Force Direction and Interlock

The Principle of Interlock:
Interlock is the inability of a paver to move independently from its neighbors. It is critical to the structural performance of interlocking concrete pavement. When considering design and construction, three types of interlock must be achieved: vertical, rotational, and horizontal interlock. These are illustrated in Figure 1.

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Vertical interlock is achieved by the shear transfer of loads to surrounding units through sand in the joints. Rotational interlock is maintained by the pavers being of sufficient thickness, meeting recommended plan and aspect ratios, placed closely together, and restrained by a curb from lateral forces of vehicle tires. Rotational interlock can be further enhanced if there is a slight crown to the pavement cross section. Besides facilitating drainage, the crown enables the pavement surface to stiffen and further lock up as the pavement undergoes vehicle loading due to traffic.

Figure 1. Types of interlock: horizontal, vertical, rotational [CLICK TO ENLARGE]

Horizontal interlock is primarily achieved through the use of laying patterns that disperse forces from braking, turning and accelerating vehicles. Herringbone patterns are the most effective laying patterns for maintaining interlock.

Testing has shown that these patterns offer greater structural capacity and resistance to lateral movement than other laying patterns (Shackel 1979 and 1980). Therefore, herringbone patterns are recommended in areas subject to vehicular traffic. Stable edge restraints such as curbs are essential. They provide better horizontal interlock among the units while they are subject to repeated lateral loads from vehicle tires.

Test your Knowledge...

• ICPI Tech Spec #4
• Shackel 1979. Shackel, B., “A Pilot Study of the Performance of Block Paving Under Traffic Using a Heavy  Vehicle Simulator,” Proceedings of a Symposium on Precast Concrete Paving Block, Johannesburg, South Africa.
• Shackel 1980. Shackel, B., “An Experimental Investigation of the Roles of the Bedding and Joint Sand in the Performance of Interlocking Concrete Block Pavements,” Concrete/Beton, No. 19.
• Shackel 1980. Shackel, B. “Loading and Accelerated Trafficking Tests on Three Prototype Heavy-Duty Industrial Block Pavements,” National Institute for Transport and Road Research, CSIR, Pretoria, South Africa, Technical Report 12.