Search

Five full-size concrete replacement slabs were constructed and tested by a heavy vehicle simulator (HVS) at 6 h after concrete placement to study the early strength requirement of concrete for slab replacement. Stress analyses of the test slabs under the HVS loads were performed using the FEACONS (finite element analysis of concrete slabs) program. Changes in the concrete properties with time and the actual temperature differentials in the concrete slab were incorporated in the analysis. The maximum induced stress is a function of the applied load and pavement design, and also a function of the elastic modulus of the concrete, temperature differential in the slab, moisture, creep, shrinkage and the coefficient of thermal expansion of the concrete. This requires that the anticipated maximum stress be properly determined. Having a flexural strength of greater than the anticipated maximum induced stress at the time the slab is open to traffic will ensure that the concrete will not fail prematurely at early age. A conceptual approach on how to determine the required property for concrete for slab replacement at different environmental conditions is presented in this paper. DOI: 10.1061/(ASCE)0733-947X(2006)132:10(781) CE Database subject headings: Concrete slabs; Rehabilitation; Rigid pavements; Stress analysis; Cracking; High strength concrete; Finite element method.

A three-dimensional air quality model proved useful in examining the uncertainty in reactivity quantification resulting from a set of reaction rate constant uncertainties. Uncertainties in rate constants for aldehyde photolysis, nitric acid formation and decomposition of peroxy acetyl nitrate and peroxy propionyl nitrate were found to have the greatest effect on relative compound reactivity values.

The effect of atmospheric aerosols and regional haze from air pollution on the yields of rice and winter wheat grown in China is assessed. The assessment is based on estimates of aerosol optical depths over China, the effect of these optical depths on the solar irradiance reaching the earth's surface, and the response of rice and winter wheat grown in Nanjing to the change in solar irradiance. Two sets of aerosol optical depths are presented: one based on a coupled, regional climate/air quality model simulation and the other inferred from solar radiation measurements made over a 12-year period at meteorological stations in China. The model-estimated optical depths are significantly smaller than those derived from observations, perhaps because of errors in one or both sets of optical depths or because the data from the meteorological stations has been affected by local pollution. Radiative transfer calculations using the smaller, model-estimated aerosol optical depths indicate that the so-called 'direct effect' of regional haze results in an [approximately equals] 5-30% reduction in the solar irradiance reaching some of China's most productive agricultural regions. Crop-response model simulations suggest an [approximately equals] 1:1 relationship between a percentage increase (decrease) in total surface solar irradiance and a percentage increase (decrease) in the yields of rice and wheat. Collectively, these calculations suggest that regional haze in China is currently depressing optimal yields of [approximately equals] 70% of the crops grown in China by at least [approximately equals] 5-30%. Reducing the severity of regional haze in China through air pollution control could potentially result in a significant increase in crop yields and help the nation meet its growing food demands in the coming decades.