As an off-shoot of our discussion about pervious concrete and curing, a second discussion developed about how pervious is treated during the winter. The prevailing wisdom is that both over-aggressive plowing with steel blades and the use of deicers is damaging pervious concrete. Following are opinions from two pervious concrete experts on this issue.
Just Say No and Use a Rubber
Brian Lutey, VP of Green Building, Ozinga Ready Mix Concrete Inc., Chicago
Both deicing chemicals and hardened steel plow blades are damaging pervious concrete. The plow blades strike the pervious surface with great force and at obtuse angles. These impacts can remove aggregates completely, remove just the protective paste coating on these aggregates, or infuse the matrix with a multitude of cracks, typically all three. And the damage increases exponentially with each impact, freeze-thaw cycle, and application of salts. The physical damage then allows salts and deicing chemicals a direct route to damage more surfaces, aggregates, and the paste bridges holding them together. Invest in a $500 rubber plow blade and pay for it with the savings from NOT applying de-icing salts. Keep it simple: Just Say No (to salts) and Use a Rubber (plow blade, that is). Can we get Federal money to teach this to owners of pervious concrete?
The aggressive chemicals now being used to keep roadways clear, and the ‘sticking agents’ like beet juice or whey from cheese-making in Wisconsin, will attack both the aggregates and the paste within both conventional concrete and pervious concrete. With pervious concrete, we have a much thinner section of paste to protect the aggregates, and a lot more nooks and crannies with large surface area for these chemicals to stick, accumulate to dangerous levels, and eat at the concrete.
There is also the natural re-application of these chemicals as the brine in the stone layer wicks back up to the surface where the water evaporates, leaving the chemicals on the surface of the paste. So even if a rain event washes the chemicals out of the pervious, they are automatically and naturally re-applied after each rain event. I think it was John Kevern who said that approximately 50% of the water that enters a pervious concrete pavement will evaporate back out through the top.
Because of these aggressive chemicals, and a low tolerance for callback claims, Ozinga puts Ecoprises Concrete Solutions' MWC (Mix Water Conditioner) in every yard of our Filtercrete pervious concrete. We have over 12 pervious mix designs in our system and the only ones that don’t use MWC are DOT mixes.
While we are constantly striving to prevent future damage with improved mix designs, offering maintenance and repair solutions, we have no control over the maintenance of the pavements. The problem is that typically, the maintenance practices (deicing application and snow removal) are controlled by a different department from the designers, and even if they are aware of the restrictions, it is difficult to enforce when the crews are half asleep and just trying to move snow, so they don’t know or don’t care about the pervious concrete. During the winter the focus is on removing snow and ice, and the crews don’t discriminate between pavement types.
It is my belief that the MWC is a key factor in producing durable pervious concrete by making it with a paste that is dense, “waterproof,” shrink-resistant, and consistent for the installer. This is in addition to the other benefits that it brings in producing consistent pervious concrete. The MWC assists in smoothing out some of the variables that arise when producing pervious concrete in the daily operations of a ready-mix operation with over 30 facilities with differing aggregates, cementitious materials, admixture combinations, and truck configurations.
At Ozinga, we are constantly trying to make our Filtercrete pervious concrete idiot proof, but they seem to keep making better idiots. I share as much of our developments as I can each year at the Bunyan Roast, and pick up valuable tidbits from others as well. The MWC-treated paste goes a long way to do this by protecting the aggregates and the paste itself from being damaged by de-icing chemicals, however damage from metal plow blades or loader buckets can damage or remove the thin paste coating, which opens the aggregate to attack, despite our best efforts. This is especially true the first winter, when the concrete is not very mature. The second winter after placement, the concrete may have 8 to 10 times more maturity than it does the first winter. I wish someone would do a study on the maturity differences between the first and second winter for a slab placed in October in Chicago
Applying a densifier after 28 days may be a way to artificially accelerate the maturity of the paste, at least as far as it will penetrate. I also recommend using The Bean, from C2 Products, before applying the plastic and leaving it on for seven days. I have, however, had jobs where the plastic was on for just one day, with the MWC in the mix and sprayed with Bean oil and these projects are still in good shape. I also have projects where we used the MWC integrally and then used their Densifier, and the Bean after a week, and these projects are performing very well. As far as I know metal plow blades have not been applied to these projects.
While most of our projects survive with little or no winter damage, we also have projects where the MWC and the Bean were used, and metal plows and loaders were used to remove snow this past winter, and the surface has been severely damaged. On these projects, we recommend that the owner clean the loose and damaged materials away so that we can evaluate the extent of the damage, and determine if it will be continuing, or if it is all from winter damage. If the damage is not continuing, I recommend that the surface be sprayed with densifier to make the existing paste at the surface as dense as possible. If we are doing any remedial patching, I recommend that we do the patching/repairs first, then apply the densifier. (Otherwise the densifier may impede bonding of the patches/repair materials.)
The project that made me a believer in MWC had suffered from 3 years of de-icing chemicals. The damage started at about 3/4 of an inch below the surface, and pieces would come off in flat chunks and in loose gravel, depending on the traffic. My theory is that the Bean oil protected the top ¾ inch of the paste, but when there was no more Bean Oil to coat the paste deeper in the pavement section the protection was absent. As the chemical de-icers attacked the paste the structural damage below the surface accelerated as the aggregates and the paste bonds were weakened.
We milled the surface 3 inches and cleaned it using a series of prototype vacuum machines from Bunyan Industries. We then had Walt Flood of Flood Testing Labs test the permeability and compare it to the pre-ground values in order to ensure we had restored permeability with the Bunyan BIRD. We then applied Ecoprises’ Densifier to the entire lot and installed a 2-inch pervious-on-pervious overlay in the two parking aisles, and 2 inches of ultrathin whitetop conventional concrete with macro fibers in the center drive aisle. I waited a week and then applied the densifier to the entire lot to add suspenders to the belt system, as I did not want this customer to ever have another issue with this lot.
In short, I think MWC as an integral admixture is vital to protecting pervious concrete from deicers. I also think that the Bean is important. Studies by John Kevern, suggest that it might be better if the Bean is applied after a week or even 28 days, as that is when more of the pores will be open and available for it to penetrate. Ecoprises’ Densifier could be substituted for the Bean at 7 or 28 days, however I think that the paste chemistry is not sufficiently developed on the day of installation for the densifier to work.
If I was in charge of installing a pervious pavement in an area where I knew that deicers would be applied heavily (like the project I just spoke of), I would produce it with MWC in the mix, cure it with plastic for seven days, and come back in 28 days to apply either the Bean or Densifier. I would re-apply these products at least one more time after they had dried, in order to get the material as deeply into the pavement as I could.
Salting is a Goof
David Mitchell, Bunyan Industries, Salt Lake City
Recent findings by Bunyan Roasters in Illinois, Pennsylvania, and Maryland have exposed an alarming new aspect to the limitation of this pavement product. Fifty sites in the path of heavy, cold storms have shown a pattern so extreme that it illustrates another way that pervious concrete differs from conventional mixtures.
Impermeable pavement behaves very differently as the first flush carries away the brine into the urban watershed, along with the rest of the urban drool. The typical outfall design of a pervious concrete storm water management system will collect the brine and continue to concentrate the brine as the system evaporates. Salt crystals regenerate as moisture evaporates from the slab section and the process continues until the entire void structure becomes affected.
We are familiar with the exponential increase in the exposed surface area of pervious concrete. The collective surfaces of all interconnected passages greatly increase the potential for evaporation of mix water during construction. It appears that this increase in surface area also includes a significant increase in exposure to deicing salts. The distressed pervious sites that were examined included many surrounding pieces of conventional concrete that show the entire surface paste has stripped off. Many of the distressed pervious pavements that were examined appeared to have similar attack throughout the slab section.
The exceptions also became more conspicuous where damage did not appear. Zeke Zinchiak built pervious pavement at the Chinese Christian Church and found it in pristine condition. This suburban Baltimore site is surrounded by some of the worst damage in the group, exposed to the same precipitation and freezing conditions. Certain ethics exist in the Chinese culture that view the act of salting the earth as a goof. This applies to commercial fertilizer as well as deicing salts.
Another example is Mayor Rahm Emmanuel's prize jewel on the Chicago canal system. It is a beautiful training and staging facility for elite rowing teams. This site is surrounded with a pervious plaza, ramps and steps to access the river system. The manager of this facility gave specific instruction to the snow removal crew that no deicing products are allowed on pervious concrete. Rowing teams, visitors, and ducks enjoy this beautiful pervious surface. This story came about because of the lessons learned just two miles away where another noteworthy pervious project was ruined by deicing salts.
The pedestrian plaza at the VA Hospital in Pittsburgh is the scene of more typical distress. We found pervious under attack so extreme that the aggregate is stripped clean! Other locations delaminated a top layer that is less than one inch thick, possibly attributable to protection from a soybean based penetrating cure that was used. Bruce Cody from Pennsylvania Aggregates & Concrete Association carried the spear on this project and should be recognized for his leadership on this difficult task.
A pervious mixture can be designed to resist salt damage, in view of the amounts being tracked onto the pavement surface by wheeled vehicles. This mixture would still stand no chance of survival under normal salt loading practices. This is the same mixture used on the logo slab during 2014 WOC Pervious Live, with five pounds of Forta, five percent fume and forty ounces of MWC.
Based on what we now know, promoters and builders of pervious pavement must declare a zero tolerance ban on any intentional application of deicing products. We must conclude that potential owners of pavement who insist on the practice of applying deicing salts, should be advised to select conventional concrete pavement for salted areas and areas located downstream from salted areas. The popular design which drains runoff from impermeable asphalt into pervious concrete pavement should also be considered as a major source of brine. Serious consideration should be given to the sources of contamination that enter a pervious system. The organic components of urban drool are considered yummy food by the 36 different individuals found in the "bug zoo" that can live in pervious pavement, the aggregate base and surrounding soils. Most of these individuals are simply not able to thrive if the system is loaded with salt.