Frictional behaviour of segmental retaining wall units infilled with recycled concrete aggregate
The use of segmental retaining wall units as the facing column for mechanically stabilized earth (MSE) retaining walls has increased noticeably all over the world. Hollow and solid, both types of modular block units are used in dry-stacked facing columns. Nowadays, Hollow concrete units are being implicated frequently because of its cost-effectiveness and other technical advantages. The cavities of hollow concrete units are filled with natural (fresh) aggregates for better shear resistance. The use of fresh aggregates as in-fillers in retaining wall constructions is expensive and unsustainable (annihilation of natural resources). This study mainly focuses on frictional behavior of newly designed and manufactured precast “I” blocks infilled with fresh and recycled aggregates. A series of tests were performed using a specially designed & fabricated direct shear apparatus to assess the frictional behavior of infilled blocks under different normal loading conditions. The tests were executed based on the exiting ASTM and National Concrete Masonry Association (NCMA) test protocols. Test results were outlined in the form of shear stress-displacement relationship to compare the effect of recycled aggregate against the fresh aggregate. Shear capacity envelopes were also plotted using Mohr-Coulomb failure criteria to find out the angle of friction for each case. Test results revealed that the angle of friction of the blocks infilled with the recycled aggregate is almost equal to those with the fresh aggregate.
Application of recycled concrete aggregates as alternative granular infills in hollow segmental block systems
Abstract Geosynthetic Reinforced Segmental Retaining Walls (GR-SRWs) are developing as well recognized earth retaining structures for their manifold economic and technical advantages. It is a matter of great concern that still now a huge volume of natural coarse aggregate is used as filler in GR-SRWs, thus depleting natural resources and presenting a challenge for sustainable development. The main objective of this study is to examine the effect of recycled coarse aggregates (RCAs) used as alternative granular infills in hollow facing column. As granular infills, two types of recycled aggregates were used along with natural coarse aggregates (NCAs). Recycled aggregates were primarily selected based on the grading of the source waste concretes to investigate its effect on the frictional behavior of recycled aggregates used as infillers. Purely frictional capacity of I-Block infilled with recycled aggregates was compared to those infilled by fresh aggregates. To eliminate the effect of shear pins on interface shear capacity of I-Block system infilled with gravels, none were used in purely frictional shear. A series of direct shear tests was performed using a specially designed and fabricated direct shear apparatus to assess the frictional behavior of infilled blocks under different normal loading conditions. The tests were conducted following the exiting ASTM and National Concrete Masonry Association (NCMA) test protocols. The test results were outlined in the form of shear stress–displacement relationship to compare the effect of recycled aggregate against the fresh aggregate. Shear capacity envelopes were also plotted using Mohr–Coulomb failure criterion to find out the angle of friction for each case. Test results reveal that the angle of friction of the blocks infilled with the recycled aggregate is almost equal to those with the fresh aggregate. The results also show that compressive strength of the source waste concretes has a little or no effect on the frictional performance of recycled concrete aggregates used in facing units.
The Effects of Certain Polymers on Tensile strength and Tension Relaxation of Wet Web
The trend in papermaking has been towards lower basis weights, decreased amounts of softwood kraft pulp and an increased use of fillers and recycled fibers. All these changes tend to result in lower strengths of both wet and dry web. To maintain necessary strength in the papers, a greater quantity of strength additives is often required. The effect of different polymers on the dry paper strength has been widely studied and the action of mechanism of different polymers is quite well known. There is much less information of their effect on wet web properties, and especially on the tensile strength and relaxation characteristics. In this laboratory scale study, the effects of adding different polymers by spraying on wet web tensile and relaxation characteristics at high strain rate were examined. The sprayings were done on wet paper sheets before wet pressing. The results showed that spraying of polymers significantly improves wet and dry paper tensile strength without any increase in dry paper density. The effect of different polymers on wet web mechanical properties was characteristic for each polymer. Carboxymethyl cellulose (CMC) and chitosan improved wet web tensile strength at dryness levels higher than 55%, while polyvinyl alcohol (PVA) improved wet web tensile strength also at lower dryness levels. The most prominent increase in dry and wet web tensile strength was obtained by sequential application of oppositely charged polymers. However, the relative increase of tensile strength was significantly higher for the wet web (80% increase) than for the dry paper (35% increase) compared to the reference without added polymer. Also the mode of polymer addition had a significant effect on wet web tensile strength. Despite a significant increase in the wet web strength, only minor effects on the residual tension (tension after 0.475 s relaxation) was observed. Based on this result, it was concluded that spray addition of the polymers used in this study increased molecular level interactions between fibers but had no effect on the fiber network activation.