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CONCRETE BLOCK PAVEMENTS

CONCRETE BLOCK PAVEMENTS

Design - Execution

Due to the large increase in small-scale concrete products, it seemsnecessary to better publicise the main rules for good execution.

Recommendations for laying of concrete block pavements are alsoessential since in addition to the classic concrete blocks, most of the so-called decorative blocks can also be delivered in various sizes with theBENOR (Belgian quality certification) label. What is more, it should benoted that in the new Standard Specifications, the regulations withregard to road pavings have been expanded considerably.

The recommendations listed in this bulletin above all aim to provideclear information about the way in which concrete blocks can be laidproperly and without damage. In addition, this must occur easily,quickly and economically.

Before discussing the actual laying of the blocks, the primary designaspects will also briefly be recalled: after all, good execution of pavingstarts with a good design.

As far as the choice of concrete paving blocks is concerned, preferenceshould be given to concrete blocks that comply with the European andthe Belgian NBN B21 311 standard “Concrete paving blocks”. Thequality of the blocks is after all guaranteed best if they bear theBENOR label.

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DESIGN

The structure of concrete block pavements usually looks as shown infig. 1.

The behaviour of the pavement is in the first place determined by thequality of the subbase.

On week subgrades, a capping layer must also be laid.

A kerb is always needed with block pavements, on the one hand tolock in the stones and on the other hand to ensure that the material ofthe laying course does not wash away.

The laying course is typical for block pavements: it is applied to thesubbase as a profile layer and serves as a bedding for the concreteblocks.

The joints are not filled until the concrete blocks have beencompacted.

The adjacent table gives a number of examples of recommendedstandard cross-sections in function of the type of traffic expected (sizeand frequency of traffic loadings) and the bearing capacity of thesubgrade. Blocks with a thickness smaller than 80 mm are usuallyreserved for pedestrian pavements.

Interlocking blocks (type B, cf. standard NBN B21 311) are better ableto cushion the stresses from breaking and swerving cars (and lorries).

It is best to choose the type of interlocking block in consultation withthe manufacturer (permissible deviations in size, laying pattern orbond to be used, availability of accessories, possibility of makingbends, etc.)

Designing concrete block pavements also includes correctly detailingkerbs, gutters, special spots, speed bumps and ramps, etc. (fig. 2 ). Thequick drainage of surface water is also an important criterion. Thisapplies not least to pavements laid on an impervious subbase of leanconcrete.

The most commonly used bonds are shown in fig. 4. Chevron,herringbone and elbow bonds are less subject to deformation as aresult of the traffic. They also cause less rolling noise than the stretcherbond.

Note:In case of exceptionally high loads, such as on container storageareas and airport pavements, and for porous pavements (that is tosay in which the surface water can seep away directly through andbetween the blocks into the subgrade) in principle other standardcross sections and details apply than are shown here

PREPARATION OF THE WORK

Subgrade

Good drainage of the subgrade (drainage of rainwater andgroundwater) benefits the behaviour of the pavement. Beforeapplying the subbase or capping layer, the subgrade must always befree of water and compacted as well as possible.

Subbase

In order to avoid deformations, generally a cement-bound subbase isrecommended.

The execution of the subbase can usually occur in the same way as fora normal road subbase. However, sometimes lighter compactingmachines have to be taken into account.

When using a stony subbase, which is presently very topical,segregation of the materials must be avoided. Continuous grading andrelatively fine granulates (max. 32 mm or 20 mm) are recommended.

The subbase must have an even and a properly closed surface. It isabsolutely necessary:

• to remove any unevenness greater than 1.5 cm. Usually it is arequirement that unevenness measured with the 3m straightedgemay not exceed 1 cm;

• to provide the necessary transverse slope. For concrete blockpavements usually a transverse slope of at least 2.5 % is required.

Only in this way a laying course with a uniform thickness can beachieved.

Kerb

Practically speaking it is always necessary to place a kerb beforeapplying the laying course and often even before laying the subbase.

The permissible deviations in the size of the blocks delivered must betaken into account for this. After all, making a small adjustment in thewidth to be paved can mean that a lot of cutting and sawing isavoided...

LAYING COURSE

The behaviour of the block pavement depends to a significant degreeon the quality of the laying course. A laying course serves on the onehand to cushion the small unevenesses in the subbase and any slightdeviations in the thickness of the blocks, and on the other hand makesit possible to compact the blocks properly and keep them in place.

That is why the ideal laying course is a slightly elastic and stable layer.It is uniform in thickness, which after compaction is 3 to 4 cm thick. It isnot deformed by traffic, nor by fluctuations in the moisture content. Inaddition, it must be relatively porous for most applications, so that nowater can stagnate between the blocks and the laying course. So it caneven have a draining function.

In case of an impervious subbase of lean concrete, there is a dangerthat the water collects in the laying course. This can be avoided bymeans of a suitable drainage system (fig. 2c).

Materials for the laying course

Taking into account the above requirements, the following materialsqualify:

• For ordinary streets and squares with moderate traffic, wellgraded, sharp, coarse sand 0/4 can be used, that preferablycontains less than 3 % fine elements smaller than 0.080 mm.

• For road surfaces with relatively heavy traffic, such as roads inbuilt-up areas with through traffic, improved bedding sand isrecommended. During the past few years mixtures have beensuccessfully applied of about 40 % crushed sand 0/2 and 60 %crushed stones 2/7 or 2/4, whose total content of fine elementsshould also be limited to 3 %.

• To achieve greater stability of the laying course, cement-stabilizedsand is sometimes used - usually at least 100 kg cement per m3-amongst other things for the construction of speed bumps but alsofor bus stops. This means that the concrete blocks have to becompacted immediately on applying the laying course andcertainly before the mixture begins to harden. For heavy trafficthis cement-stabilized sand has to be made with coarse sand (0/4).

Applying of the laying course

If the laying course is applied on a stony subbase, then the top of thatsubbase must be properly closed, which can possibly be done bycompacting in fine material and/or washing it in. If not, this can latergive rise to unevenness in the pavement. To achieve a laying coursewith a uniform thickness of 3 to 4 cm after compacting, in general 4 to5 cm of non-compacted material must be spread out, depending onthe nature and moisture content of the materials.

The bedding sand itself is normally not compacted. When compactingthe blocks placed, indirect compacting is used. If blocks are placed on alaying course that has already been compacted, they can becomedamaged during compacting.

Sometimes a thicker laying course is desired, for example as a profilelayer on an existing subbase or pavement. When certain materials areused, such as a ternary mixture or cement-stabilized sand, the layingcourse is then partly compacted beforehand. Then a layer of 15 to 20mm of loose, non-compacted material has to be applied on top of thatto cushion the small permissible deviations in the thickness of theblocks.

However it should be noted that a thick laying course is not advisablefor pavements subjected to heavy traffic.

APPLYING AND FINISHING OF THEBLOCK PAVEMENT

This involves placing the concrete blocks next to each other on thelaying course and fixing them.

Placing of the concrete blocks

Thanks to their regular shape, concrete blocks can usually be placedtightly against each other on a profiled laying course.

In principle the blocks are laid in such a way that narrow joints ofabout 2 mm are obtained. The actual joint width is affected by theshape of the blocks and the bond to be made. If certain bonds withrigid joint lines are prescribed, then it is advisable to regularly checkthe alignment of the blocks with a string to make corrections wherenecessary.

It is advisable to use concrete blocks from different packages at thesame time, so that any differences in colour are less noticeable. Layingoccurs from on top of the blocks that have been placed in order toavoid disturbing the laying course. Normally, work starts at the edges,preferably with entire blocks. Openings are filled up with filling blockscut to size, that should really not be smaller than half a block (fig. 3b).

Mechanical placing of the blocks can increase the output and thusreduce the laying costs. Good agreements have to be made with themanufacturer with regard to the bond, joint widths, connections, etc.

Special blocks with an irregular shape are fixed one by one in thelaying course, using relatively wide joints.

In order to avoid puddles of water on the sides, the blocks must beplaced in such a way that after compacting they are still 5 to 10 mmhigher than the kerbs (or the gutter elements).

It is best to place around singular spots, such as inspection pits, one ortwo rows of stones, or to place them in high-quality micro-concrete(fig. 2h, 2i, 3c).

Special attention must be given to connections between concreteblock pavements and monolithic concrete or asphalt pavements (fig.2k, 2l).

Fixing of the concrete blocks and filling of the joints

When any openings have been filled, the blocks are fixed. This is donewith vibratory plate compactors with a rubber or plastic sole. In thisway the blocks are pressed into the bedding sand, whilst anyunevenness is eliminated.

Before starting with the compacting, the surface of the blocks and theplate compactor have to be cleaned. Compacting always occurs fromthe edge to the middle of the pavement. In addition, the compactingis kept at least 1 m from the zone where the blocks are still being laid.

The evenness is measured with a 3 m straightedge. Any unevenness ofmore than 5 mm is not accepted and has to be corrected by relayingthe blocks.

Differences in height between adjacent blocks may normally notexceed 2 mm. This applies especially to blocks without bevelled edges,for which even the smallest irregularity will be noticeable in skimminglight.

Any damaged blocks are immediately replaced.

When compacting has been carried out a number of times, fine, drysand 0/1 with less than 10 % fine elements smaller than 0.080 mm, orcrushed sand is swept into the joints.

For certain types of decorative paving, in which the joints can bereasonably wide (> 3 mm), the use of crushed sand 0/2 or a ternarysand mixture is recommended.

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Even wider joints (> 5 mm) are usually filled with cement mortars. Indry weather the surface must be kept moist for a few days, in order toguarantee that the joints are properly and durably filled.

Filling the joints with sand and compacting is repeated until the blocksare completely fixed. To facilitate filling the joints in wet weather, thesand is sometimes washed into them. If the pavement has to deal withfairly heavy traffic, a complementary compaction is useful. This occurswith a multityred roller weighing at least 10 tons, but not until all theaforementioned work has been carried out.

Finally, it is advisable to top up badly filled joints with sand a fewweeks after carrying out the work. It can also be useful to inspect thejoints and if necessary top them up at a later date.

Opening to traffic

The pavement may be driven on immediately after laying and properlyfixing the stones. If the laying course is made of cement-stabilizedsand, for example at speed bumps and zebra crossings, it is advisableto keep out heavy traffic for a few days.

Deicing salts should not be spread on coloured, relatively youngconcrete blocks that are laid in the autumn for at least the first 6weeks after placing them

COLOURED CONCRETE BLOCKS

Coloured concrete blocks are not only chosen for aesthetic reasons.The following functional considerations can play a role when choosingthe colours:

• The ordinary grey, preferably dark grey, blocks or blocks with arather neutral colour (perhaps with nuances), brown-red, heathercolour and black-red are the most suitable for carriageways.

• Raised crossings require striking colours, for example dark redcombined with black and white blocks for the marking.

• To avoid misunderstanding, (light) red blocks should primarily beused for cycle paths.

• Dark blocks in black concrete, perhaps made with blackgranulates, are very suitable for speed bumps, parking spaces andbus stops. Light coloured blocks are not advisable in these placeswhere there is a danger of oil stains. And darker elements are alsobetter used for ditches in which dirt can accumulate.

• Light colours (light grey, off-white, sand colour, yellow, ochre,light red ...) are above all suitable for paving pedestrian areas:footpaths, pedestrian streets and squares.

If additional requirements about colours are included in specifications,it is always useful to follow the guidelines of the manufacturers orsuppliers. Certain colours are usually indicated with colour charts orcolour codes. It is advisable not to let the work start until samples havebeen presented for approval.

When laying block pavements with coloured concrete blocks, of coursethe same basic rules apply as for normal blocks.

Using coloured concrete blocks

Due to small variations permitted in the raw materials andinterruptions or changes to the production, slight differences in colourare practically unavoidable. These differences will be more noticeablewith coloured blocks than with normal grey ones. So this has to betaken into account when large surfaces are laid with the same colour.Nuanced colours or combinations with darker blocks can offer asolution.

However, conversely small differences in tint fade in time. Asustainable decorative effect that is based on colour thus assumessufficiently strong contrasts. Variations in shape and bond can be usedto support the colour motifs. Interruptions in colour and shape of theblocks also make it possible to correct any slight bends.

Although the recommendations in this bulletin were drawn up withthe utmost care, faults cannot be ruled out. Neither FEBELCEM, northose who have worked on this publication can be held liable for anydamage that ensues from using the information offered.

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CONCRETE

BLOCK

PAVEM

ENTS

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picture: Guy Laurent Rijmenam

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Authors :Ir. arch. N. Naertin co-operation withIr. L. HendrickxIr. A. Jasienski

"Originally published in 1996 as technical sheet n° 8 of the FEBEL-CEM "DOSSIER CEMENT".

BIBLIOGRAPHY

HENDRIKX L.Recommendations for the pose of concrete paving blocksConcrete - Magazine of the Concrete Industry Association, nr. 122,1993, p. 69-77

SHACKEL B.Design and Construction of interlocking concrete block pavementsLondon and New York : Elsevier Applied Science, 1990, 229 pp.- Concrete block paving - Essential considerations- Concrete block paving - Installation- Concrete block paving - DetailingLeicester : Interpave - The Concrete Block Paving Association,Interpave Information Sheets, January 1991

Straatwerk vergeleken (Streetwork compared)Ede (The Netherlands) : C.R.O.W. Publication n° 78, 1993, 93 pp.

Beton-Bauteile für den Verkehrswegebau (Precast ConcreteElements for Road Construction)Bonn (Germany) : Informationsstelle Beton-Bauteile, [1994?], 120 pp.