Mechanical performance and capillary water absorption of sewage sludge ash concrete (SSAC)

Disposal of sewage sludge from waste water treatment plants is a serious environmental problem of increasing magnitude. Waste water treatment generates as much as 70 g of dry solids per capita per day. Although one of the disposal solutions for this waste is through incineration, still almost 30% of sludge solids remain as ash. This paper presents results related to reuse of sewage sludge ash in concrete. The sludge was characterised for chemical composition (X-ray flourescence analysis), crystalline phases (X-ray diffraction analysis) and pozzolanic activity. The effects of incineration on crystal phases of the dry sludge were investigated. Two water/cement (W/C) ratios (0.55 and 0.45) and three sludge ash percentages (5%,10% and 20%) per cement mass were used as filler. The mechanical performance of sewage sludge ash concrete (SSAC) at different curing ages (3, 7, 28 and 90 days) was assessed by means of mechanical tests and capillary water absorption. Results show that sewage sludge ash leads to a reduction in density and mechanical strength and to an increase in capillary water absorption. Results also show that SSAC with 20% of sewage sludge ash and W/C=0.45 has a 28 day compressive strength of almost 30 MPa. SSAC with a sludge ash contents of 5% and 10% has the same capillary water absorption coefficient as the control concrete; as for the concrete mixtures with 20% sludge ash content, the capillary water absorption is higher but in line with C20/25 strength class concretes performance

Effect of mixture proportions on the drying shrinkage and permeation properties of high strength concrete containing class F fly ash

Sustainability of concrete can be improved by using large volume of fly ash as a replacement of cement and by ensuring improved durability of concrete. Durability of concrete containing large volume of class F fly ash is dependent on the design of mixture proportions. This paper presents an experimental study on the effect of mixture proportions on the drying shrinkage and permeation properties of high strength concrete containing large volume local class F fly ash. Concrete mixtures were designed with and without adjustments in the water to binder ratio (w/b) and the total binder content to take into account the incorporation of fly ash up to 40% of total binder. Concretes, in which the mixture proportions were adjusted for fly ash inclusion achieved equivalent strength of the control concrete and showed enhanced properties of drying shrinkage, sorptivity, water permeability and chloride penetration as compared to the control concrete. The improvement of durability properties was less significant when no adjustments were made to the w/b ratio and total binder content. The results show the necessity of the adjustments in mixture proportions of concrete to achieve improved durability properties when using class F fly ash as a cement replacement

Effects of silica fume fineness on mechanical properties of steel fiber reinforced lightweight concretes subjected to ambient and elevated temperatures exposure

This paper presents the effects of silica fume (SF) fineness and fiber aspect ratios of steel fiber on fresh and harden characteristics of high-strength lightweight concrete containing oil palm shell as coarse aggregates. The effect of elevated temperatures on the residual compressive strength of above concretes is also evaluated in this study. Three different SF fineness of 18400, 21000, and 28000m 2 /kg and 2 different aspect ratios of steel fiber of 40 and 80 are considered. Results show that the increase in SF fineness and steel fiber aspect ratio marginally affect the air-dry density of steel fiber reinforced lightweight high-strength concretes, however, the workability is reduced by about 9% to 14% due to increase in SF fineness. The compressive strength of steel fiber reinforced lightweight concretes at all age increases with increase in SF fineness and an improvement of about 37% is observed at 56days by increasing the SF fineness from 18400 to 28000m 2 /kg. Strong correlations are also observed between the strength improvement factor and the SF fineness. Water absorption of above concretes is also reduced by 3% to 14% due to increase of SF fineness from 18400 to 21000 and 28000m 2 /kg. The increase of SF fineness also significantly reduces the residual compressive strength loss at 300°C and 450°C. This loss of residual compressive strength is lower in lightweight concretes containing 16mm long steel fiber than 8 mm long steel fiber. The existing Eurocode model overestimates the residual compressive strength of steel fiber reinforced lightweight concretes containing no SF, however, this discrepancy is significantly reduced with increase in SF fineness

Effect of mixing methods of nano silica on properties of recycled aggregate concrete

The effect of presoaking of recycled coarse aggregate (RCA) in nano silica (NS) solution on the properties of recycled aggregates concrete is compared with the direct mixing of NS in concrete. In this study, four series of concrete are considered. The first and second series are control series containing 100% natural coarse aggregate (NCA) and 100% RCA, respectively. The third and fourth series are similar to the second series in every aspect except that 2 wt% NS is added through direct mixing in concrete and presoaking of RCA in 2 wt% NS solution of 24 hr, respectively. Results show that the measured 28-day compressive strength of recycled aggregate concrete containing presoaked RCA is higher than that containing direct mixing of NS in concrete. The volume of permeable voids, water sorptivity, and chloride ion penetration measured after 28 days curing is also reduced in recycled aggregate concrete containing presoaked RCA in NS solution than that containing direct mixing of NS in concrete. The total porosity of mortar adjacent to presoaked RCA is also found to be significantly lower than the same, where NS is mixed directly. The concentration of pores in the matrix adjacent to presoaked RCA is also shifted to medium capillary pores from the large capillary pores of matrix adjacent to RCA containing no NS. The volume of medium capillary pores reduced significantly in the matrix adjacent to presoaked RCA compared to the same, where NS is mixed directly. The scanning electron microscopic image analysis also supports this observation