Danley Systems | Products and systems for wet concrete construction

September 2, 2021

The evolution of dowel design

Whether used for service yards, industrial flooring, or other slab-on-ground applications, concrete slabs are known to be subject to a range of stresses – resulting in cracking, spalling, and faulting that can impact service life. While stories of complete structural failure are rare, issues with load transfer are known to be a common reason for failure over time. Matt Bollé, National Sales Engineer at Danley, explains how the use of dowels has evolved over time to minimise known load transfer issues and improve the performance of industrial concrete floors and hardstandings.

What do we mean by load transfer?

Industrial flooring, hardstanding, and service yards are usually made up of several panels. As load is applied to the surface, the panels need to work together to support the load. This interaction between the panels is known as load transfer and is measured in terms of Load Transfer Efficiency (LTE). Poor load transfer, when the panels deflect and don’t transfer the load from one panel to the other efficiently, is known to create high slab stresses and damage which reduces the service life of the concrete ground supported slab. In applications with poor load transfer, the costs are multiple, from downtime and costly repairs, through to damage to vehicles and materials handling equipment, and even injury caused by employees tripping and falling on uneven surfaces.

Better performance by design: dowel technology

It has long been recognised that aggregate interlock alone is often insufficient in terms of load transfer efficiency. In the US, and increasingly in the UK, industry is evolving towards the use of dowels combined with more conservative joint spacing, which has reduced known issues. However, just as with any reinforcement method, inadequate dowel design can create a failure across a joint which can be very expensive to resolve. For example, as well as providing load transfer, dowels should be designed to allow connected panels to shrink, making it important that dowels allow two or even three dimensional movement.

The history of dowels

The first dowels used in slab-on-ground applications were round. However, it soon became apparent that round dowels brought with them an inherent set of issues. The very design and shape of a round dowel creates a point load, radiating a splitting force that can cause faults in the concrete. In addition, traditional round dowels can restrain movement along the longitudinal joints causing internal stresses that lead to further cracking. In the 1980s, when the issue was first recognised, the proposed solution was to attach a compressible foam material to the dowel, allowing the slabs to move independently while also providing efficient load transfer. The shortcomings of this approach – namely the displacement and/or crushing of the foam material – were overcome by using an ABS clip to hold the foam in place; a solution still in use today.
During the nineties, dowel design and specification evolved to favour a square design over the more traditional round dowel. While the design offered superior load transfer, contractors often preferred the older round design because it could be rotated and removed far more easily than its four-cornered counterpart. Furthermore, research into the development of industry specifications and standards provided evidence that bearing stress was an issue in both round and square designs. The research revealed that a joint’s differential deflection under loading increases with a smaller dowel cross-sectional area across a joint. If the bearing surface area becomes too small, then the bearing stresses can cause failure of the concrete around the dowel.
MicrosoftTeams-image
In simple terms, the research negated the use of both round and square dowels, and helped to explain why larger dowels are preferable – and indeed necessary – for heavier loads. In turn, this led to the development of a rectangular design, followed by the more modern tapered plate and diamond-shaped dowels.

The benefits of tapered plate dowels

Danley’s PD3® design, for example, features a tapered and sleeveless plate dowel that provides optimum performance in limiting joint deflection to provide superior joint stability. The PD3® dowel provides the highest deflection control tolerance in line with ACI Standards, limiting joint spalling, facilitating load transfer and providing the lowest risk of restraint. When used at alternate intervals with plate dowels, the system maximises the percentage of steel at the joint, while the debonding material allows for close steel-to-concrete contact and allows shrinkage without restraint and without the need for a plastic sleeve. The design offers the best serviceability, lowest maintenance and longest lifespan for any slab on ground application.

Conclusion - why dowel design matters

As well as offering a reliable and predictable joint performance that overcomes the shortcomings of its round, square and rectangular predecessors, Danley’s tapered plate dowel solution complies with TR34, TR66, Britpave Concrete Hardstanding Design Handbook, ACI 330.2R-17, and ACI 360R-10 specifications/regulations. When considering a pavement with the same truck activity (1000 trucks per day), an optimised pavement with dowels can reduce slab thickness and concrete volume by almost 20% – reducing both the cost and raw material usage significantly and adding to the solution’s sustainability credentials.

Tapered plate dowel systems eliminate one of the biggest frustrations for designers, contractors and owners alike – joint spalling. By switching to tapered plate dowels, or Danley’s Strategic Reinforcement™ Design solution, all stakeholders stand to benefit from reduced cost of ownership, faster installation and improved efficiency.