Scholarly article on topic 'Proportioning, Microstructure and Fresh Properties of Self-compacting Concrete with Recycled Sand'

Proportioning, Microstructure and Fresh Properties of Self-compacting Concrete with Recycled Sand Academic research paper on "Civil engineering"

CC BY-NC-ND
0
0
Share paper
Academic journal
Procedia Engineering
OECD Field of science
Keywords
{"recycled aggregate" / "self-compacting concrete" / "recycled sand" / microstructure / proportioning}

Abstract of research paper on Civil engineering, author of scientific article — Diego Carro-López, Belén González-Fonteboa, Fernando Martínez-Abella, Iris González-Taboada, Jorge de Brito, et al.

Abstract The use of fine recycled aggregates from recycled concrete is limited due to the high absorption of the material and the subsequent reduction in mechanical performance. At the same time, Self-Compacting Concrete (SCC) uses large amounts of fines to ensure its flowability. Therefore, this type of concrete could allow the use of fine recycled aggregates. Hence, the aim of this work is to study the proportioning and the effects in the microstructure and the fresh basic properties of the use of recycled sand to produce SCC. The concrete mixes analyzed incorporate recycled sand (in percentages of 0%, 20%, 50% and 100%) and natural coarse aggregates. The mix design used an equivalent mortar, which allowed obtaining a suitable concrete that could be at the same time comparable between different replacement ratios and usable in real-life applications. During the design of the mixes with the mortars, the workability was measured from 10 min to 90 min using mini-cone and mini-funnel tests and the suitable ones were chosen to perform self-compacting concrete. Once this was done, these mixes were produced at concrete scale, and with these, basic properties were measured. The fine recycled aggregate changes the workability and the rheology of the mortar and concrete. These differences also affect the microstructure in terms of bonding and porosity distribution. There is a severe reduction of compressive and splitting strength as a result of the use of recycled sand, and this could be linked directly to these changes of the microstructure. The recommended substitution ratio with small decrease of mechanical performance is up to 20%.

Academic research paper on topic "Proportioning, Microstructure and Fresh Properties of Self-compacting Concrete with Recycled Sand"

(I)

CrossMark

Available online at www.sciencedirect.com

ScienceDirect

Procedía Engineering 171 (2017) 645 - 657

Procedía Engineering

www.elsevier.com/locate/procedia

Sustainable Civil Engineering Structures and Construction Materials, SCESCM 2016

Proportioning, microstructure and fresh properties of self-compacting concrete with recycled sand

Diego Carro-Lópeza'*, Belén González-Fonteboaa, Fernando Martínez-Abellaa, Iris González-Taboadaa, Jorge de Britob, Fernando Varela-Pugaa

a Department of Construction Technology;Civil Engineering School; Universidade da Coruña; Campus de Elviña s/n, 15071, A Coruña, Spain bCERIS, Department of Civil Engineering, Architecture and Georresources, Instituto Superior Tecnico, Universidade de Lisboa, Portugal

Abstract

The use of fine recycled aggregates from recycled concrete is limited due to the high absorption of the material and the subsequent reduction in mechanical performance. At the same time, Self-Compacting Concrete (SCC) uses large amounts of fines to ensure its flowability. Therefore, this type of concrete could allow the use of fine recycled aggregates. Hence, the aim of this work is to study the proportioning and the effects in the microstructure and the fresh basic properties of the use of recycled sand to produce SCC. The concrete mixes analyzed incorporate recycled sand (in percentages of 0%, 20%, 50% and 100 %) and natural coarse aggregates. The mix design used an equivalent mortar, which allowed obtaining a suitable concrete that could be at the same time comparable between different replacement ratios and usable in real-life applications. During the design of the mixes with the mortars, the workability was measured from 10 min to 90 min using mini-cone and mini-funnel tests and the suitable ones were chosen to perform self-compacting concrete. Once this was done, these mixes were produced at concrete scale, and with these, basic properties were measured. The fine recycled aggregate changes the workability and the rheology of the mortar and concrete. These differences also affect the microstructure in terms of bonding and porosity distribution. There is a severe reduction of compressive and splitting strength as a result of the use of recycled sand, and this could be linked directly to these changes of the microstructure. The recommended substitution ratio with small decrease of mechanical performance is up to 20%.

©2017 The Authors.PublishedbyElsevierLtd. Thisis an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the organizing committee of SCESCM 2016.

Keywords: recycled aggregate; self-compacting concrete; recycled sand; microstructure; proportioning.

* Corresponding author. Tel.: +34-881-015-249 E-mail address: diego.carro@udc.es; dcarro@udc.es

1877-7058 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the organizing committee of SCESCM 2016.

doi:10.1016/j.proeng.2017.01.401

1. Introduction

As the population grows and improves its lifestyle, a proportional increase of the consumption of natural resources and energy occurs. One of the industries with greater responsibility in the consumption of natural resources and generation of waste is the construction industry. Construction and demolition waste (CDW) is undoubtedly one of the main centers of attention in the search for waste reduction.

This research is focused on the determination of the influence of the use of recycled sand on self-compacting concrete in terms of proportioning and microstructure. With this, the variables of interest in this research are the percentage of replacement of natural sand and the workability measured over time.

There are different ways of obtaining the composition of a SCC and in this work the objective is an appropriate workability, i.e., the resulting concrete should present fresh-state SCC behavior. This research involves the replacement of 0%, 20%, 50% and 100% of natural sand by recycled sand and this type of sand presents a high absorption. Thus, it is expected that the mortars and concrete with high substitution ratios of these aggregates will present lower workability. This could be dealt with by increasing the content of superplasticizer but in the extreme case of 100% of replacement this was not enough to provide acceptable workability. However, if the amount of superplasticizer varies, the different mixes would not be comparable. Therefore, the superplasticizer quantity was adjusted for the control concrete and kept constant for all substitution ratios.

1.1. Use of fine recycled aggregate concrete

The possibility of the use of Fine Recycled Aggregates (FRA) in self-compacting concrete (SCC) was demonstrated by Kou & Poon [1], who obtained SCC with acceptable workability and compressive strength using even 100% of both fine and coarse recycled aggregates. The authors found out that the values of slump flow grew with the replacement ratio, and this indicated that the recycled aggregates did not absorb 100% of the water at 24 hours (W24h). They repeated this test after 1 hour and they found a positive correlation between the substitution ratio and the loss of workability.

Other interesting reference of the use of FRA in SCC is the work of Corinaldesi & Moriconi [2] who achieved SCC concrete with coarse and fine recycled aggregates separately in percentages of 0 and 100%. In this work the w/c ratio was established as 0.45 and the content of cement as 440 plus 100 kg of filler. These fines were alternatively limestone, fly ash and also powder from recycled aggregates. The main effect of this last type of powder was to reduce over time the workability of the paste; this was evidenced in terms of yield stress values and plastic viscosity values. Finally, this work also refers a significant reduction in compressive strength, much larger in the case ofFRA than in recycled coarse aggregates.

There are also works about recycled coarse aggregates (RCA) in SCC, recent examples of these could be the work of Silva [3], or the ones of Safiudin et al. [4] and Grdic et al. [5]. All results agree in two points:

• The effect of the absorption of the recycled aggregates must be compensated. This adjust is done by means extra water or adding superplasticizer;

• There is a reduction in compressive strength and modulus of elasticity with the substitution's percentage.

Research on the use of FRA in ordinary concrete is not extensive. The main effect that is reported in the literature

is the loss of compressive strength with the substitution ratio. Khatib [6] refers that concrete with 25% and 100% suffer reductions of 15% and 30% relative to the reference mix. Evangelista & de Brito [7] refer that with percentages of substitution up to 30% there were no significant reductions. The other important effect described is the loss of workability due to the high absorption of the FRA. Some authors add more water to compensate this effect; others add more superplasticizer and a few of them deal with the effects of the loss of workability.

Another significant question is the variability of the quality of the FRA. For example, Pereira et al. [8] refer a FRA with 13.1% of absorption; on the other hand, Khatib [6] refers one with 6.3% of absorption and Zega et al. [9] uses a FRA with an absorption of 8.5%. These differences obviously should produce significant effects on the properties of the resulting concrete. However, in all cases this absorption is higher in the recycled aggregate than in the natural one. The main idea is that there is a negative relationship between the absorption of the recycled sand and the quality of the resulting concrete. This follows the same trend that is well established with coarse recycled aggregates [10].

1.2. Fresh properties & rheology of SCC

SCC is designed and produced with fresh-state properties in mind. SCC exhibits enhanced capabilities substantially different than the ones of the ordinary concrete. The enhanced properties that are looked for in SCC are: flowability, viscosity (measure of the speed of flow), passing ability (flow without blocking) and segregation resistance [11,12]. The priority of these properties depends on the use of the concrete: in a section strongly reinforced the crucial property is the passing ability; on the other hand, when casting a slab, the emphasis is on the flowability and the segregation resistance. Thus, different SCC are designed with a specific behavior corresponding to its use [11].

The SCC fresh-state properties are used as reception criteria and one or two tests are performed when the concrete arrives at the construction site allowing acceptance or not of the concrete batch [13]. However, the evolution of the fresh properties over time is a key factor in the properties of concrete, i.e. the open time can be adjusted to the type of superplasticizer or retarders [11,13,14]. It has been observed that the rheological parameters worsen with time [15] and the open time varies in different mixes. This evolution can be observed in the equivalent mortar, where it is enhanced by the absence of coarse aggregate.

The fresh-state properties are measured by means of several specific empirical tests. For the flowability, the slump-flow is widely used; for the viscosity of the mix, T50 is used but also the V-funnel; for the passing ability, the L-box and J-ring are the most common and, finally, for the segregation resistance, the sieve segregation is one of the most widely used [11,14].

The empirical approach is useful but it is possible to study the phenomena in further detail. Fresh SCC can be modelled as a viscoelastic fluid with a model that relates the shear stress (x) and shear rate (y). There are various models for the equation that relates both properties, but the Bingham model is one of the simplest and most widely accepted [16]. This model describes concrete flow in terms of yield stress (x0) and plastic viscosity with this equation: x = x0 + n-y. Yield stress represents the stress necessary to initiate or maintain flow, whereas plastic viscosity expresses the increase in shear stress with increasing shear rate once the yield stress has been exceeded

There is a significant difference between the SCC when it is at rest for a long period of time or when it has been recently mixed, due to its thixotropy. The resistance to deformation varies substantially, so that two parameters could be defined: static yield stress, that relates to undisturbed concrete, and dynamic yield stress that relates to disturbed concrete [18,19].

2. Materials

A Portland cement, CEM I 42.5 R according to European Standard EN197-1, was used. In addition to this, the powder fraction was completed with limestone filler.

The superplasticizer used to achieve suitable SCC mixes was a modified polycarboxylate type usual in SCC production. No retarder was used to control the hydration or the open time.

Three types of aggregates were used. The coarse fraction was a natural limestone gravel. Two fine aggregate were used: a natural limestone sand that was partially replaced by a recycled sand. The source of the natural aggregates was a crushed aggregate from a northern-western Spain stone quarry. The fine recycled aggregate came from crushed concrete from a local recycling plant (RECINOR).

The natural and recycled sands need to be well graded and have a size distribution as similar as possible. To achieve both objectives, it was necessary to adjust the grain size curve of one of the aggregates. In this case it was decided to divide the natural sand in three fractions: below 0.25 mm, between 0.25 mm and 2 mm and above 2 mm. After this, these three fractions of natural sand were remixed to obtain a "corrected natural sand" with a size distribution similar to that of the recycled sand.

In addition to the regular absorption test, a continuous measurement of water absorption of the aggregates was conducted. At the usual reference time of 10 min the absorption of the recycled sand was 72% of the absorption at 24 h, and in the case of natural gravel and sand it was 50% and 70% respectively.

3. Mix proportioning of self-compacting concrete with fine recycled aggregates

3.1. Proportioning methods

There are many references about the design of SCC with different methodologies. Mainly, there are two groups of procedures to adjust the composition: on the one hand, one can set the strength level to be achieved and then the components' content necessary to meet this objective. On the other hand, one can start from a given standard formulation that will be corrected to achieve the workability appropriate to the target application. In this work, the second option was used.

Schwartzentruber and Catherine [20] proposed the use of the concrete equivalent mortar (СЕМ) to study the rheology of fresh concrete with the assumption that the rheological properties of СЕМ should be correlated with those of the corresponding concrete. For the СЕМ design it is considered that all friction phenomena take place at the cement paste/aggregate interface. Therefore, the total specific area of the aggregates is a fundamental variable to understand the level of workability of concrete.

When the composition of the СЕМ is determined the main following relationships concerning the original concrete composition should be kept constant: cement and filler content, water-cement ratio, and fine aggregate content necessary to achieve the same total surface area of coarse aggregate replaced. An example of this procedure can be seen in the Rubio-Hernández et al. work [21].

Since the start of the СЕМ method there have been various approaches to obtain the equivalent mortar of a specific concrete. In this context, the Nepomuceno et al. method [22] used in this work is a generalization of the one proposed by Ouchi et al. [23] and it centers its efforts on the characterization of the mortar and correlates the behavior of mortar and the one of concrete. This method uses two parameters, Gm and Rm measured in mortars that are correlated with the slump-flow and the T50 time of concrete.

Because of easier preparation, mixing, and sampling, СЕМ mixes consume less materials, energy, and time for testing, which can greatly simplify and speed up testing of concrete [24]. This represents a solution to adjust the mortar that would produce a SCC. In addition, as in this research the interest is centered on the recycled sand, the study of mortar is more useful in the aspects where the effect of the properties of the recycled aggregate is magnified.

3.2. Concrete composition and mixing

The design of the reference SCC is based on the Nepomuceno's method [25] using some recommendations from the СЕМ and rational methods. This methods can give guidelines of how to adjust a mix to a set of properties like compressive strength and fresh-state parameters. However, it is not known how to ensure that a reference concrete with SCC behavior, once some fraction of it is replaced, would remain in the domain of acceptable quality SCC. For instance, if the replacement of FRA is set to 100% and no water is compensated, the result is a conventional concrete with no SCC behavior (Fig. 10).

Regarding the high water absorption of the recycled aggregates, it was necessary to adjust the mixing water, which was done by adding extra water. However, not all the 24 h water absorption was compensated, rather the absorption at 10 min. This criterion was also used before by other authors [8] [26]. To ensure comparability, all materials were oven dried before their use.

In order to study the effect of FRA in the SCC proportioning and microstructure, the designed concrete included an increasing proportion of recycled sand. With this same idea, the incorporation of coarse aggregate was fixed to 30% in volume of natural gravel. This criterion corresponds to the recommendations of the rational method [23] [14]. Besides, this allows to study the mortar fraction of the SCC separately.

To start correctly the adjustment of the mix, the paste (with a previously fixed w/c ratio) was tested alongside with variable percentages of superplasticizer additive (Fig. 1). The test showed that the optimum quantity was 2% of sp. However, this high content produced segregation and finally it was set to a value of around 1.7% of superplasticizer.

The use of the mortar fraction to adjust the mix has proved to be a powerful tool to find and compare mix proportions. The batches are only of 1.6 liters and this reduces the quantities of materials to prepare: oven dry the

aggregates, mixing the fractions of the sand, weight all components, etc.; so tests are substantially faster and/or it is possible to perform a much larger number of tests. Once a mix proportion that fulfilled the Nepomuceno's recommendations was found, it was tested in full-scale concrete performing the empiric fresh-state tests: slump-flow, V-funnel, L-box and J-ring.

This mix design in two stages, first mortar and later full concrete, was helpful to achieve in a rapid way suitable compositions of SCC with FRA. Additionally, it should be emphasized that the only variable that affected the flowability and viscosity of the mix was the presence of FRA. Based on this, the other mixes were designed by replacing a given volume of natural sand with recycled sand. The replacement ratios were 0%, 20%, 50% and 100 %.

The mortar approach allowed designing a suitable SCC concrete mix. This mix was tested at concrete scale, obtaining a SCC with acceptable slump-flow results but with high values in the V-funnel test. With these data, and focusing on obtaining a robust but flowable mix, small increases in w/c ratio and superplasticizer content were introduced. Therefore, the final mix was obtained (Table 1) with only minor changes from the composition adjusted in the mortar phase.

As the research's interest is to understand the influence of the replacement of natural with recycled sand, it was important not to include other adjustment or variation in the mixture proportioning. With this, the effects could be attributed to the recycled material.

Table 1. Concrete composition ofthe mixes

Mass (kg) ofthe mixes

, % ofrecycled sand replacement (in volume) . Absorption

Material Volume (1) J F k ' Density

.. 24 n (%)

0% 20% 50% 100 % (kg/1)

Cement 128.7 400.0 400.0 400.0 400.0 3.11

Limestone filler 66.4 180.0 180.0 180.0 180.0 2.71

Water 184.0 184.0 184.0 184.0 184.0 1.00

Additional water (1) 10.36 13.39 31.74 53.12 1.00

Recycled sand (2) _ 63.6 365.9 731.9 2.30

Natural sand (2) 865.6 692.4 432.8 - 2.72

Natural gravel (2) 300.0 768.0 768.0 768.0 768.0 2.56

Water/cement 0.46 0.46 0.46 0.46

Superplasticizer 1.70 1.70 1.70 1.70

(% in mass of cement + filler)

(1) Water absorbed by the aggregates after 10 min and correspondent with the mixing time (2>All materials were oven dry before its use

To obtain all the mixes with the different substitution ratios, given volumes of natural sand were replaced with the same volumes of recycled sand. No other adjustment was introduced because the focus was on studying the effect of the use of different percentages of recycled sand. Also, a detailed protocol for the mixing of concrete and mortar was set (Fig. 2).

1,4 1,8 1,S 2,0

% additive ^cement weight

Fig. 1. Marsh cone test results

Fig 2. Mixing procedure for concrete and mortar

4. Results and discussion

4.1. Comparability of mixes with different substitution ratios of FRA

As mentioned previously, the general procedure to obtain the mortar content was based on the Nepomuceno method [25] [27]. It would have been possible to strictly follow the CEM method [20] [21] and include an extra content of recycled sand equivalent to the specific surface of the gravel. This would lead to an equal specific surface, but the absorption of this extra amount of sand would be much higher than the one of the gravel (their water absorption at 24 h are 9.3% and 1.1% respectively). In this way, the equivalent mortar would only be equivalent in terms of specific surface but the influence of the absorption would be quite more relevant. So, finally, it was decided not to include this extra sand and assume that the mortar phase correlated accurately with the concrete. It should be

noted that all mixes included 30% in volume of natural gravel, so the only variable of influence was the presence of FRA.

During the adjustment phase the only tests performed were the ones related with the fresh state, i.e. mini-cone and mini-funnel. The Nepomuceno's limits were complied with [25,27] so the target spread values in mini-cone and flow time in mini-funnel were 251-263 mm and 7.7-8.7 s respectively. These limits were unreachable for all the mixes at the same time. If it were adjusted for 0% replacement, the mixes with 50% and above of FRA would not exhibit SCC behavior. They were conventional mortars with fluid consistency and low values in spread in mini-cone; and also with high passing times in mini-funnel, when they did pass at all. On the other hand, if the mix were adjusted for 100% of FRA incorporation the result would be a severe increase in fluidity and reduction of viscosity. This would lead to severe segregation in the substitution percentages of 0% and 20%. This can be seen in the sections of Fig. 3 of a rejected mix with excessive fluidity and low viscosity that produced segregation for low substitution ratios.

TOP 0% 20% 50% 100%

Fig 3. Segregation in some mortar tests, increased with low substitution ratios

It was detected that if the 100% FRA mix was adjusted and exhibited acceptable parameter in both mini-cone and mini-funnel tests (according to Nepomuceno's criterion), the mix with 0% replacement showed complete segregation of paste and aggregates (Fig. 3). When the 0% mix was the reference, the 100% mix lost the SCC behavior. Therefore, it was clear that the complete fulfillment of the criterion was impossible, so new limits were set. In this way, it was possible to find out mixes where all batches showed SCC behavior even though with some variability in viscosity and flowability. However, at least there were groups of SCC products under comparison, not conventional concrete compared to SCC.

This new criteria is represented in Fig. 4, where it can be seen that the narrow range that Nepomuceno's method indicates is widely expanded (mini-cone: 251-263 mm & mini-funnel: 7.7-8.7 s). However, when the values of mini-

cone spread grew substantially, segregation problems started to appear. Therefore these mixes are also not valid

Fig 4. Criterion ofacceptance ofequivalent mortars to produce SCC concrete

Another question that should be addressed properly is the time at which the mix is adjusted. In this work, the f fresh-state parameters were measured just after the finishing the mixing process (at 10 minutes). However, as seen in Fig. 6; these parameters change with time and, in the case of the 100% substitution, a mix that presented complete SCC behavior lost all fluidity and passing ability after 90 minutes. Maybe, 30 minutes or 45 minutes could represent better the SCC fresh-state characteristics of a specific mix. Testing at one of these times could be more representative of the real behavior in the construction site, but this is impractical from a production control point of view.

4.2. Microstructure of SCC equivalent mortars

The selected mixes were studied under optic microscopy at the age of 14 days in a preparation of petrographic thin sections. The pictures (Fig. 5) show a significant increase in porosity with the percentage of FRA. These pores are clean with no precipitate in them and with size varying from 50 to 250 < m.

It was not possible to visualize the different interfacial transition zone (ITZ) of the aggregates, because this is easier in coarse aggregates and rather difficult with recycled sand. This ITZ has been referred to be the weak part of the microstructure [28].

However, in the 100 % replacement mix there was a fraction of natural aggregate - from the original concrete -that seemed to be separated from the original paste. This could be attributed to the processing of the aggregates.

Fig 5. Microstructure of the mortar phase with FRA in percentages of50% (left) and 100 % (right).

4.3. Mortar results

The mortars were tested over time by means of the mini-cone and mini-funnel tests (0). In the mix that includes 100% of recycled sand, at 90 minutes the mini-funnel test was unfeasible due to the rigidity of the mix.

The spread is higher with low sand substitution percentages. However, for the reference mortar, and also for the one with 20% of sand substitution there is a slight increase in the spread values from 15 to 60 minutes. Regarding the passing time (V- funnel), it could be said that there is a clear trend towards reduced values with time and with replacement percentage. This contrasts with the results reported by Jin [15], who found out a continuous decrease of spread and increase of the passing time over time.

Time (min)

Fig. 6. Left: Spread results of the mortar on mini-cone test. Right: Passing time results ofthe mortar on mini-funnel test

The density and compressive strength of the mortars are substantially influenced by the presence of the recycled sand (Fig. 7). In the case of the 100% substitution, the decrease of compressive strength at 28 days is 48%. Concerning the density, it is reduced with the same trend.

% of Recycled Sand % of Recycled Sand

Fig. 7. Left: Hardened density at 28 days and fresh density of mortars. Right: Compressive strength at 28 days ofmortars

4.4. Concrete results

Once the mix proportion was tested in the mortar phase, the next step was to produce equivalent concrete mixes. At this stage the empirical tests used were the usual slump-flow test and V-funnel. In addition, tests with rheometer were performed: stress growth test and flow curve test. In the two first all substitution ratios were performed but in the case of the concrete with a 100% of replacement the results at 90 min showed a complete loss of the SCC behavior. After these results, it was decided to go on only with 0 and 50% replacement. This allows studying the effect of the incorporation of the recycled sand.

In Fig. 8 the results of slump-flow test are presented. A reduction over time of the spread and an increase in the T500, higher with the substitution ratios, can be seen. A similar trend was detected in the case of the V-funnel results (Fig. 9), but there where substantial differences between the 0% and 20% and the 50% and 100% group. In this last group, the effect of time is stronger; it seems that the concrete loses flowing ability with the incorporation of recycled sand.

The flowability of the 50% and 100% mixes suffered a severe reduction, losing their SCC behavior. In the case of the 100% mix, it was even impossible to achieve a flowable concrete (Fig. 10).

Time (min) Time (min)

Fig. 8. Results of the slump-flow test (spread & Tsoo)

Fig. 9. Results ofthe V-funnel test

Fig. 10. Slump test ofthe mix with 100% ofrecycled sand at 90 minutes. The slump-flow test was unfeasible even though atl5 min it was a SCC with a spread of680 mm

4.5. Mechanical properties of SCC

Fig. 11 presents the compressive strength of the mixes. There are substantial reductions of the mechanical properties with the incorporation of recycled sand. At the same time, the evolution with the concrete age is slightly lower in the case of concretes with fine recycled aggregate. The trends are similar to those of mortars.

Fig. 11. Evolution of compressive strength in concrete samples

5. Conclusions

The following conclusions can be drawn:

• The use of some fixed parameters as 30% of gravel content and 1.7% of superplasticizer for all the replacement ratios (0%, 20%, 50% & 100%) leads to comparable SCC's and allows studying the mortar phase separately;

• Criteria of comparability for the mixes that included recycled sand were established: they should present SCC behavior to be comparable, if not, the reference proportion should be readapted. It is not possible to comply with all recommendations when the nature of the aggregates is changed;

• The microstructure of the pastes was homogeneous and no different ITZ was observable, some aggregates were detached from the paste. The incorporation of recycled sand resulted in an increase in porosity;

• With this mix design it was possible to produce self-compacting concrete with a substitution of up to 100% of recycled concrete sand. However, 50% and 100% mixes started to lose this behavior after 60 min;

• There was a reduction of filling capability and flow ability over time and the loss of properties was substantially higher as the percentage of recycled sand increased;

• The mechanical properties are affected by the incorporation of recycled sand: the influence is low in the case of 20% but, for larger substitution ratios, the losses are severe.

References

[1] Kou SC, Poon CS. Properties of self-compacting concrete prepared with coarse and fine recycled concrete aggregates. Cem Concr Compos 2009;31:622-7. doi:10.1016/j.cemconcomp.2009.06.005.

[2] Corinaldesi V, Moriconi G. The role of industrial by-products in self-compacting concrete. Constr Build Mater 2011;25:3181-6. doi:10.1016/j.conbuildmat.2011.03.001.

[3] Carro-López, D.; González-Fonteboa, B.; Brito, J. de; Martinez-Abella, F.; González- Taboada, I.; Silva, P.: "Study of the Rheology of Self-Compacting Concrete with Fine Recycled Concrete Aggregates", Construction and Building Materials, V. 96, Elsevier, UK, October 2015, pp. 491-501.

[4] Safiuddin M, Salam M a., Jumaat MZ. Effects ofrecycled concrete aggregate on the fresh properties ofself-consolidating concrete. Arch Civ Mech Eng 2011;11:1023-41. doi:10.1016/S1644-9665( 12)60093-4.

[5] Grdic ZJ, Toplicic-Curcic G a., Despotovic IM, Ristic NS. Properties of self-compacting concrete prepared with coarse recycled concrete aggregate. Constr Build Mater 2010;24:1129-33. doi:l0.1016/j.conbuildmat.2009.12.029.

[6] Khatib IM. Properties of concrete incorporating fine recycled aggregate. Cem Concr Res 2005;35:763-9. doi:10.1016/j.cemconres.2004.06.017.

[7] Evangelista L, de Brito J. Mechanical behaviour of concrete made with fine recycled concrete aggregates. Cem Concr Compos 2007;29:397-401. doi:l0.1016/j.cemconcomp.2006.12.004.

[8] Pereira P, Evangelista L, de Brito J. The effect of superplasticisers on the workability and compressive strength of concrete made with fine recycled concrete aggregates. Constr Build Mater 2012;28:722-9. doi:10.1016/j.conbuildmat.2011.10.050.

[9] Zega CJ, Di Maio AA. Use of recycled fine aggregate in concretes with durable requirements. Waste Manag 2011;31:2336-40. doi:10.1016/j.wasman.2011.06.011.

[10] ACHE. Use ofrecycled aggregates for production of structural concrete. (In Spanis. Madrid: Asociación Científica del Hormigón Estructural; 2005.

[11] EFNARC, BIBM, ERMCO, CEMBUREAU;, EFCA. The european guidelines for self-compacting concrete. Specification, production and use. 2005.

[12] Khayat KH. Workability, testing, and performance ofself-consolidating concrete. ACI Mater 1 1999;96:346-53.

[13] EFNARC. Specification and guidelines for self-compacting concrete, vol. 44. 2002.

[14] ACHE. Self-compacting concrete: design and appliances (in Spanish). (In Spanis. Madrid: Asociación Científica del Hormigón Estructural; 2008.

[15] lin J. Properties ofmortar for self-compacting concrete. University ofLondon, 2002.

[16] Banfill PFG. The rheology of fresh cement and concrete - A review. 11th Int. Cem. Chem. Congr., Durban: 2003.

[17] Billberg PH. The structural behaviour ofSCC at rest. In: Singapore Concrete Institute, editor. Our world Concr. Struct., Singapore: 2011.

[18] Khayat H, Omran A, Magdi A. Evaluation of thixotropy of self-consolidating concrete and influence on concrete performance. I SIMLAMCAA - IBRACON, 2012, p. 14.

[19] KovlerK, RousselN. Properties offresh andhardened concrete. Cem Concr Res 2011;41:775-92. doi:10.1016/j.cemconres.2011.03.009.

[20] Schwartzentruber A, Catherine C. La méthode du mortier de béton quivalent ( MBE ) - Un nouvel outil d'aide a la formulation des bétons adjuvantés. Mater Struct 2000;33:475-82.

[21] Rubio-Hernández FI, Velázquez-Navarro IF, Ordóñez-Belloc LM. Rheology of concrete: a study case based upon the use of the concrete equivalent mortar. Mater Struct 2012;46:587-605. doi:10.1617/sll527-012-9915-l.

[22] Nepomuceno M, Oliveira L, Lopes SMR. Methodology for mix design of the mortar phase of self-compacting concrete using different mineral additions inbinaryblends ofpowders. Am Concrlnstitute, ACI Spec Publ 2012;26:317-26. doi:10.1016/j.conbuildmat.2011.06.027.

[23] Ouchi M, Hibino M, Ozawa K OH. A rational mix-design method for mortar in self-compacting concrete. Proc. sixth east-asia- pacific Conf. Struct. Eng. Constr., 1998, p. 1307-12.

[24] Assaad JJ, Harb J, Chakar E. Relationships between key ASTM test methods determined on concrete and concrete-equivalent-mortar. J ASTM Int 2012;6:1-13.

[25] Nepomuceno M, Oliveira L, Lopes SMR. Methodology for mix design of the mortar phase of self-compacting concrete using different mineral additions in binary blends ofpowders. Constr Build Mater 2012;26:317-26. doi:10.1016/j.conbuildmat.2011.06.027.

[26] Pereira P, Evangelista L, de Brito J. The effect of superplasticizers on the mechanical performance of concrete made with fine recycled concrete aggregates. Cem Concr Compos 2012;34:1044-52. doi:10.1016/j.cemconcomp.2012.06.009.

[27] Nepomuceno M, Oliveira L. Parameters for self-compacting concrete mortar phase. Am. Concr. Institute, ACI Spec. Publ. (253 SP), 2008, p. 311-27.

[28] Sidorova A, Vazquez-Ramonich E, Barra-Bizinotto M, Roa-Rovira JJ, Jimenez-Pique E. Study ofthe recycled aggregates nature's influence on the aggregate-cement paste interface and ITZ. Constr Build Mater 2014;68:677-84. doi:10.1016/j.conbuildmat.2014.06.076.