Application of √t-type, logarithmic and half-time methods in desorptive measurements of diffusivity in narrow humidity ranges

This paper concerns the use of non-stationary desorptive measurement techniques for defining the mass diffusivity of cement based materials. Three different procedures are presented: √t-type calculation; logarithmic; and half-time procedures. Cement mortars of different water to cement ratios (w / c), equal to 0.50, 0.65 and 0.80 were selected as the model environment for testing the usability of the above-mentioned desorptive techniques. The study was carried out at the temperature (T) of 20 °C within narrow relative humidity (φ) ranges: from φ1 = 30% to φ2 = 12%, and 50% → 30%, 75% → 50%, 85% → 75%, 97% → 85%. The results obtained are used to evaluate the conformity of these methods. The conformity is analyzed with regard to each mortar in all the above humidity ranges Δφ. The values of diffusivity Dm, defined by means of the √t-type calculation and the logarithmic procedure, demonstrated rather high conformity, all relative differences between Dm(√t) and Dm(ln) did not exceeded 20%. However, the half-time procedure can be applied for rough estimation of the diffusivity only. That is because deviations between Dm(t1 / 2) and the values found by means of the two other methods were too large.