Techniques of Testing Aggregate Replacement in Hardened Concrete - Term Paper Example

Techniques of testing aggregate replacement in hardened concrete Introduction/Background: Concrete is the most widely used building material used allover the world. The major constituents of concrete are cement, aggregates and water. The cement in the form of binder holds together the natural or artifical aggregates, which comes in coarse and fine form, to get a hard solid mass capable of taking the designed loads from the structure. The combinations of cement and aggregates in various proportions have given different characteristics in concrete that have ensured wide range of strength requirement in any proposed structure. All the civil engineering marvels existing on the earth would be indebted to the power of concrete in ensuring its expected quality (Chang et al, 1999). But the rapid increase in the infrastructure development initiatives all over the world have resulted in the consumption of huge quantities of concrete which has put heavy pressure on the environment for the supply of the aggregates. The shortage of natural aggregates has forced them to turn to manufactured aggregates. The manufactured aggregates too had its own problems due to the improper quality control in the production process, poor starting raw material or rock for the manufacture of aggregates etc. The increasing demand of the aggregates may not be met with the manufactured sand too. Thus the aggregate replacement systems in concrete have been in vogue as a long-term solution for the aggregate crisis and also from the increased quality demand in the construction. Use of various pozzolanic materials like blast furnace slag, silica fumes, fly ash are few such materials that have found to be suitable in this direction (Chang, et al, 1999). The pozzolanic materials are not very reactive by themselves but in presence of cement exhibits cement like characteristics. Also, as these materials are mostly waste by-products from different industrial processes, the alternatives proposed could also help in identifying a reuses option and thus ensuring environmental sustainability. Research aims & Objectives The broad objectives of the project are to explore the potential replacement for the aggregates in concrete. The specific objectives are (i) To identify the most suitable aggregate for replacement based on the literature survey. (ii) To critically evaluate the suitability of different type of aggregate replacement in concrete for different elements like beam and column. (iii) To compare the failure stresses due to flexure and axial loads for different glass aggregate replacement ratios.. Literature Review Rubber as Aggregate Replacement Large quantities of rubber in the form of used waste tyres are often dumped and discarded that often lead to environmental problems and unhygienic conditions. Thus research is undertaken to explore the possibility of using the rubber shredded to the sizes of fine aggregates that could be used as a replacement of normal aggregates in concrete. Earlier research have shown that such type of replacements could give considerable advantage in the resistance to frost and ice melting salt. Also the advantage of adding rubber based elastic aggregates on the elastic properties of concrete under static and dynamic loads are also reported. In the experiments conducted the cement and aggregates are weigh batched while the water and the admixtures used are batched by volume. The tests undertaken on this concrete did not show any change in the slump values with that of concrete prepared from normal aggregates (BS EN 12350 - 1:2000 & BS EN 12350 - 7:2000). Also, the lower density of the rubber aggregates resulted in the lowering of the density of concrete prepared from it. Further, the higher porosity of the rubber pieces makes the rubber aggregate based concrete more porous and higher air entrainment. On the prismatic compressive strength values both the concrete did not show any significant differences. But rubber aggregate concrete showed higher ultimate strain before failure. The deformation characteristics of tyre aggregate concrete showed that they have higher set plastic deformations than the ordinary concrete. The Figure 1 shows a relative comparison of strains observed in different concrete under varying level of compressive strength. Finite Aggregate replacement with Pulverised fly ash Flyash is a waste material generated in the thermal power plants and causes considerable amount of environmental problems related to its disposal. Successful efforts have been made in finding different application for its reuse in the construction sector. The pozzolanic properties the flyash has been made use of in the manufacture of pozzolanic cement. Recently, the efforts are made for its use as an aggregate for the concrete preparation. The experiments were conducted for this purpose to determine the mechanical properties of concrete by replacing the sand with class F flyash (Siddique, 2003a). The replacements by weight are made in five different percentages: 10, 20, 30, 40 and 50. The results of the tests performed to determine compressive strength, split tensile strength, flexural strength and modulus of elasticity were determined at 7, 14, 28, 56 , 91 and 365 days. The results obtained showed that flyash could be successfully used as partial replacement of fine aggregate that can be confidently used as fine aggregate in structural concrete (Siddique, 2003a). Another set of experiments conducted showed that both the compressive strength and the abrasion resistance of the concrete increased with the increase in percentage of replacement with flyash. Nealry 40 percent increase in the abrasion resistance is reported to have achieved by replacing 40 percent of cement and with flyash (Siddique, 2003b). Attempts made in the replacement of cement with class F flyash too have showed a lower 28 day compressive strength and splitting tensile values , flexural strength , modulus of elasticity and abrasion resistance of concrete. But, these values showed a continuous improvement from 91 to 365 days mostly interpreted for the possolanic action of the flyash. The results recommends that even 50 percent cement replacements are possible in the manufacture of precast elements and also in the construction of reinforced concrete elements (Siddique, 2004). Figure 1 – Ultimate Strains on failure of concrete (Skripkiunas et al, 2007) Blast furnace slag aggregates Blast furnace slag is a non-metallic by product formed in the process of reduction of iron ore to molten iron in the blast furnace. They consist mostly of silicates, alumino-silicates and calcium alumina silicates. Depending on the cooling method used for molten slag different forms of slag products are formed. These are air cooled blast furnace slags (ACBFS), foamed or expanded slag, pelletized slag and granulated blast furnace slag (Blast furnace slag, n d). Air-Cooled Blast Furnace Slag is hard lump slag produced when liquid slag is cooled under ambient conditions . They are crushed, screened and used. Expanded or Foamed Blast Furnace Slag : When the cooling of slag is accelerated by adding specified quantities of water highly porous materials with high bulk density is obtained, this is referred as expanded or foamed blast furnace slag. Palletized slag materials are formed by cooling with water and quenching in air. The pellets are crystalline and could be used as aggregates. Granulated blast furnace slags are sand size particles formed from rapid water quenching. They exhibit cementitious properties when converted to fine particles by crushing or milling. They are found to be suitable for a partial replacement or as additive to Portland cement. The general composition of blast furnace slag is as given in Table 1 Table 1. Typical composition of blast furnace slag (Blast furnace slag, n d) Composition Mean Proportions (as % of weight) Calcium Oxide (CaO) 39 Silicon Di Oxide (SiO2) 36 Aluminum Oxide (Al2O3) 10 Magnesium Oxide (MgO) 12 Iron (FeO or Fe2O3) 0.5 Manganeese Oxide (MnO) 0.44 Sulphur (S) 1.4 Detailed research has been undertaken to asses the feasibility of using bottom ash, granulated blast furnace slag and a combination of these materials as fine aggregate in concrete (Isa et al, 2007). Series of durability tests were conducted and the results were compared with the reference concrete. Bottom ash, granulated blast furnace slag and their combination replaced the 3 to 7 mm sized aggregates and was used in their non-ground form. Each of them had five test groups representing replacement proportions of 10 %, 20 % , 30 %, 40 % and 50 % . The advantages in the replacement was assessed based on the durability test results on these ratios (Isa et al, 2007). The results showed that GBFS and BA influenced the durability of concrete positively when used as aggregates. Their ability to reduce surface abrasion and less resistance to temperature effects are the factors that added to their merit. The scanning electron microscope images have shown that chemical and physical characteristics of both GBFS and BA are the major factors that influenced their durability. Thus the research results recommends the utility of using GBFS and BA to be used as alternate aggregates in concrete (Isa, et al, 2007). Use of incinerated ash The utility of using the incinerated ash of the stabilized municipal solid waste along with natural aggregates has been investigated in detail. The flyash from the incinerator was washed , milled and then stabilized using cement - lime process . They are then reused as recycled aggregate for the preparation of mortar and concrete. These modified aggregates are used in the proportion of 200 to 400 kilogram per cubic metre of concrete. The were later tested for the compressive and flextural strengths, modulus of elasticity and also drying shrinkage. The engineering characteristics of concrete prepared from such recycled aggregates did not differ much from the results reported with those of natural aggregates (Collivignarelli and Sorlin , 2002).The potential reuse of solid waste incineration ash is till unclear due to the possibility of toxicity leaching. The ash from these units are often classified as potentially toxic materials and hence detailed experimental evidence is required to establish their acceptability levels. It is observed that the bottom ash obtained from refuse-derived fuel incineration process have shown better acceptability for use as fine aggregate when compared with that that obtained from solid waste incineration carried out without any pretreatment. Still, the compressive strength values obtained from using RDF based bottom ash have shown a reduction of 23 percent in comparison with conventional type of aggregates (Chang et al, 1999). Use of recycled aggregates The search for alternate aggregates for construction has also resulted in the concept of using recycled aggregate obtained from demolished concrete structures. The coarse and fine fraction obtained from crushed concrete is accepted only if the recycled aggregate concrete hence prepared using them satisfies all the properties of the concrete. From the preliminary studies undertaken it is understood that strength of recycled concrete is higher than that reported for concrete prepared using natural aggregate when both under the same pulse velocity . While the strength -rebound relationship established for both the type of aggregates did not show any significant difference. A comparison of the permeability characteristics between the concrete prepared using different recycled aggregates would give the relative merit of such type of materials in the building sector. The factors influencing the permeability like void space and sorption characteristics across concrete prepared using natural aggregates, recycled concrete aggregates and also crushed brick aggregates were studied with respect to the strength of concrete. The results obtained showed that the variation in the above mentioned factors decreased with increasing strength for all types of concrete (Padmini et al, 2002). Use of glass in construction. Though large quantities of waste glass often get recycled , a small portion of it is used in the construction process for the preparation of concrete. It is found that glass particles can generate unfavourable alkali-silica reactions as it turns unstable in the alkaline environment of concrete. Due to this reactive nature, the glass is ground to fine powder and is mixed with the cement as a pozzolanic material. This material have displayed favorable strength gain in concrete and could replace cement up to 30 percent when the glass fines are added to atleast 20 percent weight of the cement (The Concrete society., n.d.). Further, the addition of glass fines could also retard the aggressive alkali reaction caused either by coarse glass particles or by natural aggregates. In an another attempt to explore the possibility of glass as an aggregate option, the detailed experiments were conduced to assess it suitability in concrete in the proportions from 0 to 100 percent. The results obtained have confirmed the utility of glass as aggregate and even 10 to 25 percent replacements have given compressive strength values much higher to those reported in normal concrete (Alhumoud et al, 2008). Also , 50 percent replacement have been recommended for both coarse aggregates and fine aggregates for the target concrete strength is 32 Mpa with acceptable level of strength development behaviour (Shayan and Xu, 2004). The camparison of the results on the replacement of sand with glass is shown in Table 2. Table 2 Comparison of strength in glass sand combinations in concrete % Gain in Strength of consrete % of glass replaced with cement % of glass replaced with sand 0 52 52 10 48 52 20 40 53 30 35 54 40 28 56 (Source : Shayan and Xu, 2004) Waste glass is one of the most important waste materials that is found to have great utility in the concrete preparation. These materials were usually directly transported to landfills and its application in concrete have certainly helped in the value addition. The major challenge in using waste glass aggregates is the alkali-silica-reactivity of glass in concrete. The recently published research results report different type of mitigation strategies for alkali-silica reactivity (TRE, 2009). Most of the studies have identified the influence of factors like the type of glass, particle size of aggregate, presence of pozzolanic materials like flyash or lithium based inhibitors, addition of compounds in the form of air entrainers or shrinkage reduction agents and fibre reinforcements (TRE,2009). The important findings reported are the 50 percent replacement of sand with fine glass particle having particle size less than 0.5 mm did not exhibit any adverse strength characteristics (TRE, 2009). Also, addition of 20 percent of flyash too showed good control on the expansion caused by alkali-silica reaction (TRE,2009). Thus the information presented by the researchers could give sufficient information for the preparation of concrete mixtures using glass fibre aggregates. In an another set of experiments glass articles were tested for the replacement as a binder. In the case of such experiments the replacement as binder was tested for the proportions of 10 %5, 20% and 30 %. Also, 100 % replacement of fine aggregate with the processed glass was also evaluated. The workability tests conducted on concrete prepared using natural sand and glass sand with different binder replacement levels gave 70 to 80 percent variations in results when compared with the control samples (Perkins,2007) . Interlocking of angular particles, suction established across larger particles in the presence of water and surface tension caused by t he water on the glass particles are the reasons identified for the marked variation in the workability. In the case of strength tests, the concrete prepared from 100 percent replacement of fine aggregates by glass particles exhibited comparable performance with that of control sample. At the end of 28 days the control sample gained a strength of 49.1 N/mm2 while that of the concrete containing glass aggregates attained 49.5 N/mm2 (BS EN 12350 - 1:2000 & BS EN 12350 - 7:2000). Inspite of the favorable results obtained from the experiments, the visual analysis did not give any signs of proper bond between glass aggregates and cement paste (Perkins, 2007).In an another study , nearly 80 Kg of crushed glass was used as replacement for sand in the proportions of 10 %, 15 % and 20 % for the preparation of concrete weighing 900 Kg (Ismail and Al-Hashmi, 2009). These experiments too showed high strength attainmet by waste glass in 28 days , which is observed as nearly 80 percent of strength shown by the controls (BS EN 12350 - 1:2000 & BS EN 12350 - 7:2000). Further, the flexural strength and compressive strength of the specimens prepared using waste glass in 20 percent were higher than by 11 percent and 4 percent respectively in comparison with those of control specimen . Another striking observation was the reduction in expansion in the mortar bar tests by 66 percent from that of the control samples (Ismail and Al-Hashmi, 2009). Based on the results obtained from tests conducted on different aggregate substitutes those using glass promises to give better results in strength and performance (BS EN 12350 - 1:2000 & BS EN 12350 - 7:2000). Hence research has been planned to investigate the effect of different type of loads on such concrete to arrive at the determination of the reliability of using glass based aggregates in construction process. The literature survey has revealed that the different aggregates have been tested for the compressive strength of the concrete specimen prepared using them. But this information alone is not sufficient to establish their reliability in performance under different combinations of loads. This lacuna would be addressed by using detailed laboratory investigations into the performance of sample specimens under different loading conditions. The glass-based aggregates have been chosen as the replacement for all the above mentioned tests due to their better performance observed in comparison with the other aggregates as observed from the literature review. Methodology The methodology used would be detailed laboratory testing of concrete specimen prepared using glass-based aggregates. Various specimens of structural samples like beams and columns would be prepared using different grades of concrete using glass aggregates. The tests would include flexure tests and axial load test to assess their behavior as beam and column for different replacement ratios of the aggregate. The tests would also investigate the influence of admixtures on the strength behavior of the elements. The results would be compared with the control samples that would be based on the natural aggregates. The beams and column specimen would be prepared as per the BS specification stated for testing of beam and column elements. The testing could be undertaken on a load frame with digital strain recorders for understand the deformation characteristics of the members. Analysis The results presented is tabulated as shown in the sample table 1. The details of flexure and the effect of the axial load is reported separately. The effect of aggregate replacement on the strength characteristics of the members shall be determined very easily. Table 1 The strength characteristics of members Replacement ratio of aggregate Grade of concrete Beam element Column element Maximum Bending moment Maximum deflection Maximum load Maximum deformation 10 Gr1 Gr2 Gr 3 Gr4 20 do do do Control do do do Also the variation of failure stresses in bending and under axial loads are presented graphically as shown in Figure 2. This would help to understand the effect of the replacement percentage on the strength characteristics on the structural member. Thus these information would help to understand the potentiality of using glass based replacement in the concrete mixes for beams and columns used in the structural frame work. Figure 2 Typical form of graph to compare stresses Conclusion The detailed literature survey conducted on the property testing of different type of aggregates have shown interesting results. The artificial aggregates which were mostly produced as by product of industrial processes would soon become a essential component of building industry. Though all of such type materials cannot be used with very high confidence. Some of the aggregates from solid waste incinerations should be weighed against the potential health hazard they might cause both to the worker and occupants. Detailed research on the long term studies, in addition to their strength behavior, are essential to propose these materials as a viable option. References Alhumoud , J M, Al-Mutairi, N, Z and Terro, M J (2008) , Recycling crushed glass in concrete mixes,International Journal of Environment and waste management, 2 (1-2) : pp 111-124. Blast furnace slag (n.d.), [Online] Available from . [5 February 2009] BS EN 12350-1:2000, Testing Concrete. Method of sampling fresh Concrete on site. Chang, N. B. , Wang, H., P., Huang, W. , L.. and Lin, K.S. ( 1999) The assessment of reuse potential for municipal solid waste and refuse-derived fuel incineration ashes ,Resources, conservation and recycling , 25 (3-4 ) : pp 255-270 Collivignarelli, C and Sorlini, S (2002), Reuse of municipal solid waste incineration flyashes in concrete mixtures, Waste Management, 22 (8) : pp 909-912 . Isa, Y , Turhan, B and Omer, O (2007), Durability of concrete incorporating non-ground blast furnace slag and bottom ash as fine aggregate, Building and Environment, 42 (7) : 2651 – 2659. Ismail , Z Z and Al-Hashmi, E A (2009), Recycling of waste glass as partial replacement of fine aggregate in concrete, Waste management, 29(2), pp 655-659. Padmini A K, Ramamurthy, K and Mathews, M S. (2002), Relative moisture movement through recycled aggregate concrete, Magazine of Concrete Research , 54 (5 ) , pp- 377-384. Perkins, G.D (2007) Development of concrete containing waste glass [Online] Available from [16 February 2009] Skripkiunas, G, Grinys , A and Cernius, B (2007), Deformation properties of concrete with rubber waste additives, Materials Science, 13 (3) : 219-223. Siddique, R,(2003a) , Effect of fine aggregate replacement with Class F Flyash on Mechanical Properties of Concrete, on Cement and Concrete Research, 33 (4) : pp 539-547. Siddique, R (2003b), Effect of fine aggregate replacement with Class F fly ash on the abrasion resistance of concrete, Cement and Concrete Research, 33 (11) : pp 1877-1881. Siddique,R (2004), Performance characteristics of Class F flyash concrete, Cement and Concrete Research, 34 ( 3 ) : pp 487-493 The Concrete Society, Recycled glass aggregate [Online] Available from [6 February 2009] Shayan, A and Xu, A (2004) Value added utilisation of waste glass in concrete, Cement and Concrete Research, 34 (1), : pp81-89 . TRB(2009), Recycling and utilizing waste glass as concrete aggregate [Online] Available From http://pubsindex.trb.org/document/view/default.asp?lbid=881611 [15 February 2009] Read More