Leachate chemical attack on solid waste floors can seriously increase the rate of deterioration of concrete. This article offers advice on how to slow down that deterioration and improve the durability for both new and existing floors.
By Bob Swan
There are many types of solid waste facilities. These include transfer stations that handle standard municipal solid waste (MSW), material recovery facilities (MRFs), and anaerobic digesters. Determining the best concrete for a particular floor is highly dependent upon the function of the facility. Using a universal concrete mix design for all solid waste floors may not yield the best results. There is no one-size-fits-all. A floor subjected to a high volume of construction and demolition material should have a concrete mix design different than a high solids anaerobic digester. The American concrete institute defines high strength concrete as concrete that exceeds a compressive strength of 6,000 psi. But the appropriateness of a particular mix design should go well beyond just the ultimate strength. Consideration should be given to customizing that concrete mix to fit the specific environment. The goal is to design the concrete to maximize the life of the floor, prolonging future repairs and shut down times, which can be extensive and costly.
Where Does the Leachate Come From?
There are several ways leachate can end up on a solid waste processing floor. Environmental directives demand that waste liquids be processed within the waste stream and not leaked out into the streets of the community. To contain the liquids, garbage trucks installed large rubber gaskets around the container edges. That liquid waste, which used to be left on our streets, is now dumped at the transfer station. While this is good for the environment, it is bad for the concrete tipping floor.
The fermentation of decaying organic matter is the primary culprit. High amounts of leachate can be found in high solids anaerobic digesters, food bunkers, organic waste storage areas and recycling centers (see Figure 1). Aluminum can and plastic bottle recycle centers will see also see floor deterioration from residual carbonic acid and sugar-based beverages (see Figure 2). Designing a floor to resist a combination of abrasion and chemical attack is critical for long term performance at all these facilities.
Why is Leachate a Problem?
For many transfer stations, much of a tipping floor’s deterioration is due to a combination of MSW volume moving across the floor and mechanical abrasion from bucket loaders. It has been reported that the typical service life of a tipping floor is about eight years (the point at which the top layer of rebar begins to show).
However, that life goes down substantially if there is significant leachate attack. Accelerated deterioration due to leachate is especially true at recycling centers and high solids anaerobic digesters. As the organic waste sits on the floor waiting to be processed and moved, these acidic liquids chemically attack the floor and break down the cement paste within the concrete (see Figure 3). So, along with the impact and abrasion damage that the tipping floor receives from the waste volume and front-end loaders, the leachate corrosive liquids also accelerate wear.
At its most basic, concrete is composed of cement, aggregate, sand, and water. Admixtures and pozzolanic materials are then often added to improve the concrete’s performance for both the plastic and hardened state. When water reacts with cement, the reaction (called cement hydration) gives off some heat and produces a paste primarily composed of calcium silicate hydrate and calcium hydroxide. The quality of the cement paste is a key factor of what gives concrete its strength.
Concrete typically has a pH of 12-14 (alkaline) because it has an excess amount of calcium hydroxide. Leachate on the other hand is a weak organic acid and generally has a pH of 4-6 (acidic). When leachate and concrete come into contact with each other, the leachate (acid) reacts with the calcium hydroxide (alkali), resulting in deterioration of the cement paste. Destruction of the paste causes the concrete to weaken and lose strength.
For a tipping floor handling standard municipal solid waste (MSW) and little organic waste, most of the floor wear takes place evenly across the concrete. The aggregate and the cement paste wear at the same rate generating maximum durability of the floor (see Figure 4).
When leachate is present however, the cement paste is attacked by the acid, weakening the paste. A weak paste reduces the bond of the paste to the aggregate. This allows the aggregate to more easily pop out when material is pushed across the floor (see Figure 5).
A Leachate Resistant Concrete Floor
So, how do we increase the chemical resistance of the floor? There are two ways that can be accomplished. Making the concrete itself more leachate resistant and/or using a chemical resistant high-performance topping. Let’s first discuss the concrete and how we could alter the mix design for added durability in a high leachate environment.
First Consideration: Aggregate
The quality of the aggregate plays a large role in the overall durability of a concrete floor. In fact, the abrasion resistance of concrete is mainly a function of the hardness of the coarse
aggregate. However, not all aggregate is created equal. For instance, granite is a much harder material than limestone. Plus, limestone is far more susceptible to leachate attack than granite. There are testing procedures used to check aggregate quality. Among them are: Aggregate Crushing Value, Aggregate Impact Value and Abrasion Value of Aggregate. Your local ready-mix provider should be able to provide more information.
The volume of coarse aggregate and total aggregate in the mix is also important. There will be far better results if the volume of coarse aggregate exceeds 11 cu.ft. per yd of concrete and the total amount of aggregate approaches 70 percent by volume of the mix (see Figure 6). Additionally, if your standard mix calls for using 3/4″ top size aggregate, consider increasing the size to 1 inch or 1.5-inches (see Figure 7). Here is how these changes help the floor. Increasing the size and volume of the coarse aggregate minimizes the surface area the cement paste must cover. By reducing the amount of paste you need, you minimize the amount of water that is in the mix. That leads to less shrinkage and cracking of the hardened concrete. None of these changes should sacrifice strength.
Second Consideration: Water to Cement Ratio
Use a mix design that has a low water to cement ratio (w/c). Lowering the w/c ratio gives several advantages. Deterioration of the cement paste reduces the overall durability of the concrete floor. As the cement paste wears away, the coarse aggregate is exposed. Lowering the w/c ratio allows the cement paste to become stronger when the overall porosity of the concrete is reduced. This leads to a stronger, more leachate resistant cement paste. That stronger paste will bond to the aggregate better and that allows the aggregate to substantially contribute to the abrasion resistance of the floor.
As the amount of water in the mix decreases, the concrete is less likely to develop shrinkage cracking. These are cracks that are clearly visible to the naked eye and may allow an easy path for leachate to get through the concrete and into the substrate. Excess water in the mix also creates a more porous concrete by developing small fissures within the hardened cement paste. These fissures not only cause a reduction in the overall strength of the concrete, but also make it more susceptible to leachate attack.
You will significantly improve the durability and strength of concrete by lowering the w/c ratio from 0.50 to 0.40. Admixture technology will allow you to drive that ratio even lower, perhaps to a 0.35 w/c ratio. But as you drive that ratio below 0.40, the concrete can become very difficult to trowel and finish the floor with an acceptable appearance. In general, you need at least a 1 to 2 inch water slump (prior to admixture dosing) to get a decent finish on the concrete.
By improving the quality of the cement paste, the overall performance of the concrete is improved. This is especially true when a weaker, less abrasion resistant aggregate must be used.
What About Additives Like Silica Fume and Fly Ash?
The addition of silica fume and fly ash in the concrete mix improves the overall abrasion resistance and durability of a typical tipping floor. While their use does improve a floor’s wear resistance, silica fume and fly ash also make the concrete more susceptible to chemical attack.
The reason is that both silica fume and fly ash react with the excess calcium hydroxide in concrete to form additional calcium silicate hydrate. That is why the addition of those pozzolanic materials helps strengthen and reduce the porosity of concrete. However, since those reactions reduce the amount of calcium hydroxide in the concrete, these materials also reduce the concrete’s ability to buffer leachate
attack. So, while the use of silica fume and fly ash increase strength and wear resistance, they weaken the concrete’s ability to withstand acid attack.
Bottom Line: Use silica fume and fly ash for typical tipping floors that have little exposure to leachate. However, in a high leachate environment, like waste food bunkers and high solids anaerobic digesters, using silica fume and fly ash may not be the best choice.
What About High-Performance Toppings?
Iron aggregate-based toppings have been around since the early 1970s. For years these toppings were the industry standard for high wear and high impact environments. But experience has shown that chemical attack plays a significant factor in iron-based topping deterioration. They are particularly susceptible to acid attack.
There are several high-performance toppings on the market today that offer far better resistance to leachate attack. At least one topping has a compressive strength exceeding 15,000 psi and has shown outstanding wear and impact resistance as well as excellent leachate resistance (see Figure 8). When choosing a topping, make sure you ask the following questions:
1. Has the topping been used in high leachate environments, such as anaerobic digesters or other organic waste processing facilities?
2. Has the topping been successfully used in high volume MSW facilities (+1500 tons/day)?
3. Does it come with a warranty that includes bond of the topping to the substrate?
4. Does the warranty cover both labor and materials?
While high-performance toppings may be initially viewed as expensive, they in fact yield a high ROI when used in critical areas. A high-performance topping may only be needed in critical areas near port edges and push walls. When used as such, they can be of great value toward increasing the floor’s durability. At least one topping on the market can be placed over a weekend with the floor returned to service Monday morning. Limiting shutdown time to 72 hours may be very desirable.
Do Rubber Cutting Edges Help or Hurt?
Anecdotal accounts make the case that rubber blocks mounted on bottom of loader buckets may promote deterioration in high leachate environments. They may be hurting the floor in two ways:
1. If sufficient downward pressure is applied to the bucket, the rubber may be forcing additional leachate liquid into the cement paste. This causes the paste to deteriorate at a faster rate.
2. As the cement paste is weakened, the rubber may be pulling the aggregate from the concrete matrix. If that happens you never get the full advantage of having the aggregate protect the floor against surface wear.
It should be noted that both of the concerns stated above are highly dependent upon bucket pressure applied by the loader operator.
Not all solid waste concrete floors should be designed the same. Different environments may dictate changing the mix design to better accommodate the type of facility to maximize overall floor performance. Key concrete mix design criteria considerations for a leachate resistant concrete floor include:
1. Use the highest quality coarse aggregate available
2. The total amount of aggregate should be close to 70 percent by volume
3. Use 1″ to 1.5″ top size aggregate rather than 3/4″
4. Use as low a water to cement ratio as practical. Specify at least 0.45 (0.40 is even better)
5. Limit the use of silica fume to 3 percent or less. Limit the use of fly ash to 15 percent or less. (It should be noted that both silica fume and fly ash increase the durability of concrete if leachate is not present).
6. Consider not using rubber cutting edges on loader buckets in a wet, high leachate environment
7. If a significant increase in floor performance is required and/or a very short repair shutdown time is desired, a high-performance topping may be the right choice
Not all solid waste floors should have the same concrete mix design. Because the individual environment can vary depending on the facility type, consideration should also be given to individualizing the concrete. Spending extra money to upgrade the aggregate and to lower the water/cement ratio is typically a good strategy. It will reduce future maintenance costs and future shutdown time for repairs. Finally, using a high-performance topping in selected areas of the tipping or processing floor may have an excellent return-on-investment. | WA
Bob Swan is President of HD Concrete Floors. Bob has more than 35 years of concrete industry repair and restoration experience. He has an extensive background in concrete mix design and formulation of materials used in extreme high wear environments. He is a long-time member of the American Concrete Institute and the International Concrete Repair Institute. Bob is a current member of SWANA and focuses on improving the durability of concrete floors in the Solid Waste Industry. He can be reached at (702) 239-1027 or e-mail [email protected].
A significant amount of information for this paper came from the following articles and reports:
• Design of Waste Transfer Station Concrete Overlays against Premature Deterioration by Sungwoo Park, Morton Barlaz, Mohammad Pour-Ghaz (North Carolina State)
• Keeping Transfer Station Costs In Line by Jim Miller (J.R.Miller & Assoc.)
• Extending the Useful Life of Your Tipping Floor by Bruce Clark (SCS Engineers)
• Tipping Floors – Making Them Healthy Again by Jim Andrews (American Restore Contractor)
• Creating Durable Concrete Floors by Bob Swan (HD Concrete Floors)
• Facility Design: Food Waste Preprocessing by Jim Miller (J.R.Miller & Assoc.)