Analysis of the utilization of blast furnace slag as recycled aggregates in self-compacting concrete

Equation Chapter 1 Section 1 Trabajo Fin de Grado Grado en Ingeniería Civil Analysis of the utilization of blast furnace slag as recycled aggregates in self-compacting concrete Autor: Adelardo Vahí Sánchez de Medina Tutor: Héctor Cifuentes Bulté Carlos Leiva Fernández Dep. Mec. Medios Continuos y Tª de Estructuras Escuela Técnica Superior de Ingeniería Universidad de Sevilla Sevilla, 2017 2 3 Trabajo Fin de Grado Grado en Ingeniería Civil Analysis of the utilization of blast furnace slag as recycled aggregates in self-compacting concrete Autor: Adelardo Vahí Sánchez de Medina Tutor: Héctor Cifuentes Bulté Profesor contratado doctor Carlos Leiva Fernández Profesor titular de Universidad (Dpto de Ing. Química y Ambiental) Dep. de Mecánica de Medios Continuos y Teoría de Estructuras Escuela Técnica Superior de Ingeniería Universidad de Sevilla Sevilla, 2017 4 5 Abstract In this article, a study for investigating the effects of replacing the aggregate of selfcompacting concrete by blast furnace slag has been carried out. Different mixes have been made by substituting the fine and coarse aggregates. The fracture energy, the tensile strength and the compressive strength have been tested. The remaining properties of selfcompacting, or the absence of them, have been observed. This research can find goals such as decreasing the price of aggregates, reducing the industrial waste and attenuating the rate of consumption of natural resources. The results show that the self-compacting property is gradually lost as the slag content is increased, so – when the ratio of replacement is low – the concrete keeps the selfconsolidating properties. However, these losses affect to the mechanical properties. 6 1. Introduction Blast furnace slag (BFS) is a nonmetallic industrial by-product, formed in blast furnaces with the melting of the iron ore, producing molten pig iron [1]. More than 500 million tons of blast furnace slag (BFS) are produced every year worldwide [2]. The blast furnace slag can be granulated slag (GBFS) or air-cooled slag (ACBFS). Granulated slag is cooled rapidly by using water, resulting in a vitreous slag, while air-cooled slag is slowly cooled by ambient air. The air-cooled slag solidificates and crystallizes, creating a rock-like slag [3]. The main use of blast furnace slag is cement production [4], which can be done by using granulated slag, but BFS can be used also as any other additive to concrete or as part of alkali activated materials [5] in the form of GBFS or ACBFS. Today, in the European cement regulations, there are 14 types of cement containing blast furnace slag [4]. In Spain, the 70% of slag produced in 2014 is recycled. However, the remaining 30% wasted - that represent half million of a tone – is deposited in landfills due to unknowledge about the recycling opportunities of this material [6]. Blast furnace slag can be used also in road construction [4], which is it main use in the USA. In there, in 2012, 68.6% of the ACBFS was used as road base, road surface layers and asphalt concrete, whereas only 13.1% of the ACBFS was used as aggregate replacement for concrete in the USA [7]. However, the demand for concrete has increased rapidly due to the development of industrialization and urbanization in the last decades. It is estimated that the world demands over 10 billion tons per annum of construction aggregates [8]. The use of blast furnace slag in the production of concrete can have economical motivations – acquiring blast furnace slag as a waste industrially generated is cheaper than obtaining natural aggregates from a quarry. Moreover, the reuse of blast furnace slag has implications in the enviromental preservation and promotes a sustainable development. In this way, is 7 important to investigate the use of slag in the production of concrete, either as an aggregate or as a binder [4]. The convenience of slag usage as aggregate in concrete will depend on the properties that have to be achieved, in fresh and hardened states. In this way, according with some authors [9], the concrete produced with blast furnace slag has more workability when fresh, facilitating the vibration and making a better compaction with less entrapped air. When investigating hardened properties, some researchs assure that a 50% replacement of cement with blast furnace slag can improve the mechanical properties and durability of concrete [10]. On the opposite, blast furnace slag replacing cement provokes lower strength during the early days due to a low initial rate of hydration [4]. This handicap can be fixed by the usage of chemical activators, by increasing its specific surface or by raising its temperature [11]. Consequently, depending on the application of the concrete, as well as on climate conditions, the rate of cement replacement by blast furnace slag will be different. However, the depletion of the natural aggregates as well as the consumption of large amounts of energy on the production, transportation, and use of raw materials, should be considered when producing concrete [12]. With this consideration, the use of ACBFS as aggregate in concrete, must be taken into account. According to [1], “during the past two decades, a series of studies have been undertaken to understand the behavior of concrete containing ACBFS aggregates [13–15]. These studies have shown that ACBFS aggregate concrete (SAC) has a great potential to be a feasible alternative to natural aggregate concrete (NAC) in the construction industry”. The goal of this research is to investigate the effects of adding air-cooled blast furnace slag (from now on, recycled aggregate) in substitution of the natural aggregate on the properties of the self consolidating concrete. Some concretes samples have been made in order to test their compressive and tensile strength, to study how the fracture proccess happens and to 8 see how the bilineal tension softening diagram changes as the slag content increases. It is used both coarse and fine aggregate, and the effect of each one of them is studied separated and mixed. 9 2. Materials and methods 2.1 Materials For the production of the different compositions studied in this work, several components have been used. The cement used was CEM-II/B-L 32,5N, fabricated according with EN 197-1[16]. The superplasticizer used was MasterGlenium 355C and MasterGlenium ACE 325. The fabrication of it has been done according with the European Standard EN 934-2 [17]. The natural fine aggregate (NFA) used was siliceous sand, while the natural coarse aggregate (NCA) used is was lime-filled. The recycled coarse aggregate (RCA) and the recycled fine aggregate (RFA) was all air-cooled blast furnace slag (ACBFS). The chemical composition of all of them is shown in Table 1. The measured density of NFA was 2.65g/cm3, the density of NCA was 2.66 g/cm3 and the density of ACBFS was 2.7 g/cm3. The similarity between densities make this ACBFS to be an appropiate replacement of this aggregate. 16 (ft). This test is defined according to the specifications in the Spanish Standard UNE-83515 [24], which assign this test for fibre-reinforced concrete. However, according with previous authors [25], this test is also valid for mass concrete. Cylindrical steel punches with a height of 20 mm and a diameter of 25 mm were placed at the centre of the top and the bottom surfaces of the specimen. The specimens used were cubic with faces of 10 cm long in each side. Four different samples for each kind of concrete have been tested. The displacement of the metallic cylinder over the top face and the load applied to the concrete specimen were recorded each 0.2 seconds. During the test, the cylinders suffer compression against the surfaces where they were colocated, until they penetrate on the concrete specimens. This penetration provokes a tension state inside the concrete, until the specimen gets broken. In this point, the maximum tensile stress (ft) that the concrete can support, is reached. Once the maximum load applied for breaking the specimen is known, the ft can be measured by the formula of Chen and Yuan [26]. 3.4 Three-point bending test This test was done in order to measure the fracture energy according to the work-of- fracture method of RILEM. The fracture energy of concrete is an important parameter in the analysis of the mechanical behaviour of concrete structures [27]. The test was done directly on the initial prism specimens, with a length of 44 cm and squared bases of 10 cm long in each side. Before doing this test, a notch is done to the concrete. This notch goes parallel to the bases, with 5 centimeters of depth, and cuts completely a lateral face of the prism. The notch is done in the center of the prism, separated the same length (22 cm) to each base. The lateral length of each prism is 10 centimeters, so the ratio between 17 depth of the notch by total depth of the specimens was 0.5. In the middle cross section with the notch, the remainig area uncutted has a height of 5 cm and is 10 cm wide. This brings to a ligament area of 50 cm2. The notch is collocated looking downwards, so the face cut by it will be called the bottom face from now on, and in the opposite– the top face – will be where the load is applied. Since the modifications proposed by Guinea et al. [28-30] has been followed, the bending test is done compensating the self weight – some rubber bands were holding the prism for this. The load is applied on the top face of it, in the center of that face, exactly over the notch, with a cylinder parallel to the bases of the prism. The load applied provokes displacements and strains, which are measured along with the value of the load. There is a record of data for each 0.25 seconds. In that way, there are four measurings taken each second for each of the four following magnitudes: the load applied to the top face, displacement of that face, strain of the bottom face according with a reference taken on the top face and the aperture of the notch in the bottom face. There are two different stages in the test: firstly, the load applied is increased by making constant the increment of the displacement of the top face, which increases 0.2 mm each minute, until the load reaches a value of 0.2 kN. After this point, there is a second stage where the evolution of the test is controlled by the speed of opening of the bottom part of the notch, until the aperture of the notch has increased in 1mm since the beginning of the test. In this moment, the test finishes. When the aperture of the notch reaches 1 milimeter, there is a crack that can be seen going from the front of the notch to the top face. When the rubber bands are removed, the prism breaks following the route marked by the crack. All prisms have been tested, so the final results for each material can be taken by an average of more than one sample (four samples as much), making it possible to discard the results of any test done wrongly. 18 3.5 Bilinear softening curve With the results of the bending test, some parameters can be obtained with an iteration proccess. By using the real softening curve (which correlates Load and Notch widening), the Elastic modulus and the ft can be calculated. Also, three more data (a1, a2 and b2) which define the slopes of two straight lines (a1, a2) and the zero crossing of one of this lines with vertical axis in the diagram (b2). With this five data, it can be represented the bilinear approximation of the theorical correlation mentioned before – Load-Notch widening – as can be seen in Figure 5. The final result is a three-line graph that represent some of the main properties of each concrete in a very simple way, just with linear correlations. Figure 5. Load (F) – Notch opening (d) bilineal correlation 19 4. Results 4.1 Density test The results obtained are shown in the Table 3. Mix I II III IV IV.b V Density (kg/m3) 2.28 2.21 2.17 1.97 2.15 2.31 % standard deviation 0.98% 1.88% 2.64% 1.22% -- 2.63% Table 3. Density of mixtures As can be seen in Table 3, the adition of slag leads to a decrease in density. Cement and water content have not changed between different mixtures and density of different aggregates is quite similar, so density depends mostly in porosity. Mix II, where replacement of aggregate is equal to 50% of fine aggregate, shows a decrease of 3% in density, while the replacement of 50% in the coarse aggregate shows a decrease of 5% in density, when compared with composition I. In fact, mix II has a slightly higher granulometry that explains the higher porosity which causes this decrease on density, meanwhile mix III is made with coarse slag aggregate, which – as could be seen in Figure 2 – had irregularities and cavities in the surface of the grains, causing the lower density on the mixture when compared with mix I. Moreover, a wider particle size distribution causes a higher packing density and decreases water demand, while a narrower particle size distribution gives higher hydration rates for equal specific surface area [19-21]. Mix III has a narrower particle size distribution, however the proportional amount of water was not changed between different mixes, which provokes a decrease in density. However, the density of composition IV, which had a full replacement of aggregate by slag, is 13.5% lower than density of composition I, and the absolute data was 1.97 kg/m3, which is extremely low compared with an average concrete density. This matches with the 20 excessive porosity that it showed in Figure 3. The variation of density between mixes I and IV are an emphasis of the variation of density between mixes I and II or mixes I and III. All the reasons why density was lower in mix II and in mix III have a combinated effect in mix IV. This excess of porosity was tried to be corrected by adding more water to the mix, which corrected the density by increasing it a 9%, which means having a density 6% lower than mixture I. With this information, we can sentence that a full replacement of aggregate by blast furnace slag, affects to density. However, mix V did not show any porosity. Therefore, mixes I and V have very similar densities. The difference of density in mix V is only around 1.5% in relation to mix I, which suggests that the self consolidation of mix I worked as good as the vibration. Aggregates in mix V are smaller than aggregates in mix I, which explains the slightly less porosity that it have. 4.2 Compressive strength The result obtained on this test consist on the nominal stress produced by the maximum load applied to the specimen, when it is supposed a uniform distribution of the stresses. The results are shown in Table 4. Material I II III IV IV.b V fc (MPa) 41.16 26.48 26.83 11.82 27.89 56.69 % standard deviation 4.5% 7.9% 20.2% 12% -- 4.2% Table 4. Results of Compression Test It can be seen that the concrete made with both coarse and fine slag replacing half of the aggregate has better compressive properties when vibrated than the self-compacting 21 concrete made only with natural aggregate – this improvement is over the 35%. Therefore, it can be assured that fewer costs in the material can create a significantly better material, but assuming the renounce of a self-consolidating concrete, which can be an interesting fact considering the increasingly concern of the waste of materials. Without vibration, the slag provokes a descent on the compressive strength when compared with the mix made only with natural aggregate. If the replacement of aggregate is not accompanied with an extra vibration, the fc of concrete is reduced by 35% when the amount of replaced aggregate is 50% of one of the two kinds of aggregates. There is no evidence of difference between replacing coarse aggregate or fine aggregate. This descent is situated around the 70% when all the aggregate is replaced. The excess of porosity reflected in a lower density is a cause for having less compressive strength in a concrete [31]. The sample of composition IV.b, which was made by adding an extra quantity of water to the mix, was tested also for checking its compressive strength, resulting in a sample with a value for the fc of 27.89 MPa, more than twice the value of the average of sample IV. This fact can lead us to considerate that the replacement of aggregate by slag should be done along with a higher use of water. 4.3 Tensile strength When Barcelona test is applied to concrete without fibres, the ft can be obtained by applying the formula proposed by Chen and Yuan [26]. This formula uses geometrical parameters of the test and the maximum load applied before the concrete cube breaks. The average maximum load and ft obtained with this test for each kind of concrete Is shown in the Table 5. Material I II III IV IV.b V Maximum load (N) 81175 71250 69550 20016 32500 80325 22 ft [26] (MPa) 3.32 2.91 2.84 0.67 1.32 3.28 % standard deviation 4.8% 3.0% 12% 13.9% -- 15.3% Table 5. Results of Barcelona Test As a conclussion, we can say that the addition of slag creates concretes with lower tensile strength. This reduction is located between 12% and 14% when only 50% of one of the kind of natural aggregates is replaced (mixtures II and III). When the replacement of aggregate is total and the amount of water added does not change, the ft falls down an 80%. The external appearance of this mix, suggested that it could not be used for structural usage, and the low data of ft reached is a hard evidence of this. However, this composition improves its properties notably by adding more water to the mix. When the replacement of aggregates by slag is superior than it in mixes II and III, but the concrete is vibrated, the ft remains in similar values than that obtained with just natural aggregates. 4.4 Fracture energy The results from three-point bending tests are shown in curves correlating the load applied with the vertical displacement produced in the bottom face (face with the notch) of the prism. The area covered by this graph is the so-called work-of-fracture. In Figure 6, the average Load-displacement curve for each material is shown. 23 Figure 6. Load - Vertical displacement for different mixtures The average fracture energy per area for each material (GF) and the maximum load reached for each concrete is shown in Table 6. As said before, the uncutted area in the cross section with the notched has 50 cm2. Material I II III IV V Maximum load (N) 1847 1861 1728 966 1831 GF (N/m) 51.28 51.28 56.7 75.29 46.47 % standard deviation of GF 19.4% 10.3% 16.5% 10.2% 33.8% Table 6. Results of Bending Test According with previous authors [28-30], the specific fracture energy of each mix can be calculated by the method RILEM with some modifications. The RILEM method needs various ratios of depth of the notch and does not consider the compensation of self weight. The fracture energy obtained is not a real constant on the material because it depends on the size of the sample. With this modifications, the fracture energy is calculated as the area 0 300 600 900 1200 1500 1800 0,00 0,20 0,40 0,60 0,80 1,00 Load (N) Displacement (mm) III III IV V 24 covered by the curves shown in Figure 6 plus an extra area that can not be measured when the self-weight is compensated, which correspond to the asymptotic area. This extra area can be estimated by adjusting the tail of the load-displacement curve [32]. Taking a point (δu) in the right-side of the curve – where the test was stopped – and considering the load in this point (P’(δu)) to be fictionally 0, the non-measured work of fracture that can not be measured directly is considered to be 2·𝐴 δu , where A is the value that gives a best approximation of the shape of the curve at the left of δu, when this shape is calculated as an hyperbole 𝑃′= 𝐴 · (1 δ2−1 δ𝑈 2), where δ is the displacement and P’ is the load depending on displacement and taking as a virtual reference P’(δu)=0. As shown in Table 6, the GF of mixtures I and II is equal, meanwhile the substitution of coarse aggregate leads to a increase in the GF, being this increasement of 10.5%. With the highest usage of slag (mixture IV), when it replaces all the natural aggregate, the GF reaches its top, increasing a 47% more than concrete with natural aggregate. The average GF obtained for mixture V is the lowest of all mixtures, which maybe shows that this concrete has less ductility. The GF is a 9% lower than in the reference concrete, however the deviation calculated of the different samples which have been tested is too high (33.8%), which shows hesitation over this data. One aspect to take in account is the fact that mixtures with lower GF has a higher maximum load. All curves of the different samples vary from sharped to wider. As can be seen in Figure 6, the highest peak loads are obtained with a lower vertical displacement, and as the peaks of the curves gets lower, they tend to be more to the right. The extreme situation is mix IV, which despite having a maximum load of around 50% of the maximum loads of the other mixes, it covers a wider area, with the pre-peak curve less vertical and the decrease of postpeak curve softer. 25 4.5 Bilinear softening curve The bilinear approximation of the real softening curve (Bilinear softening curve) presents the average results for each mixture shown in Table 7. Material I II III V ft 3.09 3.36 2.81 3.37 a1 39.9 40.2 30.8 39.4 a2 0.42 1.79 0.92 2.18 b2 0.100 0.199 0.149 0.193 E 29181 25412 24651 25627 Table 7. Parameters for Bilinear softening curve. The load-displacement curve obtained in bending test for mix IV is very different of the curve that should be obtained for an adequate concrete. The non-linearity of pre-peak part of the curve and the similarity between the two slopes in the post-peak zone made it impossible to find a bilinear softening curve with a low enough tolerance. For all the other mixes, bilinear softening curve are shown in Figure 7. It can be observed that slag aggregate causes always a disminution on the value of the modulus of elasticity. This disminution varies between 12.9%, 15.5% and 12.2% for mixes II, III and V respectively. The similarity between mixes II and V remains in the other parameters, making both curves very similar one to the other. The mix III, however, shows a shorter peak (lower ft) and a more horizontal slope at the right of the peak, meaning that mix III is a weaker material in the elastic phase but more energy is needed for the propagation of the cracking [33]. Tensile failure in concrete can be caused by tensile failure of aggregates or by cracking in the surface between aggregate and cement paste [34]. Considering this, the size of aggregates in mix III was smaller than in mixes I, II and V, and this make shorter the route for propagation of cracking 32 7. References [1] Ozbakkaloglu, T., Gu, L., Fallah Pour, A. 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