We have discussed necessity of analysis of fresh concrete; the analysis means mainly determination of water/cement ratio and cement content. Cement-content have been discussed in previous post, here we will discuss about water/cement ratio.
ASTM C1079 suggested us a procedure to determine free water in concrete and with the help of ASTM C1078 we can determine cement content, both will produce water-cement ratio. Both prescribe us chemical procedures which require sophisticated equipment and skilled operator as well which is not usually available in laboratory.
A chloride titration test have suggested by U.S. Army for determining water content which is a very rapid procedure but calcium-titration procedure for determination of cement content have some limitation (as discussed in previous post) and together they may produce erroneous result.
BS 1881: Part 2 suggested a method based on buoyancy for determination of water/cement ratio of fresh mix, but later this method was withdrawn by them. Ramme and Naik also used same principle for determining water/cement ratio which need a parameter, aggregate/cement ratio. This ratio may sometimes be unreliable or uncertain.
When water content of fresh concrete has to be determined only, a more accurate method based on scattering of neutrons emitted by influence of heat can be used. This procedure requires dry density of aggregate used in concrete mix.
The source of emission of thermal neutrons should be placed in bulk of aggregate incorporated in the mix or within test sample of concrete mix. The principle is- hydrogen has property of retardation and scattering of thermal neutrons; as hydrogen is an important element of water, this nuclear method offer us an accuracy of around ±0.3% in determining water content in concrete mix.
Regarding dry density, another source of gamma radiation (back scattered) is used to determine this. Thus the procedure requires two sources
• Thermal neutron source
• Gamma radiation source
The apparatus as a whole consist of
• Thermal neutron and gamma source
• Scintillation and neutron detectors
• Related counters
The limitation of this method is it takes time to measure as calibration has to be done in situ. So in determining water content there have a reliable method but not practicable in all situation. Again only water content determination not offer accurate water/cement ratio, the accuracy of cement content is also important. Thus analysis of fresh concrete is difficult; indeed there have no convenient test method; and not enough to certify a mix before placement.