Construction World June 2016

PROJECTS AND CONTRACTS

PUMPING with

ADMIXTU

Contruction sites

Just as important as understanding the concrete pumping process is the role played by admixtures in controlling the way in which concrete behaves frommixing to final curing. Eddie Correia, executive vice president at Chryso Southern Africa, says that without the use of admixtures pumping concrete would not be an economically viable construction methodology. “Without admixtures the concrete mix would have to be modified to make it pumpable and this could include altering the sand/aggregate ratio or even the water content. This is most undesirable as it could adversely affect the concrete’s physical characteristics,” Correia says. “The key to the overall success of concrete pumping is all about getting the right mix in terms of paste, paste volume and admix- ture selection, and this is where it is important to secure access to a company with the necessary level of applications knowledge to ensure that the most appropriate admixture is selected for the mix design and the construction project,” Correia says. One of the most important objectives that will ensure successful pumping is to achieve a dense, cohesive concrete mix with sufficient paste volume. This will enable the correct level of workability of the concrete. Good aggregate coating is essential with the primary goal to attain a high level of plasticity which will allow the material to be moulded and shaped to hold the required form, thereby facilitating the pumping operation. Correia says that even where the available aggregates are not of a high standard it is possible through the introduction of an appropriate admixture to produce medium to high workability of the concrete. An example would be use of Chryso’s Quad 20 which enables greater cohe- sion and is not sensitive to mix designs. Where admixtures are sensitive to mix designs the risk of material segregation is ever present. “Segregation can be a serious risk and must be guarded against at all costs. Where material segregates and the water separates out of the mix – in other words where water washes out of the paste – there is a strong possibility of blockages occurring in the pipeline,” Correia explains. Superplasticisers from the Chryso® Fluid range will assist in creating a favourable water/cement ratio by reducing the amount of water, and this also prevents shrinking, cracking and porous concrete. Pumping concrete is perhaps one of the most challenging aspects within the construction environment, and with the increased move towards taller structures, it is one which contractors need to understand. >

Significantly, Chryso® has developed Fill Free® technology which assists contractors to produce concrete that is cohesive and yet still has a moderate viscosity. This modified PCE-technology was developed to have non-thixotropic properties and makes use of Chryso® superplasticisers. Superplasticisers, otherwise known as water reducing agents, enable good aggregate coating by dispersing the cement grains thereby minimising agglomeration of the mix and increasing plasticity and therefore workability of the concrete. Pumping pressure and material grading Other factors that could contribute to segregation are pumping pressure and material grading. If the pumping pressure used is too high excessive pressure is placed on the mix and this could force segregation of the concrete during pumping. Likewise if the grading of the aggregates is incorrect, they will be pushed out during pumping and this will result in blockages in the pump and/or pipeline. Correia explains that when being pumped concrete moves in the form of a cylinder and is separated from the pipeline wall by a lubri- cating layer made up of water, cement and fine aggregate or sand. It is essential that this lubricating film is achieved on all sides of the pipe and that the requisite workability is attained so the concrete can be pushed or transported through this channel. Chryso® Fill Free® technology facilitates a cohesive concrete paste which is less sticky. This is important as stickiness can produce lower cleaning efforts in the pipeline and also increase the resistance of the flow of the concrete in the pipeline. A paste that is too sticky will also make it difficult to maintain a constant rate of pumping. “Again, this is where superplasticisers are invaluable is achieving the plasticity and workability needed for the concrete paste to pass easily through reducers and move through bends in the pump and pipe- line without causing blockages,” he says. Paste volume Typically paste volume is determined by the aggregate type and shape as well as the grading curves of the mix. In addition to this, both the water/ cement and water/binder ratios of the mix design and appropriate admix- ture selection play critical roles. Correia cautions that simply changing the ratio of components in a mix design is not the answer. One of the ways to increase the volume of the paste is to increase the air content in the mix through the use of an air-entraining admix- ture. This will allow small stable bubbles to be developed in the paste and enable sand particles to move by each other more freely, increasing the plasticity of the mix without the need to add additional water. Caution must be used, however, in the appropriate selection of the air-entraining admixture. The introduction of too much air into the mix can create a ‘concertina effect’ in the pipes that will affect pumping distances.

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Chryso® Fluid Optima 100 was used on the Impala Platinum No 16 shaft to give the mix extended workability without affecting the setting time.

Without the use of admixtures pumping, concrete would not be an economically viable construction methodology.

CONSTRUCTION WORLD JUNE 2016