Tag Archives: American Concrete Institute

Concrete Slab-on-grade Reinforcement

Long time readers will recall concrete finishing does not rank amongst my favorite building tasks. It is an art form with a gene I was not blessed with. Most pole barns, post frame buildings and barndominiums utilize slabs poured on grade.

Reader KYLE in KAPLAN writes:

“In your pole barns, do you typically use wire mesh, or rebar when doing a slab?”

For areas where heavy vehicles or equipment may be placed, our independent third-party engineers will specify rebar reinforcement for concrete slabs on grade. Much of your need for reinforcement will be dictated by how well your site was prepared. Properly compacted sub-grades can reduce needs for reinforcement – as well compacted site is less prone to adverse effects from uneven weight distributions. Sadly, most clients and builders just do not spend enough time and effort to arrive at good site preparation.

Choosing concrete for a residential or commercial construction project is a great way to ensure you are using a strong, durable material. There are several ways to make sure your concrete has proper strength for your building. Concrete changes density when it sets, making it vulnerable to cracking. Concrete also can crack due to changes in temperature or unevenly distributed weight or stress. When pouring concrete for driveways, foundations, or floors, three common ways to reinforce concrete are to use wire mesh, rebar or fibers.

Using wire mesh is a common method to reinforce poured concrete. Wire mesh makes a square grid pattern and is laid down before concrete gets poured. Wire mesh is usually one layer of a two-dimensional grid running along length and width of poured concrete, but not height. During concrete pouring processes, you or your workers will raise previously laid down wire so it runs along concrete height (thickness) middle. This reinforcing material inside helps to prevent cracking during temperature changes and while concrete is setting.

Instead of laying down a wire mesh before concrete is poured, using fiber mesh involves mixing in different fibers such as glass, steel, synthetic fibers, or natural fibers. Fiber mesh reinforces concrete throughout its entire structure rather than just one plane. This comprehensive reinforcement protects against not just cracking due to fluctuating temperatures and changing densities from setting, but also helps prevent water from bleeding out of concrete and gives concrete’s surface a higher impact resistance.

In addition to providing a more thorough protection for your concrete pour, fiber mesh typically takes less time than wire mesh to use. This is because wire mesh has to be carefully measured to fit the pour site and needs to be held up at a certain level during pouring processes. Conversely, fiber mesh can be added straight to mix, removing the need for an extra step while pouring. Fiber mesh is also more cost-effective since there is less time involved in pouring and material is used more efficiently. There has been concern among some as this fiber mesh method can create a “hairy” finish due to some fibers protruding from the slab’s surface. However, this is only temporary since they are often laid down flat when trowels flatten concrete’s surface, and any fibers still protruding are quickly worn down or burned off by the sun if outside, or a torch if indoors.

American Concrete Institute (ACI) lists factors playing a role in how thick covering concrete must be to support rebar.

Cast-in-place concrete requires placement of wet concrete around rebar, then holding it in place as it sets and dries around it. This is usually done with rebar supports helping hold it at correct depths, but this does open it up to a certain level of operator error.

For cast-in-place concrete in contact with ground permanently, recommended covering concrete thickness is three inches.

This means for cast-in-place slabs less than 5 inches thick, in most circumstances, there should be no rebar involved. Slabs at this thickness are simply too thin to adequately cover and protect rebar while still exploiting its reinforcing nature.

One thing certainly helping is to check your local regulations, as they take into consideration local environment for optimal construction.

Variances in Surface of Pole Building Concrete Slab

Variances in Surface of Pole Building Concrete Slabs

Reader RON in MARYSVILLE writes:  “What is an acceptable variance in the finished level of the concrete floor in a new 24×36 pole building? It is 1/4 inch out of level, 1 1/8 inch slope, 3/4 inch hump all acceptable numbers? My floor has all of these and more.”

Like fences make good neighbors, contracts make happy clients.

You will need to go back to the contract you signed with the concrete finishing company to see what allowable tolerances you agreed to.

According to ACI (American Concrete Institute) 117-10 Section 4.8 your finisher could probably argue the point from the Commentary found in R4.8.4:

“The purpose of establishing floor surface tolerances is to define surface characteristics that are of importance to those who will be using the surface. The two surface characteristics thought to be of greatest importance for concrete floors are flatness and levelness. Flatness can be described as bumpiness of the floor, and is the degree to which a floor surface is smooth or plane. Levelness is the degree to which a floor surface parallels the slope established on the project drawings.  Two methods are identified for use in the evaluation of floor surface finish tolerances. The F-Number System uses data taken at regular intervals along lines located in random locations on the test surface. The described methods use different criteria to evaluate the as constructed data. Therefore, it is important that the Specifier select the method most applicable to the end user of the floor. “

Here is a simple ACI check for flatness:

“ The gap under the straightedge and between the support points shall not exceed either of the values as listed for the specified Floor Surface Classification in Table”

In the Table, for a conventional floor, the maximum gap for 90% compliance samples should not exceed ½ inch, for 100% ¾”.

“ The following minimum sampling requirements shall apply for test surfaces evaluated using this tolerance method: A test surface is deemed to meet specified tolerances if it complies with the maximum-gap-underthe-straightedge limitations given in Section above. The maximum disparity between a taut string stretched between the bottom corners at the ends of the straightedge and the underside of the straightedge shall not exceed 1/32 in. at any point. The minimum number of samples = (0.01) area for floor areas measured in ft2. A sample is a single placement of the straightedge. Orientation of the straightedge shall be parallel, perpendicular, or at a 45-degree angle to longest construction joint bounding the test surface. An equal number of samples shall be taken in perpendicular directions. Samples shall be evenly distributed over the test surface. Straightedge centerpoint locations for samples shall not be closer than 5 ft. Test results shall be reported in a manner that will allow the data to be verified or the test to be replicated, such as a key plan showing straightedge centerpoint location and straightedge orientation.

R4.8.6 The manual straightedge approach evaluates the flatness of a floor surface by placing a 10 ft long straightedge on the floor surface and measuring the maximum gap that occurs under the straightedge and between the support points.

R4.8.6.1 Measurements should be taken between straightedge support points and perpendicular to its base. Smaller gaps between the straightedge and supporting surface are indicative of higher flatness quality. The use of this approach requires that 90% of the data samples should comply with values in the second column, and 100% of the data samples should comply with values in the third column.”

Apply the above to your floor, in lieu of specifics spelled out in your contract, if these tolerances are not met, you may yet have a cause of action.