Industrial residues such as sludge from water treatment plants (Swtp) from centrifuged method; electrical porcelain residues (Pw); silica fume (Sf1 and Sf2); tire-rubber waste were evaluated in order to be used in concrete structures of electrical energy and environmental sectors, such as utility poles, crossarms, and reef balls technology. The results showed the necessity for evaluating different recycling concentrations in concrete, concomitantly to physicochemical tests allowing to diagnose natural and accelerated aging.
One of the most recent challenges of modern society is the research of new alternatives of environmentally responsible technologies for the final disposal of residues generated by industrial, domestic, and commercial sectors.
Governmental entities and international communities have been acting together to promote environment protection and pollution reduction through environmental laws concerning well-stablished residue limits and intensive fiscalization.
These actions result in viable application to diverse residues, which can be converted in useful raw materials. An example is the silica fume byproducts, which were initially considered industrial residues, but nowadays are largely used in civil construction due to its greater pozzolanic reaction capacity compared to most of hydraulic cements. Many other different byproducts can be used, like furnace slag, metakaolin, tire-rubber residues, and others [
This paper emphasizes the importance of the study of recycling conditions and/or residues disposal in civil construction, by using engineering analyses and science materials evaluation. Thus, four different residues (silica fume, sludge from water treatment plant, electrical insulator porcelain and tire-rubber wastes) were tested in concrete core samples; in reef balls and concrete blocks technology for fishing habitat and for creating biomass (animal or plant life); in typical concrete structures of the electrical energy sector, such as crossarms, utility poles, and dams.
All used materials were submitted to physicochemical analyses and were pretreated prior to concrete samples casting.
Filler-modified and sulfate-resistant (type CPII-F 32), high-early strength (type CPV-ARI RS), pozzolan-modified concrete (type CPII-Z 32), and sulfate-resistant pozzolanic (type CPIV-32 RS) Portland cements were used. Each of these was used in order to meet local standards and extend the durability of the structure in aggressive environments such as seawater and coastal regions.
Fine and coarse aggregates consisted of washed natural medium sand and crushed basalt stone with maximum nominal size of 4.8 and 19 mm, respectively. They were tested according to the recommendations of Brazilian standards. Synthetic aggregates from waste samples were crushed to both fine and coarse nominal size and also tested.
The search for economically and environmentally advantageous solutions for the treatment and sludge disposal of WTP remains a challenge, especially for developing countries, living in severe economic constraints and where health problems require emergency solutions. Monthly, approximately 4,000 tons of WTP sludge dry matters are produced throughout the state of Paraná, southern Brazil. In the city of Curitiba (population of 1.9 million people, approximately), capital of Paraná, the potable water supply is provided by Iguaçu, Tarumã, Irai, and Passaúna WTPs, which account for over 50% of all the production of sludge in the state. Passaúna WTP produces, by centrifugation method, about 360 tons/month of sludge. It is the WTP object of study in this work. For this research, collections of hebdomadaires centrifuged sludge were conducted during two months of the year. Afterwards, the final content was homogenized, oven-dried at 110°C, and disaggregated [
Artificial aggregates of medium and high voltage electrical porcelain waste were obtained by grinding the product in a hammer mill type. The crushed material was classified into different particle sizes after grinding. The fine particulate portion was used to study its potential alkali reactivity by mortar-bar method [
Condensed silica fume (Sf) is a byproduct usually originated from induction arc furnaces in the silicon metal or ferrosilicon alloy industrial processes, where the reduction of quartz to silicon at temperatures up to 2000°C produces SiO vapors, which oxidize and condense in the low-temperature zone to tiny spherical particles consisting of amorphous or noncrystalline silica. The amount of SiO2 present in this pozzolan is, invariably, close to 80% and is directly related to the existent production process [
Currently, Sf was widely used as a supplementary cement material to enhance the strength and durability of concrete. In this research, Sf was also used in order to lower the pH of the resulting concrete to facilitate settlement of marine organisms.
The use of rubber waste in concrete is important from the ecological point of view. Population growth and increased use of disposable materials such as packaging, tires, and PET bottles, among others, have caused the accumulation of large quantities of solid waste, which are limiting the capacity of landfills. In 2005, the city of Rio de Janeiro, southwest of Brazil, tires and rubber products accounted for about 0.5% of urban waste and in São Paulo, this quantity is near 3% [
Rubber band scroll waste from retreading tires was used without any pretreatment. The composition of the predominant residue was characterized as styrene butadiene rubber by infrared Fourier transform. The average particle size distribution was 4.8 mm.
Ideal concrete mix proportions (by mass for a concrete mixture (w/w)) are listed in Table
Mix ideal proportions (by mass for a concrete mixture) and properties of fresh concrete with admixtures.
Mixture/sample | Materials code | |||||
---|---|---|---|---|---|---|
Swtp | Pw |
|
|
|
Tw | |
Cement | 1 | 1 | 1 | 1 | 1 | 1 |
Sludge (water treatment plant): Swtp (%) |
|
0 | 0 | 0 | 0 | 0 |
Porcelain waste (Pw) (%) | 0 |
|
|
0 | 0 | 0 |
Silica fume (Sf) (%) | 0 | 0 | 0 |
|
|
0 |
Tire-rubber waste (Tw) (%) | 0 | 0 | 0 | 0 | 0 |
|
Fine aggregate (natural sand) | 1.860 | 1 | 347 | 1.837 | 1.541 | 1.820 |
Fine aggregate (artificial) | 0.16 | 1 | 347 | 0 | 0 | 0 |
Natural coarse aggregate (19 mm) | 2.980 | 1.497 | 512 | 3.306 | 1.541 | 2.620 |
Artificial coarse aggregate (19 mm) | 0 | 1.497 | 512 | 0 | 0 | 0 |
Water/cementitious materials ratio | 0.51 | 0.50 | 0.5 | 0.46 | 0.41 | 0.50 |
Superplasticizer | 0 | 0 | 0 | 1 | 0.3 | 0 |
Slump (mm) | 18 | 25 | 25 | 50 | 50 | 14 |
Unit weight (kg/m³) | 2,247 | 2,219 | 2,219 | 2,357 | — | 2,220 |
Air content (%) | 3.5 | 0.5 | 0.5 | — | — | 2.4 |
Notes: Portland cement types: filler modified (CPII-F 32); *high-early strength (CPV-ARI RS); **pozzolan modified (CPII-Z 32); ***sulfate resistant (CPIV-32 RS).
For each mixture and material, different types of specimens were casted: (100 × 200) mm cylindrical concrete specimens for determining compressive and flexural strength and elastic modulus at 3, 14, and 28 days after casting [ (150 × 300) mm cylindrical concrete specimens for permeability testing and determining specific density, absorption, and porosity of concrete after 28 days of curing; (300 × 100) mm cylindrical concrete plates for abrasion resistance of concrete according to ASTM C1138 [
Also, six concrete utility poles and crossarms were casted with Pw1, RC, and Sf1 and tested for flexural strength after 28 days of concrete curing and also for electrical properties and visual surface inspection during natural ambient exposition. The poles were double-tee cross-section shaped, B type, 11 m long, and with 300 daN of nominal strength, complying with a Brazilian standard [
Cement and natural and artificial aggregates were characterized by physicochemical analyses, previously to their use in the mixtures. Elemental chemical composition and chemical phases were obtained from energy dispersive spectroscopy (EDS), X-ray fluorescence (XRF), and X-ray diffractometry (XRD) methods. PW 2400 Philips fluorescence equipment was used to determine the elemental chemical composition. XRD of specimens were measured using a Philips (X’Pert MPD) diffractometer with Cu-K
Potential alkali reactivity of cement and aggregate was evaluated according to ASTM specification [
Nondestructive method, such as electrical half-cell potential with copper-copper sulfate reference electrode, CSE, was used to verify the service life performance of reinforcing steel in concrete in a 3.4% w/w chloride solution and of concrete utility poles submitted to natural aging condition, according to the literature [
A marine ECS was built at Caueira beach, in Itaporanga D’Ajuda district, near Aracaju, SE, in northeastern Brazil. It was located at about 2 m above the high tide line of the Atlantic Ocean [
Sf2 concrete admixtures were cast as thick plates (20 × 270 × 300) mm to be previously tested in a marine ECS located at 17 m depth, following a perpendicular line of Praia de Leste beach coast until 30 m deep (25°30′ S). Sample plates were periodically tested by flexural strength and by microstructure concrete surface investigated by Scanning Electron Microscope (SEM) and energy dispersive system analyses (EDS). Afterwards, this composition was cast as reef balls and 1 m3 concrete blocks technology forms. Both of them were tested in a marine shallow shelf at approximately 30 m of depth for approximately five years [
The pH of this resultant concrete mix lowered from 12.3 to 11.4. Concrete with this pH generally needs to age in the ocean for 3–6 months before the pH in the surface region approaches the 8.3 pH of seawater and favour marine organism’s settlement [
The physicochemical analyses of cements were in accordance to manufacturer specifications and Brazilian standards.
The major chemical components obtained by XRF tests from the sludge sample were: 16.55% of silica, 13.07% of alumina, 4.15% of ferrite, 49.79% of volatile materials, and 16.44% of humidity. In natura water treatment sludge was identified as kaolinitic group by XRD as shown in Figure
XRD pattern of Swtp phases: “in natura” and after 3 h/800°C treatment.
The result of average compressive strength of 8% (w/w) Swtp concretes at 28 days of curing was 27.6 MPa, being superior to setup limit used for concrete structures as poles for electric energy distribution network. The average flexural strength at 28 days was 3.0 MPa, which is in accordance to the literature data for similar admixtures [
The average permeability resulted from 8% (w/w) Swtp was 0.9 × 10−10 cm/s. As the permeability of concrete depends on mix proportions, compaction, curing, and microcracks in the core, and also there is a close relationship between the strength and its durability, the results indicated a good performance mixture resultant from kaolinitic group and cement phases.
Porcelain waste concrete admixture resulted potentially in alkali reactive with CPII-F 32 cement type, as shown in Figure
Expansion of Pw admixture mortar sample.
The expansion tests resultant from different picked-up porcelain material parts and CPII-F 32 cement type showed that porcelain with glazed and sulphur cement phases (as obtained and grounded) is the most damaging to the concrete materials, followed by porcelain parts, porcelain material with glazed but without sulphur cement phase parts, followed by porcelain materials without glazed and with sulphur cement phases. This latest phase presented the lower expansion results, as demonstrated in Figure
Expansion of mortar admixtures containing porcelain insulator waste.
SEM of sulphur cement phase micrography in porcelain concrete admixtures.
Pw compressive strength resulted in 30.4 MPa at 28 days of curing, being classified as restrained resistance [
Because of the reduced lifespan resulted from the potential reactivity essay with porcelain materials and CPII-F 32 cement type, the Pw mortar sample was in a second time cast with special sulfate-resistant cement. Portland high-early strength cement (CPV-ARI RS) reduced its expansion limit in 16- and 28-day ageing test to the recommended values (innocuous consideration) as shown in Figure
Expansion results of Pw mortar casted with high-early strength sulfate-resistant cement (CPV ARI-RS).
In Figure
The image shows 3 utility poles and crossarms. Two of them were cast with RC and Pw admixtures (right positions). In detail, is illustrated a CSE system used to measure the seasonal reinforced steel corrosion potential performance.
Electrical half-cell potential measured during these first five months on urban ambient ECS condition indicated no corrosion activity for RC and Pw structures. As previously reported, both structures are exposed at low aggressive atmosphere.
The electrical results made in RC and Pw reinforcing steel in concrete partially immersed in 3.4% w/w NaCl aqueous solution as function of ageing time are showed in Figure
Electrical half-cell potential results of RC and Pw reinforcing steel in concrete as a function of aging time in 3.4% NaCl solution.
Silica fume admixtures in concrete have been presenting better lifespan performance of concrete structure submitted to salt aggressive environment, as viewed in Figure
Electrical half-cell potential results of RC and Sf1 utility poles submitted to the natural aging for 480 days in northeast Brazilian Caueira beach ECS.
Tests made in the reinforcing steel in concrete submitted partially immersed in a 3.4% NaCl aqueous solution have been indicating that Sf1 samples are having a double lifespan in comparison with RC reinforcing steel, both of them tested at the same laboratory salt aggressive condition.
Visual inspection on Sf1 and RC utility poles submitted to northeast Brazilian Caueira beach ECS demonstrated that RC structures have been presenting, too, rebar corrosion surfaces with consequently concrete microcracks. Any corrosion surface defects have been viewed on Sf1 concrete structures exposed at same environmental condition.
Besides the increase of lifespan concrete structures, the silica fume material causes economical positive effects when compared to a reference concrete. The economical differences lowered from 34% in 28 curing days to 24% in 90 curing days, at the same compressive strength results. This phenomenon had been attributed to pozzolan-modified Portland (CPII-Z 32) cement used to cast both samples [
Sf2 mixes casted in thick plates and tested previously in a 17 m depth marine ECS demonstrated good lifespan performance by flexural strength results in the function of ageing time, as well as in terms of biological marine material habitat, as shown in Figures
Flexural strength of Sf2 concrete admixture in terms of ageing time in a 17 m depth marine ECS.
Sf2 concrete admixture and other materials thick plates tested in marine ECS located at 17 m depth, at Praia de Leste beach.
Sf2 cast as reef balls technology and 1 m3 blocks, as presented in Figure
Sf2 reef balls technology and block forms of Sf1 concrete mixture before installation in a marine shallow shelf at Parana state, southern Brazil. The detail shows nowaday jewfish in the block habitat.
Tw concretes cast with up to 20% w/w were tested to be used as paver and curb pieces and as repair materials for hydraulic concrete structures of hydroelectric power plants [
In Figure
Paver block with 10% w/w Tw concrete mixture. The paver surface view of 20% w/w Tw concrete mixture is shown in detail.
Compressive strength tests in 10% w/w Tw resulted in 20 MPa, presenting at the same time economic benefits and environmental advantages for materials repair in hydraulic structures [
Abrasion-erosion test results of Tw samples showed that the wearing was 75% lower than RC samples, meaning that they have better performance. In Figure
Abrasion-erosion Tw samples image tested and the schematic wearing surface plot results. The wearing surface resulted from three different repair materials concentration: 5% w/w Tw; 10% w/w Tw; 15% w/w Tw.
The Swtp obtained by centrifuged method can be used as concrete admixture up to 10%. The best mechanical behaviour was achieved with 8% w/w Swtp concrete mixture, indicating that it can be used in concrete utility poles and crossarms.
Pw concrete admixture cast with CPII-F 32 cement type had its lifespan reduced due to the large potential reactivity values in all partial chemical phases analysed. These alkali aggregate reactions were reduced after the change of the cement type to a high-early strength and sulfate-resistant one (CPV-ARI RS). This composition could be cast in utility poles and crossarms applied in electrical distribution energy until 25% w/w porcelain waste. Even so is strongly recommended laboratory tests previously cast it in civil structures.
Sf1 and Sf2 silica fume mixtures cast as utility poles and crossarms, and reef balls technology and cubic concrete blocks showed good resistance performance when used in high salt Brazilian northeast coastal areas and into 17 m depth marine environment, respectively. Poles, crossarms, reef balls structures and cubic concrete blocks with silica fume admixtures (Sf1 and Sf2) showed good chemical resistance performance in high salt Brazilian Northeast coastal areas and, into 17 m depth marine environment, during 500 days and approximately 5 years of tests exposition, respectively.
Tw composition up to 15% w/w concrete mixture had good mechanical performance during tests, showing that it can be used as paver, curb pieces, and as repair materials for hydraulic structures concrete dams. The 20% w/w Tw composition presented large quantities of rubber-tired concrete surface efflorescence and porous defect, causing poor visual market aspect and low mechanical resistance.
As observed, the worry on the use of recycled materials is not limited to structural stability, but also their durability in concrete structures, meanly submitted to salt aggressive environment.
This work was sponsored by Companhia Energética de Sergipe (ENERGIPE), Companhia Paranaense de Energia (COPEL), Instituto de Tecnologia para o Desenvolvimento (LACTEC), Agência Nacional de Energia Elétrica (ANEEL), UFPR/CEM, and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq, Lei 8010/90). The authors wish to thank the (UFPR/PIPE and UFPR/PGERHA), LACTEC/IEP/PRODETEC.