Aspects of concrete mix design
The consistency of raw materials is most important to concrete quality and it’s essential to select the most appropriate test methods to evaluate and monitor those materials, says BRETT BEATSON, director of Beacon Technical Services.
The process of designing concrete mixes might appear relatively simple to the casual observer. The chemistry of concrete on the other hand, is very complex and there are many methods of approach to concrete mix design. As in all other aspects of concrete, practical experience is an essential requirement in that there is always more to learn.
Most mix designers develop their own methods over time, which often imparts a characteristic to the concrete or a style that makes it different from that of another mix designer. Knowledge of the raw materials used, the batching plant and the particular concrete market all play a role in designing fit for purpose concrete.
Once a concrete mix has been designed, trialed, verified and approved for production, it is not the end of the story, maintaining the mix integrity to withstand the constant material variation requires constant monitoring.
The sampling of aggregate and sand is the most important process before any tests are performed in order to design mixes. NZS 3111:1986 Section 5 provides the methods for sampling concrete aggregate. It makes reference to having a random sampling plan that can be based on the time, or particular place, a sample is taken from. It suggests using one of a number of well-tried methods such as drawing lots of numbered marbles or using tables of random numbers, then relating this to a grid reference of each stockpile bin. In my experience with concrete plants this doesn’t happen very often if at all.
While there is a requirement in NZS 3104 for a concrete plant to do a certain number of aggregate tests based on its consumption, the emphasis is often more on this than the validity of the actual results. Nor are concrete plants ideal places to sample aggregate from for a variety of reasons, including difficulty and safety issues accessing aggregate from drive-over bins and conveyor belts. NZS 3104 permits a concrete plant to accept results from the aggregate supplier.
Many of the larger aggregate suppliers have proper QA procedures in place, and often an IANZ registered laboratory as well. I believe samples taken at the quarry source by trained technicians gives a truer representation of the actual quality and variability of materials. Obtaining test values that are representative of the materials is important in the initial mix design and for ongoing monitoring.
While the aggregate suppliers test results provide a measure of the production consistency, further variation can be expected from the influences of stockpile management, loader and truck handling, the weather etc. It is important when aggregate is delivered to a concrete plant that it is visually inspected several times a day. Concrete plant staff should look for any obvious signs of difference between loads delivered. The type of things to look for are colour difference, clay lumps, clay coating on aggregate, appearing bigger or smaller than normal and any forms of contamination. Notifying the aggregate supplier of anything visually different can help trace the source of the problem sooner and assist with fixing the problem.
Future trends in concrete making
Manufactured Sands (MS) are likely to be a reality in New Zealand in the near future as consents and environmental issues are making natural sands more scarce and expensive. Transportation of natural sand (further away from the end use) often makes it the most expensive raw material.
In other countries manufactured sands are now the only option because natural sand reserves are depleted or off limits. While MS have both benefits and negative aspects, with specialist equipment processing it is entirely possible to manufacture sand that will perform equal to that of natural sand.
The rapid development of chemical admixtures is currently having a huge impact on concrete applications and new uses for concrete. Poly carboxylate water reducers and superplasticisers, which are more powerful, are becoming the norm that have advantages over the previous generation naphthalene types.
For the concrete mix designer there is now a much wider choice of products available and there can be many solutions in order to produce a particular special purpose concrete. Sometimes there can be compatibility issues with admixtures, cement and other raw materials. Laboratory trials are always the best means of evaluating a new admixture and comparing this, where possible, to a control mix with a proven history.
While this gives a good starting point the next step is to scale up from labcrete to realcrete with field trials to better evaluate the setting and finishing properties or other special requirements.
The concrete mix design, the quality of the aggregates and the cement to a large degree influence how the concrete will behave in the fresh state. Admixtures are becoming more and more able to enhance specific concrete deficiencies to provide practical and economic solutions in many situations.
Probably the most significant admixture discovery some time ago was air entraining agents. While their original intention was for use in concrete for freeze thaw environment protection there are many other situations where they are useful.
Air entraining admixtures are very effective water reducers and can improve harsh aggregates/sands and concrete workability generally. In certain types of special mixes they can make the difference between a concrete mix or process working and not working.
Probably the biggest driver of changes in cement making in the future will be environmental issues and government legislation. With cement production producing CO2 as a by product we are likely to see an increase in the amount of fillers our cements contain, as has occurred in other countries, to reduce the environmental impact
Finding suitable fillers that enhance concrete properties, and are cost effective, will be an ongoing quest. It must also be remembered that there are many manufacturing industries that use cement, some of which can be very sensitive to cement composition changes, especially fillers, hence it is very difficult to produce one cement to suit all applications.
Similar to the admixture industry, there are many other products now available that are finding their way into everyday usage in concrete such as fibres (steel, polypropylene, macro polymeric); SCM’s (silica fume, natural microsilica); fillers (flyash, limestone, rock flour); and recycled materials (crushed concrete, glass, plastic etc).
Then there is the whole decorative concrete market which can allow a mix designer to be creative in producing freestyle concrete with personality and expression. This often includes such additives as colour pigments, specially sourced aggregate, shell or glass. This can be a challenge to a mix designer, balancing the aesthetics and performance with the necessary building requirements.
Exciting new materials for concrete making will continue to be developed, at the same time there are small discoveries that can be made everyday from practical observation and listening to what customers say. Often the best way of solving a particular concrete problem, when the cause might not be clear, is to just try something different and following up by measuring the response and getting reliable feedback from the end users.
Contractor Vol.33 No.2 March 2009
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