This work describes the designing stage of the experimental campaign for the production of permeable pavements as a future application on an urban scale using CDW. The effects of recycled aggregates on the mechanical properties of permeable concrete are evaluated in the fresh state (embedded air content and workability) and in the hardened state (axial compressive strength, tensile strength by diametrical compression, tensile strength in bending and modulus of elasticity). The produced concrete slabs, composed of five types of permeable concrete mixtures, can be applied as a final coating on soils with permeable profiles.

The experimental campaign includes the following parts: collection and crushing of recycled construction and demolition aggregates, characterization of these aggregates, the definition of traces, moulding of specimens, mechanical tests for the evaluation of concrete strength, analysis of the results of strength tests of the specimens, and moulding of permeable concrete slabs which consist of recycled aggregates.

The following is the design stage of the experimental campaign for the production of permeable pavements using construction and demolition waste (CDW). The effects of recycled aggregates on the mechanical properties of permeable concrete were evaluated in the fresh state and the hardened state [30]. Based on the results obtained, concrete slabs were produced using five types of permeable concrete mixtures. The slabs can be applied as a final coating on soils with permeable profiles. For the production of permeable concrete specimens, five mixtures were considered: (I) 100% coarse natural aggregate – AGN, (II) 100% coarse aggregate from demolition waste – AGD, (III) 100% coarse aggregate from construction waste – AGC, (IV) 50% AGN and 50 % of AGD, (V) REF – fine and natural aggregate. Test specimens were moulded with each of the five mixtures to perform the tests of compressive strength, tensile strength by diametrical compression, and tensile strength in flexion and to produce permeable concrete slabs.

Considering that one of the objectives of this work is to evaluate some of the mechanical properties of permeable concretes produced with recycled aggregates from construction and demolition waste, the following response variables were defined: 1) Air content incorporated into the concrete [31]; 2) Workability measured by the concrete slump rate [32]; 3) Resistance to axial compression [33]; 4) Tensile strength by diametrical compression [34]; 5) Tensile strength in flexion [35].

The water used in making the concretes, mortars, and pastes came from the supply system of the city of Rio de Janeiro - Companhia Estadual de Água e Esgoto (CEDAE). For the production of mortars, this work opted to use Portland Cement Common CPII E-32. The choice for this type of cement was made due to the similarity of the present study with research work on mortar with recycled ceramic waste aggregate, which was in progress in the laboratory. The use of the same type of cement allows for future comparisons between studies, considering the difference in the kind of waste recycled used. The aggregates used in this study were obtained from the disposal of construction waste from the new Building Materials Testing Laboratory at the Polytechnic School of the Federal University of Rio de Janeiro (LAMAC). The laying of the ceramic floor generated the construction waste (RC), composed solely of ceramic waste and the demolition of the concrete base of the old cyclic loading test machine for concrete elements, which in turn gave rise to the demolition waste (RD), composed only of concrete waste as shown in Figure 2.

Figure 2.

Construction waste (a) and demolition waste (b)

Coarse aggregates were produced in the structure laboratory (LabEST-COPPE / UFRJ), after collecting the waste samples, where there was no need for processing, as there was a very small amount of contaminants, such as cardboard, glass, plaster, wood, plastic, earth, among others, probably due to the finishing stage that the work was in. The jaw crusher, model Queixada 200, was used for crushing the waste. The equipment was regulated for the production of coarse aggregates, with average dimensions of the No. 1 gravel. The adjustment was made by successive attempts based on tactile-visual analysis of the aggregates produced (Figure 3a). After completing the calibration of the equipment, the waste was crushed using different procedures for each type of waste. After crushing, parts of the materials were bagged for the characterisation of recycled construction and demolition aggregates (Figure 3b). A part was stored in 200-litre PVC containers (Figure 3c) as raw material for the production of the specimens for testing the mechanical properties of permeable concrete, as well as for moulding slabs of that concrete. Table 1 presents a schematic summary encompassing all parameters of the experiment execution, including the number of specimens and their respective tests.

Figure 3.

Coarse aggregate selected (a), coarse aggregate after separate grinding for characterisation (b), and coarse aggregate after grinding for making the plates (c)

Air incorporated into concrete is defined as air introduced intentionally by means of an appropriate agent. This air must be clearly distinguished from accidentally trapped air; the types of air differ by the dimensions of the bubbles, with those of embedded air having a diameter of about 0.05 mm, while those of accidental air usually form larger bubbles, some as large as the common surface flaws of concrete [31].

The control of the content of incorporated air is fundamental to the control of the quality of the concrete, either to verify maximum and minimum desirable limits of incorporated air or to identify the contents of voids of air in the concrete. The air content in concrete is, therefore, a topic of utmost importance to its final quality. The control of the percentages of air in fresh concrete allows the measurement of the dosages, the additions of additives, and, as a consequence, the guarantee of the quality of the material. Air values above those predicted in the material dosage indicate that the concrete may suffer mechanical damages, such as reductions in compressive strength and modulus of elasticity, or aesthetic ones, such as the formation of surface macro bubbles [36].

Conventional concretes contain in their interior, even without the use of air-incorporating additives, 1 to 3% of their volume in trapped air due to the mixing process and its consistency. In the case of concrete produced in plants and transported by concrete mixer trucks, this percentage can reach the order of 4%. Percentages of incorporated air above 5% can damage the mechanical performance of the material [37].

In Brazil, NBR NM 47/02 - Fresh Concrete - Determination of Air Content by the Pressometric Method, is the test used to obtain the value of air incorporated and/or trapped in concrete [38]. Figure 4 shows the device used to measure the air content in concrete, which consists of a hermetically sealed container filled with fresh concrete. Water and air under pressure are injected through the holes in the cover to expel the air contained in the fresh concrete. The pressure gauge detects the pressure loss and indicates the equivalent percentage in the mixture.

Figure 4.

Built-in air meter

The air content is calculated by the difference between the actual volume of concrete (volume considering air) and the theoretical volume (without considering air and calculated based on the specific masses of the components), expressed as a percentage of the actual volume. The accuracy of the obtained value will depend on the precision of the density values of the components. The results are shown in Table 2. It can be reported, observing the results of the test as mentioned above, that the values found were very close to the mixtures of conventional concrete and permeable concrete with natural aggregate (gravel No. 1), with an average variation of a little more than 10%. These values are below the maximum limit of 3% of air incorporated into conventional concrete [38]. Air values above those predicted in the material dosage indicate that the concrete may suffer mechanical damage, such as reductions in compressive strength and modulus of elasticity, or lowered aesthetics, such as the formation of surface bubbles [40].

There is unanimity in stating that concretes with recycled aggregates have a lower Consistency Index than mixtures executed with natural aggregates of the same trait. This statement is justified by the greater porosity presented by the recycled material, a fact that ends up increasing the water absorption of the same and decreasing the amount of free water of the mixtures. Also, the more angular shape of the recycled aggregates can cause a decrease in the workability of the concretes produced with this material [40]. The cone trunk slump test is the test that will be used in this study, considered a qualitative index of the stability and fluidity of the concrete mixture in the fresh state, being the most widely used for uniformity control of concrete production worldwide [37].

The experimental program included the study of the workability of recycled concrete. It was decided to avoid using plasticizers or superplasticizers for production of permeable concrete at a lower cost and to evaluate the workability of concrete without the use of additives [38].

The workability of the permeable concretes produced was determined by measuring the reduction of the cone trunk according to the prescription of NBR NM 67 for the determination of consistency by cone slumping in conventional concrete, with natural aggregates (a) and slumping of concrete, with recycled aggregate from demolition (b), as shown in Figure 5. The values of the measurements of the cone trunk slumping of the concrete mixtures carried out in this research, defined in the experimental study, are shown in Table 3.

Figure 5.

Slump of conventional concrete with natural aggregates (a) and slump of concrete with recycled aggregate from demolition (b)

Regardless of the slump values obtained for concrete with recycled aggregates, the greater or lesser ease with which they can be compacted is what can be considered as a parameter to limit their use [41]. Based on the studies cited, the porosity of recycled coarse aggregates and their heterogeneous and angular shape are the properties that can most influence the characteristics of concrete in its fresh state.

Some important aspects were observed in the measurements of the cone trunk slump. The content of substitution of natural coarse aggregate for recycled coarse aggregate had a significant effect on the permeability of the permeable concrete, where there was a very large variation between the mixtures. the smallest difference between the abatements of the concrete with recycled aggregates and conventional concrete occurred in the concrete with 100% recycled aggregate from the construction waste, reaching 118% greater. However, the most significant difference occurred in the concrete with 100% recycled aggregate from demolition waste, reaching 260%. The recycled aggregates, because they are more porous, incorporate a greater amount of air into the mixtures and, despite the greater roughness given to the concrete, and the physical properties of the aggregates, which favor the locking of the mixtures and the reduction of the abatement value, these are present more workable, because the greater the amount of trapped air, the lower the resistance to densification [42].

The results obtained in the cone slump test for the concrete mixtures studied indicate that some procedures can minimise the adverse effects of recycled aggregates on the consistency of the concrete. One can increase the amount of water in the mix by a value equal to part of the total absorption of the aggregate or pre-wetting it before starting the mix. Another option is increased consumption of cement, which will result in the modification of the w/c ratio. Other procedures include using plasticizing additives or increasing their dosage, alternatively using less porous (that is, denser) coarse aggregates with less water absorption.

The mechanical properties concern the potential of the permeable concrete to resist the stresses that are requested. Among the properties analysed, compressive strength is most frequently used on all study fronts, given the relative ease of carrying out the tests [40].

The strength of a material is defined as the ability to withstand stress without breaking. In concrete, therefore, strength is related to the stress required to cause rupture, defined as the maximum stress that the concrete sample can withstand. In compression, the specimen rupture occurs even if there are no visible signs of external fracture, but the internal cracks are in such an advanced state that the specimen will not withstand a greater load. Strength is generally considered to be the fundamental property of concrete. It gives a general indication of concrete quality because it is directly related to the structure of the hardened cement slurry [43].

A Shimadzu servo-controlled press, model UF-F 1000 kNI (Figure 6), as prescribed to NBR 5739/2007, with a loading speed of 0.0075 mm/min [38], was applied in the evaluation of the compressive strength. The rupture tests used cylindrical concrete specimens with dimensions of 10 cm in diameter and 20 cm in height.

Figure 6.

Servo-controlled Shimadzu press to perform the tests of resistance to axial compression

The characteristics of stresses versus strains were measured using electrical sensors in the specimens to measure longitudinal displacements. The specimens had irregular upper and lower surfaces in such a way that they impaired the application of the loads during the test. To solve this problem, these surfaces were regularised with a layer of sulfur as can be seen in Figure 7.

Figure 7.

Capping of the upper and lower surfaces of the specimens

The axial compressive strength of concrete is one of the most important properties when evaluating the performance of a structure [44]. Resistance is linked to the ability of materials to withstand stresses without breaking. Broadly speaking, resistance is closely related to the porosity of materials. The more porous these are, the lower their resistance tends to be. In concrete, in addition to the porosity of the cement matrix and coarse aggregate, the porosity of the transition zone between the matrix and the aggregate must be considered. In the study of the strength of concrete with natural aggregates, generally, the use of dense and resistant aggregates causes this property to be influenced basically by the porosity of the matrix and the transition zone. However, when experimentally studying the strength of concrete with recycled material [44], it is believed that the porosity of the aggregate will play an important role in determining the strength of the concrete. The authors of [44] compared, in their study, the compressive strength of permeable concrete composed of recycled aggregates with permeable concrete composed of natural aggregates. They found that this strength directly decreases probably due partly to the reduction of the cement interface and the recycled aggregate and partly to the increase in the number of concrete voids. The axial compression strength is calculated by Eq. (1) [38].

Where: f c – compressive strength [MPa], F – maximum load obtained in the test [N], and d -diameter of the specimen [mm].

The tests of the specimens were performed as established by NBR 5739/2007 and the results obtained are listed in Table 4. It appears that the results of compressive strength are within the limits, lower and upper, for the compressive strength of permeable concrete with natural coarse aggregates varies from 7 to 14 MPa [40]. The cases where sharp reductions have occurred show the negative influence of the porosity of coarse CDW aggregates on their own strength and on the strength of the concrete. Concrete mixtures composed of recyclable aggregates, both 50% replacement and 100% aggregate replacement, had their axial compression strength around 26% less than the mixture of concrete with crushed stone No. 1.

Although the tensile strength of concrete is a secondary characteristic, since concrete does not resist well to tensile stresses, however, as porous concrete consisting of recycled coarse aggregate still has a small number of research, it is necessary to study this mechanical property [45]. In the tests of tensile strength by diametrical compression, the concrete specimens were subjected to compression loads along two axial lines that are opposed. The load must be applied continuously at a constant speed within a range of 0.7 to 1.3 MPa, until the specimen ruptures [44]. The test was carried out according to NBR 7222/2010 - Mortars and Concrete: Determination of tensile strength by diametrical compression, for cylindrical specimens, with dimensions of 10 cm in diameter and 20 cm in height and aged 28 days. The tests were performed on the Shimadzu press, model UF-F 1000 kNI. The test was carried out on 06 (six) specimens, for each of the 05 (five) mixtures established in the experimental project [38]. Figure 8 shows the permeable concrete specimen positioned in the press.

Figure 8.

Test specimen positioned in the press to perform the diametrical compression test

The tensile strengths obtained by diametrical compression were compared with conventional concrete and with permeable concrete with natural coarse aggregate (crushed stone 1). The diametrical compression values (f t,D), found through the test described in NBR 7222/2011, at 28 days of cure are shown in Table 5.

The tensile strength by diametrical compression is calculated by (Eq. 2) [38], as follows:

Where;

f t,D: tensile strength by diametrical compression [MPa], to the nearest 0.05

F: maximum load obtained in the test

d: diameter of the specimen

l: specimen height

It was found that all permeable concrete mixtures had lower stresses than conventional concrete. The results of mixtures with recycled aggregates were slightly higher, but very close to the result of mixing with natural aggregate, around 8%, even with different water/cement factors, that is, this factor did not change the tensile strength by compression diametral. The angular and lamellar shape, as well as the rough surface of the aggregates, contributed to this result.

The modulus of elasticity of concrete depends on the porosity of its phases (paste, aggregate and transition zone). In the case of the aggregate, its maximum dimension, shape, surface texture, granulometry and mineralogical composition can also influence the modulus of elasticity by influencing the microcracking of the transition zone. However, porosity is more important by virtue of being linked to its rigidity, and strength.

Based on the characteristics that influence the deformation modulus, it can be said that the deformation modulus of recycled aggregates is very close to the modulus values presented by the cement slurry matrix, since the composition of construction and demolition waste is basically from cementitious base materials (concrete mortars, coating and masonry execution) and very porous ceramic components. In this way, as the porosity of the aggregate is what controls the restriction of the deformation of the matrix, in the case of recycled aggregate the concrete produced will be more deformable, since this control is incipient. Matrix and aggregate of recycled concrete are much more porous when compared to concretes produced with natural aggregates. The decrease in the specific mass of recycled concretes also leads to reductions in modulus values. There remains only the influence of the transition zone which may have had an improved adhesion, as already seen previously, but which alone is not sufficient to raise the modulus values of the recycled concretes [46]. The modulus of elasticity (Ec) found at 28 days of curing were compared with permeable concrete with natural coarse aggregate (brita1) and are shown in Table 6.

The modulus of elasticity was calculated by (Eq. 3), which follows below.

Where;

Ec = modulus of elasticity [GPa].

σb = the greater stress in MPa (σb = 0,4fc).

0,5 = the basic stress in MPa.

εb = medium specific deformation, under greater stress [mm]

εa = average specific strain, under the basic stress of 0,5 MPa [mm]

Observing the graph shown in Figure 9, it can be seen that all permeable concrete mixtures with recycled coarse aggregates had lower moduli of elasticity than those of permeable concrete mixtures, composed of natural aggregates.

Figure 9.

Test result graph-elasticity module

The greater the amount of dense aggregates in a concrete mixture, the higher the modulus values obtained. Thus, the statement that the increase in the amount of porous aggregates in concrete mixtures decreases the modulus of elasticity, is also valid. It is believed that this is one of the characteristics of concrete with recycled aggregates, where the decrease in the modulus of elasticity occurs as the type of CDW is replaced, and the concrete residue is more porous than the ceramic.

Although the tensile strength of concrete is a secondary feature, since concrete does not resist tensile stresses, however, as porous concrete consisting of recycled coarse aggregate still has a small number of research, it is necessary to study this mechanical property [44]. The prismatic specimens were marked and tested as prescribed by NBR 12142/2010 - Concrete: Determination of the tensile strength in flexion in prismatic specimens - Test method, with dimensions of 10 cm × 10 cm × 40 cm (Figure 10), aged 28 days, loaded at a speed of 0.8 to 1.2 MPa / min [38].

Figure 10.

Positioning of the specimen for the flexural tensile test.

This test was performed on the Shimadzu press, model UF-F 1000kNI, as shown in Figure 11.

Figure 11.

Shimadzu press, model UF-F 1000kNI.

The tensile strength in the permeable concrete bending (fctM) can be determined in two different ways, depending on the location where the rupture occurs. The standard NBR 12142/1991 establishes two equations for the calculation of resistance, one if the rupture of the traction surface occurs in the middle third of the span length and another if this rupture occurs outside the middle third, but less than or equal to 5 % of span length. The tensile surfaces of the specimens tested in this study broke in the middle third, so the resistances were calculated using (Eq. 4) [38].

Where;

f ctM: flexural tensile strength [MPa]

P: maximum load obtained in the test [N]

L: span length [mm]

b: specimen width [mm]

d: specimen height [mm]

The results found after the ruptures of the specimens, with 28 days of curing, are listed in Table 7. The conventional concrete test was not presented, as there was a significant loss in the acquisition of its data.

The results tended to be higher, the lower the water / cement factors. The mixtures of concrete with demolition aggregates and gravel No. 1, had the results of tensile strength in flexion greater, around 25%, than the results of mixtures with 100% construction aggregates and the mixture with 50% demolition aggregates and 50% construction. The results of the tensile strength tests in this research reached higher values in mixtures of permeable concrete with 100% natural aggregate and 100% recycled construction aggregate, 2.40 MPa and 2.44 MPa respectively, probably due to the cubic shape and rough surface texture of these aggregates [38], where it is inferred that the shape and surface texture of the aggregates have a considerable influence on the strength of the concrete. The tensile strength indexes are slightly lower for concretes with recycled coarse aggregate when compared with conventional concretes [41].