BETTER SLAB
PERFORMANCE

By Brent Long, Vice President Carolinas
Preconstruction, and Mark Demaree,

Project Director, Charlotte, NC

Clear heights rise to meet the demand, averaging 32 feet in 2020. The increase in online shopping has catapulted companies like Amazon to never-before-seen success in the industry – but this doesn’t come without its own share of challenges. To remain competitive, companies are now offering blinding shipping speeds and two-day delivery promises to their customers, leading to the need for taller warehouses across the country.

In the world of industrial construction, project goals and business ambitions are climbing higher and higher - as in clear heights! Increased product demands in industrial facilities, from warehouses and distribution to manufacturing, food processing, and adaptive reuse, means spaces are growing.

As the saying goes, a building is only as strong as its foundations. The taller the building becomes, the greater the pressure exerted on the foundational concrete slabs. As such, slab design and performance are even more critical when working with an increased clear height. This increase in weight requires both a stable and long-lasting slab foundation to maintain the structural integrity of the building itself for years to come.

Success in industrial construction projects rests on proper slab performance. Assessing the risk factors that influence slab performance during the preconstruction phase and leveraging a skilled contracting partner can help you mitigate high costs and headache down the road.

A key factor to consider for slab success is the type of concrete flooring being used, and whether the cost and durability are appropriate for the project.

Contrary to common belief, most ground slabs laid in the U.S. are unreinforced. Depending on the stresses and use intended for the concrete foundation, this may not be the best option for your industrial construction project over the long term.

When it comes to reinforcement, there are many options to choose from – including welded wiring, steel bars, and steel and synthetic fibers. All of these reinforcement choices, however, do not actually prevent the concrete from cracking or splitting. Cracking is a normal, natural occurrence in concrete. Instead, they lie dormant in the concrete slab until a crack forms, then work to control the crack and maintain the integrity and load-bearing potential of the slab.

While both steel and synthetic fibers create a more homogeneous concrete mixture, there are subtle differences between the two that may sway the ultimate decision on which is appropriate for a project.

NeXT:

Weighing the Pros and Cons of Steel vs. Synthetic Fibers

Pros:

• Greater ability to reinforce large structures
• Sustain transfer of weight over time
• Better control over cracks in the concrete
• Can resist creep or shifting due to prolonged transference of pressure

Cons:

• More expensive than synthetic
• Require reduced spacing of joints
• Needs higher fiber dosage per cubic foot of concrete mixture

• Limits workability of concrete mixture

Pros:

• Come in low-volume (LVSF) or high-volume
  (HVSF) varieties
• Reduce possibility of cracking and shrinkage
• HVSF mixtures can accommodate a higher
  density of fibers while retaining workability

• Allows for greater spacing between joints

Cons:

• Does not eliminate curling and / or creep
• Encourages greater slab thickness
• Does not totally control cracking caused by
  external forces
• Should not replace moment-resisting or
  structural steel reinforcement

Mobility of Products / Distribution of Weight on Slabs

Larger distribution facilities will naturally have more frequent transferences of weight across their flooring as forklifts and other heavy machinery move product to and from shelves and into trucks.

Taking into consideration the planning of not just the structure, but the functions and unit groupings within the building are critical when evaluating where and how to lay the concrete slabs and reinforcement requirements. The team must analyze the facility’s intended layout and assign proper joint stabilization, steel fiber reinforcement and possibly even rebar for a stronger base where needed.

To determine which fiber choice is right for your project, consider the amount and frequency of weight transference across slabs; longevity of project; pricing and budget; quality of soil and moisture levels; and intended joint spacing.

Ensuring the proper quantity (dosage) per cubic yard of concrete in any industrial warehouse, manufacturing, or distribution project will help keep cracks as tight as possible and minimize spread or spalling in the event they do occur. Depending on the type of soil discovered when analyzing the subgrade, joint spacing will also be a factor to consider. Even if the subgrade is prime, joints can extend the life of concrete flooring by helping it stay flexible as the earth shifts below and the environmental factors begin to take effect.

As the pressure from the taller building extends downward, even beyond the concrete foundations, the quality of the subgrade and whether any treatment is required must be taken into consideration. Additional investigations into the soil must be conducted to determine its organic structure.

Understanding the soil composition and potential reactions early can help determine the best course of action to ensure the project quite literally stands the test of time.

Read more on extended floor joints here!

Historically, column spacing along dock doors was approximately 50’ to allow for the average forklift and other man-powered machinery. However, as the world makes way for a new wave of automation, so too must industrial construction developers. Warehouses for e-commerce companies like Amazon have begun utilizing robots to assist in moving product through these massive facilities. As a result, horizontal clearances to rack spacing has grown to 52’ and even 54’ to provide enough room for these automated helpers.

Coupled with subgrade conditions and concrete floor compositions, exploring these questions will present the best solution for the right cost to lead to a successful -- on-budget -- project.

Looking ahead to the final function of the facility and intended floor loading will ensure smart design and proper construction.

Problem: When doors are open, exhausts are running, or there is no temperature control in an industrial project, humid air can seep in and create condensation on the concrete flooring. These “sweating slabs” are slippery and can cause a major safety threat to employees moving product around the floor. The porousness of the concrete mixture should be considered, as slightly porous surfaces allow water to be absorbed more efficiently to remove slow-forming condensation. Porousness is determined by several factors, including the sealer application and finishing techniques used by the concrete team.

Preventative Measures: To mitigate sweating on large surface areas of placed concrete, regular cleaning and scrubbing of the floor will remove dust and debris that can clog the concrete’s pores and prevent optimal absorption.

Adjust the A/C and keep the interior temp as close of the external air temperature as possible. Large fans or other equipment to maintain efficient air flow throughout the building and in between racking will generate air movement and prevent moisture from accumulating. In some instances, a commercial dehumidifier may be needed to eliminate condensation on the concrete surface.

Considerations of long-term outlooks when it comes to slab design is key to maximizing performance and minimizing future issues. Having the right partners in the room to consider elements in the preconstruction phase will enable the team to foresee common concrete challenges and offer affordable, preventative solutions.

Problem: When a concrete slab floor splits into horizontal layers, cracks, or separates, delamination has occurred. These changes often occur near the surface of the concrete once the mixture has been placed, compacted, and settled. Any excess water and air trapped in the mixture are displaced and the less-dense materials drift toward the top. Due to the trapped air and moisture, voids develop below the surface and will eventually crack open under use. In addition to excess water and air in the concrete, if steel is used as a reinforcement tool, corrosion of the steel can contribute to delamination.

Here are some of those common problems and the best ways prevent them from occurring post-pour:

Preventative Measures: Separation of concrete slab layers can be avoided by sealing the concrete at the appropriate time and allowing proper time for airing and bleeding to occur. Setting of the slab mix should be uniform and placed on a previously-covered subgrade – this prevents the ground from being too cold to place the concrete, as a cold subgrade slows down the setting process.

Problem: While the rate of slab curling is affected by how fast or slow the slab dries, one of the main causes of slab cracking is the temperature at which it cools and the resulting contraction of the mixture when setting.

Preventative Measures: If there is a change in section size in the concrete slab, a joint should be placed at that change to mitigate cracks and allow the concrete to expand and contract freely.

In addition to a proper water-to-cement mixture for the slab placement and accurate dispersion of reinforcement fibers or bars, excess moisture should be limited to prevent water seeping into the slab. While some smaller cracks are unavoidable, waterproofing the slab will help prevent excess moisture from getting into the cracks and corroding the reinforcement within.

Problem: Curling happens when the upper and lower portions dry at different rates and/or temperatures, which tends to occur faster on the upper layers and slower towards the bottom. Because of the difference in rate of drying, the top layer begins to contract and lose volume, causing the slab to bow upwards.

Preventative Measures: Although some minor curling is to be expected, there are ways to mitigate extensive curling in a concrete slab and promote uniform changes throughout. Avoid concrete mixtures with high water content and pursue one with a washed, high coarse-aggregate content.

Limit excess water on the jobsite and consider reinforcement near the top layer to help restrain curling.

Curling is a natural part of concrete slab placement. The question is whether the curling has occurred to a degree that the slab is no longer usable or safe.

If you’ve inherited a slab issue from an existing structure or otherwise faulty concrete foundation, proper prevention practices may no longer be on the table. This is when partnering with an experienced general contractor is going to bring expertise. They will be able to help you assess these post-placement challenges by securing the expert specialists to implement repairs quickly and efficiently, avoiding surprise issues down the line.

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