2024-25_NTCA_Reference_Manual_dig_dwnld

Ceramic Tile and Stone Installations over Concrete Substrates

THE DIVISION 3 AND 9 DIFFERENCES CAUSE MAJOR PROBLEMS FOR FLOORING INSTALLERS Method for measuring with a 10 ft. (3 m) straightedge. The straightedge method specifies that the gap observed under a freestanding or leveled 10 ft. (3 m) long straightedge shall not exceed 1/8 in. (3 mm). The problem with this method is that there is no standard method for taking measurements (i.e., number of tests, location, direction) or quantitative procedure for establishing compliance of a test surface. MEASURING F-NUMBERS ACROSS CONSTRUCTION OR ISOLATION JOINTS Don’t measure F-numbers across construction or isolation joints. FLATNESS VARIATIONS WITH ELAPSED TIME Flatness varies with time. The flooring installer doesn’t see the floor the concrete contractor produced. The floor the installer sees isn’t as flat as it was when the flatness was first measured. Flatness is time sensitive because of curling. Curling changes an initially flat panel into a curved panel with raised edges. CAUSES OF CURLING Changes in slab dimensions that lead to curling are most often related to moisture and temperature gradients in the slab. When one surface of the slab changes size relative to the other, the slab will warp at its edges in the direction of relative shortening. This curling is most noticeable at the sides and corners. One primary characteristic of concrete that affects curling is drying shrinkage. Anything that increases drying shrinkage of concrete will tend to increase curling. The most common occurrence of curling is when the top surface of the slab dries and shrinks with respect to the bottom. This causes an upward curling of the edges of a slab. Curling of a slab soon after placement is most likely related to poor curing and rapid surface drying. In slabs, excessive bleeding due to high water content in the concrete or water sprayed on the surface; or a lack of surface moisture due to poor or inadequate curing can create increased surface drying shrinkage relative to the bottom of the slab. Bleeding is accentuated in slabs placed directly on a vapor retarder (polyethylene sheeting) or when topping mixtures are placed on concrete slabs. Shrinkage differences from top to bottom in these cases are larger than for slabs on an absorptive subgrade. Thin slabs and long joint spacing tend to increase curling. For this reason, thin unbonded toppings need to have a fairly close joint spacing. In industrial floors, close joint spacing may be undesirable because of the increased number of joints and increased joint maintenance problems. However, this must be balanced against the probability of intermediate random cracks and increased curling at the joints. The other factor that can cause curling is temperature differences between the top and bottom of the slab. The top part of the slab exposed to the sun will expand relative to the cooler bottom causing a downward curling of the edges. Alternately, during a cold night

when the top surface cools and contracts relative to the bottom surface in contact with a warmer subgrade, the curling due to this temperature differential will add to the upward curling caused by moisture differentials. Slab Surface is Cooler and Drier than Base - Upward Curling – Typical in Internal Slabs Slab Surface at a Higher Temperature and Moisture than Base – Downward Curling

CURLING AFFECTS ON FLATNESS/LEVELNESS (ACI 117R-90/ACI 117-06/ACI 117-10)

ACI 117R-90 said curling will not significantly change a floor’s FF value. Later data showed this isn’t true. Curling also changes the gap under a straightedge. ACI 117-06 and ACI 117-10 do not say curling will not significantly change a floor’s FF value. But it doesn’t say why that statement from ACI 117R-90 was eliminated.

FACTORS THAT AFFECT SHRINKING a) Water and cement content b) Aggregate size and grading c) Aggregate stiffness d) Admixtures e) External restraints f) Environment

EFFECTS OF HUMIDITY ON SHRINKAGE AND CURLING

If the floor surface is exposed to a high relative humidity, it doesn’t shrink much. If the surface doesn’t shrink much, the floor doesn’t curl much.

GRANULAR WETTING CAUSES a) Vapor retarder not carried up the side b) Poor drainage c) Reverse slope d) Settlement at the slab edge e) Roof water not carried far enough away f) Irrigation water g) Broken water pipe

Curing (ACI 360 Design of Slabs on Grade) – “Extended curing only delays curling, it does not reduce it”

INCREASED CURING POTENTIAL a) Don’t cure by ponding

b)Avoid most wet curing methods because passage of water through cracks and joints increases moisture content at the bottom of the slab. This increases curling potential or can be a source of internal moisture that may affect the adhesion of moisture sensitive underlayments, primers or membranes .

Concrete should be placed directly on an intact vapor retarder (no blotter layer).

NTCA Reference Manual | 2024 / 2025

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Chapter 2 | Substrates