregate. Prairie designs the amount of lightweight aggregate to be added to each batch by volume and batches by weight to meet that volume. The weight of the lightweight will vary in order to maintain that correct volume. This change is due to moisture, porosity, and microstructure of the lightweight aggregate.