We often talk about “setting” of concrete, but the word means different things to different people, and each definition is based on a different test method.
One of the most useful definitions of setting is in the context of finishing concrete flatwork. A finisher knows that concrete has reached “initial set” at about the time that standing on the concrete leaves a bootprint only about ¼ inch deep. This is about the time that you can begin the first machine-float pass. As the concrete continues to stiffen, the finisher finally observes that his or her boots only leave the slightest scuff on the concrete, indicating “final setting.” Beyond final set it is difficult to smooth out rough spots without sprinkling water on the surface and working-it-in. When this ritual of the “blessing of the slab” is successfully performed, the miracle of bringing apparently set concrete back to life occurs because water has been forced in-between cement particles, breaking their rapidly developing bonds, and increasing porosity of the paste at the concrete surface.
For an indoor slab to be covered with tile or carpet, getting rid of bumps in the floor while gaining a somewhat softer, more porous top surface may be an acceptable trade-off. For a bare floor subjected to traffic and abrasion, or exposed to water, deicing salts, and freezing weather, “blessing the slab” can be a curse. To provide an appropriate and durable finish, work beyond bull-floating shouldn't start until after the concrete has reached “initial” set, and should be complete prior to the onset of “final” set. ACI Committee 302 (Construction of Concrete Floors) has named the time period between initial and final set as the “Window of Finishability.”
Time the finishing. Time the curing.
Just as the timing of finishing is dictated by setting, so is the timing of curing. The concrete surface can be harmed, in some cases, when a saturated wet cure, such as burlap or soaker hoses, is applied to concrete that has not yet reached its final set. Water applied forcefully to a green concrete surface can penetrate that surface, damage the paste, and degrade the concrete. One can be more aggressive in applying water to cure the concrete the further along the concrete is in the setting process. There is a danger, however, in waiting too long to apply a wet cure as the peak rate of evaporative water loss occurs immediately after finishing. This is where spray-applied curing compounds are extremely valuable. They can be applied without walking on or marring the freshly finished surface, and then can be followed-up with a more substantive curing method a few hours later.
Cold weather and hot weather
Setting characteristics of concrete are so important that when cold weather and lower concrete temperatures threaten to slow down the rate of setting, set accelerators are added to the concrete. (These are not the same as “anti-freeze agents.”) When hot weather and warm concrete cause concrete to set faster, “set retarders” are added to slow it down. While set retarders in the right dosage can delay the time of initial set, buying the finisher time to get-on the slab, retarders do not always extend the length of the window of finishability. So finishers might have to work just as hard and just as fast even though the retarder allowed a delayed start of finishing operations.
Setting is the result of hydration
What we call “setting” is the result of the hydration of portland cement, producing microscopic mineral products that link adjacent cement grains to each other, much the way Velcro strips attach to each other. As hydration progresses, each cement grain is bound more tightly to its neighbors, so that it becomes more difficult for an outside force to deform the concrete. This is why the depth of a finisher's footprint becomes shallower as hydration proceeds. If we looked at it another way, we would see that the pressure required to press a finisher's boot into the concrete surface to a depth of ¼ inch gets greater with time.
We can extend this same principle by sticking, jabbing, plunging, or dropping any number of objects onto cement paste, mortar, or concrete and measuring either the resulting depth of penetration, or measuring the force required to reach some pre-defined depth of penetration. Since cement hydration drives concrete setting, it makes sense to measure setting characteristics of the cement by itself. This is done by dropping Gilmore needles (ASTM C 266) into a fresh cement paste and recording the times at which needles of a specific size and weight penetrate to a particular depth. (Figure 1)
Times to “initial” and to “final set” for cement are routinely measured this way and are reported on cement mill test reports. For testing mortar, larger probes are pushed to a depth of 1 inch (ASTM C 403. (Figure 2)
When it takes a contact pressure of 500psi to embed the needle to 1-inch depth we say the mortar has reached initial set. When it takes 4000 psi to do the same thing (100 lbs force on a plunger with a 1/40 in2 contact area), we say the mortar has reached final set.
Pick a test method
There are literally dozens of variations on these test methods. A technician at the cement plant might observe that a particular cement reached initial and final set in the ASTM C 191 Vicat Needle test in 100 and 250 minutes, respectively, after mixing. But if she used the ASTM C 266 Gillmore Needle test instead, she might report 75 and 400 minutes for initial and final set. At the concrete plant a quality control technician might mix that same cement with water and sand to produce a mortar for which the ASTM C 403 mortar setting test might show initial set at 6 hours (360 minutes) and final set at 7 hours (420 minutes). Finally, on the real job site, the finisher foreman might observe that concrete made with the same cement and same sand reached initial set (when he started his float machines) at 3 hours (180 minutes) after batching, and final set was reached just one hour later. We end up with at least three different versions of setting times, differing by as much as 3 hours for time of final set!
An additional point of confusion is that ASTM C403 is officially called, “Standard Test Method for Time of Setting of Concrete Mixtures by Penetration Resistance.” But don't be misled by the word “concrete” in the title. This test is performed by first sieving-out the coarse aggregate from a sample of fresh concrete, leaving a mortar that is tested for setting time. The resulting value is then a mortar set time which can be different from the concrete set time by a number of hours.
The good news is that any factor that speeds-up or slows-down setting will have a similar effect in cement paste, mortar, and concrete. But, testing set time of the cement alone cannot tell you about the influence of flyash, slag, silica fume, superplasticizer or any other admixture. Neither can a mortar setting test tell you about the effect of coarse aggregate, or the drying and crusting effects of sunshine or wind. Lab tests can't predict effects of variable field conditions, but neither can field tests isolate the effects of various factors to enable you to diagnose or solve a setting problem.
Field test method proposed
A field test of concrete setting has been proposed to ASTM for evaluation. The test measures the force required to indent a steel plate into concrete to a depth of ¼ inch. The plate is fabricated to have the same contact area as a size 11-1/2 boot, an embedment force of 100 lb. the pressure under the plate is about the same as under the boot of a 200 lb. concrete finisher. (Figure 3) (Abel, Jon D., Hover, K.C., “Field Study of the Setting Behavior of Fresh Concrete,” Cement, Concrete, and Aggregates, Vol. 22, No. 2, December 2000, pp. 95-102.) Figure 4 compares setting of concrete vs. mortar, and it is clear that the concrete mixture “sets” far sooner than the mortar sieved from the same concrete.
On one job the setting data in Figure 5 appeared to support the contractor's complaint of variable set time over several consecutive loads of concrete. However, the time scale in Figure 5 is based on time since concrete placement. When the same data are plotted vs. actual batch time (Figure 6) all truckloads had about the same setting behavior except for the first truck of the day, carrying cooler concrete that set more slowly. Variable haul time and truck time on-site prior to placing critically affect setting time as judged by the finisher!
About the Author: Ken Hover, Professor of Structural Engineering, Cornell University
Earned Ph.D. at Cornell University. Faculty member in Cornell's School of Civil & Environmental Engineering for 18 years, teaching courses in reinforced concrete design, concrete materials, and construction management. Research concentrated in concrete durability and the impact of construction operations on concrete quality. Earned the top teaching awards at departmental, college, and university levels. Appointed to rank of Full Professor in 1992. Undergraduate Dean of College of Engineering, 1996-1999.
Received ACI's National Educators Award (the Kelly Award) in 2001. ASCE Best Basic Research Award, 1992.
Current member of ACI 305-Hot Weather, 308-Curing (Past Chairman), 309-Consolidation, 318A-General Concrete and Construction, Elected to ACI Board of Directors, 1999. Chair of 308 Subcommittee for new “Guide to Curing Concrete.” Fellow since 1992.
Prepares and presents seminars and short courses nationally and internationally. Developed Federal Highway's short courses on Concrete Materials and on Concrete Mix Design and Proportioning. Rated top technical speaker at World of Concrete every year since 1994.
Licensed Professional Engineer since 1974.
© 2008 L&M Construction Chemicals, Inc. | ConcreteNews September 2008.