24
14.1.1 Types of structure suitable for slipform construction After a hundred years of operations, slipform is still the fastest method of construction for designated vertical structures, whether the system is operated on a 24-hour basis or on a dayshift-only basis, yet there are many in the construction industry today who have never had the opportunity to become involved with this specialized system. The technique has been used to create some of the world’s tallest and largest structures ever built in reinforced concrete, including: Straight and tapering chimneys Observation towers

slip formwork

Embed Size (px)

Citation preview

Page 1: slip formwork

14.1.1 Types of structure suitable for slipform construction

After a hundred years of operations, slipform is still the fastest method of construction fordesignated vertical structures, whether the system is operated on a 24-hour basis or on adayshift-only basis, yet there are many in the construction industry today who have neverhad the opportunity to become involved with this specialized system.

The technique has been used to create some of the world’s tallest and largest structuresever built in reinforced concrete, including:

• Straight and tapering chimneys• Observation towers

Page 2: slip formwork

14/2 Slipform

• Concrete gravity structures (oil platforms)• Bridge pylons and piers• Mine-shaft headgear towers

It has also been used to build innumerable basic structures, such as:

• Silos• Linings to shafts below ground• Surge shafts• Liquid containment vessels• Service cores for commercial buildings• Lift and stair shafts

Typical structures, suitable for slipform construction, are usually over 25 m in height,for economical reasons, stable in the free-standing condition, or are able to be stabilizedwith temporary or permanent bracing, and benefit from a very short construction period.The economical qualifying height for each structure may be less if repetition is involved,which may be the case for bridge piers or caissons.

Slipform construction is an essential method for constructing concrete silos and chimneysover, say, 4 m in diameter, quickly and economically, with other structures needing to beassessed for plan configuration and height. Tapering structures with wall reductions,either gradual, over a short height, or stepped, can be accommodated. However, it isadvisable to seek the opinion of specialist contractors at an early stage in the proceedingsto establish the suitability of structures for slipforming.

14.1.2 Standard equipment

The equipment generally used for slipform is similar in format to that used for normalwall formwork, consisting of face panels backed by walings and supported by strongbacks,for either single- or double-face ‘shutters’. However, the height of the face panels isusually limited to 1.2 m, and the strong backs are formed into yokes straddling the wallwhich are mechanically lifted by the jacks. These jacks are attached to the crossbars ofthe ‘H’-shaped yokes which are in turn supported by rods or tubes positioned within thewall or structure. The tubes are supported from the foundation and are laterally ‘braced’by the formed concrete during the slipform operations, by the frames in openings or bytemporary columns formed specifically for the purpose.

The early designs consisted of simple two-deck systems with timber face panels andwalings. The yokes were also constructed from timber and required low crossbars to caterfor the induced stresses. The slipforms were raised manually by screw jacks, all necessitatinglabour-intensive operations. These systems prevailed until after the Second World War,when the lifting systems were replaced with synchronized hydraulic circuits and hydraulicjacks. The later development of stronger steel yoke frames allowed the critical distancesunder the cross-bars to be increased substantially, making reinforcement fixing and generaloperations far easier.

Two main types of system have evolved over the years,

• A lighter two-deck system, generally utilizing 3 tonne capacity hydraulic jacks andclimbing on 25 mm diameter rods (Figure 14.1)

Page 3: slip formwork

Slipform 14/3

Figure 14.1 Typical section through light-duty two-deck slipform.

Page 4: slip formwork

14/4 Slipform

• A heavy-duty three-deck system, using 6 tonne capacity jacks and climbing on 48 mmdiameter tubes (Figure 14.2).

The former lightweight system is only suitable for the construction of small simplestructures, whereas the heavy-duty system is suitable for more complex structures andprovides numerous advantages, including:

• Greater safety.• Additional storage• Weather protection• Greater accuracy• Good access• Easier distribution• Wider spacing between lifting positions• Better and easier support for ancillary services

The systems are composed of standard components, resembling Meccano sets, thatcan be built to various plan shapes and re-used many times for maximum economy. Steelhas replaced timber for all the major components providing better strength and durability.Only the decks are made up in either softwood or plywood for ease of shaping and fixing,although in certain circumstances even these areas can be arranged with standard panels,composed of metal decking or similar.

Complex plan shapes can be built using modular panels and standard slipform equipment,only occasionally requiring specially fabricated items to complete the arrangements, butat the same time allowing economical re-use for the majority of the component parts.

These systems, employed on non-tapering projects, are assembled at the commencementlevel, usually a base or foundation structure, and are plan-braced before commencementto maintain the correct profile throughout the operations. Minor changes, such as wallreductions, can easily be accomplished during slipforming. The introduction of additionalwalls at higher levels or the termination of walls before completion of the structure canalso be accommodated, albeit with certain restrictions.

14.1.3 Tapering equipment

The heavy-duty systems referred to earlier have been adapted to cater for tapering structuresand are significantly different from the standard arrangements for uniform sections. Thelifting unit of the slipform operates in a similar manner for both systems. The formworksections between the lifting units are rigid for non-tapering work, whereas they need tocater for reductions in wall length, as the tapering takes effect. This is achieved withoverlapping panel arrangements, usually positioned between each lifting point, whichtake up the variation in each section. Large reductions may necessitate the removal offormwork panels as the slide progresses or even the removal of certain lifting units. Astop to the sliding process may be necessary for this purpose.

For circular tapering structures, a system of radial beams or a special truss arrangementallow the lifting positions to be traversed towards the centre, effectively reducing thediameter. Wall reductions are brought about by ‘squeezing’ either the inside shutter, or theoutside shutter, or both towards the centre of the wall. A force is applied, either mechanicallyor hydraulically, to move the shutter, on either face, away from the yoke leg, whilst still

Page 5: slip formwork

Slipform 14/5

Figure 14.2 Typical section through heavy-duty three-deck slipform.

Page 6: slip formwork

14/6 Slipform

supporting the main sections of walings. Alternatively, the positions of the vertical membersof the yoke frame are moved towards the centre of each wall achieving the same effect.All incremental adjustments are absolutely critical, need to be fully synchronized andneed to maintain the correct degree of taper on the shutter, as referred to in the nextsection.

H-shaped, rectangular, or hollow structures may also be tapered, but in every case thetapering operations must maintain equilibrium, or gain independent support to remain online. The reductions must be carefully monitored and this is normally achieved by makingthe incremental reductions against fixed, accurate scales.

Tapering structures are usually in excess of 80 m high and consist of observationtowers, chimneys, bridge pylons and piers. They reduce for aesthetic reasons as well asfor design consideration.

14.2 Design of the slipform

It is not proposed to detail design calculations for slipform rigs in this chapter. The designof slipform shutters has mainly been empirical. Operating the slipform produces a tide ofchanging forces, but only when the operations are poorly carried out or the concrete mixis poorly designed do the forces become unacceptable. Filling the forms with concrete isa slow process and does not produce the normal pattern of pressures associated withstandard concrete pressure diagrams. Initially, the shutter is propped at the base, changingthe yoke legs into propped cantilevers. As soon as the slipform is lifted off the ground, theyoke legs revert to their normal operating situation.

The main concern is the friction at the interface of the concrete and the shutter and thatany induced forces that may occur do not become excessive. Other main considerationsare the adequacy of the decks to support the live loadings, and the combination of theloads, plus any frictional loads that may overpower the jacks. This is one more reason forusing a heavy-duty system that is better able to cope with a combination of loadings.However, it is reassuring to note that slipforming is a failsafe method of operations. It isimpossible for the slipform to fall as it would only attach itself to the structure, irrevocablyas far as the operation of the jacks is concerned, should this ever happen.

14.2.1 Standard system operations

The jack capacities have evolved from the strength of the rods or tubes used with thesystems – 3 tonne for 25 mm rods or 6 tonne for 48 mm tubes – and the style of slipformrig is related to the inherent carrying capacity. Jacks are generally composed of lower andupper clamp heads supported by twin or annular rams. The rams open and close hydraulicallyto climb the jack up the support and lift the slipform. The lifting units are usuallycontrolled by one hydraulic circuit, enabling the system to lift in increments according tothe ‘set’ of the rams.

The tubes or rods are generally withdrawn and re-used for economical reasons. Toachieve this, sleeves extending slightly below the shutter and hanging from the crossbarsare drawn through the concrete as the system climbs, leaving circular voids for the tubes

Page 7: slip formwork

Slipform 14/7

to sit in. This allows them to be withdrawn upon completion or at intervals for highstructures, without adhesion to the concrete structure.

Positioning of the lifting units is critical for the correct operation of the slipformsystem. Whereas the lighter systems are limited and the jack positions are usually placedat 1.8 m centres maximum along a wall, the heavy-duty system allows the centres of thejacking units to be safely increased to 2.5 m, making steelfixing and other activities somuch easier. Many factors influence the design of the jack layout and retribution wouldbe swift for an ill-conceived arrangement.

The shutters are assembled to the plan layout, preferably without the aid, or otherwise,of kickers. The yoke frames are installed, followed by the working decks, before thehydraulic circuits and jacking positions are completed. The electrical supply and thecircuits for the lighting, poker vibrators and power tools, but not least, the power units forlifting the slipform also have to be installed.

When the system is complete and ready to operate, the shutter is filled with concrete,in layers, over several hours and in accordance with the setting time of the concrete, sothat once the climbing begins, it becomes a continuous operation. As the climbing commencesthe inspection platforms can be completed.

The placing of the concrete is a systematic layering system based on maintaining theinitial set of the concrete at the mid-position of the shutter. A critical operation beforecommencing the concreting is to set the ‘taper’ on the shutter. This involves accuratelyadjusting the verticality of the shutter, relative to the line of the wall, so that the formpanels lean in at the top and lean out at the base position to provide a ‘batter’. This hasthe effect of reducing the wall width at the top and increasing the width at the bottom ofthe shutter by a few millimetres and the wall is formed halfway down at the true thickness.

The success of a slipform relies upon several factors including a well-assembled rig,good planning to match plant and material requirements, good discipline throughout theexecution of the operations, but more importantly, the design and provision of the correctconcrete mix.

14.3 Concrete mix

14.3.1 Performance and development

The taper on the shutter, referred to in section 14.1.3, is the key to the slipform technique.Concrete is systematically placed in the forms in layers of approximately 150–200 mmdeep and immediately vibrated. The concrete slumps and starts to form the shape of thestructure, half-way down the shutter, as the initial stiffening commences. The concreteforms as it separates from the face panels due to the taper on the shutter. It is thereforeessential that the initial stiffening occurs in the centre of the shutter, to form the wall atthe correct thickness.

The concrete emerging at the base of the shutter, for a regular slipform, is often only4 or 5 hours old and in a green condition.

Traditional construction requires that the concrete is sufficiently workable to ensurefull compaction and that it meets the minimum strength requirement at a given age.Slipform construction, however, not only demands that the concrete remains workableduring distribution and placement to ensure full compaction, but it must be cohesive to

Page 8: slip formwork

14/8 Slipform

avoid segregation or bleeding. It must remain plastic in the top half of the shutter with acapability of internal mobility plus external deformation without suffering planes ofweakness or actual fracture. Furthermore, it must also provide a laitence for the lubricationof the forms at the interface to prevent anything other than nominal friction.

Until the advent of pumping, there was always an air of mystique surrounding slipformconcrete design, but today a standard ‘pump mix’ often requires very little adjustment toprovide a suitable slipform mix, as they both have to meet similar criteria, with theexception of the setting times.

A typical slipform mix, therefore, requires a higher volume of fine material than wouldnormally be required for maximum cement economy with the effect of providing a matrixin which coarse aggregate can rotate, whilst also providing a zone of low shear strengthat the shutter face. It must also retain the correct amount of water for the hydrationprocess to be completed, without any tendency to bleed. This is particularly importantconsidering the method of vibration that is adopted for slipforming.

The speeds of the slipforms in the early years were purely related to the performanceof the cements, aided and abetted by the ambient temperatures and the amount of wateradded to the mix. OPC in the UK, for instance, provided a concrete mix that wouldnormally attain slipforming casting rates of between 150 mm/h and 300 mm/h. The watercontent was the sole ‘accelerator’ or ‘retarder’ for the mix concerned. The casting rate andthe minor control that was available fortunately matched the outputs of the workforce andthe available plant.

The 1960s and 1970s provided a more adventurous era, when larger, more complexstructures were undertaken by slipform construction. Typical examples that were constructedby slipform were the Toronto CN Tower, a tapering structure 350 m high, the ‘NatWest’building in London, a complex plan arrangement with a height of 190 m, and oil platformsof large diametric proportions. The oil platforms were slipformed in shallow coastalwaters in one operation, to provide flotation, whilst maintaining an even keel.

This period also saw an increase in the use of admixtures. Retarders were mainly usedfor slipforming, allowing the speeds to be adjusted to match the distribution and placementof materials. With large floating structures the speed of sliding was reduced, in someinstances, to as low as 75 mm per hour, which necessitated careful monitoring of theconcrete for successful results.

New materials, particularly cement substitutes, were also developed and later incorporatedinto slipforming operations, providing long-term advantages for the concrete and somedisadvantages for the slipforming operations. PFA, with its low heat generation and slowgain in strength, produced varying results with the initial set of the concrete and provedeven more unpredictable when combined with plasticizers to meet the lower w/c ratiosdemanded during the 1980s and 1990s. Although ggbs is a little more predictable, it isnow usual to limit the cement substitution for slipforming, whenever possible, to 30 percent of total cementitious content in order to achieve some consistency in the settingtimes.

PFA and ggbs produce good finishes due to the migration of the fines to the surfaceduring vibration. Any other fillers, such as kaolins, if approved for use, have the sameeffect.

Microsilica is an extremely fine pozzolan which is used to improve the properties ofconcrete. Unfortunately, it has a very resinous effect and does not assist the slipformmethod of operations. Its use, therefore, must be carefully monitored, if specified, and it

Page 9: slip formwork

Slipform 14/9

is advisable to limit the quantity used to 5 per cent of the cementitious weight. Like allpozzolans it reduces the heat of hydration and slows down the strength development inthe early stages.

14.3.2 Mix design

The concrete mix for slipform should be designed to comply with the contract specificationand the relevant standards, that would be expected of any normal mix. As referred toearlier, an easy reference would be akin to a ‘pump mix’ based on a 75 mm or 100 mmslump, depending on the likely ambient temperatures to be encountered.

The initial stiffening of the concrete should occur at a given time after batching, toallow for delivery, distribution, and the time taken to slide to the half-way position in theforms – for a single silo, say 18 m diameter, with normal site conditions, 3.5 hours wouldbe anticipated.

14.3.3 Cement

The cement and aggregate selection may be determined by regions, but must comply withthe job specifications. The cement used will also have a bearing on the possible use andtype of admixtures.

It is usual practice to use solely OPC, or OPC combined with either pfa or ggbs. Onlyoccasionally is sulphate-resisting cement specified as this has been largely superseded bythe use of pozzalans. The setting time of the cement, given on the certificate, is a guideas to how the mix may react, but the standard paste is very different from the cementwhen combined with the rest of the materials. Other important factors affecting theperformance of the mix are:

• The cement temperature• The 24-hour strength• Ambient temperatures.

14.3.4 Aggregates

Coarse aggregateCoarse aggregates should be considered for the overall design of the concrete mix and notfor the slipform requirements. Rounded aggregates are certainly not a prerequisite, asmany successful slides have been carried out using sea-dredged materials or crushedrock. The normal size for the aggregate used is 5–20 mm. Occasionally, the size has beenlimited to 10 mm for areas that are very congested with reinforcement, and in specialcircumstances lightweight aggregate has been used, although this application requires theaddition of sand to ensure compatibility with slipforming. A structure that is suitable forslipforming is unlikely to require aggregate in excess of 20 mm.

Fine aggregateThe success of the slipform mix revolves around the combination of the fine aggregates

Page 10: slip formwork

14/10 Slipform

and the cement. Of the total aggregate (fine + coarse) a good slipform mix should requirebetween 38 per cent and 48 per cent of fine material depending on the other parameters.The fine aggregate grading should meet the medium category, as too coarse a gradingwould permit bleeding, whilst too fine a grading would retain too much water for too longa period and cause slumping. For the fine aggregate grading the sieve analysis shouldindicate around 50 per cent passing the 600 micron sieve with a good distribution throughoutthe lower banding, to provide the correct mass for retaining the water, to ensure goodworkability, a good finish and complete hydration. Fine aggregate achieved from crushedrock would probably be too coarse and would have to be complemented with a dune sandor similar fine material. If cement substitutes are chosen, the higher proportion ofcementitious material will often compensate for a dearth of fines.

14.3.5 Admixtures

The development of high-strength concretes has brought about the prolific use of plasticizingadmixtures to control the w/c ratio, increase workability and maintain high strengths. Thestrength of concrete used in slipforms has increased considerably in recent years to meetthe demands of the designers. Normal core design now generally requires concrete strengthsof C35 to C40 and civil engineering projects often call for strengths ranging up to C80 orgreater.

The plasticizer often has a retarding action particularly when overdosed, and whenused in conjunction with pozzolans. However, pozzolans are often required as partialcement replacement for large projects to lower the heat of hydration and improve durability.In some of these cases a plasticizer still needs to be used in conjunction with a retardingagent. The recent use of plasticizers formed from polymer systems has improved theperformance of concrete with cement replacement and reduced the variability with regardto setting times. As in all cases with admixtures used under such stringent conditions, trialmixes are necessary to establish addition rates and setting times. Other admixtures, suchas air entrainment are rarely used or required for vertical slipforming.

14.3.6 Distribution on the slipform rig

The concrete is placed in layers, systematically around the slipform, and distribution canbe achieved in many ways. The plant chosen for servicing the slipform combined with thelayout of the structure will often dictate the methods to be employed. Logistics will oftenaffect the thinking as well as the labour rates prevailing in the area of operations.

For medium-sized cores or circular structures requiring between 4 and 8 cubic metresper hour, concrete could be delivered towards the walls, by chute, directly from the crane-skip with a small clearing-up operation required afterwards. Alternatively, the concretecould be deposited on the working deck, by skip or pump, with the labour force beingresponsible for final distribution, by shovel to the allotted areas (Figure 14.3). Thissystem would cater for in excess of 6 cubic metres per hour. A rate of over 6 metres perhour would allow distribution to the walls to be carried out directly by pump. This methodis often ideally suited for circular structures, providing the pump boom is able to reach.

For silos or cores serviced by mobile crane, the provision of one or two wet hoppers,

Page 11: slip formwork

Slipform 14/11

Figure 14.3 Typical section through heavy-duty three-deck slipform.

Top deck

Working deck

Hanging scaffold

Page 12: slip formwork

14/12 Slipform

strategically placed, would allow final distribution to be achieved by wheelbarrow. Thisis an old-style method of distribution that is still proving useful.

For any method chosen, it is still necessary to clean up after each operation to avoidold concrete being placed with the next fresh layer.

14.3.7 Vibration

Vibration is a major contribution to the success of a slipformed structure, as it affects thefinish, the strength and the curing. The placement of concrete for a slipformed structureis very different from the method associated with normal pours. To reiterate, the concreteis placed systematically in layers, with each layer formed from a similarly aged batch.Each layer should be placed and immediately vibrated by immersing the poker vibratorat approximately 600 mm centres for several seconds, purely to compact the concrete.The vibrator must not be used for placing the concrete, or penetrating too far beyond thesurface of the proceeding layer. Prolonged vibration will cause the concrete to act as aliquid and will gradually affect the lower levels, where the process of setting has started.A well-designed mix should prove easy to place and should compact relatively easy.

14.3.8 Curing

Curing of concrete is a process designed to provide an environment that will allow thehydration of the cement to be completed satisfactorily, and is affected by the local climatesand seasons. A slipform operation demands a cohesive concrete mix and thus a greaterdegree of fine material than that required for normal design. It therefore has a betterability to retain the water required for the total period of hydration.

The specification generally demands a free w/c ratio of between 0.4 and 0.6, whichsatisfies the strength requirement and also provides sufficient water for the hydrationprocess under controlled conditions. Due to the nature of the vertical slipform process, itis not practicable to ‘flood’ the structure or to cover it in wet hessian, polythene or similar,so as the concrete emerges from the forms it is, therefore, necessary to protect the outerface to prevent water loss for a prescribed period until the ambient humidity is sufficientto provide the moisture required.

Shrouding the slipform rig, from the top deck to the underside of the hanging scaffold,will provide a natural curing zone immediately around the shutter and the exposed element,maintaining a high humidity, and preventing shrinkage cracks. It will protect the surfacefrom the direct sunlight and wind at a critical period, or in cold climates it will alsoprovide an insulation barrier to maintain a reasonable temperature gradient. It has adistinct advantage over traditional construction, as the protection is in place beforehandand there is no time lag, allowing the concrete to be exposed to the elements at a vulnerabletime.

Extremes of climate must be given careful consideration and examples of such detailedplanning have allowed slipforms to be carried out successfully in temperatures rangingfrom –14°C in the UK to 48°C in the Middle East, whilst in areas such as NorthernEurope and Scandinavia, slipforms have been completed in far lower temperatures.

Spray-on curing membranes are often used in warmer climates in addition to shade

Page 13: slip formwork

Slipform 14/13

netting, but they require careful consideration, not only to monitor application but alsobecause following trades may be affected. White dyes are often added to sprayed curingcompounds, to ease the application visually and reflect the sunlight when the surface isexposed. Curing membranes work on the principle that the hydration process requires theamount of water retained to provide maximum efficiency, as no additional water is usedin the curing process.

Another method of curing, which is often specified in hot climates, is a mist of sprayedwater. This can be achieved by suspending perforated pipes from the inspection platformwith a gravity-fed or pumped water supply, that sprays the outside surface after a suitabletime. This method requires considerable organization and maintenance throughout theslipform operation.

For average ambient conditions experience has shown that shrouding the slipform hasprovided adequate curing conditions.

14.3.9 Methods of concrete distribution

Tower craneAdvantagesThis is generally the best and simplest solution for delivery and initial distribution. Thecrane driver usually has total vision and hence saves time on delivery. A one-cubic-metreskip would be used for most operations as the weight of a full skip is within the normalminimum capacity of the crane. The ‘hook-height’ would need to be 7 metres above thefinished height of the slipform to allow for delivery and dismantling.

DisadvantagesIt is a very costly item of plant with expensive set-up costs. Generally it is underutilizedfor servicing small one-off structures. For very tall structures, it is often necessary to raiseand ‘tie in’ the crane (possibly above 50 m in height).

Mobile craneAdvantagesIt is easier to gain access to various parts of the site and generally far cheaper to use thana tower crane. Hire periods relate to the actual requirements on-site.

DisadvantagesThe driver is often working blind and relies on instructions from the banksmen, thereforeit is slower to operate than a tower crane. It is limited in height and, more importantly, inreach. This method often requires a different method of distribution for the concrete onthe platform, i.e. wet hopper and barrows.

HoistAdvantagesFor small structures on plan, requiring a maximum of 3 m3 of concrete per hour a ‘self-erecting’ rack and pinion hoist can be used quite successfully. Concrete can be transportedin dobbin barrows in the goods/passenger cage or by a special skip that travels on one sideof the mast with the cage operating on the opposite side. This method is ideal for servicingsmall to medium chimneys and other small towers.

Page 14: slip formwork

14/14 Slipform

DisadvantagesThis system is limited for quantities. Ties are required between the mast and the structuresat approximately 6 m centres (half the normal distance required) because of the heightrequired above the last tie to discharge onto the slipform shutter. Rolling ties attached tothe slipform assist the reduction of the spacing for the static ties but can present problemson small structures due to large forces introduced by a cantilever action.

Note: Rope-guided hoists can be used supported from ‘cat heads’ on the slipformshutter. However, this type of plant is not readily available and faces stringent regulationswith regard to set-up and operation. Each application would have to be designed to meetindividual needs.

PumpAdvantagesStatic and mobile pumps can be used successfully because of the similarity of the concretemixes. It is very useful for supplying concrete to slipforms carried out in difficult locations,and for slipforms requiring relatively large volumes of concrete. Height of pumping is notusually a governing factor for static pumps.

DisadvantagesThe system is not suitable for small shafts where the volume of concrete required is lowon an hourly basis. The lower limit should be 6 m3/h and ambient conditions must becarefully considered. Mobile pumps have a limited range, mainly due to height versusreach, posing similar problems to mobile cranage.

In selecting systems for distribution of concrete it is vitally important to choose themost direct method to avoid:

• Time delays• Loss or gain of heat from the concrete• General loss of moisture.

Obviously the cost of setting up a system must be taken into consideration. This wouldbe based on availability of plant and general running costs and utilization during theperiod on-site.

14.3.10 Problems that may arise with slipforming

The slipform method of construction is a highly skilled operation and is more akin to theprocess industry than the construction industry. It is a combination of mechanical operations,material supply and human resources that may give rise to occasional problems.

However, as slipform is an unusual method of construction for most teams, there is aninducement for greater research, planning and preparation which tends to reduce complacencyand thus help to eliminate major defects. As with all operations, monies allocated to thiseffect would be substantially less than costs attributable to remedial operations.

Problems can emanate from the main operations listed below:

• Steering of the forms• Mechanical or hydraulic operations

Page 15: slip formwork

Slipform 14/15

• Concrete placement• Fixing of reinforcement• Positioning of inserts

Quality assurance, method statements, risk assessments and well-trained labour shouldeliminate the majority of problems.

The provision, operation and steering of the equipment is usually entrusted to specialistcontractors, as only the experience of fully trained personnel is able to ensure that satisfactorystructures are constructed.

The poor performance of the equipment, poor quality control and distribution of theconcrete, poorly fixed reinforcement, and badly distributed loads can affect steering ofthe forms.

Embedments, particularly of a heavy nature, fixed proud of the correct position andnot taking account of the taper of the forms, can also affect the steering and correctoperation of the forms.

The design of a suitable concrete mix should allow the slipform to progress within arange of speeds that take account of:

• The complexity of the structure• Volume of concrete and reinforcement• Amount of embedments

As the laitence on the surface of the formed wall provides the lubrication for the steelforms, it is not surprising that problems evolve from the failure to produce the correctconsistency with the concrete mix.

Minor defects in the concrete range from:

• Bleeding• Surface honeycombing• Limited slumping• Surface tearing

to more serious defects such as:

• Loss of external arisses• Larger areas of slumping• Loss of sections of wall• Severe surface tearing• Lifting of sections of wall

The reinforcement fixing can affect the rate of climb and the steering of the shutter aswell as producing problems that equally exist with traditional construction, such as misplacedor omitted starters.

Box-outs or inserts can also be wrongly positioned or omitted, but a misplaced heavyinsert could impede the path of the shutter and severely affect the progress of the slide.

14.3.11 Remedial action

Good supervision, strict disciplines, correctly maintained plant, well-selected crews and

Page 16: slip formwork

14/16 Slipform

contingency arrangements all combine to help eliminate remedial work. There is nosubstitution for good knowledge of the equipment and a wealth of experience. If majorfaults occur then the slipform operations can be halted in order to facilitate repairs orremedial work.

When assessing the mode of operation for remedial action for the concrete, there aretwo types of structure to take account of:

• Elements of buildings where access can be gained, at a later date, to most levels asbuilding work progresses.

or

• Free-standing structures with completely exposed faces, where access would be difficultat a later date, namely towers, silos, chimneys, etc.

A well-designed mix and a steady rate of progress go a long way to producing a goodstructure with an excellent finish.

Adjustments to the mix should be made to cater for:

• Changes in ambient temperature• Variations in material content including moisture• Changes in the rate of progress

Admixtures, such as plasticizers/retarders, may be required to facilitate the above andprovision for their inclusion should be made at the design stage. However, the easiestadjustment to make is by varying the slump within the allowable limits. The water contentshould be adjusted and used as a retarder or an accelerator. Bleeding is a function of themix and should be eliminated, although some bleeding could be caused by excessivevibration.

The speed of the shutter should be balanced with the slump of the concrete distributedthroughout the slipform and all the associated operations.

The concrete emerging from the shutter after a few hours is in a ‘green’ condition andas it is easily accessible from the hanging scaffold or inspection platform, provides theperfect opportunity to make good minor surface blemishes. A mixture of similar finematerials can be dressed onto the surface to cater for honeycombing or other imperfections.Bonding agents should not be required at this stage and if used will discolour the concrete.

Slumping below the shutter should not occur, as it is the result of incorrect speed ofsliding, poor batching, or poor materials. However, should minor slumping occur, it ispossible to rectify this from the hanging scaffold, by removing the offending section andrendering the void in similar material.

It is also possible to cut away poor areas of concrete and use localized static shuttersto reform the affected parts whilst the concrete is still in a green condition. Thus a naturalbinding action would be induced.

To maintain progress it may be wiser to leave behind some repair work to be carriedout at a later date. This would apply, in particular, to the repositioning of starter bars orinserts that would require concrete that had gained sufficient strength to cater for drillingand fixing. After slipforming, the structure would have gained sufficient strength for theworks to be carried out in a similar manner to in-situ work.

The general philosophy for any remedial action is ‘prevention is better than cure’ andevery effort should be made to uphold this theory.

Page 17: slip formwork

Slipform 14/17

14.3.12 Slipform mixes – examples

Slipform mix used in the south of England in winter conditionsSpecification. C40. Free w/c 0.5 OPC. Slump 75 m.

OPC 390 kg (Sp. Surface 365 units Initial Set 144 min.)20 mm marine flint 664 kg10 mm marine flint 287 kgSand marine 800 kg

Grading % passing BS sieve5 mm 992.36 mm 821.18 mm 69600 micron 55300 micron 31150 micron 775 micron 2Free w/c ratio 0.47 (water heated for winter conditions)No admixtures

Slipform mix London average autumn conditionsSpecification C35 Free w/c 0.56 OPC slump 100 mm

OPC 360 kg20 mm land flint 765 kg10 mm land flint 255 kgSand land flint 780 kg

Grading % passing BS sieve5 mm 992.36 mm 811.18 mm 67600 micron 51300 micron 18150 micron 3Free w/c ratio 0.56Water-reducing agent Cormix P108 16 ml

14.3.13 Operations in varying temperatures

Slipform operations are carried out in most areas of the world, presenting a vast range ofclimatic conditions. A slipform is able to cope with a far greater range of temperaturesthan conventional construction as the slipform rig can be shrouded for shade and thereforecooled or heated in tent-like conditions. Thus if the concrete is delivered to the formswithin the specified limits, the operating or ambient temperatures can be controlledaccordingly.

Page 18: slip formwork

14/18 Slipform

14.3.14 Hot weather concreting

Precautions with the concrete before and after batching:

• Protect cement storage areas from sun, induce cool air flow around silos.• Cool aggregates by shading and inducing air flow, or spray coarse aggregate with water.• Use refrigerated water in batching plant.• Protect delivery vehicles from sunlight.• Wrap readymix vehicle drum in hessian and spray regularly.• Dampen distribution plant sufficiently to compensate for evaporation to avoid moisture

loss from concrete.• Vertical stand pipes for pumping should wrapped in hessian and dampened.• Avoid time delays in delivery and placing.

All, or a combination of the above actions can be activated according to the severity of theheat and amount of humidity. Cooling by air flow or evaporation has a great effect inreducing temperatures with little cost.

Cooling the batching water by flaked ice can produce variable results due to timefactors and size control and requires the utmost quality control.

The slipform decking can be sprayed with cool water to lower the ambient temperatureand motorized plant should be avoided where possible to reduce heat build-up – or theeffect of fumes in restricted areas. The shrouding should be created from shade nettingand fans can used to induce air flows, if necessary. The use of low-heat cementitiousmaterials is an advantage. Air circulation can be induced in vertical shafts by creatingducts or vents in the working decks.

14.3.15 Cold weather concreting

Precautions before and after batching:

• Protect aggregates from winds and frost.• Heat batching water – insulate tank.• Steam heat or use jet-air heaters on aggregates.• Avoid delays in delivery and distribution.• Use the minimum amount of plant for distribution.

The shrouding to the slipform rig should be complete to the underside of the hangingscaffold (inspection platform) and thoroughly tied or otherwise fixed to form a completeshelter avoiding all draughts. In extreme conditions, heaters can be deployed on theplatforms providing they do not present hazards.

Generally in climates similar to the UK heated water for the batching plant is the mostthat is required to provide suitable concrete for the slipforming operations.

14.3.16 Catering for horizontal connections and openings

It is not the intention to provide detailed information for connections or void formers, butprovision is required for constructing openings and horizontal slabs or steelwork on even

Page 19: slip formwork

Slipform 14/19

the simplest of vertical structures. This can range from a few openings for pipework oncivil engineering structures, with provision for accepting roof beams, to complexarrangements for providing slab and beam connections, door openings, service ducts andthe like on large-core structures for high-rise office blocks.

14.3.17 Connections for concrete

SlabsThis is usually achieved by the use of proprietary pull-out starters, which also provide arebate, formed by expanded metal, to receive the slab. These are inserted into the slipformedwall ahead of the concrete. The maximum size of bar that can be successfully accommodatedin this arrangement is 16 mm. For diameters above this, it would be necessary to usereinforcement couplers.

BeamsLeave voids to receive the main reinforcement from the beams, or revert to couplers.Couplers are often attached to a former of plywood for more accurate positioning, andalso to form a nominal rebate.

14.3.18 Connections for steelwork

SlabsIf in-situ slabs are required, possibly supported by metal decking, it is usual to providesingle pull-out starters as anti-crack reinforcement in the top half of the slab.

BeamsPockets formed in the walls or plates are the preferred methods to the receive steelwork.There are a variety of ways in which these can be achieved. For small beams, and edgesupports, it is preferable to ‘drill and fix’ afterwards as the main beams or slabs areinstalled.

Box-outs or void formersAll formers for openings are fixed into the reinforcement at the working deck level, withreinforcement required to pass through some of the smaller formers. Small formers canbe made from polystyrene or similar, whilst the majority are manufactured in timber.Large openings may be slipformed. It should be noted that due to the taper on the shutter,the boxouts should be narrower than the width of the wall in order to pass through. By thesame token, all plates or other face inserts will be positioned back from the surface by afew millimetres.

Page 20: slip formwork

14/20 Slipform

14.3.19 Example of slipformed project

The flour mill – Doha – QatarMain contractor – Taylor Woodrow International. Slipform subcontrator – RMDThis was a fine example of slipforming, demonstrating economical construction over ashort period – programme time 9 months. Sixty-four grain silos, consisting of 4 no.blocks of sixteen were slipformed in eight sections and a further 3 no. blocks of rectangularsilos were slipformed in five sections, necessitating vertical slipformed joints on allstructures. The arrangements allowed two sets of equipment to be used alternately overthe construction period to provide good logistics. Each block was slipformed from thebase level to the roof level to include the external walls and internal support columns.Temporary columns were used internally at the top to maintain continuity and support forthe slipform. The two sets of equipment required 146 no. heavy-duty jacks, to completethe works. A climb rate of between 5 and 6 metres per day was achieved (See Figures 14.4and 14.5).

The extremely smooth finish required for the flour silos was achieved despite temperaturesreaching 48°C. Concrete was batched using air-cooled aggregates and refrigerated waterfrom an automated batching unit set up on-site.

For vertical sliding joints, referred to above, the chosen positioning usually allows astub section of wall, with starters projecting, on which the panels of the adjacent slideconnect by approximately 100 mm. The panels are then guided by the existing section ofwall.

The mix details were as follows:

Contents/m3

Cement SRPC 400 kg/m3

Coarse aggregate 1110 kg/m3

Fine aggregate 650 kg/m3

Water 180 litres/m3

Retarding plasticizer 1.6 litres/m3

Slump 75 mm

14.3.20 Dayshift-only sliding

This system, often referred to as ‘intermittent sliding’ was introduced thirty years ago bySlipform International Ltd for use in environmentally sensitive areas, or where the supplyof concrete proved difficult or costly on a 24-hour basis. It has been used for the constructionof many service cores in cities throughout the world from the UK to Australia.

The method of construction allows for concreting with the slipform throughout thenormal daily working hours, avoiding additional costs, and for the slipform shutters to bepressure washed in the early evening. This creates a ‘dayswork’ joint and, hence wouldnot be suitable for liquid-retaining structures, or exposed surfaces such as chimneys,although linings for storm water overflow tanks have been constructed by this method.

Allowing for the intermittent nature of the work, this method is still faster than othersystems such as climbing forms, as ‘Dayshift Only’ still achieves 15 m in height perweek.

Page 21: slip formwork

Slipform 14/21

14.3.21 Conclusions

The versitility and economy provided by concrete will ensure a long future for thematerial in the construction industry, whether utilizing the present ingredients, or by-products from future manufacturing. Slipform construction cannot be ignored as it is thecheapest and fastest method of construction for many types of structure. It is an operationthat places demands on everyone associated with the work, but the results are there to see.Structures built forty to fifty years ago by the slipform process remain in better conditionthan similar structures constructed conventionally. Like every building, poor workmanshipor materials will manifest themselves, but the slipformed monuments are there as livingproof – oil platforms, telecommunication towers, large cores, bridge pylons. The systemswill develop with the benefits provided by the electronic age.

14.4 Horizontal slipforming

Developed in the USA in the early 1970s and born of a need to rebuild mile upon mile ofhighway, horizontal slipforming in two modes – paving and kerbing – is now carried outin many countries throughout the world. This concrete construction is an instantaneousextrusion that requires the moulded shape to be self-supporting and thus has limitationsin shape. The two operations require different machines to carry out the constructionwork, but both are propelled forward very accurately leaving in place the formed concrete.

Figure 14.4 Second group of circular silos slipformed.

Page 22: slip formwork

14/22 Slipform

14.4.1 Paving

Used for constructing concrete aircraft landing strips, roadways, railtracks, aprons andother large areas, paving machines are generally capable of laying up to 90 linear m/h.Panel widths are usually from 3 m to 10.5 m wide although 4 m to 6 m are more typical.

The paving machine is constructed on a transverse beam design with drive units at

Figure 14.5 Second half of rectangular block slipformed.

Page 23: slip formwork

Slipform 14/23

each side, providing the support, height control and guidance. Depending on the application,the paver may have two, three or four drive units, usually of the tracked variety. For largecontracts ‘paving trains’ are set up, consisting of three separate items of plant – a spreader,a placer and a finisher. Concrete is fed directly to the fore of the spreader by readymixtruck(s). This concrete is distributed by a transverse worm (auger) on the spreader, withthe placer following a few metres behind completing the placing and vibrating. The basicfinished surface is achieved with a hydraulically controlled skid. The finisher providesany surface texture that may be required and puts in place either a sprayed or plasticcuring membrane. For smaller contracts all the actions are carried out using one machine.

Each machine is driven by its own diesel engine which is also responsible for supplyingthe power to the hydraulic systems, and supplying the electrical power. Electronic sensorssteer the paving machines, guided by string or laser lines set up ahead of the concretingoperations. Hydraulic vibrators are set up in a bank and are controlled to provide thecorrect level of vibration, which is critical for the durability of the concrete.

14.4.2 Kerbing

More compact versions of the paving machines are used to form edge kerbs, barriers,drainage channels, irrigation ditches or similar. A special mould is designed to suit therequired shape of the kerb or barrier. This is attached to the rear of the machine andextrudes the concrete to the given shape. Guidance of the slipformer is again achieved byelectronic sensors to lines. The height and pressure to control the mould is governed byhydraulic rams. The concrete is fed from the readymix truck to a hopper via a chute orelevator, and then by auger to the mould. Controlled vibrators are again used in theplacement of the concrete. A vast variety of shapes can be formed providing that they areself-supporting immediately upon exposure.

The present limitation on height is approximately 1.85 m, and rates of pour havereached 250 linear m/h. The choice of a smaller slipforming machine allows the formationof kerbs to very tight radii – 600 mm has been achieved using wheeled machines, and1.2 m using tracked machines.

14.4.3 Concrete mix

Within an hour of batching, the concrete supplied for horizontal slipforming may berequired to be freestanding in its final location. It therefore requires to be a cohesive mixthat is able to provide a suitable finish with one pass. As with a pump mix and a verticalslipform mix, a higher fines content is required to provide these characteristics. However,whereas vertical slipforming can be achieved with high slumps of 50–150 mm, dependenton ambient temperatures and operating speeds, it is essential for horizontal kerbing tomaintain very tight slumps of 20–40 mm to cater for this immediate exposure, free of anyexternal side supports. The slump for paved areas can be increased accordingly, and maydepend on edge restraints.

To expedite the placing of the concrete by this extrusion method, an air-entrainingadmixture is used which improves workability and cohesion, whilst in the long term alsoimproves durability through increased resistance to de-icing salt and frost attack.

Page 24: slip formwork

14/24 Slipform

14.4.4 Vibration

Vibration is critical for vertical and horizontal slipforming. Experience has shown thatover-vibration with horizontal slipforming has reduced the air containment to less than 5per cent and has caused premature deterioration of the concrete. This has led to theintroduction of special individual controls to vibrators which are fixed in banks, in orderto monitor the operation of each unit.