26 27April 2019 M12

// Concrete & Precast Solutions

Semi-precast is the core of our business, ahybrid between traditional in-situconcrete and traditional precast.

Although sometimes seen as an under-developed off-site manufactured solution, thesemi-precast design philosophy bringsenormous improvement to cost andconstruction efficiency.

The semi-precast approach aims to deliversolutions fully compliant with the operationaldesign requirement. The ideal configuration isnot adapted to prefabrication. Whereas we willidentify cost and time saving opportunities duringthe design development phase, we can adapt tothe most precise configurations for operationalaccuracy. In principle, any structure imagined inin-situ concrete can be delivered in semi-precast.Approving Authority confidence is engenderedby implementing designs which cannot bedisproved by failure to comply in any respect withthe specified codes and standards, whethernational, international or customer specific. Weplace wet concrete against hard concrete, theway it’s always been done. The difference is thatsome of the concrete was manufacturedelsewhere and the location of joints andinterfaces are unconventional. Regardless, theintegrity of joints and interfaces remainsuncompromised and verified by design.

The Benefit of PrefabricationWe complete the difficult parts of construction inour factory, under ideal conditions and underquality supervision. Features including pipe-fittings, nibs, corbels, launder-channels, formworkattachments and stability-footings (among others)are eliminated from the site works. Products aredelivered to site on a just-in-time basis thentaken from the delivery vehicle to their service

position in one simple operation without fuss ortemporary propping. Small crews achieveamazing productivity by following simple stepsand using well designed components anddelivery systems.

TolerancePrecast concrete units can weight in excess of20 tonnes. Under normal manufacturingtolerances, it could be very difficult to ensure theprecision required to maintain accurate alignmentand watertight fit. The in-situ joint provides atransition between elements which ensures acomplete and perfect fit (in it’s liquid phase) and acompletely ‘relaxed’ structure at introduction toservice. We don’t stress pieces into alignment orposition. The design assumptions are fullyrealised.

Waterproofing IntegrityAt every interface a scabbled surface is prepared.In addition, a smooth dense slot is preserved forthe application of hydrophilic strip. This providesongoing self-healing capability in service. Weuse only one hydrophilic product; DensoHydrotite. Hydrotite is resilient to inflation prior toencasement in concrete. It can compressagainst the surrounding concrete with a pressureof up to 3MPa on contact with water and hasbeen approved by Tokyo Underground for designlife up to 100 years. It is also DWI and Materialsin Contact approved for potable water

FLI Carlow are the premier total service provider of engineered structural solutions to the Water, Energy, Storm Attenuation and Bespokemarkets. Our capacity to design not just the precast units, but the structure into which they integrate and the manufacturing tools used tomake them has kept us at the forefront of innovation in our industry.

Leaders in Semi-Precast ConcreteDesign for Manufacture & Assembly

applications in the UK. That’s 100 years to firstsignificant maintenance of the structure – nocompromise. Sealants used at the mechanicalinterfaces of traditional precast concrete tankingstructures rarely have a service life in excess of20 years. These features are particularly relevantto storm attenuation, storage, treatment andbasement applications,

Structural ContinuityAlthough relatively short, the in-situ joint is usedto enable two-way bending of the structure. Thiscapacity is not available using any other precastapproach. Reinforcement lap lengths aredesigned on a bond-stress basis to ensure fullcapacity in smaller spaces. Even the interfacebetween the in-situ concrete stitch and theprecast unit is designed for the same crack-widthcontrol as the body of the structure. Thisensures that all elements of the structure providethe minimum standard of waterproofing integrityand full compliance with concrete structuresdesign standards.

By bending in both orthogonal directions,structures are thinner, lighter, economical, requireless transport, less craneage and have a lowercarbon cost.

CarbonOur particular concrete mix design delivers veryhigh early strength for efficient production,typically, 25 Newtons at 16 hours. This ensuresmaximum safety and maximum value byextracting products daily. We use very highconcentrations of GGBS (66%) which inconjunction with other energy and carbon savingmeasures has reduced our carbon cost from278kg/tonne of concrete manufactured to182kg/tonne. To ensure the high early strengths

despite the use of GGBS we use thermalactivation. By adding mixing water at up to 80°Cthe disadvantage of slow strength developmentof GGBS is eliminated.

Sulphate ResistanceHigh levels of GGBS when used in conjunctionwith limestone cement and limestone powders(for self-compacting behaviour) produce a designchemical resistance class DC4. That’s sulphateresistant concrete at no additional cost.

ValueThe precast unit is delivered to site withprojecting reinforcement often from five of it’s sixfaces. All components are 3D modelled prior tomanufacture and assembled in model-spaceprior to fabrication. This eliminates the risk ofclashing reinforcement and disruption toprogramme. Projecting reinforcement leavesvery few bars to be placed on site.

Typically the vertical in-situ stitches represent30% of the volume of the perimeter and internalwalls. While precast concrete products arerelatively expensive (although value-adding), theconcrete used in the joints is locally sourcedreadymix at approximately £40 per tonne.

The formwork required is very light. Shutteringply facing with vertical stiffeners is locked againstthe structure using steel braces and MKK coneanchors. The advantage in this low costapproach is that the formwork is torsionallyflexible adapting easily to the surfaces on whichthey bear. Smooth transitions and tightinterfaces are achieved. Through-ties arecompletely eliminated. These are often aproblematic feature of conventional in-situ works.

Length of JointsThe number and length of joints in our solutionsare often questioned. Our industry sees joints asthe most problematic elements of waterproofconcrete construction – more joints, more risk.The rebuttal is that it’s not the number of jointsyou fear, but the distance between them.Traditional construction methods utilise joints at6m – 10m centres. Thermal and dryingshrinkage accumulate over these lengths and areconcentrated at a single interface. The precastunit has long completed its shrinkage whenplaced on site therefore the shrinkage to beaddressed is that occurring over only 500mm. Inaddition, each interface has the benefit of afactory prepared scabble, reinforcementcontinuity designed to the higher standards forcrack-width control and a self-healing strip ateach end. The semi-precast interface is subjectto only between 5% and 10% of the movementoccurring at conventional joint. As a result, it issignificantly better preforming than conventionalconcrete in this respect.

Rate of ConstructionSemi-precast structures are typically completedin 60% to 40% of the time taken to deliverconventional structures. This is where the valuelies, both in terms of reduced preliminaries andhugely increased productivity. During theconventionally difficult construction of verticaland suspended works, equivalent productivity perperson on site is ten times greater.

For more information, call us on +44 (0) 1279423303 or email us at enquiries@flicarlow.com.You can find us on the web at the below address.

www.flicarlow.com

Bryn Colwyd RGF and DAFStructures Snowdonia

Semi-precast structures aretypically completed in 60%

to 40% of the time taken to deliver conventional

structures. This is where the value lies, both in terms

of reduced preliminaries and hugely increased

productivity.