The reinforced floor in a light industrial plant is curling. The floor was placed in March and April and curling was perhaps first noticed the following autumn. Now that the winter has passed and the atmosphere in the building has been unusually dry, the slab has risen as much as 1/2 inch at the corners. We are consulting for the owner about the problem, and we plan to recommend grinding in some key locations; in other places we will recommend sawing diagonally across the corner to allow the corner to drop down. The owner is reluctant to repair before all the curling has taken place. Is there any way to determine if the process of curling has stopped?
The first thing to do is to periodically measure elevations of edges and corners with an engineer's level to determine if the elevations are changing and the rate of change is diminishing. However, this may tell only a part of the story. With the arrival of spring comes a period of higher relative humidity, and this increase in relative humidity may temporarily slow down the rate of drying and curling. We therefore suggest, as a second step, keeping a running record of the relative humidity in the plant. The measured humidities and floor elevations should be plotted on one or two calendar graphs. If the reduction in the rate of curling corresponds to an increase in relative humidity, curling may resume when the relative humidity begins to go down again. If there is eventually no increase in elevation (even when the relative humidity is low) the curling should have ceased. Another way to get an indication of what is happening within the slab is to take three 6-inch-diameter cores to obtain the amount of material required for "Standard Method of Test for Moisture Condition of Hardened Concrete by the Relative Humidity Method," ASTM C 247. The cores should be immediately enclosed in polyethylene bags and taken to a laboratory for dry sawing (with as little exposure to the atmosphere as possible) into three layers representing the top, bottom and middle sections of the slab. After the three top pieces of cores are combined to make one sample, the three center pieces to make a second sample and the bottom pieces a third, the samples are sealed temporarily in plastic bags until the ASTM test is made to measure the relative humidity for each layer. The relative humidity is then plotted against the center height of the layer above the bottom of the slab. The plotted curve will show the moisture gradient in the slab. All three readings can be compared with the average ambient relative humidity within the building to see if the moisture content of the slab is approaching a steady state.