Durable floors are very important. The dictionary states that durability is the ability of something to resist wear and tear, and a floor surface needs to resist wear and tear year after year. The wearing surface is the top 1/16 to ¼ inches of the slab thickness. Although flatness is important, it is not an indication of the durability of the wearing surface or the overall quality of the floor.
Many things affect the ability to achieve long-term durability. Concrete quality, finishing procedures, weather conditions/placing environment, joints, and subgrade just to mention a few. Quality concrete is certainly one of the most important parts of the equation for durable floors. The best contractor in the world will not be able to meet expectations without it. Conversely, a good concrete mix design will not overcome improper procedures at the batch plant or in the field. So for this article, the discussion of issues related to concrete mix design will be followed by good construction practice.
Concrete Mix Considerations
The components in all concrete mix designs are basically the same. However, durable floors demand special requirements. Priority should be focused on developing a low-shrinkage mix that minimizes cracking and curling while maintaining design strength. The concrete also should be reasonably easy to place, finish, and pump.
The components of a concrete mix are sand, stone, cement, water, and entrapped air. As indicated in Figure 1, the total aggregates, as in this example, are typically about 70% of the total volume of materials. With this in mind, it is easy to understand why the quality of the aggregates has the greatest impact on the characteristics of the concrete. So this is where you have the best chance to meet your goals.
The type, hardness, shape, and gradation of the aggregates all affect performance and should be considered. Gradation is the first important consideration. Computer programs help to analyze and improve gradations (see Figure 2 and Figure 3). Most of the aggregates for concrete are “gap graded,” meaning that there is a gap between the largest size of fine aggregate and the smallest size of course aggregate. Problems usually occur in the intermediate sizes and deficiencies or excesses easily are identified when plotted on a materials distribution chart as in Figure 2. This chart illustrates the combined aggregates that are retained on each individual sieve. When there are gaps or deficiencies in individual aggregate sizes, there will be voids in the concrete that must be filled with concrete paste.
If you optimize the aggregate gradation, you reduce the paste in the mix and therefore reduce the cement requirement and water demand. Lowering the total water reduces shrinkage. ACI 302 recommends a uniform distribution of 8% to 18% for large top-size aggregates such as 1½ inches, or 8% to 22% for smaller maximum-size aggregates such as 1 or ¾ inch retained on each sieve below the top size and above the #100 sieve. In the real world, this is not always achievable and some deviation is acceptable. In my experience, you don't want the percent retained to fall below 6% on more than two adjacent sieves. This may require some blending of aggregates. In fact, most of the mixes require the addition of an intermediate size.
Another method for determining aggregate gradation is a coarseness factor chart (see Figure 3). By itself this is not nearly as meaningful as the aggregate distribution chart but is very useful when the two are used together. The coarseness factor chart is a method of analyzing the size and uniformity of the combined aggregate distribution, balanced with the fine aggregate content of the mix. The coarseness factor (the x axis in Figure 3) defines the relationship between the coarse and intermediate particles. It is the percent of combined aggregate retained on the #8 sieve that also is retained on the 3/8 inch sieve. The y axis in Figure 3 represents the percent of combined aggregate passing the #8 sieve. There are five zones identifying regions for acceptance or rejection. If the plot of x and y falls within the optimal zone, this indicates that the mix is acceptable but it does not tell you exactly what to fix if it is not acceptable. This is useful as a quick check and the plot can be changed with modifications in the fine aggregate. However, it needs to be used in conjunction with the materials distribution chart in order to fine-tune the mix.
Another thing to consider is the mortar fraction. This is an extension of the coarseness factor chart. The mortar fraction consists of all materials in the mix that passes the #8 sieve. This is a good indicator of the amount of paste in the mix. The key is to keep the paste at a minimum while having enough available to produce a durable finish. Specific mortar fractions for various construction types and aggregate sizes are found in ACI 302.1R. The lowest mortar fraction possible is recommended—52% to 54% for 1½ inch maximum-size aggregate and 53% to 55% for 1 inch maximum-size aggregate.
For an ideal mix, expect the aggregates to be graded within the limits mentioned: a coarseness factor close to 70, workability factors of 35, and mortar factors less than 54. This is a fairly “boney mix,” but for an industrial floor it provides for high durability. The intended use is for projects placed in a controlled environment inside a building where the concrete is usually struck off with a laser screed. Different placing environments and/or floor types would of course change the material requirements.
A few other suggestions to keep in mind:
- Use the largest size of properly graded aggregate available
- Coarse aggregate should comprise approximately 60% of the total aggregate in the mix
- Crushed limestone is preferable to natural stone
- A natural sand with a fineness modulus of 2.70 to 2.90 is preferable Of course there is a lot more to all this than discussed here. More information is available in ACI 302.1R.
If the paste or mortar fraction is reduced, so is the amount of cement and water. But this may not always be the case. Some ready-mix producers may design mixes with more cement than needed to overcome the water they anticipate will be added at the jobsite. Using 520 to 560 pounds of portland would be a reasonable range for cement content for a floor exposed to wear and tear.
Lately the trend has been toward very low cement contents. This is a good thing, but there are limits. Floor slabs with as little as 470 pounds of cement are possible. With good well-graded aggregates, strength may not be an issue but finishability may be when durability is a priority. Regardless of other factors, finishing procedures demand at least 520 pounds of cement to produce a hard, dense, burnished trowel finish. A straight cement mix often is preferred to provide the most durable surface. Replacing a portion of the cement with fly ash has become popular and certainly is useful in many applications. It is important to note that you may not get as dense a surface when fly ash is used as a cement replacement in the mix.
Floor mixes usually contain Type I or Type II portland cement. Shrinkage-compensating concrete (SCC) mixes made with expansive cement is another option for durability. Instead of initially shrinking like conventional concrete, SCC initially expands. It is designed to expand enough to offset later drying shrinkage and therefore eliminating contraction joints and minimizing cracks in floors.
The amount of water in the mix is often an issue. With a properly graded aggregate system and the cement amounts previously mentioned, look for a water-cement (w/c) ratio of 0.50 to 0.54. Concrete slump always is a topic of conversation and controversy. For floors, a slump range of 4 to 5 inches works well. Many flooring contractors like to place concrete in this slump range. But a low-slump concrete produced with properly graded and proportioned materials will be much easier to place and finish than a higher slump mix made with improper material ratios. Slump is not a true indication of the water content of a mix but it is often the driving force for the addition of too much of it.
The workability of concrete is improved and water content is reduced with the use of water reducers. They are necessary in all mixes but should be kept to a minimum. Requirements will vary depending on the quality and proportion of the other materials in the mix, as well as weather conditions and concrete temperature. When possible, use only Type-A water reducers using hot water to accelerate, and ice or chilled water to control high temperatures. This may be impractical or too costly in some locations. If that is the case, use of retarding or accelerating admixtures in moderation may be the better choice. Usually only 3 to 4 ounces per 100 weight of cement should be used. If aggregate gradation is less than desired, the use of a midrange water reducer may be acceptable. The use of high-range water reducers is almost never necessary for floor slabs.
The last mixture component to discuss is air entrainment. You should expect to see 1% to 2% of entrapped air by volume in the mix, but it should never exceed 3%. Entrained air reduces strength and does not finish well. A slab that is to be hard toweled should not contain entrained air.
Good Construction Practice
Good construction practices should be controlled by the floor contractor. In order to do this, the contractor must have knowledge and understanding of all aspects of the construction process including the concrete. It is important to understand how the materials in the concrete mix affect its performance during construction. You also need to know how materials and construction procedures affect the long-term durability of the floor. Much of this information is found in the text and graphs in ACI 302.1R, “Guide for Concrete Floor and Slab Construction.”
A job does not always go as planned on paper. It is very helpful if the contractor is able to look at each component in the mix design and know what to expect. Quality control people at most ready-mix operations are happy to help with this. Aggregates, especially, should be studied and optimized if possible. Mortar fraction is important because it is an indicator of how well the concrete will place and finish. Ready-mix producers sometimes become nervous when contractors investigate their mixes but usually will try to accommodate when they know what you are trying to achieve.
Water content and slump control are very important. This is easy to do with the help of the ready-mix producer. A target slump should be established based on conditions and goals. This should be maintained by the ready-mix producer and monitored by the contractor. Water should not have to be added to every load at the jobsite. Slump can be very consistent when the total moisture in the mix is known and controlled. Moisture on the fine aggregate must be known—not guessed. When coarse aggregates are dry, they will soak up a portion of the mix water and this should be anticipated. Wash out water from previous loads should be discharged before batching. Adding water at the jobsite is sometimes unavoidable but should be kept at a minimum. It is not a problem if the maximum allowable water in the mix is not exceeded but it's a waste of valuable time.
In addition, slump tests should be performed by a testing lab to confirm consistency. Test cylinders should be collected daily to confirm compressive strength. Too often it is assumed that the air content is within limits, but this needs to be confirmed with monitoring tests at the jobsite.
It is essential that that everyone involved in the development of mixes and construction understand what the goals are. Proper design, good construction practice, and attention to the details are the formula for durability.
— Terry Fricks is president of The Fricks Co., a Fort Worth, Texas, firm specializing in designing and constructing industrial floor slabs nationwide.