T raditionally, there have only been two water-reducing performance categories of chemical admixtures available to the concrete producer,

namely WRAs (water-reducing admixtures/plasticisers) and HRWRs (high-range water-reducing admixture/superplasticisers).

WRAs tend to be lignosulfonate based and have a very small effective dosage range, offering only around 10% water reduction. The higher-performing HRWRs tend to be comparatively dosage sensitive and difficult to use and, unless the application calls for a very high-quality concrete where high strengths and workability are required, are generally unsuitable for ‘daily’ use.

However, today’s concrete producers now require a higher level of performance from their product at a time when all production costs are being scrutinised. Advances in admixture technology have helped in

the production of these products, so meeting these increased and often technically challenging demands.

One of the results of this advancement is a more ‘user-friendly’ class of admixture – the mid-range water-reducing agent or MRWRs. While there is no formal definition of a MRWR within BS EN 934-2(1), it is generally agreed that this product type will provide a water reduction of between 12 and 18% when used within its target dosage range. As the name implies, this admixture group fills the gap between normal (WRA) water reducers and (HRWR) superplasticisers.

The use of MRWR over normal WRAs holds many benefits. They can be used in significantly higher dosages without the risk of any retardation of the concrete. The higher dosages permit greater water reductions, allowing the benefit of either a cost saving from using less cementitious products or higher strength gains when using the same volume of cementitious material.

Compared to traditional WRAs, mid-range water reducers will give higher early and ultimate strengths. They are very user friendly, which makes it very easy to control the workability of the concrete when produced at the batching plant due to the unique blend of raw

ADMIXTURES

12 concrete DECEMBER 2010 www.concrete.org.uk

Chemical agent cuts water useAbove and overleaf: The Finzels Reach project in Bristol, looking along the River Avon.

Justin Ransted of Sika looks at the water-reducing performance of admixtures and the introduction of a new chemical agent.

Today’s concrete producers now require a higher level of performance from their product at a time when all production costs are being scrutinised.

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materials used in its manufacture. This leads to the reduction in any water addition on-site. This added control gives the producer and end user a greater peace of mind that the required specification will be achieved.

As the concrete is generally produced with a lower water:cement ratio (w/c), drying shrinkage will be reduced, hence the chances of cracking are minimised.

The use of a MRWR is also of benefit where the ‘aggregate package’ used for the production of the ready-mixed concrete maybe poor, leading to a high water demand, giving lower strengths or poor consistence and slump retention. The addition of the MRWRs could help improve the workability without increasing the w/c ratio, leading to more ‘user-friendly’ concrete on-site and a better surface finish.

SikaPlast powered by Viscocrete technology offers a multi-functional approach; without any adverse effects of retardation it can act either as a MRWR or HRWR. This enables many ready-mixed concrete producers to cut down on the amount of admixtures needed to be stored at each site.

So versatile is the MRWR that SikaPlast is the product of choice in Sika’s Watertight Concrete System to enable the watertight concrete to be placed and compacted correctly in all structures. Table 1 overleaf summarises the results of the use of SikaPlast MRWR concrete CEM III (350kg/m3).

As the results show, when the admixture is added to the concrete there is an increase in the 28-day compressive strength. This is due to the fact that it enables the producer of the concrete to lower the w/c ratio in its mix design, while retaining or increasing the workability of the concrete produced.

www.concrete.org.uk DECEMBER 2010 concrete 13

For more information

1.800.961.4477 • www.xypex.com

INTEGRALCONCRETEWATERPROOFING

PRECAST

READYMIX

SHOTCRETE

Xypex Admix is blended into the concrete at the time of batching to produce a non-soluble crystalline structure that blocks pores and seals micro-cracks throughout the entire concrete matrix. The result? Your precast pipes, manholes, vaults, foundations, slabs, and more will be waterproof from the moment they’re poured. Xypex also resists chemical and sulfate attack.

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The uses of MRWRs are wide and varied. Where a self-compacting concrete is required it will reduce the risk of segregation or bleeding and if it is to be pumped any distance it will make the product more cohesive and improve the rheology of the mix.

Recent projectHanson Concrete has currently supplied over 2000m3 of Sika Watertight Concrete to the Finzels Reach Project, where an underground car park for a new-build residential apartment building is being constructed. The car park is adjacent to Bristol’s floating harbour and is below the water level.

Hanson Concrete used SikaPlast 15RM mid-range water reducer as the plasticiser, enabling the subcontractor to place the concrete anywhere on-site, whether it be pumped, skipped or by hand. The subcontractor and concrete company were able to rely on the quality of the MRWR to ensure a consistent workability for any of the mixes that were required for any given application on the project.

Cost-effectiveThe addition of a MRWR will not only save the producer storage space and time, it will also provide

a cost-effective alternative to using separate admixtures for different applications or concretes. It will also give the end user a quality product that will handle easily and provide the aesthetic finish required. ●

14 concrete DECEMBER 2010 www.concrete.org.uk

ADMIXTURES

1. BRITISH STANDARDS INSTITUTION, BS EN 934. Admixtures for concrete, mortar and grout. Part 2 – Concrete admixtures. Definitions, requirements, conformity, marking and labelling. BSI, London, 2009.

Reference

Table 1 – The use of SikaPlast MRWR CEM III (350kg/m3) concrete

Dose w/c ratio Consistence 28-day (v/wt of slump, mm compressive cement %) strength, MPa

Control – no None 0.66 135 39 admixtureSikaPlast 15RM 0.50 0.53 180 60.5SikaPlast 15RM 0.80 0.47 210 72.5

Over 2000m3 of Sika Watertight Concrete has

been supplied to the Finzels Reach Project, where an underground car park for

a new-build residential apartment building is being

constructed.

Right: The car park is adjacent to Bristol’s

floating harbour and is below the water level.

Finzels Reach, Counterslip, Bristol, Avon

Engineer ClarkebondContractor John Sisk and SonSubcontractor KildownetConcrete supplier Hanson Concrete

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