Danley Systems | Products and systems for wet concrete construction

September 1, 2021

Load Transfer Efficiency: What is it and what do I need to know?

Load Transfer Efficiency (LTE) is an important parameter affecting the performance of industrial pavements, hardstandings and slab-on-ground design. As load-transfer efficiency decreases, many types of distress rapidly increase, including pumping, spalling, faulting, and slab cracking. With LTE an important factor for initial specification and ongoing maintenance decisions, Matt Bollé, National Sales Engineer at Danley, shares his insight into how different types of load transfer solutions can influence the LTE, overall efficiency and lifecycle of concrete hardstandings.

What exactly is LTE?

Known as slab-on-ground construction, industrial concrete hardstandings are usually made up of a number of individual panels that are created by either forming or inducing joints across the pavement. As load is applied to the concrete pavement near the joints, the loaded slab and unloaded adjacent slab will deflect differentially to one another as the panels try to distribute the load between them as it moves across the joint. This interaction between the slabs – and their ability, or inability, to share the load equally between them and limit differential deflection, is directly linked to the LTE mechanism selected and joint width opening.
Poor load transfer is known to create high slab stresses and reduce the service life of the slab, with the American Concrete Pavement Association (APCA) stating that “most performance problems with concrete pavements are a result of poorly performing joints”. For this reason, concrete ground-supported slabs should be designed and installed with the appropriate load transfer mechanism specified to ensure maximum long-term performance – including aggregate interlock, enhanced aggregate interlock with mesh, and dowels – to the effectiveness of which is measured in terms of LTE.

Why is poor load transfer efficiency an issue?

Unstable joints – or those with a poor LTE – exhibit excessive deflection that leads to spalling, chipping, pumping and faulting of the pavement – very common for external hardstandings.
In material handling applications, for example, where forklifts carrying heavy loads repeatedly travel along a specific route, loaded slabs must carry the majority of the load, which can result in them deflecting significantly more than the adjacent slab. This exposes the edge of the joint, which then degrades under repeated loading and impact from the forklift or other material handling equipment.
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The amount the unloaded slab deflects is directly related to joint stability. If a joint is stable and performing well, both the loaded and unloaded slabs deflect in near equal amounts , reducing the likelihood of issues. To achieve this, or limit differential deflection, the joints need a stabilising mechanism that will provide efficient and consistent load transfer.

Common load transfer solutions

The most common Load Transfer mechanisms include aggregate interlock, enhanced aggregate interlock with mesh, and dowels.
Solutions that rely on aggregate interlock as the load transfer mechanism can lose load transfer efficiency over time through fatigue and repetitive loading conditions In applications that have tight joints and cracks, typically <1mm, it is possible to reliably maintain a high degree of aggregate interlock and load transfer. However, this level of reliable aggregate interlock is only possible when specifying very small joint spacings (approx. 1.8m) as the amount of shrinkage for such small panels prevents the joints from opening beyond 1mm. When first installed, aggregate each side of the crack below the joint maintains a level of physical interlock, enabling the successful transfer of a load from one side of the crack to the other. As a joint opens through shrinkage of the concrete, the aggregate within the concrete loses contact and the level of interlock and ability to transfer loads reduces.
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For this reason, as you start to increase the joint spacing towards 4.6m, the load transfer efficiency begins to reduce by approximately 50%. In the UK, joint spacing is typically around 5-6m, often larger, reducing the LTE and increasing the likelihood of spalling, pumping and cracking yet further.
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In fact, statistics from the Portland Cement Association show that aggregate interlock is reduced by 85-95% and load transfer efficiency is lost when the joint opens up 1.5mm. The graph below also highlights how LTE is reduced more rapidly through higher load repetitions and is therefore highly variable in the performance it offers.

LTE and the role of dowels

Dowels from suppliers such as Danley are known to offer improved load transfer efficiency of 90% for joint openings as wide as 20mm and help reduce maintenance costs without negatively impacting the project timeline or installation cost. The physical dowel acts as the mechanism for distributing load between adjacent slabs and can bridge the gaps when joints open wider than 1.5mm to provide a consistent level of performance for the intended design life of the concrete pavement.
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“Theoretically, if the dowel is 100 percent efficient, the dowel will transfer one-half of the applied load from one slab to another. This is true if each slab at the joint deflects an equal amount and each assumes one half of the applied load.” Principles of Pavement Design by Yoder and Witczak.
However, while dowels undoubtedly provide improved LTE versus aggregate interlock, the shape of the dowel can also have a huge bearing on its effectiveness – read our blog on the evolution of dowels here. Plate dowels offer less deflection than round dowels, spreading the sheer load across a wider surface and allowing for variation in design that can accommodate the slab’s lateral and longitudinal horizontal movement, helping to minimise the size and number of restraint cracks.
Danley’s Strategic Reinforcement™ Design combines PD3® tapered plate dowels at the sawn joints with Danley® Dowels at the construction joints. The Danley® dowel tapered plate dowel and sleeve systems promotes load transfer across the joint and minimises differential deflection between adjacent slab panels. When used with the PD3® tapered and sleeveless plate dowel, the system offers best-in-class performance in limiting joint deflection. In addition to the dowels, joints are strategically placed closer to mitigate shrinkage cracking, thereby eliminating the need for mesh or fibres. The geometry and 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.
The 90% load transfer capacity at every joint means that individual panels can be designed to accommodate less load than in conventional mesh slabs. This enables the use of thinner slabs, offering a more sustainable solution by significantly reducing the amount of steel and concrete used to deliver the project.
The result provides the highest deflection control tolerance in line with ACI 360R-10 standards, as well as complying with Concrete Society TR66: External in-situ Concrete Paving, Britpave Concrete Hardstanding Design Handbook and ACI 330.2R-17: Guide for the Design and Construction of Concrete Site Paving for Industrial and Trucking Facilities.

Improved load transfer efficiency: Reaping the benefits

With aggregate interlock offering variable results, with an LTE of zero for larger panels used over a period of time, dowel systems are proven to offer far superior performance and greater value for customers. For specifiers and installers, projects costs and time on site can be reduced by the ease of installation. For end-users, the overall lifecycle costs of the solution are attractive, with fewer maintenance requirements, a longer-service life, superior aesthetics, and less likelihood of vehicle damage caused by uneven concrete yard slabs.
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