Endocrine disrupting chemicals in wastewater treatment plants in Kenya, East Africa: Concentrations, removal efficiency, mass loading rates and ecological impacts.

This study investigated the levels, mass loadings, removal efficiency, and associated ecotoxicological risks of selected endocrine disrupting chemicals (EDCs), namely, dibutylphthalate (DBP), diethylhexylphthalate (DEHP), dimethylphthalate (DMP), linuron (LNR) and progesterone (PGT) in wastewater, sludge, and untreated dry biosolid (UDBS) samples from twelve wastewater treatment plants (WWTPs) in nine major towns in Kenya. Analysis was done using high-performance liquid chromatography coupled with triple quadrupole mass spectrometry (LC-MS/MS). All the wastewater influents had quantifiable levels of EDCs with DBP being the most abundant (37.49%) with a range of 4.33 ± 0.63 to 19.68 ± 1.24 µg L-1. DEHP was the most abundant in sludge and accounted for 48.2% ranging between 278.67 and 9243.49 ng g-1 dry weight (dw). In the UDBS samples, DEHP was also the most abundant (40%) of the total EDCs detected with levels ranging from 78.77 to 3938.54 ng g-1 dw. The average removal efficiency per pollutant was as follows: DMP (98.7%) > DEHP (91.7%) > PGT (83.4%) > DBP (77.9%) > LNR (72.2%) which can be attributed to sorption onto the biosolid, biological degradation, photolysis, and phytoremediation. The pH was negatively correlated to the EDC concentrations while total dissolved solids (TDS), chemical oxygen demand (COD), biochemical oxygen demand (BOD5), and electrical conductivity (EC) were positively correlated. The mass loadings were as high as 373.33 g day-1 of DBP in the treatment plants located in densely populated cities. DEHP and PGT had their Risk Quotients (RQs) > 1, posing a high risk to biota. DMP, DBP, and LNR posed medium risks as their RQ values were between 0.1 and 1. EDCs are therefore loaded to environmental compartments through either the effluent that loads these pollutants into the receiving aquatic ecosystem or through the UDBS, which are used as fertilizers in agricultural farmlands causing potential toxicological risks to aquatic and terrestrial life.

Properties of concrete mixes containing tire rubber and brick powder exposed to sulfuric acid and cured in water: A comparative study.

The existing literature shows that rubberised concrete suffers from reduced mechanical properties when it is compared with normal density non-rubberised concrete. This is due to the underlying reduced bonding between tire rubber and other concrete ingredients. The massive sulfuric acid attack in rubberised concrete must have additionally discouraged researchers from attempts to assess the phenomenon of improving performance of rubberised concrete. A research was undertaken to compare the properties of concrete mixes containing tire rubber replacing coarse aggregate and waste clay brick powder (WCBP) replacing cement exposed to sulfuric acid and cured in water. Concrete cubes and cylinders of concrete grades of 20 MPa, 25 MPa and 30 MPa were immersed in 5% sulfuric acid solution up to 90 days following moist curing of 27 days. Other concrete cubes and cylinders were cured in water for comparison. The compressive strength findings indicated that all the specimens exposed to sulfuric acid had lost more than 57% of their compressive strengths after 90 days with reference to the corresponding samples cured in water. In contrast, out of all concrete mixes investigated for all concrete grades, never were the split tensile strength losses of the specimens exposed to sulfuric acid greater than 43.1% compared with those cured in water. In each exposure condition, concrete mixes with 5% WCBP showed slight improvements in compressive and split tensile strengths in contrast with the conventional concrete mixes. Visual inspection of the specimens illustrated depositions of flaky or white substances on the outer layers of specimens exposed to sulfuric acid compared with specimens cured in water. Moreover, the split tensile strengths of specimens were not severely affected with exposure to sulfuric acid in comparison with compressive strengths. Eventually, the research identified the existence of WCBP in rubberised concrete as a promising criterion of minimising strength losses of rubberised concrete.

Simplified shear equation of deep concrete beam considering orientation effect of opening and mechanical properties of fibre-concrete interface.

In the present era of technology, the design of structural deep concrete beams is highly modernized through the use of computer-aided tools. However, theories for such design are the paramount aspects to be understood particularly when the beam has openings. To improve the mechanical properties of the deep beam with openings, recycled tyre steel fibres are required. To estimate the bond strength of the fibre-concrete interface, the concrete shrinkage strains for fibre lengths 30, 50 and 60 mm with a content of 0.5% were considered. To mitigate the conservativeness of some available shear models and improve the design of deep beams, the simplified shear equation model was developed. The model was established using a simplified compressive stress block, forces in steel reinforcement and shear stress at the fibre-concrete interface. The combined effect of opening height and length was also considered in the model. The results show that incorporating fibres in concrete increase the shear performance of deep concrete beams with openings due to high strains in the shear zone indicating high loads being transferred. For instance, beam BS2 had a strain of 0.0153 in the lower load path compared to 5.6 × 10-5 for beam BC2 There was a good correlation between measured and proposed shear capacities with t-test values of 0.46, 0.996 and 0.003 for beams without fibres, and with fibres and mesh respectively. Results also showed the model shear equation performed better compared to other equations with mean absolute error (MAE) and coefficient of variation (COV) of 9.3 and 18.9%, respectively for the control beams with openings. The model also showed a mean absolute error (MAE) and coefficient of variation (COV) of 11.6 and 7.4%, respectively for the beams with fibres. The COV and MAE for the proposed model were small than those in the database, therefore, the proposed model can provide the precise design of deep concrete beams with openings and fibres.

Behaviour of rubberised concrete with waste clay brick powder under varying curing conditions.

Recently, there has been a worldwide scarcity of pure water for curing concrete and this has called for alternative curing conditions including utilisation of sea water. An experimental study was conducted to examine the mechanical behaviour of rubberised concrete with waste clay brick powder (WCBP) under different conditions of curing including water and sea water. The samples of rubberised concrete incorporated with WCBP were cured in water and sea water for 90 days curing period. The findings showed that the conventional and modified concrete mixtures which were cured in sea water illustrated reduced compressive and split tensile strengths compared with corresponding mixes cured in water. Among specimens cured in each curing condition, concrete mixes with 5% WCBP showed increased compressive and split tensile strengths compared with the control concrete mixes. The lowest compressive and split tensile strength findings were noticed with rubberised concrete incorporated with WCBP. The comparisons of densities of specimens cured in water and sea water showed no significant distinctions between the curing conditions. Compressive strength seemed to be less sensitive to conditions of curing compared with split tensile strength. From the findings, minor reductions in compressive strengths for samples cured in sea water compared with those cured in water were suggested to be reflections of possibility of utilising sea water as a curing agent in areas where pure water is very scarce. The findings in this study seem to suggest that the use of sea water in concrete curing should not be feared and could be welcome, particularly in offshore constructions and isolated islands.

A study on mechanical properties of rubberised concrete containing burnt clay powder.

An experimental study was conducted to investigate the mechanical performance of rubberised concrete containing Burnt Clay Powder (BCP). Waste Tire Rubber (WTR) and BCP were used to replace coarse aggregate and Ordinary Portland Cement (OPC) respectively. Class 20, 25 and 30 concrete mixes based on British Research Environment (BRE) were cast and tested for compressive, split tensile and flexural strengths. The findings of the tests revealed reductions in compressive and split tensile strengths for concrete mixes with 5% BCP compared to control concrete mixes for 7, 28 and 56 days curing periods. However, inclusion of BCP in concrete seemed to increase the compressive and split tensile strengths of concrete compared to control concrete at 90 days curing period. The findings also demonstrated that WTR content as high as 20% by aggregate total volume could be used to generate rubberised concrete containing 5% BCP with compressive strengths of 18-33 MPa for class 20, 25 and 30 concrete mixes. The flexural strength of unreinforced beams decreased due to inclusion of 5% BCP compared to control concrete after 28 days of curing. Rubberised concrete with BCP was observed to promote ductile failure of concrete cubes while control concrete cubes exhibited brittle failure. The inclusion of 5% BCP in concrete seemed to decrease compressive and split tensile strengths at lower curing periods while still presenting improved results at longer curing period.

Response surface methodology-based optimisation of cost and compressive strength of rubberised concrete incorporating burnt clay brick powder.

Modified concrete is increasingly being produced by substituting concrete constituents with waste materials. Among waste materials are powder from waste clay bricks replacing Ordinary Portland Cement and Waste Tire Rubber (WTR) replacing natural coarse aggregate. However, the use of modified concrete is controlled by its cost-performance balance. This paper investigates the cost advantages of using rubberised concrete incorporated with Burnt Clay Brick Powder (BCBP) where findings are evaluated in comparison with conventional concrete. In this study, compressive strength of rubberised concrete containing BCBP was investigated using mixes generated by Response Surface Methodology (RSM). Central Composite Design (CCD) based on RSM was used to assess the influence of replacement variables of BCBP (0-5%) and WTR (0-20%) on concrete production cost and concrete compressive strength responses. First order and second order mathematical models were developed by RSM with findings from experimental design. The accuracy of the mathematical models established by CCD was tested using Analysis of Variance (ANOVA). Desirability analysis was then employed to optimise BCBP and WTR contents yielding maximum compressive strength at lower cost. Moreover, under the established optimum conditions, the performance of the optimum independent variables was experimentally verified by testing 6 cubes. Production cost of concrete containing these waste materials reduced up to 4.23% compared to conventional concrete. RSM evaluation demonstrated that the empirical findings were well suited into linear and quadratic models for cost and compressive strength responses respectively. The coefficients of determination of greater than 0.85 for all responses established that the models were capable of explaining variability in the responses. 5% BCBP and 6.875% WTR were optimum contents establishing maximum 7-days compressive strength of 27.607 MPa at lower cost of KSh 13 718.43. Optimisation of cost and 28-days compressive strength from desirability analysis gave 5% BCBP and 5.844% WTR contents as optimum values. This optimum combination resulted to maximum compressive strength of 33.970 MPa and lower cost of KSh 13 734.64. Verification of the model findings indicated considerable agreement with the verified values. From the findings, it was confirmed that a reasonable cost-performance balance for modified concrete can be achieved using BCBP and WTR.

Ductility performance of reinforced rubberised concrete beams incorporating burnt clay powder.

Application of rubberised concrete in earthquake prone areas is of significant importance. Although investigations have been conducted to research on the ductility of rubberised concrete, the behaviour of rubberised concrete with Burnt Clay Brick Powder (BCBP) is not well understood. This paper captures the ductility behaviour of rubberised concrete containing BCBP. In this study, 3 beams were investigated in flexure while the other 3 beams were made to fail in shear and flexure. For the beams that failed in flexure, ductility of concrete beams containing 5% BCBP and 10% Waste Tire Rubber (WTR) improved by 23.47% compared to control beam. This increase in ductility was evidenced with only 15.31% reduction in flexural load. Moreover, the beam containing 5% BCBP and 10% WTR failing in shear and flexure exhibited 14.59% ductility improvement with 16.33% load reduction in comparison to the control beam. Eventually, the study concluded that it is possible to achieve improved ductility without substantial loss in ultimate failure load by using 5% BCBP and 10% WTR. Such properties demonstrated that this rubberised concrete with 5% BCBP can be used in seismic applications.