Honeycombing occurs during concrete placement when aggregate and sand in the concrete bunch into clusters. Pieces of aggregate and sand stick together and fail to mix with the cement paste. These clumps of rock and sand trap air, forming air pockets. After curing, the air pockets weaken the slab.
If honeycombing is evident in visible parts of the foundation, chances are that invisible areas of the foundation are also honeycombed. Honeycombed areas are more susceptible to water infiltration. Water infiltration can cause mold growth in absorbent flooring materials, like carpeting and mortar joints. It can also cause rigid flooring materials to warp and buckle.
Using fresh concrete
When concrete is placed, it has to be fresh and workable. The aggregate, sand, cement, and water have to be combined to form a strong foundation structure. After concrete is placed in the slab foundation formwork, the mixture has to be consolidated; the large aggregate and sand have to be distributed evenly throughout the slab, not only in the center of the slab, but also in the grade beams, edges, and corners.
Consolidating the concrete
A vibrator is used to consolidate the concrete after it's placed in the formwork. Consolidating the concrete means to combine the aggregate, sand, and cement paste so that each material is distributed evenly throughout the slab. The vibrator is attached to the outside of the formwork and agitates the concrete at high frequencies. Portions of the slab that are far from the grade beams are usually consolidated with a rake or other handheld device, since the slab is thin enough in that region to do so.
Cause 1: Stiffened concrete mix
If the concrete mix sits in the truck for a prolonged period, it will begin to stiffen, causing the aggregate inside it to clump. The aggregate clumps will fail to mix with the cement paste and will create air pockets that lead to honeycombing.
Solution
Don't use wet concrete that has been sitting in the truck for a prolonged period. If the concrete ticket indicates that the mix has been sitting for a long time, reject it, and request a new batch of concrete.
Cause 2: Incorrect concrete placement
If the foundation crew pours the concrete out of the chute from too high of an elevation, the aggregate will fall to the lower areas of the slab core, where it will clump and lead to honeycombing.
Solution
Ensure that the concrete is poured from the chute no more than 3 feet above the formwork.
Cause 3: Inadequate concrete consolidation
If the foundation crew doesn't consolidate the mix correctly during placement, aggregate may clump inside the slab, leading to honeycombing.
Solution
Use a mechanical concrete vibrator manufactured specifically for the consolidation of freshly poured concrete.
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