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Under pressure: How a strategic approach to concrete reinforcement can overcome load transfer issues and extend the service life of industrial floors and pavements

As one of the world’s most widely used construction materials, concrete is well known for its durability and versatility. Extensively used in high volume areas, such as the internal floors of warehousing facilities and factories and external pavements, including parking areas and service yards, it’s vital that concrete floors and pavements are designed with longevity and serviceability in mind. Matt Bollé, National Sales Engineer from Danley® explores the best solutions to optimise the performance of industrial concrete floors and avoid known issues associated with load transfer.

What do we mean by load transfer?

In industrial environments, concrete flooring and pavements are commonly made up of a number of individual panels measuring approximately 6m x 6m. The point where one panel meets another is known as the joint. As a load is applied to the concrete, whether it is a static load from racking in a warehouse environment or a moving load from materials handling equipment or lorries in a service yard, the slabs must work together to share and support the load. This interaction between the slabs is known as load transfer  – i.e. the ability of the slab to safely and effectively transfer a load between adjacent panels.

According to the American Concrete Pavement Association (APCA), most performance problems with concrete pavement are a result of poorly performing joints.They state: “Distress, such as faulting, pumping and corner breaks occur in-part from joints with poor load transfer efficiency. All of these problems worsen when joints deflect greatly under loads.”

Put simply, poor load transfer creates high slab stresses and reduces service life, while adequate load transfer is vital to ensure rigid pavement performance and longevity.

Concrete reinforcement - what, why and how?

One of the main issues when working with concrete is the formation of cracks. For site managers, the most important first step is to establish whether the crack is aesthetic or might impact the structural integrity of the surface. The reality is that cracks can be caused by a variety of factors, from drying shrinkage and thermal contraction through to insufficient coverage or high water content – all of which will be managed by your supplier or contractor. A dominant cause of cracking in concrete slabs is improper joint design, particularly sawn joints. This can be easily addressed with an improved understanding of natural concrete behaviours and the correct product solutions.

When it comes to reinforcing concrete flooring against issues to do with load transfer, there are a number of solutions:

Mesh reinforcement

Mesh reinforcement is a popular choice in the UK construction industry. Sawn joints in mesh-reinforced concrete slabs rely primarily on aggregate interlock for load transfer. TR34 – The Concrete Society’s guide to the design and construction of indsutrial floors – stating that any mesh reinforcement that runs across the sawn joints yields under concrete shrinkage and does not provide any load-carrying capacity. The result is an approximated load transfer capacity of just 15%, meaning the loaded slab needs to be designed to carry 92.5% of the total load.

Installation of mesh can be highly variable, with excessive lapping of mesh sheets creating localised areas of high restraint. If positioned near sawn joint locations, this can prevent joints from activating, leading to random cracking within the slab. In addition, the process can be costly, with the purchase, transportation, storage, cutting and fitting of steel mesh adding to the project cost and installation time.

Fibre reinforcement

Fibre is an increasingly popular way of reinforcing concrete. Rather than using a rigid sheet of reinforcement located at a specific depth within the concrete slab, fibres of steel and synthetic materials are instead distributed throughout the entire concrete matrix. The process is designed to improve the structural integrity and tensile strength of the concrete, with increased ductility and abrasion resistance.

Fibre Reinforced Concrete (FRC) typically avoids the use of sawn joints within the slab due to industry concerns over the load transfer performance of aggregate interlock coupled with fibres at these locations. As a result, FRC is most often used in “jointless” solutions with significantly larger panels, where steel construction joints are located at 30-50m centres. These joints open significantly more, due to the fact that they are accommodating the shrinkage of the much larger panels. TR34 recommends that where a jointed FRC solution is specified, dowels are used to overcome any concerns around load transfer capabilities.

Sawn-Joint-Fibre

Overcoming known issues with mesh and fibre reinforcement

With poor joint performance and ineffective load transfer being a known issue in slab-on-ground and industrial pavement applications, greater attention should be given to the joint design.

By using Danley’s PD3® tapered plate dowel and sleeve system, which offers a load transfer capacity of 90%, it is possible to overcome many of the issues associated with mesh and fibre reinforced concrete, providing more consistent joint performance over the full design life of the concrete floor or pavement. The dowels are specifically designed to ensure that the concrete can shrink freely in both the lateral and longitudinal horizontal plane, without inducing restraint that leads to out-of-joint cracking.

Traditional mesh or fibre reinforced solutions rely on aggregate interlock as the load transfer mechanism in sawn joints between adjacent panels. However, due to the variable distribution of reinforcement throughout the concrete – particularly in fibre reinforced concrete – the interlock isn’t always reliable and can change over time and under pressure, making the joint unstable and susceptible to deflection and failure.

Strategic reinforcement - when structural integrity counts

By leveraging the high load transfer capacity of the PD3® tapered plate dowels at the sawn joints, and combining them with the equally high performing Danley® Dowels at the construction joints, Danley® is able to offer our optimised Strategic Reinforcement™ Design solution. This solution provides the highest deflection control tolerance in line with ACI 360R-10 standards, as well as complying with Concrete Society TR66 Rev. 1: External in-situ Concrete Paving and ACI 330.2R-17: Guide for the Design and Construction of Concrete Site Paving for Industrial and Trucking Facilities,

Joints are strategically placed at closer centres to mitigate shrinkage cracking in the early age concrete, thereby eliminating the need for mesh or fibres. The geometry and alternating orientation of the tapered plate dowels allow forgiving installation tolerances at the joint, while limiting spalling and reducing the risk of cracking caused by restraint.

Corner and edge loading conditions are the controlling factors in concrete slab design. The 90% load transfer capacity that Strategic Reinforcement™ provides at every joint means that individual panels can be designed to accommodate less load than in conventional mesh slabs, resulting in thinner slabs. This is how Strategic Reinforcement™ Design optimises both materials and labour, keeping costs down and reducing carbon. The design also ensures a fast and hassle-free installation with reduced time on-site when compared with standard alternatives. The final result is a superior floor or pavement, with an extended service life.

For more information on Danley’s Strategic Reinforcement™ Design, or to see how the company’s solution has reduced installation time and solved load transfer issues for a number of customers.