eLibrary Ready Mixed Concrete Concrete Fresh Density
| Concrete Fresh Density |
| Friday, 20 March 2009 19:51 |
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Concrete fresh density should not be confused with hardened cube density, they are not the same. However, a relationship can be established between concrete fresh density and the associated 28 days saturated hardened cube density. As cement hydration takes place it will continue rapidly up to approx 28 days for PC cements and up to approx 56 days for cement replacement concrete mixes (GGBS or PFA). As the concrete gets stronger it also increases in physical density (look up, ' the effects of concrete hydration' on the internet for a more detailed discussion about hydration). A typical relationship is, for example, a quartzitic naturally rounded gravel aggregate combined with a ordinary Portland cement may increase in weight by approx 40 kg per cubic metre or 1.6% after 28 days curing time. However, this may differ slightly throughout the cement content range for the same mix design and therefore becomes a variable. Relationship analysis finds it difficult to deal with variables be they known or unknown. Why go to all the trouble trying to find out the relationship between fresh wet density and hardened cube density when the comparative analysis is already available with non-variable data, i.e. the hardened cube density of the original trial mixes and production concretes. The whole idea about concrete family relationships is that we have a benchmark established against which we analyse current product performance. Concrete density is no different, why not simply compare hardened cube density to the original trial data cube density. The variations measured will be confident as the benchmark is a known constant which is the original raw data of the hardened cube density! All things being equal, by using hardened cube density we can accurately and more importantly, confidently, detect abnormal trends and deviation from the intended design. The one thing that would reduce confidence in any density retro-analysis is the cube storage conditions. The curing conditions are critical for consistency of any analysis. Confident consistency can only be achieved in a controlled laboratory environment where daily records are kept on curing conditions both ambient and curing tank water temperature. If things go wrong with the concrete density analysis the lab records will be first place to check. |
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