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BIS LEY Silica Fume

Extrema-dur & CONCRETE DURABILITY

One of the most pressing problems for the global construction in dustry is the rapid deterioration of concrete struc:ures caused by aggressive environmental condrtions including hiftl chloride and/or sulphate levels. AS a result or the substantial costs involved In re-storing affected structures, emphasis is now increasingly placed on designing for durability from the outset.

Over the last 30 years the incorporation of Silica Fume has become widely recognised as an effective and reliable strategy to enhance the durablllty of concrete. Typically Extrema-dur Silica Fume Is an ultrafine powder consisting of nearly spherical particles around 100 times smaller than a grain of cement. When introduced into con crete these particles assist to fill the voids between the cement/ aggregate matrix. Additionally, Extremadur reacts with the free llme which Is present In OPC concretes to form acklltional quantities of strong-binding CSH (calcium silicate hydrate). CSH is a principal binding component in OPC concrete. By replacing weakbinding lime with more CSH, Extrema-dur .. significantly enhances the over-all compressive strength of the mix. This extra CSH further assists in refining the pore structure of the concrete.

By adding 5 12% Extrema-dur by weight of cemEnl, In a properly designed concrete mix it is possible to produce a finished concrete of significantly higher strength and dramatically lcwer permeability. Weight for weight Extrema-Dur Silica Fume Is 2 to 5 times more efficient than cement in terms of its contribution to compressive strength. In a high quality mix, Extrema-dur4 ls capable of reducing permeability by a factor of 100 t imes or more.

How does this assist concrete durability ? Two of the most signifi cant contributors to concrete deterioration are ccrrosion of the underlying steel reinforcement and sulfate attack.

Extrema-dui4 Inhibits corrosion first of all by sub!tantlally decreas-ing the permeability of the concrete, which in turn reduces trans-port of chloride-laden moisture. Secondly, Extremadur increases the resistivity of the concrete severalfold. Electron flow is an im portant element of the electro-chemical corrosion process, and an increase In resistivity serves to Inhibi t corrosion. Anally, Extrema-du!"' improves the adhesion of the concrete to the rein forcement.

Sulfates can attack concrete by reacting with the 'ree lime in the mix. A series of chemical reactions then ensue, th~ final product of which occupies more volume than the pre-reaction components. This expansive chemical reaction forces the concr~te apart, ulti-mately causing cracking and spalling. Extrema-dut improves sul fate resistance by reducing the permeability of the mix, which lnhlb its the diffusion of sulfate through the concrete. Secondly, it re duces the amount of free lime in the concrete thereby deterring init iation of this form of expansive chemical react on. In an appro-priate mix, replacement of 10% ordinary Portland Cement by Ex trema-dur can achieve similar, or better, levels of sulphate resls-

Extrema-dur" Slllca Fume

lance than conventional SRC (sulphate resisting cement), with the additional benefits of improved chloride resi.stance and enhanced strength.

Although Extremadur is capable of substantially improving the performance of conventional concretes, Its benefits will only be realised if sound basic concrete practices are pursued. A properly optimised mix design is essential, as is accurate weigh batching of ingredients, appropriate mixing, placing .3nd, most importantly, curing. With careful attention to these a1eas Extrema-dur provides an extremely effective means of enhancing the longevi ty of struc-tures.

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BIS LEY Silica Fume

Like ~II high performana concrete, conaete maide Mm Extrem~dur-4' Silica H.Jme requires care in the selection of raw materials and mix design, coupled with sound preparation, application and curinR practices. The benefits of Extrema-Our will onlv be realised W sound basic concrete iractices are pursued. Extrema.Our Silica

~ume is not a remedy for deficient conaete procedures.

STORAGE AND CONVEYING

In general Extrema-dur' must be kept dry and away from pos-Sration of the bulk bag fabric it is strongly recommended that the product is stored under cover away from direct sun exposure.

~or large projects, Extrema-dur0 may be stored in silos and conveyed pneumatically or by screw conveyors as per cement. The silo can be considerably smaller than the site cement, GGBS or fly ash silos, due to the lower addition rate of the Sil-ica Fume. As Exirema-dur Silica Fume is considerably finer than cement it is essential that all valves and fittings in the conveying line are well sealed. Sharp bends and steep inclines in conveying lines should be avoided to minimise the possibil-ity of blockages. Conveying lines fabricated from rubber tubing are particularly effective, as these can be easily vibrated (manually or mechanically) to clear any restriction. Flange fit-tings and hose joiners should be minimised or, where possible, eliminated from the conveying path. Spreaders should be used to maintain the curvature of the rubber pipelines.

For smaller projects Extrema-dur0 may be fed directly from the bulk bags or small bags to the weigh hopper or the truck mixer itself. Bisley will be pleased to put customers in contact with specialised Silica Fume debagging/ conveying equipment manufacturers on request.

Extrema-Our may also be slurrified with water, using a high shear mixer. Undensified Extrema-dur silica fume is some-times preferred in this application, however many users have reported excellent results with densified Silica Fume, using appropriate slurrification equipment.

Working with Extrema-dur

WEIGHING AND BATCHING

For most projects, dosage rates for Extrema-dur silica fume will be in the range of 5 to 15% by weight of cement. Regardless of the required dosage level it is essential that the product is accurately weigh batched. Extrema-dur is a highly efficient pozzolan and relatively minor deviations in addition rates can materially influence finished fresh and cured concrete characteristics.

Oensified Extrema-dur silica fu me consists of loosely bound agglomerates of extremely fine, near spherical, par-ticles. To ensure optimum performance these agglomer-ates must be broken down and distributed uniformly throughout the concrete mix. Thorough mixing is therefore essential. Relative to OPC concrete it is prudent to allow for additional mixing time, regardless of the batch mixing proc-ess used. Poor mixing often manifests itself in the form of significant slump variations. In extreme cases balling of the concrete may occur. Such difficulties are readily addressed by increasing the mixing time.

WORKABILITY

Following mixing, t he pozzolanic action of the Extrema-dur Silica Fume leads to an accelerat ion of CSH gel formation. As with all com mercial Silica J=ume this - together w ith other factors including the high surface area of the produc1 - leads to an increase in water demand and a reduction in slump. Silica Fume concretes are both highly cohesive and thixotropic. The use of Superplasticisers (SP), with or with-out conventional water reducers, is essential to correct for the increased water demand and assist slump retention. Today, most major concrete admixture companies are able to furnish SP-'s that are well optimised to the characterist ics of Silica Fume concrete.

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