The 2003 construction season is in high gear, and it's now safe to say that summer has arrived. I'm going to exercise my writer's prerogative and discuss concrete. I'm a part of the movement here in the Metro to produce more durable concrete for use in urban infrastructure and I'm pleased to report that folks in the concrete industry today don't grimace or flinch today when we talk about 50 year concrete. A few years ago, 20 or 25 was the number discussed and we were finding problems after 5 or 6 years in some cases. A part of this movement is the Kansas City Metro Materials Board whose website is at http://www.kcmmb.org. The primary change for more durable concrete has been that we now specify a harder, more durable coarse aggregate (rock) than the locally available limestone. We also have specified other materials and procedures in an attempt to prevent several other concrete maladies that can shorten its life.
A concrete problem that has surfaced recently is "mortar-flaking." Mortarflaking causes the very thin layer of cement paste which is covering the coarse aggregate particles very near the surface to fail and come off as "flakes." While this is more of a cosmetic problem than a structural one, given the longer life-expectancy of the concrete, folks will hate the looks of this for a long time especially since the hard-rock beneath the flakes is a contrasting black color.
The problem is caused when the cement paste near the surface dries out before it's had a chance to develop strength and form a strong bond to the aggregate particles through the hydration process. It is caused by a failure to properly cure the concrete. I've seen this problem in both old limestone and new "hard-rock" concrete, but the new concrete has several characteristics that exacerbate the problem.
- "Hard-rock" is less absorbent than the local limestone. Local limestone therefore will contain a small amount of water that can replenish moisture lost to evaporation at the surface.
- "Slag cements” are blended with Portland cement in the KCMMB concretes to reduce the likelihood of failure due to Alkali Silica Reaction (ASR). They have the unfortunate side effect of retarding the set and lowering the early strengths. Days later in the curing period, slag cement concretes gain strength more rapidly than straight Portland mixes, and ultimate (more than 28 day) strengths may be higher, but for the first few days strengths are lower.
- Those of us that use the KCMMB mixes tend to be (I hope) more vigilant in allowing the addition of water, and in general use mixes with low water cement ratios. As a result, there is very little "bleed" water and surfaces tend to have less moisture throughout the process. Since concrete gets its strength from the chemical reaction that takes place between the cement and water, this smaller amount of moisture needs to be preserved through proper curing.
The project that brought this to my attention was constructed by a reputable, respected and capable local construction firm. The mortar-flaking problem affected curbs and flatwork, was prevalent throughout the entire job, and appeared after the job endured one winter. The curing compound was applied adequately, but over a surface that had probably dried, thus preventing water from reaching the dry surface. It's a case where doing all the right things to produce more durable concrete (all but one) amplified the problems caused improper curing. By comparing construction records with weather data at http://www.stormwatch.com it appears that the concrete was placed when the weather was both hot and windy. I've seen mortar-flaking (to a lesser degree) on other jobs where the coat of curing compound wasn't heavy enough.
The solution?
- Pay attention to weather conditions. Hot, low humidity and windy conditions contribute to the problem, so consider pouring when those conditions don't exist, or if you must, consider using one of the wet-curing methods which are still the very best way to cure concrete.
- If you are using a curing compound, be sure that the surface hasn't dried (no sheen) before applying the curing compound. I'd recommend two coats to provide better coverage (and thus better protection).
After examining this project, I started looking more closely for this problem, and had no problem finding it throughout the
metro. With limestone (because the paste and the aggregate are the same color) it's harder to spot, but it is still common.
Incidentally, the owner of the project elected to give the adjacent homeowners the option of having the concrete replaced.
