Finishing Lightweight Concrete Floors

Finishing Lightweight Concrete Floors

Over the past 80 years, more than five hundred thousand (500,000) floors have been constructed with structural lightweight concrete made with Expanded, Shale, Clay and Slate Lightweight Aggregate. The satisfactory performance record of these floors is accepted and well known. In the past few years delamination issues have been reported on both normalweight and lightweight concrete floors when a riding trowel with float pans has been
used. The purpose of this paper is to offer a better understanding of the construction, finishing and use of lightweight concrete floors.

General

Designers specify lightweight concrete floors because they are cost effective and environmentally efficient.Lightweight concrete has compressive strength compara-ble to normalweight concrete, but it is typically 25% to 35% lighter. Lightweight concrete floors offer design flexibility and substantial cost savings by providing less dead load, improved seismic structural response, longer spans, better fire ratings, thinner sections, smaller size structural members, less reinforcing steel, and lower foundation costs.

Building codes mandate the requirements for fire rated floor assemblies and are explicit with regard to the use of structural lightweight concrete. The concrete used in the floor assembly, and tested in accordance with ASTM E 119, must meet the specified physical properties of density (unit weight) and air content. For UL fire rated lightweight concrete floor assemblies, entrained air contents are specified in the concrete property criteria.

By definition, lightweight concrete is lighter than normalweight concrete. This is made possible by replacing heavy, ordinary aggregate with expanded shale, clay or slate lightweight aggregate, and by maintaining entrained air at approximately 6%. Air entrainment in concrete improves durability and workability, reduces bleeding, and is recommended for lightweight concrete by both ACI 211.2 and ACI 302. For workability and weight reduction, ESCSI recommends 4 to 7 percent air entrainment.

The typical lightweight suspended floor is used with floor coverings for foot traffic in office, commercial, multi-unit residential and institutional buildings. ACI 302 calls this type of floor a Class 2 Floor with a flat and level slab suitable for applied coverings, and having a "light" steel-troweled finish. The floor flatness/levelness tolerances for this floor are F_F25/F_L20. On some occasions, flatness/levelness tolerances are higher to meet specific design requirements. The "light" steel-trow-eled finish is not the same as a "normal" or "hard" steel-troweled finish recommended by ACI 302 for commercial or industrial floors subject to vehicular traffic.

Concrete Finishing

The increasing call for faster construction and flatter tolerances has increased the use and development of ride-on power trowels with float pans. This equipment is capable of providing flat floors with a minimal amount of labor, and has been used exten-sively on non-air entrained slab-on-grade concrete. It is now being used successfully on many elevated floors which are usually constructed with lightweight concrete. The user of this equipment needs to recognize and adapt to the fact that lightweight concrete is always air entrained at about 6%, and often has a different timing sequence during finishing.

Ride-on power trowels with pan floats impart more energy to the concrete surface at an earlier age than walk-behind power trowels. All power trowels with pan floats exert much lower surface pressures, thereby allowing the contractor to commence finishing sooner with this equipment. This fact is a major contributor to delamination issues.

Concrete Construction, March 1998, pp. 277-283, reported surface pres-sures of 0.36 to 0.98 psi for walk-behinds and ride-on power trowels
equipped with blades, 0.16 to 0.42 psi for pan floats, and 3.3 to 6.0 psi surface pressure for a person walking on the concrete. ACI 302 recom-mends that machine floating be started when the concrete will support a finisher on foot without more than a 1/8 to a 1/4 inch indentation. As a general rule, ACI 302 also recommends that when flatness tolerances are not high, power floating should be started as late as possible. This is indi-
cated when a foot print is barely perceptible.

Problems develop when the floor is power floated prematurely and consequently over worked. This is not a new development. For many decades, delamination has been known to apply to inappropriately timed hand troweling.

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