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labsPRECAST CONCRETE

PAVING SLABSLarge rectangular concrete slabs (“two bythrees”) have been used widely in urban areas inSuffolk from the 19th Century onwards and assuch are a traditional feature of many of ourtowns and larger villages. The use of thismaterial is entirely appropriate in many locations,including sensitive areas. Many of those inexistence were made in Suffolk using locallysourced gravel or granite imported fromelsewhere. These slabs should be retainedwherever possible, relaid if necessary or usedelsewhere in the vicinity. Care should be takenin choosing second-hand slabs to ensure thatthey match others which exist in the area.

New concrete paving slabs are available in avariety of textures and colours. Thicknesses varyand some slabs are designed so that they can beoverridden by traffic. Sizes range from ‘smallelement’ to larger rectangular slabs. From avisual point of view rectangular slabs are almostalways preferable to square. The largest size slabthat is practicable should be used. This mayrequire mechanical laying which may increase thecost.

Concrete slabs are relatively inexpensive andmost slabs are now sourced from large nationalcompanies. Those from reputable manufacturerswill be produced using materials from certifiedsources.

Above:Old concrete paving slabs in Woodbridge

Right:New rectangular concrete paving slabs laid inBury St Edmunds

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labs Due to the range of products available there will

generally be slabs to suit all locations andrequirements. However, it is preferable to restrictchoices to a limited palette for visual andmaintenance reasons. Slabs which derive theirnatural colours from aggregates, for examplegravel, Yorkstone or granite, complement theSuffolk environment and are preferable to stronglycoloured products dependent on dyes.

The appearance of concrete slabs will alter overtime with weathering and this couldimprove or detract from their appearance. Thisshould be considered when designing a scheme.Although cheaper than natural stone, concreteslabs normally require replacing after a shorterperiod.

A wide range of concrete slabs are available, howevercare should be taken when choosing colours.Strongcoloured products will not be appropriate in manylocations.

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labsSlabs should be laid to an agreed pattern carefully

cut to fit the required space. Small ‘left over’spaces which necessitate the excessive use ofpieces of cut slab should be avoided.

Traditionally slabs are laid with the continuous lineof the joints at right angles to the kerb andbuilding line. Infilling using concrete paviors toavoid cutting is not acceptable as it results in a ‘zipfastener’ effect.

A well laid footway using concrete slabs with anexposed pink granite aggregate (Woodbridge)

Rows of slabs should not be laid parallel with thekerbline

Infilling using concrete paviors is no longer anacceptable practice

Small pieces of cut slab and infilling with concreteshould be avoided.

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1. Normally precast concrete paving is laidwith a staggered bond for visual andtechnical reasons. Other laying patternsmay be used, for example in morecontemporary environments.

2. Slabs should usually have slightlychamfered or rounded edges in order toavoid spalling.

Flags larger than 450mm x 450mm3. These flags should not normally be used in

areas that can be accessed by vehiculartraffic. If they are used then specialrequirements of unit thickness andbedding will need agreement with SCC.

4. The flags shall be laid on a 1:3 lime sandmortar bed. A method of working and

inspection should be agreed with SCC.90% of the flag shall be in contact with thesurface under the flag.

5. The flags shall be provided with widejoints and if the length exceeds 12m in anydirection, provision shall be made forthermal expansion. If work carried out inbetween June and September the lengthmay be extended to 20m.

Flags 450mm x 450mm or smaller6. These flags shall be laid on a sand laying

course.

7. Narrow sand filled joints are to be usedexcept where the flags are to bond in withlarger units. In this case the jointrequirements of the larger units are to beused.

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Cobble

sCOBBLESCobbles are rounded stones which are usedunworked in their natural (irregular) state and asthey reflect local geology, they vary significantly incolour.

In Suffolk, local flint cobbles are quite commonand were traditionally used to improve anunmetalled surface, or as a curtilage treatmentfor properties fronting a highway.

In the past, cobbles were taken from riverbeds,fields, beaches, or boulder clay deposits.Nowadays they are a by-product of the sand andgravel extraction industry where they arereferred to as ‘rejects’. Care should be takenwhen sourcing cobbles to ensure they are notbeing taken illegally from beaches or riverbedsbecause of the impact of their extraction on thelocal environment. Although the percentage ofcobbles in any deposit varies across the regionthere should be no difficulty sourcing locally,therefore reducing transport costs. In addition,secondhand cobbles are available.

Cobbles extracted as a by-product of gravel workings (Wangford)

Old cobbles in Needham Market

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Cobble

s Today, cobbles are mostly used to deterpedestrians from walking on certain areas astheir irregular surface makes it awkward to walkon or push prams or wheelchairs across.However, in suitable locations well laid cobblesprovide an attractive hard and resilient surfacewhich harmonises with the Suffolk environment.

Compared with some other forms of paving,cobbles are moderately priced. However, theyare time consuming and labour intensive to lay.Cobbles can be supplied ready graded accordingto size but some sorting will still be required onsite to ensure an appropriate laying pattern. It isimportant to ensure that contractors areexperienced in laying cobbles as the skillsrequired are not the same as those required forblock paving.

Cobbles must be laid butt jointed and pushedinto the wet concrete with their longest sidevertical so that a minimum of 75% of the lengthis below the finished level. The cobbles shouldbe selected and arranged so that they make upat least 75% of the total area to be covered. Thisis essential to avoid the impression of an area ofconcrete with a few stones added in arbitraryfashion.

Historic examples in Suffolk include NeedhamMarket High Street, where extensive areas ofcobbles form the ‘private’ forecourt of manyproperties. New cobbles have been laid where the historiccobbles have previously been removed.

Large cobbles can be used to discourage use bypedestrians

Well laid small cobbles

Traditionally cobbles were used to demarcateforecourts

Cobbles neatly laid between an historic building anda riven Yorkstone footway

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Examples of poorly laid cobbles

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Cobble

s Technical Information1. If the cobbles are to be laid in a small area

(less than 400 m2) then the concretesupport should have a minimum overalldepth of concrete of 200mm.

2. If the area exceeds 400m2 then a specialistdesign will be required.

Concrete3. In order to ensure full closure of the mix

around the cobbles the maximumnominal aggregate size in the concrete willbe 8mm

4. The consistency of the concrete shall bechosen to allow the cobbles to behandplaced and packed without sinkingand to allow full closure around the sideof the cobbles.

5. Full details of the concrete specificationare included in Appendix B.

Cobbles6. In Suffolk flint cobbles are preferred as they

are a local material.

7. Cobbles shall be clean and from areputable quarry source.

8. They should generally be of rounded orsub-angular shape and of 63mm/90mmnominal size. Full details of cobble sizewithin Appendix B.

Laying9. If subgrade is permeable it should be

sprayed with bitumen road emulsion orcovered with polythene sheeting.

10. Bottom crack inducers should be providedat 3m centres.

11. Cobbles are to be pushed into theconcrete with their longest axis vertical.

12. At least 75% of the cobble should be belowthe level of the finished concrete.

13. The cobbles should be arranged such thatthe cobbles cover at least 75% of the areabeing surfaced.

14. The surface between the cobbles shouldbe finished to allow the free drainage ofsurface water and be within 15mm of thedesigned level

15. The highest point of any cobble shall bewithin 20mm of the designed level.

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tsSTONE SETTSStone setts are a traditional material, used inSuffolk since the 18th Century. Setts were used topave surfaces which took some form of vehiculartraffic; they were also used for gutters andaccesses across footways.

Old stone setts vary greatly in terms of texture,size and colour, depending on their source. InSuffolk granite, Yorkstone and limestone setts areall found, but they are used much more sparinglythan in other parts of the Country where thestone can be quarried locally. Existing stone settsshould be retained insitu or, if that’s not possible,relaid on site or reused in the same vicinity. Old granite setts in Ipswich

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Cropped pink granite

Butt jointed limestone

B Different coloured rectangular setts with mortarjoints

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tsNew setts are readily available from quarries inthe UK and abroad. Transport and handling costsare high due to their weight. Setts from countriessuch as Portugal and China are often cheaper thantheir UK equivalent but sustainability issues shouldbe considered before setts are ordered fromabroad.

New stone setts are available in a variety offinishes both rough and smooth. Sawn setts maybe tumbled to give them a weathered look.

The stone type and surface finish should beappropriate for the location and its use.

Laying setts often requires on-site sorting, and islabour intensive and therefore expensive butonce laid, very little maintenance should berequired.

Care should be taken to ensure that wherevehicle turning movements occur the size of thesetts and the specification for laying them issufficient to prevent the setts working loose.

Tumbled Yorkstone setts (Eye)

Granite setts used in a parking bay(Bury St Edmunds)

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Smooth pink granite

Cropped, silver grey granite cubes

Butt jointed tumbled Yorkstone setts

Rectangular cropped dark grey granite

New stone setts

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ts Technical Information1. Frost resistant material will be required for

all bedding materials within 300mm of thefinished surface on sand and gravelsubgrades; 350mm on clay; and 400mmon chalk.

2. If reclaimed setts are used they should befrom a reputable source and suitablycleaned prior to laying.

3. Granite setts are to be laid on 200mm ofsubbase.

4. Setts are to be bedded into a minimumthickness of 70mm of fine concrete.

5. Joints between setts should not exceed20mm

6. Careful selection of setts is required priorto laying to give uniform coursing andjoints.

7. Joints should be finished with smoothpointed cement/sand mortar (4:1) toapproximately 10mm below sett surface toallow water runoff below the runningsurface.

8. All mortar staining shall be cleaned fromsetts on completion to give a cleanpolished finish.

9. Sett paving shall be fully cured beforetrafficking.

10. Yorkstone setts shall be sawn or tumbledsawn yorkstone.

11. Limestone setts shall not be used intrafficked areas due to their low skidresistance.

Bedding and joint filling12. As an alternative to sand, crushed and

ground glass can be used for bedding andjoint filling if it has the same characteristicsas that of the specified sand.

Laying13. Rigid laying systems to BS7533 Part 7 shall

only be used where the pavement isdesigned to carry greater than a millionstandard axles.

14. A membrane to restrict migration of sandshall be required where a flexible layingtechnique is used over an existing crackedsurface such as a road..

15. Cement or hydraulic binder shall not beused in any materials for the laying coursesor joint filling in flexible systems.