We occasionally run across concrete conundrums that lead to acres of headaches. One of them is surface distress, usually blamed on finishing or cyclic freezing or both. Its cause depends upon circumstances; responsibility for it flip-flops depending upon to whom you are talking.
Surface distress can be caused by a variety of things including cyclic freezing helped by de-icers (or de-icers alone under certain circumstances), inadequate air-void systems, improper finishing that destroys entrained air-void systems in concrete surface regions, inadequate curing, rapid surface drying, admixture-induced surface stiffening, premature carbonation, “hard” troweling, prolonged troweling, the addition of water to surfaces during finishing, and finishing too early or too late.
Each of the potential causes of surface distress can be grouped within one of the following categories: (1) nonair-entrainment, (2) inadequate entrained-air-void system, (3) surface crusting, (4) improper timing of finishing, and (5) over-finishing. Air contents and air-void systems are functions of air-entraining chemicals and may be influenced by aggregates, supplementary cementitious materials, and chemicals in admixtures. Improper timing and over-finishing are responsibilities of the finishers who are at the mercy of the rheological and setting properties of the concrete they are finishing. Surface crusting is a chemical and physical phenomenon. Identifying the causes can be messy.
Perhaps the simplest surface distress to decipher is scaling, especially if the concrete is nonair-entrained and exposed to a cyclic freezing environment. If it is air-entrained, then it may have been over-finished so that entrained air voids have been “worked” out of the surface region. Along with that may be “planes” of weakness created by early finishing that entraps bleed water below surfaces—or causes air voids to coalesce, creating stringy voids aligned below and sub-parallel to finished surfaces, which may define the planes of weakness.
The “simple” has now become a little more complex. So let's make it even more complex and include late finishing of concrete where there is no air entrainment, no loss of air, no coalescence of air voids, and no thin, long, stringy air voids. Instead, fine but long separations can be created below the surface that are oriented sub-parallel to finished surfaces. They result because of compactive and shear forces from prolonged or severe and heavy finishing used for providing those very flat and hard-burnished floor slab finishes.
Although long a problem, but not of very frequent occurrence, is delamination of air-entrained concrete made using lightweight aggregate, currently a dilemma undergoing the scrutiny of ACI Committee 302. The dilemma results because air-entrainment is needed to reduce concrete mass in order to reduce dead loads of elevated slabs, with an additional requirement of hard, flat surfaces. In these cases, air entrainment should be at the lowest level needed, and finishing manipulations should be as short and as “light” as possible. And even then, under certain circumstances, there may be delamination.
The degree of concrete rigidity is the key to when to finish, and the makeup of the concrete is sometimes a contributing partner. But the art of when to finish rests with the finisher. Just as a pilot turns a plane over to the navigator during a bombing run, concrete is turned over to the finisher for the finishing run. Each is the master of timing, each relies on a team of experts to get them to the point of execution, and each relies on a series of ground marks. The navigator has a bombsight and celestial data as a guide. The finisher does not have those sophisticated things to help him—usually only the information the concrete surface provides, such as water sheen and imprint hardness of a heel or a thumbprint.
With new types of finishing devices from laser and vibrating screeds and pizza pan floats, walk-behind or sit-on power trowels, one thing that has never changed is the almost arbitrary ideal time to finish, which changes from concrete to concrete. The key is the concrete, and there is a great need to find a better way to identify when it is ready to finish—something more accurate than man's good intentions. And that is one of our great challenges.
Our good intentions have created such a near-apocalypse regarding the surface distress of interior concrete slabs that confusion now abounds about its causes. But, whatever the discussion, the peculiarities of concrete itself underlie the problem. And it all goes back to the rheological and setting characteristics of concrete, which are greatly influenced by the cementitious components and their interactions with admixtures.
The time has come to establish better finishing guidelines, as best as can be done, to minimize surface distress problems.