Durability of cement pastes modified by polymer dispersions

The time-dependence of some of the properties of Portland cement pastes modified by several polymer dispersions has been investigated up to one year under two curing conditions namely, (a) in air of 65 per cent rh at 20°C and (b) immersed in water at 20°C. Five different classes of polymer have been tried so far; they are polyvinyl acetates, polyvinyl propionates, butadiene styrenes, polyvinylidene dichlorides and acrylics. Two or more examples of each type of polymer have been incorporated into cement to produce pastes with a polymer to cement ratio of 0.1 and water to cement ratios of 0.3 and 0.35. The density and elastic properties of the pastes have been measured non-destructively and strength values have been determined using a universal testing machine. Attempts have been made to correlate the results of this comparative study with data on the reactivity of the polymers towards alkalis.     

Creep of Kevlar 49 fibre and a Kevlar 49-cement composite

The creep strain responses of Kevlar 49 fibres and a Kevlar 49 — cement mortar composite board to sustained stresses have been studied over an extended period in excess of four years at ambient temperature. Single filaments of Kevlar 49, 900 mm in length, were stressed in tension in the range 830 to 1830 MPa. The relationship between creep and elapsed time is represented by the power functionAt ⁿ whereA is a function of stress andn is a constant. The creep strain in Kevlar 49 was low compared with other polymers. For example after 1000 days at a stress of 1830 MPa the creep strain was 13% of the initial elastic strain and is predicted by the power function to increase to 14.6% after 4000 days. The Kevlar 49 — mortar composite was subjected to bending stresses in the range 6 to 35 MPa and the creep deflection was monitored. The relationship between creep and time could again be represented by the power functionAt ⁿ withA dependent on stress andn constant. The creep was similar to that expected from the matrix alone. The ratio of the creep deflection to the initial deflection after 1000 days at a stress of 6.15 MPa (well below the matrix cracking stress) was 1.31 and at 23.5 MPa (well above the matrix cracking stress) was 1.63.     

Designing against low-cycle fatigue at elevated temperature

Factors that are important in determining low-cycle fatigue damage at elevated temperature are discussed. The linear damage rule for computing creep-fatigue damage is shown to be unsatisfactory in many situations. The damage-rate equations developed at Argonne National Laboratory have been generalized to include multiaxial creep-fatigue under complicated loading histories. Available creep-fatigue data under combined axial-torsion loading can be explained in a consistent manner by the damage-rate approach.     

Viscosity variation of N-cetyl pyridinium chloride—high concentration range

N-cetyl pyridinium chloride in aqueous solution is found to obey the equation of fluidity, lnη_rel = (kN)/[N₀(N₀ ? N)] up to a high concentration, almost saturation. Despite micellar association the plot of ln η_relvsN/(N₀ ? N) was found to be linear for N₀ = 5.0, the hypothetical concentration at which glass transition would occur. The linearity of the plot is maintained in the presence of an added electrolyte, sodium chloride producing lower values for N₀ as the concentration of sodium chloride is increased.     

The durability of glass fibre cement-the effect of fibre length and content

The properties of glass reinforced cement composites (grc) containing 2–8 vol % of alkali resistant glass fibres of lengths 10–40 mm have been studied for periods of up to 5 years in various environments. Fibre volume fraction was found to be an important factor influencing the strength of grc at all ages, while fibre length was of decreasing significance as storage periods in wet environments increased. In relatively dry conditions, little change with time of bending, tensile or impact strengths was observed, but the matrix cracking stress was reduced. In wet environments, the cracking stress tended to increase but the ultimate strength to decrease.At 28 days maximum strength was achieved with composites having 6 to 8 vol % fibre 30 to 40 mm long. Composites with similar formulations were found to have the greater strength after 5 years' storage but, after water storage or natural weathering a strength reduction had occurred. Bending strength was approximately 70% to 86% of its 28 day value, tensile strength between 55% and 84% and impact strength 32% to 78%. Young's modulus is largely dependent upon the degree of hydration of the cement matrix and in the long-term was greater for water-stored material than for that stored in dry air.     

Active cellular preform derived from stems of jute and sticks of cane and their suitability for bulk porous Si/SiC ceramics

Stems of Jute (Corchorus capsularis L.) and sticks of Cane (Calamus rotang L.), plants of immense economic importance in the Indian subcontinent, were converted into carbonaceous perform (C-preform) maintaining the circular cylindrical shapes in lengths of 0.02–0.05 m by controlled thermal processing. Plant material precursors were characterized by analysis of elemental (C, H, N) and molecular (cellulose, hemicellulose, lignin) compositions, by determination of Bulk Density (BD) and ash content and by optical microscopy and X-ray diffractometry (XRD). C-preforms were also characterized by measurement of BD and by Scanning Electron Microscopy (SEM) and XRD. The C-preforms were further subjected to infiltration with Si-melt (1823–1923 K) under vacuum. Spontaneous infiltration and reaction yielded composite ceramics preserving the morphology of native Jute Stem (JS) and Cane Stick (CS) precursors on macro and micro scale. The materials were found to be duplex composites with Si and β-SiC as crystalline phases. The end ceramics were characterized by measurement of BD, and also by SEM and by XRD. Measured mean BD of the Si/SiC composites derived from JS and CS were 2190 Kg m⁻³ and 2250 Kg m⁻³. The respective volume fractions of large diameter (>100 μm) bulk pores were 0.134 and 0.204, in the composites derived from JS and CS. Taking into account the measured volume fraction internal pores of 0.11 and 0.149, the volume fractions of SiC were calculated to be 0.136 and 0.307 in the composites derived from JS and CS respectively, closely tallying with those calculated from the C-preform bulk densities. The cellular Si/SiC ceramics derived from JS and CS having special morphologies with long and large porous channels and oriented growth of constituent phases are likely to be suitable for devices such as high temperature insulators, catalyst support structures for gas phase reactions at elevated temperatures, molten metal filters and others.     

Comparative analysis of in vitro ultraviolet radiation protection of fabrics woven from cotton and bamboo viscose yarns

Ultraviolet protection factor (UPF) of fabrics made of 100% cotton and 100% bamboo viscose yarns were studied and a comparative analysis carried out using curve fitting technique. Bamboo viscose fabrics showed higher shrinkage, cover percentage, areal density and UPF compared to its cotton counterpart woven with identical yarn counts and fabric sett. However, the predictive model of cotton fabric UPF using fabric areal density as the input was able to estimate the UPF of bamboo viscose fabrics with very good accuracy. Furthermore, the 100% cotton and 100% bamboo viscose fabrics showed the same UPF if their cover percentage and areal density is similar. It is inferred from the analysis that the apparently higher UPF of bamboo viscose fabrics can be attributed to their higher cover percentage and areal density instead of bamboo’s inherent UV protective property which has been claimed in various literatures.     

Mechanics of filled jute bag

A simplified model of a filled-in jute bag in the form of composite shell of three segments, each of which is generated by surface of revolution, is assumed. The development and distribution of stresses in the different portions of the composite shell under the action of uniform internal pressure is calculated using membrane theory. The portion subjected to maximum stress and the controlling factors in the failure of bag, such as bag dimension and width/length ratio of bag, are identified. These findings corroborate the observations related to failure of filled jute bags in actual use.