1. Introduction
Approximately 90% of construction and demolition wastes (CDW) are currently going to landfills even though they are potentially recyclable [
1]. The use of this waste should be a priority to achieve the sustainable development objectives set by the European Commission, although this action is hindered due to lack of facilities and standards, lack of support from governments or lack of users’ confidence [
1,
2]. The use of CDW as aggregates in concrete production, mostly coarse recycled aggregates (CRA), not only means a saving of natural resources derived from the extraction of aggregate, but also economic savings. Analogously, concrete with electric arc furnace slags (EAFS) as aggregate is based on the use of waste (from the steel industry) that would otherwise be deposited in landfills. In this case, the reduction of CO
2 emissions in the processes without taking into account the transport and manufacturing of the materials can be as high as 35% [
3]. On the other hand, CO
2 emissions induced by concrete crushing are not very different from those generated in the production of natural aggregates [
4].
To improve the understanding of the paper, the acronyms used and their meaning are shown in
Table 1.
Most CRA are produced by crushing concrete that has ended its service life, i.e., they are composed mainly of natural stone and attached mortar. Typically, CRA are materials with lower density and porosity than natural aggregates (NA) because the attached mortar is less dense and more porous than the natural aggregate that it covers. The average density can be 8% lower and the average water absorption 5–6 times those of the natural aggregates [
5,
6]. According to the current Spanish concrete standards, the aggregates’ water absorption must be less than 5% to be used in structural concrete [
7,
8]. According to Etxeberría et al. [
9], the shape index of NA is 25% and 28% for CRA produced in quarries, although its value depends on the crushing process. Typically, laboratory-produced CRA are made with a single crushing stage (usually a jaw crusher), whilst NA are produced with multiple crushing (primary, secondary and sometimes tertiary). De Brito et al. [
10] found that when CRA go through the same crushing process as NA their shape index is expected to be lower than that of NA. The water absorption and the shape index are vital for the calculation of the compensation water to determine the total water/cement (w/c) ratio [
11].
There are many studies on the use of recycled aggregate (RA) in the production of structural concrete. Most of them only consider replacement of the coarse fraction of the aggregates, because the fine fraction has a great cohesion and water absorption that make it difficult to control the quality of the aggregates [
12] and reduce the workability in the fresh state [
13]. The risk of contamination of the finer fraction is also higher [
14].
The use of CRA is more common also because it has less porosity and adhered paste. Typically, CRA concrete is around 4%–8% less dense [
15,
16,
17], although this effect can be the opposite for high density CRA (e.g., CRA based on EAFS). The water absorption of these CRA concretes can be 500% higher than that of NA concrete (NAC) [
18,
19], although it tends to decrease due to the crystallization of hydration products, depending on the crushing age of the source concrete and curing conditions [
20,
21,
22]. The fresh workability with CRA is lower than that of NAC for equal w/c effective ratios, so it is important to correct the water content to achieve similar slump without the help of admixtures [
16,
23,
24,
25,
26,
27,
28].
One of the main characteristics of this concrete is the presence of three interfacial transition zone (ITZ). One is between the original aggregate of CRA and the cement paste in the source concrete and it is formed by dense hydrates, and in the case of EAFS concrete is of higher quality than with NA [
29]. Another is between the old cement paste and the new paste, and the third ITZ is between the NA of the recycled aggregate and the new cement paste. The two ITZs between recycled aggregate (stone and mortar) and the new paste are where the chemical reactions between both generate loose and pore interfaces [
30,
31]. These ITZs are thus weaker and limit the mechanical properties of CRA [
30]. Concrete with 100% CRA shows a loss in compressive strength with respect to coarse NA at 28 days, for the same effective w/c ratio and amount of cement, from 10% to 37% [
9,
17,
27,
32,
33,
34], approximately proportional to the replacement level [
35,
36] and depending on the relative strength of the new paste and CRA [
37]. This decrease in compressive strength may make it necessary to use about 5% more cement to achieve the same strength as with NA, thus compromising the cost-effectiveness and sustainability of CRA [
9,
12]. From 28 to 91 days, the relative increase in compressive strength with recycled aggregate concrete (RAC) is sometimes greater than with NA due to the hydration of unhydrated cement grains [
37]. The splitting tensile strength is typically lower but often not by as much as compressive strength, but there are studies where the tensile strength can even be higher with CRA than with NA [
9,
38], while the modulus of elasticity can decrease by 15% for 30% replacement and 45% for 100% replacement [
35,
39]. This reduction is because CRA has a lower modulus of elasticity than NA and due to the increase in the effective w/c ratio to maintain workability constant. Reductions in the modulus of elasticity result in increases in the peak strain of concrete under monotonic compression—[
32] reports an increase of 20% for aggregate replacement of 100%. It is also known that the use of CRA but also the new cement paste affect the fatigue behaviour of concrete, reducing the fatigue limit and fatigue life [
40,
41]. Other authors have shown that the multi-recycling of the concrete that contain CRA is limited and that after three recycling cycles, CRA are mostly composed of mortar and new NA are needed in the mix design [
42].
The drying shrinkage of concrete with CRA can be 50% higher [
43] than with coarse NA, while chloride ions’ penetration can reach up to 150% increases [
24,
44] due to the high permeability of this concrete. Resistance to carbonation is linked to the porosity of concrete [
15] and, therefore, to the porosity of CRA, although it is also strongly linked to the chemical composition of concrete [
24]. The high porosity of CRA makes carbonation depths increase between 22%–187% for 100% replacement [
24,
33,
45] in comparison to NAC. Some authors propose the use of more crushing stages to eliminate the attached mortar and thus obtain rounder and less porous aggregates [
45,
46], the use of acids or heat to disaggregate the mortar of RCA [
47,
48], thermo-mechanical processes [
49,
50] or the use of several mechanical systems for on-site processing [
51]. Another solution presented by the literature is the use of crushed bricks or steel slag [
13,
52] to compensate the loss of strength and durability, due to their pozzolanicity. However, these beneficiation techniques add new steps to the aggregate production process and increase production costs and the environmental impact of CRA production.
Currently, there is an increasing trend towards the use of steel slag in concrete [
23]. The use of EAFS as CRA in concrete is a novelty (to the best of the authors’ knowledge, it has never been done before) and its use is justified by the potential benefits of this aggregate and by the boom of its use in recent years mostly in road pavements or hydraulic structures [
53]. The characteristics of the source concrete determine the behaviour of the recycled aggregates concrete. Concrete with EAFS offers an improvement in compressive strength by 50% compared to concrete with NA, a slightly higher modulus [
54] and a generalized improvement in durability (low water absorption and permeability) [
55,
56], whereas the roughness of the aggregates allows improving the quality of the new ITZ of CRA. The quality of the CRA from EAFS concrete will compensate the loss of mechanical and durability properties that more common CRA provides, saving natural resources. This concrete has potential applications in foundations, plain concrete walls, or structures where a high self-weight is important (e.g., radiation-proof structures). The results of the physical-mechanical and durability tests, for concrete with coarse replacements of 0%, 20%, 50% and 100% by CRA, will be analysed and discussed, establishing the suitability of their use.
2. Materials and Methods
2.1. Materials
In this research, the NA used were: limestone gravel (2/6, 6/12 and 12/20 mm), and silica sand (0/2 and 0/4 mm) to produce the reference concrete. For the manufacture of RAC, CRA obtained from the crushing of concrete with EAFS (using a jaw crusher) 2 months old has been used. The resulting crushing material has a range of grading of 0/25 mm. This EAFS concrete has been manufactured with cement (CEM) I 52.5 R and a w/c ratio of 0.47. This source material presents at 28 days a compressive strength of 88 MPa, a modulus of elasticity of 52 GPa and an oxygen permeability of 6.48 × 10
−18 m
2. The physical properties of both the NA and RA are shown in
Table 2, after performing a measurement. The specific gravity and the water absorption have been determined according to EN 1097-6, the shape index was obtained following the EN 933-4 and Los Angeles wear has been determined according to EN 1097-2. The Portland cement used in RAC is CEM I 42.5 R (European standard), whose density is 3.15 g/cm
3 according to UNE 80103, and the mix has been made with tap water.
The obtained water absorption of the CRA meets the requirements of the Spanish standard EHE-08 for structural concrete [
7] and is more than twice that of coarse NA. However, this value is very small compared to more common CRA that normally exceeds 5% [
5,
33,
57], although it depends on the size range. The shape index of the CRA is slightly lower than that of the NA and approximately one and a half times the values obtained by Etxeberría et al. [
9] for conventional CRA. In terms of workability, these aggregates show a low shape index, but EAFS exposed surface is very cavernous and cause mesh between aggregates, demanding an extra volume of cement paste or mortar to fill the holes in their surface. However, the solid fraction of EAFS is generally much less absorbent than that of NA. Due to its properties, good mechanical and durability properties are expected [
39].
2.2. Mix Design
The design of the mix has been made using the Faury method, to obtain maximum compactness. The maximum aggregate size has been set at 20 mm, in accordance with EHE-08. The coarse aggregates (>4 mm, EN 13139) of the reference concrete (NAC) have been replaced at several ratios (20%, 50%, and 100% vol.) with CRA. The content of cement has been set at 350 kg/m
3 and the effective w/c ratio of the reference concrete (NAC) at 0.5. The total w/c ratio has been determined by adding compensation water equal to that estimated for the mixing time (10 min) from the water absorption over time test, according to the method proposed by Rodrigues et al. [
58]. The strength class of concrete has been defined as C30/37 in accordance with EN 1992-1-1. The slump has been defined as 70 ± 10 mm (S2) according to EN 12350-2 and without plasticizers (in order not to introduce more variables) for all replacement ratios. To maintain the same slump in all the mixes, the effective w/c has been slightly modified. RAC has been manufactured from the theoretical curve of NAC (
Figure 1), maintaining between mixes the same volume of aggregates of each sieve fraction, so the mix grading for NAC and for RAC is the same. The mix proportions used are shown in
Table 3.
The aggregates were dried at 100 ± 2 °C until constant weight before mixing and the mixing process consisted of a sequence of 4 min with the coarse aggregates and 2/3 of the water, 2 more minutes after adding the fine aggregates and a further 4 min after adding the cement and 1/3 of the water. The concrete was demoulded after 24 h of manufacture and it has been cured in a humidity chamber at 20 ± 2 °C and 95 ± 2% humidity (except for drying shrinkage testing specimens).
A scheme illustrating the successive steps of RAC’s manufacturing process is shown in
Figure 2. The process describes the possible multi-recycling of RAC.
2.3. Physical Properties Tests
The concrete’s absorption by capillarity has been determined on four cylindrical specimens with 150 mm diameter and 100 mm in length per mix proportion, after 28 days of curing in a humidity chamber and 14 days in an oven at 60 ± 5 °C. The test consists of measuring the mass evolution after 3, 6, 24 and 72 h of immersion, according to LNEC (National Laboratory for Civil Engineering) standards following the LNEC E-393. The absorption by immersion has been determined on four 100 mm cubic samples after 28 days of curing in a humidity chamber according to LNEC E-394. In addition, the apparent bulk, bulk, and saturated surface dry (SSD) density have been determined according to EN 12390-7 and the open porosity according to UNE 83980, for all the mixes produced.
2.4. Mechanical Properties Tests
The compressive strength () has been obtained on 150 mm cubic samples (EN 12390-1) per mix at ages of 7, 28 and 91 days. The test specimens have been tested using a load application rate of 0.6 MPa/s in a servo-hydraulic press of 3000 kN capacity and in accordance with EN 12390-3. The ultrasonic pulse velocity test has been performed on the specimens intended for the compressive strength test, prior to testing the compressive strength and just after their surface is dry. The measurement has been carried out according to EN 12504-4 with the transducers in direct transmission placed in collinear directions between two parallel faces with Vaseline on the contact surface between transducers and the concrete surface. The pulse velocity is calculated as the ratio. The compressive modulus of elasticity (E) has been determined in a servo-hydraulic press of 250 kN capacity, on three cylindrical specimens with 150 mm and 300 mm in length per mix, after 28 days of curing in a humidity chamber. The upper and lower faces of the test specimens have been levelled before the test and a compressometer/extensometer equipped with high precision displacement transducers is used to measure the micro-deformation. Four loading/unloading cycles have been used, applying an initial stress of 1 MPa (17.6 kN) and a load application speed of 0.5 MPa/s (8.8 kN), using a maximum load of according to LNEC E-397. The splitting tensile strength has been determined on the three specimens used in the modulus of elasticity test, for all mixes. A servo-hydraulic press of 3000 kN capacity and a load rate of 0.05 MPa/s (3.5 kN/s) was used, according to EN 12390-6.
2.5. Durability Tests
The resistance to chloride-ion penetration was determined by calculating the diffusion coefficient by means of the depth of chlorides penetration into concrete, according to LNEC E-463. Three cylindrical specimens with 100 mm diameter and 50 mm in length per mix have been used for each of the ages (28 and 91 days) and mixes. The specimens were cured in a wet chamber and moved to a dry chamber (20 ± 2 °C and 60 ± 5% relative humidity) in the last 14 days before testing. Carbonation resistance of the concrete was determined on three cylindrical specimens with 100 diameter and 50 mm in length per mix and per exposure time, stored 14 days in a humidity chamber followed by 14 days in a dry chamber (20 ± 2 °C and 60 ± 5% humidity) before being placed for 7, 28, or 91 days in the carbonation chamber. The conditions of the carbonation chamber and the test methodology are those proposed by LNEC E-391 (temperature of 23 ± 3 °C, relative humidity of 60 ± 5%, and CO2 concentration of 5.0 ± 0.1%). The determination of the carbonation depth was carried out with the help of a pH indicator (1% phenolphthalein solution in ethanol), cutting the specimen in quarters, spraying the solution and measuring the depth of carbonation penetration (the average depth measured in the eight contact surfaces of the broken specimen). Drying shrinkage was measured on two 100 × 100 × 500 mm prismatic specimens per mix and according to LNEC E-398, from 24 h to 91 days of age. The specimens were placed in a chamber at 20 ± 2 °C and 55 ± 5% relative humidity after demoulding and during the 91 days of testing.