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Evaluating high wear floor systems in the solid waste industry.

The solid waste industry places tremendous demands on floors, walls, and facility equipment used to process the waste stream. Floors in particular must withstand extreme impact and abrasion loads.

Tipping, push pit, and ash room floors wear rapidly and require the highest quality floors the industry has to offer. These floors are found in waste transfer stations, material recovery facilities, and waste-to-energy plants, where they are subjected to severe wear.

Owners and operators of these facilities must reduce the life cycle cost of these floors. They have to choose a floor system that will offer the longest possible service life, eliminating frequent repairs and avoiding lost revenues that result from facility shutdown for those repairs.

Wear surfaces that require fewer repairs shorten downtime, reduce overall operating costs, and increase the revenue generated at that facility. But finding and specifying a floor that meets these criteria is a challenge.

Here we will examine various floor alternatives for solid waste facilities by discussing performance properties of several common alternatives and presenting a method of evaluation based on a life cycle cost per ton of refuse processed in individual facilities.

High Wear Floor Alternatives

For purposes of this discussion the wear portion of the floor slab, subsoil compaction, and the structural load carrying capacity will be assumed sufficient. High wear floors in the solid waste industry require a wear plate of material at the surface in addition to the structural slab. Wear plate performance is critical when in selecting a floor. Once it has worn through, the floor must be repaired and retopped or the floor slab's load carrying capacity will be compromised.

Some waste transfer facilities operate on soil or asphalt floors. Others have tried imbedded rail floors, steel plate armored floors, or trowel-applied epoxy systems. These alternatives generally prove to be ineffective and/or expensive.

Industrial concrete, high strength concrete, silica fume concrete, and portland cement iron aggregate toppings are now the most commonly used floor surfaces in the solid waste industry. Industrial, high strength, and silica fume concrete are all mineral aggregate systems. Iron aggregate toppings, as the name indicates, contain iron aggregate in a cementitious matrix. The best way to compare these systems is to look at their physical performance properties and the wear rates that have been experienced in the field.

Physical Performance Properties

Data were collected from laboratory tests designed to compare these four commonly used floor surfaces. Silica fume concrete is generally considered to be 8000 psi or greater. In this recent test, silica fume was used to modify 6000 psi concrete. Table 1 shows the concrete mix designs used in this analysis. Each of these concrete mixes employed 1-in. topsize aggregate.

To determine the suitability of these materials for solid waste applications, physical performance properties such as compressive strength, impact resistance, and abrasion resistance were used to evaluate high wear floor capabilities.

Compressive Strength. Table 2 shows the compressive strength for plain industrial concrete, 6000 psi high performance concrete, concrete modified with silica fume (10 percent), and an iron aggregate portland cement topping. The numbers show that the compressive strength of an iron aggregate topping is far superior to the strengths of plain concrete, high strength concrete, or concrete modified with silica fume. This higher compressive strength improves the floor surface's abrasion resistance when compared to the alternatives.

Abrasion Resistance. Abrasion resistance data has been compiled for more than 50 years with very consistent results. Figure 1 shows the wear rates of the floor alternatives when subjected to abrasion using ASTM C 779 Procedure "A."

As these data indicate, the portland cement iron aggregate topping exhibited a greater resistance to abrasion than the other floor alternatives. The wear rate due to abrasion was eight times greater than that of plain concrete, four times better than high strength concrete, and three times better than silica fume modified concrete. The cementing materials found in all these systems are similar and provide similar properties. Why then are the abrasion resistance results so different?

Scripture, Benedict, and Bryant present findings in The Journal of the American Concrete Institute (December 1953, pages 305-316) of a study that evaluated different aggregates and the related abrasion resistance of concrete floors when these aggregates were used.

Results of their work 40 years ago are similar to laboratory analysis of current cementitious products. One conclusion reached by Scripture, Benedict, and Bryant was that abrasion resistance increases with higher compressive strength. It was also shown that no relation exists between the aggregate hardness and resistance to abrasion. All aggregates in a given type of mix, with the same compressive strength, give the same resistance to abrasion. However, malleable iron aggregate has an abrasion resistance entirely TABULAR DATA OMITTED TABULAR DATA OMITTED different than mineral aggregates. Iron aggregates, although relatively soft, exhibit about 400 percent greater resistance to abrasion than mineral aggregates. The reason is that mineral aggregates break under shear due to their intrinsic hardness and iron aggregate, being a ductile material, will deform under shear.

Impact Resistance. Also important when evaluating these floor surfaces are the values collected for impact resistance using a modified ASTM C131, L.A. Abrader test. The test consists of placing cube specimens into a rotating steel drum and rolling the drum to create a crushing-impact effect. After 2000 revolutions, the cubes are removed and weighed and a total percentage weight loss is determined. Figure 2 shows the ability of each material to resist impact.

The top portion of Figure 2 shows that the iron aggregate topping provides superior impact resistance. The bottom portion shows the amount of material lost through impact as measured by weight loss. Similar to the abrasion results, the iron topping performed better than the concrete mixes that contain mineral aggregate. Iron aggregate is a very ductile, malleable material, meaning it can deform under load. It does not chip or crack and can absorb impact and abrasion rather than fracturing and pulverizing as will mineral aggregates.

Wear Rates. Surface wear provides another source of comparative data. For this discussion, we have used cores gathered over the past nine years from several tipping floors. From these cores a wear rate was derived in inches of wear per ton of waste processed. This rate is useful in projecting future wear and repair frequency. Surface wear measurements based on tonnage of waste processed are vital when calculating a total floor system cost over the life of the floor.

Several measurements were taken from different floor sections, including normal wear areas as well as hot spot areas. Hot spots usually develop 15 to 20 ft from the front of a loading port. They are the highest traffic areas on a tipping floor and wear much faster than the rest of the floor. It is important to obtain measurements from both areas. Hot spots were found to wear twice as fast as other areas of the floor. In all cores evaluated, those taken from floors topped with iron aggregate products exhibited a resistance to surface wear up to four times greater than cores from 4000-psi concrete. This was true in samples from both average wear floor areas and hot spot areas.

Life Cycle Cost Analysis

Along with confirming the laboratory results of surface wear testing, in-field analysis can be used to compare wear rates for floor alternatives and total system costs per ton of waste processed at specific facilities. A process has been developed known as life cycle cost analysis to help owners compare different high wear floor systems.

To fairly evaluate the performance of floor system alternatives, information relative to physical facility usage and operation must be gathered and analyzed. Amount of waste processed annually is used to determine the wear requirements of solid waste facilities. Material and labor cost of alternate systems provide initial installed cost data required for the analysis.

Using data on in-service wear rates of alternative floor surfaces, owners can project how long a floor will last given their specific service requirements and usage rates, and how often that floor will require repair. Differences in wear rate will result in differences in the on-going cost to repair a floor surface over a given life cycle.

Installing, repairing, and maintaining a high performance floor system is only part of the equation. In addition to labor and material costs, owners must also be concerned with the lost revenue experienced due to facility shut-down required for repairs. A floor system that requires frequent repairs will negatively impact the profitability of a waste transfer station by necessitating shut-downs. A floor system that requires longer downtime due to differences in the repair process further impacts profitability.

Combining installation costs, on-going repair costs, and downtime costs gives a total system cost for a solid waste floor. Applying the usage and tonnage processed information allows owners to compare various floor systems on a cost per ton basis.

This value of life cycle cost per ton is a useful comparison for high wear floor surfaces. Although installation cost may be less for the initial repair of some flooring alternatives, this cost can escalate dramatically over the life of the facility if frequent repairs are required.

With all the variables involved in selecting high wear floor systems, owners, engineers, and architects need help comparing flooring alternatives for solid waste facilities. These facilities vary greatly. The one factor shared is that they all process solid waste, and that the unit of measure is the ton. Physical properties for the different types of floor surfaces can be tested for compressive strength, abrasion resistance, and impact resistance to differentiate product performance.

Wear rates are important in calculating a life cycle cost for each type of floor surface. With this information, owners and operators of these facilities are in a better position to make informed choices in selecting the most cost-effective high wear floor system.
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Publication:Public Works
Date:Aug 1, 1993
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