The history of pervious concrete up until now has been as series of failures and jobs that have given pervious concrete a less than honorable image. This is so distressing because pervious concrete has the potential to be the best paving material ever devised. The biggest problem has been that contractors up until now have taken a poor attitude when using pervious concrete, thinking that it can be handled like regular concrete. We have seen the results of this mentality. Fortunately there are many serious minded contractors across the US who are taking pervious concrete seriously and pervious concrete is now moving in the direction it should have been from the very beginning. Now don’t get me wrong!

Every pervious contractor has had their fair share of challenges and possible failures. The difference between my challenges and failures is that Enviro-Crete Inc. and many like minded companies, is that we figured into our game plans the possibilities of the challenges during each and every pour which has prevented us from having failures. The following white paper discusses and looks at the most glaring topics that have challenged us time and time again. We now look at most of these as our “Hot List” of things that can and will go wrong before, during, or after we have placed the project. Fortunately there are many serious minded contractors across the US who are taking pervious concrete to heart and pervious concrete is now moving in the direction it should have been from the very beginning. Now don’t get me wrong. Every pervious contractor has had their fair share of challenges and possible failures. The difference between my challenges and failures is that Enviro-Crete Inc. and many like minded companies, is that we figured into our game plans the possibilities of the challenges during each and every pour which has prevented us from having failures. The following white paper discusses and looks at the most glaring topics that have challenged us time and time again. We now look at most of these as our “Hot List” of things that can and will go wrong before, during, or after we have placed the project.

Heat and Wind! We’re in trouble.

On the day of the pour, the major success of the pour is really only determined by one thing and one thing only, Ambionic conditions. Temperature, humidity and wind are the single biggest factors that will affect the outcome of your pour. According to the NRMCA the ideal temperature range for pouring pervious concrete is 40’ to 90’. I personally do not like to exceed 80’ when pouring, but if there is no wind than 90’ would be fine. If there is a wind, any temperature can put your pervious concrete pour in jeopardy. The rule of thumb for wind is if there is a 10 mph. wind, do not pour or stop the pour. The risk of surface raveling is just too great. Everyday is a new condition for pouring and your experience needs to tell you what, where and when you should or should not be pouring. It is better to not pour or stop the pour than to have to replace the pour. Your primary responsibility on the day of the pour is to see to it that the pervious concrete is not compromised in any way prior to getting it covered with the sheet plastic. The curing process is the single biggest affect of the success of the pour. And then ensuring that the plastic remains in place for the full seven day duration without any disruption. Raveling occurs when the top ¼” to 3/8” dry out prior to the hydration process which can only occur when there is water present for that chemical reaction. The conditions we have talked about will dehydrate the free water necessary for hydration. This is when the cement, which binds the rocks together, will fail causing the bond between the rock to crumble. So my final recommendation is to calculate the time when it will be 80’. Then work backwards on the clock. So if it is 80’ at 11:00 am then you would want to start you pour at 3:00 in the morning. This will give you your greatest chances of success.

Mixes

Without hesitation it is in my humble opinion that the pervious concrete mix is the single biggest factor for success when planning your pervious concrete project. And the two biggest factors when designing a proper mix is to use the mindset of a “balanced mix.” What does this mean? A balanced mix is one that uses the correct relationship between the rock, the cement ratio, and the amount of water available for the proper hydration of the cement. In our earlier careers as pervious concrete practioners, we used the mindset of the more cement the better. Reason has it that the more cement the stronger the mix would be, right? Wrong! If the cement paste between the rock is too thick than the strength of the bond is only the strength of the cement. When the proper thickness of cement paste between the rock is properly designed, than the strength comes from the geometrical interlocking of the rock and the rock itself, not the paste by itself. We are believers in using the design graph developed by Mr. Dave Mitchell. This graph balances the amount of cement in relationship to the specific weight/gravity of the rock. We have had absolute success with our mixes when using this graph.

Suppliers

Choosing the right supplier is the next biggest factor when planning your pervious concrete project. The mix design can be perfect, but if the company supplying the mix does not handle and deliver the materials that make up that mix properly. You will have either a failed project or the day of the pour will seemingly never end. So what makes for a good supplier ? I will give you a list of items that I feel are of great concern and consideration.
1-Distance from plant to the job site
As with any concrete project. The time of travel from the batch plant to the project is a key factor is the selection of your supplier. Pervious concrete should be batched and placed on the ground within one hour and thirty minutes.
2- Type of fins in the drums
Low slump fins are very helpful when discharging pervious concrete. These fins are typically taller. They have the ability to discharge low slump materials such as pervious concrete much quicker and more completely. Trucks with high slump fins will discharge much slower and it is always difficult to get them to discharge completely. So when you need to get a specified number of yards placed that day it is important to know just how long each truck should take to discharge and to get them discharged as quickly as possible.
3- Quality of Rock
This is a very important part of any mix. Suppliers use many different quarries for their rock sources. The cleaner and more consistent graded the rock the better the mix will be for pervious concrete. “Dirty rock” is what we call a rock gradation that is inconsistent and has dirt or too many fines in the rock (poor gradation). This will cause your pervious mix to “Splotch.” These are areas in the pour that are consolidated and unsightly. If the rock is too dirty the mix may end up being unusable. We allow a tolerance of 5% of the surface can have splotches.
4- Experience of the supplier
From the batch man, to the drivers, to the quality control people, experience using pervious concrete is necessary. Like experience the contractor needs when dealing with pervious concrete is so important. It is likewise important that the concrete company has had many yards of experience dealing with the materials and mix designs. Since the mix is more than 50% of the success ratio when dealing with pervious concrete it only makes sense that you supplier be as competent as possible.

Admixtures. Is it really in there?

Admixtures are the newest break through in the development to pour Perfect Pervious… Every Time tm. First let’s describe the perfect mix. It would come out of the truck easily and completely. It would be pourous allowing water to drain through with a rate of approximately 210 to 300 gallons per hour. It would be tight, easy to handle, you would have plenty of time to put the perfect finish on it and it will be able to withstand surface abrasion. The strength of the material would be 3000 psi. or better. Sounds unbelievable? it does! Pervious concrete without admixtures will be a bomb waiting to explode. It will not come out of the truck easily. No matter how hard you try there will always be a yard left in the truck that will just not discharge. The material will want to flash once it hits the ground and the time you need to put the finish on it will be reduced to 20 minutes or less because the water has evaporated from the surface which will prohibit you from providing a smooth surface. In other words pervious concrete in its un-additive form will be difficult to handle at best. There are many cocktail mixes out there that use a variety of additives to get the desired qualities that they are looking for. The mix that we use has several different additives in it. We typically utilize three additives. They are a hydration stabalizer, Pervious Plus, and a VMA. The need for the three additives is as follows:
1- Hydration Stabalizer- This additive will put the cement hydration process to “sleep.” The life of the concrete, or the time needed, to place and finish it should be extended. The life of pervious concrete is typically 20 minutes. With the use of HSA the life of the concrete should extend out to 40 minutes.
2-Pervious Plus- This is the best pervious concrete additive on the market today. It does a variety of things for the pervious concrete. It allows the material to come out of the truck faster and completely. Other benefits include increased flow allows for quicker placement and screeding. It will extend the “life” of the mix so you have more time to edge, finish and place the plastic on the surface. It lowers the temperature of the initial phase of hydration thereby giving the concrete high early strength properties. It hydrates more cementious particulates which increases the strength, bond and reduces the chances of surface raveling. The best advantage though is the fact it is non-toxic.
3- VMA- This additive plasticizes the concrete so it has better flow, placement properties, and finish capabilites.

Who takes care of the plastic?

As I’ve mentioned before curing is the most critical step in the whole placement process. Getting the plastic placed as quickly as possible is critical, but just as critical is the maintenance of the plastic for the entire 7 day period of time. It is typical that the maintenance of the plastic be placed onto the general contractor because of the cost issues. This however, has been a less than acceptable way of making sure the plastic is to be maintained because the general has got lots of other issues and they fail to make sure the plastic is properly secured. We suggest that the contract be properly funded to have one of your workers inspect and maintain the plastic at least once a day during the 7 day curing process. This is a very expensive proposition when work is out of town. If this labor cost was factored into the job it is more than likely that you were not the low bidder. In the perfect world we could have the cost covered and the plastic would then be look after the way it should. But, in most cases this is done by the general contractor. Another major issue during the cure period is the use of the area. We recommend that there be no activity on the surface at all. However, we have witnessed all kinds of problems with raveling and failure when access is allowed, vehicles use it too soon, equipment and materials are staged on the surface. It is best to document when this happens and be prepared to defend any failure with these unacceptable activities.

Get the right Contractor.

Selection of the most qualified contractor available is probably the best thing any owner, project manager, contractor or person in charge can do for the ultimate success of the project. Experience is the single biggest factor separating contractors when it comes to placing pervious concrete. There is no substitution for placing hundreds of thousands of square feet of pervious concrete in all conditions and in every possible site condition. The NRMCA has certifications for individuals installing pervious concrete. There are three levels of expertise. Technician, which is basically an entry level certification. The criteria for this certification is to pass a written test after taking a 4 hour class covering basic information on pervious concrete, its properties, and installation techniques. With this certification a person should not attempt to pour a project without more qualified people to guide and help with the process. Installer, is the second level of certification. The criteria for this is to already be a technician and then be on the crew that has placed ten thousand square feet or more. There is a field placement test and observation that must be passed to achieve this level of certification. This is usually administered by an NRMCA representative. Craftsman, is the highest level of certification. It is extremely difficult to fulfill all of the requirements for this level of certification, but this is why it is the highest level. An individual must prove 1500 hours of installation experience, pass the ACI flatwork finisher