Facility managers have excellent choices to improve the quality of their tipping floor. Whether you go with concrete or a specialty topping, both of those choices have significantly improved over the years.
By Bob Swan
Improving the overall durability of concrete floors that must function in a difficult environment is of interest to many facility managers. This article specifically deals with wear, impact and chemical attacks that a tipping floor may be subjected to, and considerations regarding improving the overall life of a floor in a high-volume facility.
Anybody who has spent much time in a solid waste transfer station, a material recovery facility or a dry anaerobic digester understands the deterioration that takes place on those concrete floors. The combination of loader buckets sliding along the floor and chemical attack from deteriorating organics takes its toll on the wearing surface. It could be the original concrete itself wearing out or there may have been a previous topping that has now worn through to the substrate or delaminated in some areas. Either way, the floor will begin to need maintenance at the point where coarse aggregate in the base concrete starts showing or worse yet when you start to see rebar. In Figure 1, some rebar has actually been cut out of the floor while it is still in use, which is not advised. That steel is a structural part of the slab and is necessary to maintain the floor’s load carrying capacity.
Why the Need to Upgrade the Quality of These Floors?
Over the last 20 to 30 years there have been market trends that forced the solid waste industry to look for even greater concrete durability. One of those market trends was the move toward larger equipment driven by a need for greater productivity and reduction in labor cost. However, larger equipment equates to increased stress on the floor.
We have also seen an increase in the amount of chemical attack on tipping floors. There is now more additional recycled green waste being processed, an interest in local communities having less waste runoff water on our streets (from garbage trucks) and increased regulations from the Clean Water Act. All of this leads to more liquids on the tipping floor and a greater opportunity for organic and inorganic acids to deteriorate the concrete surface (see Figure 2).
Fixing the Floor
So, what are the options to fix the floor? Perhaps, more importantly, how can you repair that floor in a way that would make it more durable and longer lasting between repairs? Whether it is new construction or a full depth repair (see Figure 3), a good place to start is with the base concrete. Most concrete mix designs consist of large aggregate (rock), fine aggregate (sand) with the cement paste typically composed of Type I/II cement and admixtures to add workability. As shown in Figure 4, page 32, we can break down the various components to determine the volume composition of the concrete. We see that the largest component is coarse aggregate which runs about 41 percent. The fine aggregate in this particular mix runs about 33 percent. Cement content is about 9 percent with the water content at 17 percent. It should be noted that is actually more water than what is needed to hydrate the cement and make the concrete harden. That extra water is needed to make the concrete more workable and help the mix flow into place. The problem with extra water is that after the concrete hardens, the water evaporates and that leads to shrinkage of the overall slab. This is one of the primary reasons that concrete cracks. That is why a mix design will always note the water to cement ratio (w/c). This ratio is critical in determining the ultimate strength of concrete. The lower the w/c ratio, the higher the strength of the concrete, which results in less opportunity to exhibit shrinkage cracking.
A very common mix might be designed around a w/c ratio of 0.55 and would be expected to achieve 4,500 PSI at 28 days. However, a good starting mix design for a high-volume tipping floor might be specified at a water to cement ratio no greater than 0.40, giving a strength that would run about 6,000 PSI at 28 days. Many mixes today will also include replacing 20 to 25 percent of the Portland cement with fly ash. There are good environmental reasons for using fly ash (it is a byproduct of coal power plants), but it also makes good structural sense for a couple of reasons:
• Fly ash particles tend to be round. That shape allows concrete to flow (and pump) better using less water, which helps lower the w/c ratio
• Fly ash is cementious in nature and, therefore, adds strength to concrete somewhat similar to Portland cement
One of the potential downsides of using fly ash is that those mixes tend to set slower than normal Portland cement. While that may reduce the overall heat of hydration, the slab may also require extra steps be taken to provide adequate early stage curing.
How Else to Improve Concrete?
For many floors in the past, the aggregate of choice would have a top size of 3/4 inch and ideally been tested for wear and impact resistant. We have learned that the use of larger top size aggregate (1.5 inch diameter) has helped overall performance a couple of ways. It reduces the amount of cement paste required for a given mix because larger aggregate reduces the total surface area that has to be coated. This helps reduce overall shrinkage and subsequently reduces the concrete’s potential for cracking. Larger top size aggregate also gives us better aggregate interlock when random cracks do happen or where we actually plan on the concrete cracking at saw cut/ control joints.
Adding synthetic fibers to the mix may also help with overall performance. There are two primary types:
1. Micro fibers generally run about 1/4 inch long and help to control plastic shrinkage cracking. That is the cracking that may occur in concrete within the first few hours of placement and is primarily due to the loss of excess (bleed) water from the concrete mix. Micro fibers tend to slow bleed water loss thereby reducing or minimizing early stage cracking. Typical addition rates are 0.6 to 1 pound per yard.
2. Macro (structural) fibers, on the other hand, are much larger, generally, about 2 inches in length and much thicker compared to micro fibers. They can be used to replace some or all of the structural steel in the floor and they tend to do a better job of long-term crack control than conventional rebar. However, two items of note: there is a practical limit to how many pounds of fibers can be added to a concrete mix to replace the structural rebar. It could well be that the load carrying capacity required of the floor will dictate and limit the fiber addition. Also, it is imperative that any fiber added to the concrete be evenly distributed throughout the mix. Typical addition rates for structural synthetic fibers range from 3 to 8 pounds per yard.
Steel fibers can be used in concrete for tipping floors, but if chemical resistance is an overall consideration, they may not be the best choice. While steel fibers do considerably increase impact resistance, they also tend to increase the rate of deterioration due to chemical attack if the facility is processing a lot of organics.
Increasing the Chemical Resistance
With increased organics on the floor, how do we increase the chemical resistance of the concrete? The first consideration should be to lower the w/c ratio of the concrete mix design. As we have mentioned before, the more water in the mix, the more likely the concrete is 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. However, excess water 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 chemical attack. By lowering the w/c ratio, the cement paste becomes stronger and the overall porosity of the concrete is reduced. You will significantly improve the density of concrete by lowering the w/c ratio from 0.50 to 0.40.
A second consideration should be to use silica fume (micro-silica) in the concrete mix. This byproduct of the electric arc furnace industry is valuable in concrete for a couple of reasons. First, while initially inert in concrete, silica fume combines with other materials produced when Portland cement hydrates. This reaction provides an increase in compressive, flexural and bond strength. Second, the particles themselves are hundreds of time smaller and rounder than that of cement. As such, silica fume increases the concrete density and reduces the concrete’s permeability. Consider replacing 2 percent to 5 percent of the Portland cement with silica fume to add strength and increase durability.
What About High-Performance Toppings?
Iron aggregate-based toppings have been around since the early 1970s. Most of the ones on the market today contain about 70 percent iron, 30 percent cement with an admixture plasticizer added for workability. For years these toppings were the industry standard for high wear and high impact environments. But experience has shown us that chemical attack plays a significant factor in the rate of a tipping floor’s deterioration. One of the downsides of iron aggregate toppings is resistance to organic acids and road salt. When exposed to this environment, not only does the cement paste of the system begin to weaken, but the iron aggregate itself also begins to oxidize (see Figure 5).
In addition, these toppings are heavy and therefore challenging to place. Normal concrete weighs in at about 145 pounds per cubic foot. However, because these toppings have high iron content, they can weigh as much as 230 pounds per cubic foot. From a contractor’s standpoint that makes them difficult to mix and pour (see Figure 6). If the contractor does not have an experienced crew, there may be a tendency to over water the mix, allowing for easier placement. But that, of course, weakens the product and results in lower compressive strength and a decrease in performance.
Because of increasing demands within the solid waste industry over the years, other topping systems were introduced. In the 1980s, high performance toppings came on the market that were either quartz, trap rock or emery aggregate based. When compared to iron aggregate toppings, these products tend to offer reduced wear resistance, but better chemical resistance. By and large they have proven to be an upgrade for the industry and fill the niche of providing a good topping for low and medium volume transfer stations. However, for high volume transfer stations or facilities that were interested in limited repair shutdown time, there was still a need for something better.
In 2003, a topping came on the market that used both iron and calcined bauxite aggregate along with other admixture advances that addressed these industry needs.
This combined technology gives the market a 15,000 PSI topping which can be easily placed and allow the floor to be put back into service within 48 hours of placement (see Figure 7). Calcined bauxite is a nearly ideal aggregate for use on tipping floors. Most notably it is the aggregate of choice on high friction surface treatments for many state DOT pavement enhancements. Federal DOT testing deemed it the best aggregate for long term durability. The topping product has a w/c ratio less than 0.30 and offers exceptional chemical resistance as well as wear and impact.
Making the Decision
So how do you make a decision regarding which system (including concrete) to choose? The first question to address is: how long to you want your floor or topping last? Most owners believe it is time to take a look at replacing the floor or resurfacing the floor when you are down to rebar, which is typically a couple of inches below the original concrete elevation.
So how long will that take? The easy answer is: it depends. The success (or life) of the floor or topping is really related to a combination of three things: 1) the quality of the flooring, 2) the quality of the installation and 3) how the facility is used. It depends on:
• The tons per day that move across the floor.
• The content of the waste stream. What percent of it is C&D versus MSW?
• The size and nature of the equipment used to push the garbage around taking into account other factors including rubber cutting edges and loader operator experience.
• The materials and quality of the workmanship used during the install. A fair question might be to ask the installer “What kind of warranty comes with the floor?”
The ROI Value of Concrete Versus a High-Performance Topping
In many facilities, quality concrete holds the edge because it is less expensive to install and the advent of better mix designs many times makes the ROI favor concrete. The disadvantages when compared to high-performance toppings is the lack of a track record for that particular concrete mix, less wear and impact resistance, and no warranty regarding durability or suitability. Also, concrete requires a longer shutdown time. A mix designed at a 0.45 w/c ratio using Type I/II Portland cement will achieve about 4,500 psi at seven days of the intended ultimate strength of 6,000 psi at 28 days. So, a minimum of 10- to 14-day curing is advised after the concrete is placed. You can design a mix using Type III Portland cement for a faster strength gain, but studies have shown you will likely give up some long-term durability. Some high-performance toppings will allow the resurfacing of a tipping floor over a weekend with the service life measured in years beyond that of high-quality concrete. They also normally come with experienced, qualified installers and a warranty. The installer can take possession of the floor Friday afternoon and have the floor back into service Monday morning.
Finally, when considering a high-performance topping:
• Does the product have a good track record and can the manufacturer provide references?
• Does the installation crew have enough experience to complete the job over a weekend?
• Does the performance of the product allow the floor to be put back into service within a couple of days after placement?
If the manufacturer offers a warranty, does it cover the following:
• Bond of the topping to the substrate?
• A ‘no wear through’ clause for the duration of the warranty?
• Both labor and material?
The good news is that facility managers have excellent choices to improve the quality of their tipping floor. Whether you go with concrete or a specialty topping, both of those choices have significantly improved over the years. | WA
Bob Swan is President of HD Concrete Floors, LLC (Las Vegas, NV). He has more than 35 years of concrete 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 has served on several national committees for the International Concrete Repair Institute, including the Concrete Repair Materials and Methods Committee. Bob can be reached at (702) 239-1027 or e-mail [email protected].