Bump bump. Bump bump. Bump bump.
This sound is common in a warehouse or plant where forklifts are in operation. Each of those bumps is an impact as the wheels cross the joints. It’s also the sound of damage — to the slab, the vehicle and possibly the operator as well.
In recent years, more interest has focused on developing “seamless” or near-seamless floors, particularly for use in industrial and distribution facilities. These products need less repair and provide smoother surfaces for forklifts and other vehicles. They also ensure that goods stacked on pallets don’t fall over.
Reducing or eliminating joints yields savings in various ways. For example, the joints do not deteriorate, the vehicles traveling across the slab do not receive as much vibration and impact, and the entire slab is available to resist loads, not just the areas defined by the joints.
They are safer for workers, because they can prevent forklifts from tipping over or wire-guided lifts from coming off track. They can also reduce trip-and-fall injuries. Both are among the biggest hazards in warehouse settings. According to the Occupational Safety and Health Administration, about 85 forklift operators are killed in accidents each year, and nearly 100,000 are injured. Statistics from the National Safety Council show that more than 12,000 workers in warehousing and transportation were injured in falls in 2017, and 47 died.
However, it’s important to note that the advantages of seamless floors can be difficult to achieve in practice, and they demand careful design and construction.
Soil-supported concrete represents almost a third of all concrete used in real estate. Much of it is for pavement and interior slabs that distribute vehicle, storage and machine loads, and it is found in a wide range of commercial properties.
Slabs typically come with sawn or preformed joints spaced 24 to 36 times the slab thickness. Spacings larger than this can lead to uncontrolled cracking between the joints.
Even if that does not occur, the joints themselves are cracks. Joints address the shrinkage that occurs in concrete as it dries, resulting in stresses as the slabs are restrained. Where these stresses exceed the tensile strength of the concrete, the concrete will crack. Cracks also occur due to bending of the slab under loading, which is dependent on the stiffness of the concrete and the soil below. The cracks can act as hinges, reducing the stresses caused by loads.
These joints and cracks cause problems with the maintenance of the facility and equipment, as well as with the health and safety of the building’s users. The maintenance occurs at the joints where loads move across them. In warehouses, these joints exacerbate forklift maintenance and repetitive-stress injuries among workers, and they can also be a tripping hazard. Additionally, the presence of incompressible materials in the joint such as small pebbles can cause raveling (progressive failure of the joint) and joint damage.
In “seamless” or near-seamless floors, the joint spacing extends to 100 times or more the slab thickness. The reduction in the number of joints means the slabs need fewer repairs. This reduces joint impacts as well.
While there are several technical approaches to create seamless floors, there are two main methods to specify the floors. Each has benefits and risks.
The first is a traditional approach in which the building plans specify technical details in the documents and contractors submit bids in a traditional manner. The second approach is to highlight the slab areas in the site plan, indicate the loading conditions and require a specialized design/build operation to install a slab that meets the performance requirements.
The advantage of a detailed prescription specification is that the owner can exert a high degree of control. This approach is best when the designer has familiarity with the systems and the teams doing inspections. It's riskier, though, because the designer is responsible for the performance.
A performance-based specification is a metric such as crack frequency, extent or width that defines how well the concrete works. The advantage of this approach is that the owner does not need a specialist engineer, and expectations are clearly mapped out. However, the costs can be higher.
Several methods can maintain the stresses in the slab below the failure point. These include reducing shrinkage, reducing curling stresses, increasing the tensile strength of the slab and improving the distribution of stresses. Some projects employ more than one method. Regardless of the method, proper soil support is critical, because the curling stresses increase as the span between joints increases.
The simplest method of joint extension is to reduce the shrinkage of the concrete. Low-water-content mixtures or admixtures can reduce shrinkage, along with two-course wet-on-wet systems such as Ductilcrete that can extend the joint spacing up to approximately 50 feet.
Slabs can be post-tensioned so that they are in compression at all times under load. This method is best specified by indicating the required forces for post-tensioning and leaving the stressing to the contractor.
Adding fibers can increase the concrete’s tensile strength and boost joint spacing by up to 100 feet. Low-modulus fibers don’t have enough stiffness to carry more load than the concrete they replace, so they have little effect. Stiffer polymer fibers are more effective, as are steel fibers. Special equipment can place the fibers into ready-mix plants or trucks without balling, and the use of water reducers helps in distribution.
The most effective way to avoid shrinkage is to design cement that expands upon setting. When constructed properly, it works well. However, the placement of individual reinforcing steel bars and the detailing requires a degree of effort that is far higher than what is normally seen in slab construction. That’s why specialist contractors do most of this work.
Systems exist that can be placed with essentially no limit on the span between joints. The combination of shrinkage-compensating concrete and steel fibers has shown promise in execution.
These are often installed as proprietary systems. Examples include Ductilcrete or the Primx system. They remove some of the risk of using seamless joints by passing on the design and construction requirements to specialized contractors.
Many of these systems have specific warranties. Some products are more generic, supported by admixture or ready-mix concrete providers, and they can be specified as bid-build installations. These systems can be installed at nearly the same cost as traditional slabs in projects where the design takes advantage of the reduced curling stresses to provide a thinner floor with the same strength.
Regardless of the construction method or the technologies employed, it’s possible to build large-span slabs that provide all the strength of conventional slabs without any issues arising from the use of joints. If facilities need to expand, the slabs can be added on to as readily as any conventional slab.
However, there are bigger risks associated with these slabs than those encountered in normal slab construction. Where joints are installed, some out-of-joint cracking can occur without affecting the performance of the slab in any meaningful way. Where there are no joints, and the slab begins to crack due to design or construction errors, there can be frequent and extensive cracks. This can have a major impact on the performance of the slab and its maintenance costs.
Anecdotally, it seems that these installations are either very successful or very problematic. This typically occurs because either the designer or the installer is not aware of the fundamental aspects of the structure’s performance. An important factor in avoiding poor performance is to have both designer and contractor aware of the key performance requirements and how to measure and achieve them.
Kevin MacDonald is the founder and president of Beton Consulting Engineers.