Experimental and Theoretical Analyses on the Density and Modulus Development of Concrete Under Continued Hydration

In this paper, new models of the density and modulus development of concrete under continued hydration were studied. Experimental study was performed for different mixes of concrete. To avoid considering the effect of variation of Poisson＇s ratio, the one-dimensional ultrasonic technique was adopted to detect the modulus development of concrete under continued hydration. The experimental results indicate the nonlinear characteristics of density and modulus evolution. At the initial stage of continued hydration, the density and modulus increase quickly, and then the increases slow down and finally tend to be constant. The mechanism of modulus enhancement is that the newly produced C-S-H gel in the continued hydration process not only leads to the decrease in porosity, but also repairs the initial defects of concrete. Based on this mechanism, simple differential equations for the density and modulus development of concrete were established by considering the chemical reactions of continued hydration, and new simple models for density and modulus development were proposed.

Urbanisation Footprints and the Distribution of Air Quality in Nairobi City,Kenya

Various postulations on the relationship between urban morphology and air quality are qualitative.This fails to establish the strength of the contributions of each morphological parameter in the spatial distribution of the air quality.It is this gap in knowledge that this study sought to fill by modelling the correlation existing between the urban morphological variables of development density,land uses,biomass index and air quality values of Nairobi city.While 30 development zones of the city constituted the target population,IKONOS satellite imagery of the city for the year 2015 was utilised in establishing the development densities,land uses and biomass index.The parameters were transformed into numerical surrogates ranging from 1 to 10 with lower values accorded to zones with low biomass index,the highest development density,noxious land uses,high gaseous concentrations and vice-versa.Pearson’s correlation coefficients(r),coefficients of determination(R),t-tests and the Analysis of Variance(F-tests)with levels of significance being 95%were used to determine the strengths,significances and consistencies of the established relationships.The study established that development density is the most significant morphological variable influencing the distribution of air quality.This is followed by biomass index and to a weaker extent,land uses.