Nitrate content in lettuce (Lactuca sativa L) heads in relation to plant spacing,nitrogen form and irrigation level

A field study was conducted at two locations (Jordan valley and Al‐Jubeiha) with different rainfall levels, altitudes and temperature ranges. The study was established to evaluate the optimum planting density, nitrogen (N) form and irrigation level to attain the best quality of lettuce crop in terms of minimum nitrate (NO₃) content and to minimise the impact on the environment. Seeds of ‘Amar’ lettuce were sown 1 month before transplanting. Three forms of N fertiliser (Ca(NO₃)₂, (NH₄)₂SO₄ and CO(NH₂)₂) were applied at three times at a total rate of 100 kg N ha^?1. Three in‐row spacings (15, 20 and 25 cm) were assigned. Two irrigation levels were applied: level 1 had twice the amount of irrigation water as level 2, which was achieved by doubling the number of irrigation lines. The results indicated that N form significantly increased both N and NO₃ contents. Ca(NO₃)₂ was the most effective in increasing the N and NO₃ contents in lettuce leaf tissues, followed by CO(NH₂)₂ and then (NH₄)₂SO₄. The outer leaves (green colour) had about five times the NO₃ content of the inner leaves (pale yellow colour). However, the effect of N form on production, total N absorption and N recovery was significant in the following order: (NH₄)₂SO₄ > Ca(NO₃)₂ > CO(NH₂)₂. Increasing the plant spacing resulted in a significant increase in N and NO₃ contents in the lettuce leaves. Copyright © 2004 Society of Chemical Industry     

Strength reduction factors for structural rubbercrete

Many researches have been carried out to study the fresh and hardened properties of concrete containing crumb rubber as replacement to fine aggregate by volume, yet there is no specific guideline has been developed on the mix design of the rubbercrete. The experimental program, which has been developed and reported in this paper, is designed and executed to provide such mix design guidelines. A total of 45 concrete mixes with three different water to cement ratio (0.41, 0.57 and 0.68) were cast and tested for fresh and mechanical properties of rubbercrete such as slump, air content, unit weight, compressive strength, flexural strength, splitting tensile strength and modulus of elasticity. Influence of mix design parameters such as percentage of crumb rubber replacement, cement content, water content, fine aggregate content, and coarse aggregate content were investigated. Three levels of slump value (for conventional concrete mixes) has been selected; low, medium and high slump. In each slump level, water content was kept constant. Equations for the reduction factors (RFs) for compressive strength, flexural strength, splitting tensile strength and modulus of elasticity have been developed. These RFs can be used to design rubbercrete mixes based on the conventional mix (0% crumb rubber content)     

Evaluation of rubbercrete based on ultrasonic pulse velocity and rebound hammer tests

Several research works have been carried out to study the fresh and hardened properties of concrete containing crumb rubber (rubbercrete) as a replacement of fine aggregate. The outcomes of these studies have highlighted the advantages and disadvantages of rubbercrete compared with conventional concrete mixtures. In view of the fact that rubbercrete is being used in the construction industry for a variety of purposes, evaluations of the rubbercrete mixtures using non-destructive tests such as rebound hammer (RH) and ultrasonic pulse velocity (UPV) to establish valid relationships is worthwhile. Fifteen mixtures with different w/c ratios (ratios of weight of water to weight of cement) and crumb rubber content percentages were prepared, cast and tested using RH and UPV at different curing ages. Models were proposed and statistically validated to predict the relationship between compressive strength with UPV and rebound number (RN) for rubbercrete mixtures at 3, 7 and 28 days.