Key wo reis Urea formaldehyde resin Reaction pH Molar ratio Molecular weight I3 C NMR spectra Bond strength . Formaldehyde emission SummaryUrea formaldehyde resins were formulated with combination variables of three reaction pH (1.0, 4.8, and 8.0) and four molar ratios of formaldehyde to urea (2.5, 3.0, 3.5, and 4.0). The resins were prepared by placing all formaldehyde and water in reaction kettle and pH was adjusted with sulfuric acid and sodium hydroxide, respectively. Urea was added in 15 equal parts at 1-minute intervals. The proportion of high molecular weight products in the resin increased substantially äs the reaction pH decreased. Furthermore, the F/U molar interacted with reaction pH to effect resin molecular weight. At acidic pH, the high molecular weight products increased äs F/U ratio decresed; while, at alkaline pH, little difference was evident between the high or low molecular weight products at various F/U ratios. The formation of a high percentage of uron derivatives under strong acidic conditions also indicated these resins differed considerable from conventional UF resins formulated in the past. Panels bonded with resins catalyzed at strong acidic conditions resulted in lowest formaldehyde emission but slightly lower bonding strength. Of the three pH conditions evaluated in the study, both weak and strong acid catalysts Systems are not commonly used in conventional UF resin formulation. Based on the bond strength and formaldehyde emission data, however, the weak acid catalysts seems to provide the best compromise between the strong acid and the conventional alkaline-acid catalyst System currently used for formulating UF resin wood adhesives. Procedure Resin preparationAll UF resins were prepared in the laboratory. Each resin preparation was replicated one time. To prepare each resin, all formaldehyde and water were placed in a reaclion kettle and pH was adjusted with sulfuric acid and sodium hydroxide, respectively.Holzforschung / Vol. 48 / 1994 / No. 6
The present work is aimed at studying the determination method and implementation process of reasonable completion state for the Hunan Road Bridge, which is currently the widest concrete self-anchored suspension bridge in China. The global finite element model and the cable analytic program BNLAS were integrated. The synthesis algorithm of completion state determination was proposed. The contact relationships between the cable and saddles were captured using the refined FE discretization method. The concrete shrinkage and creep effects during the construction and operation periods were predicted using the CEB-FIP 90 model and the age-adjusted effective modulus method. The cable alignments under the free cable state, system transformation condition, and completion state were obtained. Moreover, the multiple-control method for the whole process of system transformation was proposed. The multiple parameters included the hanger tensioning force, exposed amount of hanger anchor cup, and tag line position. A detailed system transformation procedure was formulated and well preformed in the construction site. In addition, the further optimization analysis of final hanger force was conducted based on the actual completion state. The influence on the stress and geometry evolution of girder brought by the final girder alignment was investigated. The measured results of structural alignment and stress show that the target completion state was well implemented. The accuracy and efficiency of the proposed multiple-control method were verified by checking the tag line position of each step. In addition, the optimized final hanger force and girder lifting amount were obtained, which can provide feedback and reference for the construction control and service safety of the similar concrete self-anchored suspension bridges.
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