Interlocking Concrete Block Pavements: Design, Construction and stability

Introduction

Historically, interlocking concrete block paving technique originated from small-element surfacing which involved stone setts, wooden blocks, and bricks used for early city pavements in some parts of Europe in the nineteenth and twentieth centuries.

Today, interlocking concrete block pavement is extensive and covers the following:

  • Paving of container terminals at ports and harbors
  • Paving of courtyard
  • paving of driveways
  • Paving of parking areas
  • Paving of pedestrian Walkways
  • Paving of large public precincts
  • Construction of speed humps to calm traffic
  • Paving surfaces subjected to oil spillage

The blocks offer Easy maintenance access to underground utility Lines.

Structurally, concrete block pavements are similar to typical flexible pavements which either the whole or part of the base course replaced by a block layer underlain by a thin layer of sand.

Load spreading is achieved by interlocking of the blocks and is improved significantly when the underlying layer allows a degree of “bedding-in” to take place.

As more traffic uses a newly-constructed block pavement there is a progressive settlement which leads to full mobilization of block interlock or block “lock-up” thus allowing the block units to act as a coherent layer.

block pavement structure
Block pavement structure

Despite this initial non-elastic behavior, it is agreed universally that concrete block pavements behave as flexible pavement rather than rigid pavements and are designed as such despite being composed of an array of interlocking rigid elements.

Design of interlocking concrete block pavements

Block pavements just like any flexible pavement should be designed to resist excessive rut deformation or structural failure from the cumulative damaging effect of the vehicles using it.

A widely-used approach to design is to determine the equivalent thickness of conventional paving material that would provide an equivalent performance as the block in sand Layer.  

The equivalent thickness relations are a convenient method of adapting existing flexible pavement design methods to include block pavements.

The pavement is initially designed using the existing flexible pavement design method.

The block surface and sand bedding layer are then converted to an equivalent thickness of a conventional flexible pavement material.

The required thickness of payments under the blocks is simply given by the difference between the conventional design thickness and the block surface’s equivalent thickness.

Several design charts suited to particular conditions exist. In the absence of rigorous design a simple approach is to replace 165 mm of base course thickness with a bedding sand layer and 80 mm block Surfacing.

Block characteristics

Concrete paving blocks may be manufactured to a thickness of 60, 75, 80, etc., the 80 mm size being the most extensively used. There is no limit to the shapes to which blocks may be manufactured.

Block shape may vary from rectangular to hexagonal, dentated, and other proprietary shapes. The image below provides some example blocks shapes.

Some examples of concrete block shapes
Some examples of block shapes

It is thought that shaped blocks tend to perform better than rectangular blocks as the former have the ability to mobilize wedge action and geometric interlock on all their sides.

Construction

  1. Bedding sand

Concrete block pavements must be constructed on a layer of bedding sand. The Sand layer does not contribute to the structural capacity of the block pavement but serves the following purposes:

  • It serves as a regulating layer to provide an even surface on which to lay the blocks
  • It accommodates the manufacturing tolerance on block height.
  • It provides a uniform layer and acts as a stress attenuating Layer (cushion) preventing stress concentrations which could damage the blocks.

Current design trend is towards using 20-25mm compacted thickness of sand, a thicker sand layer only results in greater surface deflection.

It is believed that the performance of the bedding sand is related to the shear strength of the sand measured in terms of the angle of Shearing resistance (angle of friction, φ). Sands that exhibit higher values of φ tend to rut less.

Sands having maximum particle size of 5 mm and not more than 3% of fraction less than 75μm are recommended.

For heavy channelized traffic, it is recommended to use sand devoid of fractions less than 75μm in size. Sands that are sharp rather than rounded have been found to lead to better pavement performance.

2. Laying and compaction

Blocks are laid with a joint spacing of 2-3mm and to any several possible patterns, namely, stretcher, herringbone, basketweave, Chevron, etc.
The herringbone pattern has been found to result in the highest block stability. Using pigmented (colored) blocks it is possible to lay the blocks to any desired patterns to improve environmental aesthetics, especially in large public precincts.

After laying, the blocks are compacted with a vibrating plate compactor to seat them properly in the bedding sand and in the process force some of the bedding sand to migrate upwards to partially fill the joint spaces between, thus creating an initial interlock between the blocks.

Block Laying patterns
Block laying patterns: A-Herringbone, B-Stretcher, C-Chevron

3. Jointing Sand 

Sand is swept into the joints between the blocks from the surface of the pavement in order to fill the joints.

It is the presence of the jointing sand that ensure vertical and horizontal interlock and makes it possible for the blocks to act as a coherent layer rather than as individual units. 

The jointing sand and deposit of detritus in the joints as the pavement is used add some degree of imperviousness to the surfacing which is necessary to prevent the bedding sand from being gradually washed away bt infiltrating water.

Stability

The blocks must be stable and remain in their placement positions when subjected to vertical and horizontal loads.

Stability is achieved mainly by interlock developed through friction, wedge action, and the geometric arrangements of the block so that a load applied to a block is supported by the block, its immediate neighbours, and to some extent its distant neighbours. 

This way, the block paving is mobilized to behave “flexibly” and to act as a continuous mat to provide load dissipation.

Altogether, there are four ways by which paving concrete block pavements achieve stability.

  1. Edge restraint

This is obtained by constructing at the edge of the pavement some form of solid firm structure such as a curb to prevent the blocks and bedding sand from migrating outwards at the edges.

The blocks must be laid o abut (touch) the firm structure, or where there is space between a block and the structure because of geometric limitations, the space must be filled with lean concrete.

2. Vertical interlock

This is the restraint to vertical (usually downward) movement of vertically loaded blocks due to the friction created by jointing sand between blocks.

3. Horizontal interlock 

This is achieved through friction between adjacent blocks as a result of the presence of the jointing sand the wedge between adjacent blocks, especially for shaped blocks.

4. Geometrical interlock

This is the restraint to vertical and horizontal movements developed as a result of the geometry (shape) of the block and the manner in which the blocks have been laid. Shaped blocks provide a higher degree of geometric interlock and wedge action unshaped blocks.

Examples of possible restraint to block movement
Examples of possible restraints to block movements

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