Characterisation of recycled ceramic mortars for use in prefabricated beam-filling pieces in structural floors

P. Rubio de Hita^a, F. Pérez-Gálvez^a, M.J. Morales-Conde^b*, M.A. Pedreño-Rojas^a

a. Departamento de Construcciones Arquitectónicas 1, Escuela Técnica Superior de Arquitectura, Universidad de Sevilla, (Sevilla, Spain)

b. Instituto Universitario de Arquitectura y Ciencias de la Construcción, Escuela Técnica Superior de Arquitectura, Universidad de Sevilla, (Sevilla, Spain)

*mjesus.moralesconde@gmail.com

1. INTRODUCTIONTOP

Today in Spain, as in other European Union countries, waste valorisation is considered a priority, which has given rise to initiatives to minimize the impact of the waste that is most prevalent and hardest to recycle. According to the National Statistical Institute, in Mediterranean countries such as Spain the construction sector generated 32.7 million tonnes of waste material in 2011, which represents 51.17% of the nationwide total (1). Moreover, of all construction and demolition waste (CDW), ceramic materials accounted for 54% (2), partly from coating manufacturing (vertical and horizontal) and, to a lesser extent, from the production of bricks and roof tiles.

In addition, around 80% of ceramic CDW comes from building demolition, whereas 5–10% is material rejected because of defects or imperfections (3), offcuts or adjustments. The former are, therefore, materials with a high level of impurities, especially in the case of non-selective demolitions; and the latter are usually much more homogeneous and uniform, and which contain a significant amount of waste that can be reused, thus contributing to a reduction in the high rate of CDW.

The common goal of reducing CDW has led researchers to undertake numerous studies on the potential for recycling building waste. In Spain, the National Waste Plan (PEMAR) (2016–2022) (4) aimed to promote the reuse of CDW, especially recycled aggregates, which yielded new products that complied with current regulations on construction, specifically in the case of recycled aggregate concrete. The feasibility of using this type of waste and its use in the manufacture of new products has impacted favourably on the environment, and this area has been a source of ongoing research in the last few years.

However, adding recycled aggregates to the manufacture of resistant products, such as structural concrete, comes with a reduction in their resistance properties, by limiting their compressive strength to 150 kg/cm². For this reason, the Spanish Regulation for Structural Concrete (EHE 2008) (5) restricts the use of recycled aggregate to 20% of the total aggregate, as well as limiting aggregate replacement to oversized particles due to the high water absorbing capacity of fine aggregate, because it diminishes the mechanical properties of the final product. Some studies in this field have examined recycled aggregate concrete as partial cement substitute to a level of 50% (6, 7), as well as an aggregate substitute of up to 25% (8–12).

Research has tended to focus more on mortar, in which investigators have been able to manufacture products with good mechanical and physical properties for reuse in non-resistant structures, such as mortar for masonry. When so used, recycled materials were added as replacement for sand (13–16), for gravel as natural aggregate (17,18) and for cement (7,19–21), in several cases reaching a replacement ratio of 50%.

Other studies have analysed mortar with recycled ceramic aggregate from production waste, in particular bathroom ceramic, producing mortars with excellent properties at a replacement rate of 25% of coarse aggregate (8,10–12,18).

Most other articles in this field centre on the physical, mechanical and chemical properties of samples containing recycled material. However, few studies describe the practical application of recycled materials for construction purposes. Some refer to the use of mortars with recycled ceramic material for non-resistant products and elements such as concrete bricks and blocks (22, 23), pedestrian paving stones (22–25) or sub-base material for the construction of roads (26–28). In these papers, the waste used is mechanically shredded for later use in the manufacturing of the cement, as a replacement for the cement itself, in order to obtain mortars or additives.

The research in this paper aims to take ceramic waste, discarded as defective, as offcuts or surplus to requirements, and add it to the manufacturing process for new mortars to substitute natural aggregate. The objective is to achieve the maximum possible percentage of waste content that allows to obtain a mortar with adequate physical and mechanical properties for developing a beam-filling piece for wooden structural floors. The aims is for these infill blocks to be used in rehabilitation processes, in order to recover the traditional structural and construction systems applied to historic buildings. This will also contribute to construction sector sustainability, with the use of waste materials, and to the recovery of traditional construction systems.

2. EXPERIMENTAL PROCEDURETOP

This study is divided into different stages:

2.1. Characterisation of the materialsTOP

The following materials were used in the manufacture of the mortar (Figure 1):

No additives were used in the mix.

To determine the main elements in the materials, a mineralogical analysis was conducted by X-ray diffraction (XRD), using PANALYTICAL-Axios equipment (with rhodium excited at 4kW); an X-ray diffractometer with a BRUKER-D8 Advance A25 (DI81I-90) was also used, as well as a Cu anode excited at 40 kV and 30 mA. In this way, the crystalline phases of the different materials were obtained (cement, natural aggregate and ceramic aggregate), and the natural aggregates were compared to the ceramic content. In order to study the similarity between the different elements of ceramic production two types of ceramic waste were compared: unglazed ceramic tiles and simple hollow ceramic brick.

Quantitative data obtained with X-Ray fluorescence spectroscopy (Table 1) revealed that both the commercial aggregate and the ceramic waste have silica as their main component, and in similar proportions. Furthermore, it can be observed that the composition of the remaining elements is similar in the commercial aggregate and in both ceramic materials, so the substitution of part of the natural aggregate with recycled content would seem to be appropriate for the manufacture of the mortar.

Results from the XRD are presented in Table 2. All the crystalline phases of the ceramic waste are very similar to each other in the case of bricks and unglazed ceramic tiles. This implies that ceramic bricklaying waste could be used in both new construction and rehabilitation, along with discarded material from industrial ceramic production.

2.2. Mortar production. Preparation of test piecesTOP

Ceramic waste mortars were produced at different ratios: 10, 20, 30, 40 and 50% (PC10, PC20, PC30, PC40, PC50). A reference mortar made from commercial aggregate and with no additional waste content (PC) was also manufactured for use as a sample to compare the results obtained in each case.

The procedure for mortar production includes the following stages:

In Table 3, the amounts and proportions of each material are represented for the different mixes analysed. It can be observed that the cement-to-aggregate (natural and recycled) ratio is always the same. However, the water-to-cement ratio varies in accordance with the percentage of recycled ceramic used, rising as the recycled aggregate content increases, in order to ensure the workability of the mortar.

For each mixture, 18 prismatic test pieces of 40x40x160 mm³ were produced and later evaluated (Figure 3). When ready, the test samples underwent a curing process at 20±2ºC for 28 days. During this period, relative humidity was maintained at 95±5% for the first 7 days, and at 65±5% for the remaining 21 days, according to the UNE-EN 1015-11 standard (31). After curing, the pieces were analysed, as established in the test plan (Table 4).

2.3. Mortar characterisation tests TOP

In order to characterise the mortars, the following tests were conducted, according to regulatory procedures (Table 4). Mineralogical tests were also carried out on two samples, PC and PC-30, using XRF and XRD, to study how the addition of ceramic aggregate affected the chemical composition of the mortars and the formation of the crystalline phases.

3. RESULTS AND DISCUSSIONTOP

3.1. The consistency-density relation of fresh mortarsTOP

Figure 4 shows the effect of adding recycled ceramic with regard to the workability of the mixture. The higher the percentage of recycled ceramic in the mortar, the greater the amount of water needed in order to guarantee adequate consistency.

The fresh density tests (32) (Figure 5) yielded mortars with a density higher than 1200 Kg/m³, and since no fluidiser additive was used, the consistency value should be between 165 and 185 mm (33). This explains the need to change the water ratio in mixtures with the highest percentage of recycled ceramic.

3.2. Density of hardened mortarTOP

Figure 6 shows the average bulk density values for hardened mortars at 28 days (34). It is observed that density decreases as the percentage of recycled ceramic rises, reaching values close to 1850 Kg/m³ for mortars with 50% recycled content, which entails a reduction of 7.89% compared to the reference mortar. The decrease in the mortars’ density benefits the manufacture of prefabricated pieces because a reduction in the net weight of the elements, in this case structural floors, is achieved, which makes it suitable for rehabilitation works.

3.3. Capillary water absorption TOP

Capillarity water absorption (35) (Figure 7) decreases considerably in test pieces PC-20 and PC-30 in contrast to the reference mortar, while it clearly increases in mixtures with a high percentage of ceramic content. The ceramic material’s absorption capability is a crucial factor in the choice of the best mortar for the manufacture of pieces, as well as its porosity values and mechanical resistance.

3.4. PorosityTOP

Computed tomographies were conducted on all the test piece series to determine their morphology and to gather information on their porosity (Figure 8, Figure 9). The tomography equipment used was the YXLON Cougart model, which produced 2D images that showed the ceramic distribution within the mixtures as well as the proportion of pores, which increases as the amount of ceramic material rises, leading to a less homogeneous distribution.

The quantitative study of the porosity reveals a progressive rise in this parameter when the percentage of ceramic aggregate increases. This trend holds for mortars with up to a 50% substitution ratio (Figure 10). In test pieces PC-40 and PC-50 linear progression is maintained in relation to the ratio of pores, with a different distribution in mixtures with low ceramic content, generating areas of pore concentration around the ceramic aggregate and, consequently, a less homogeneous distribution.

3.5. Mechanical behaviour. Flexural and compressive strengthTOP

Figure 11–12 show data obtained in flexural and compressive strength tests (31), which were determined for all the mixtures at 7 and 28 days.

It can be observed that flexural strength increases up to a recycled ceramic ratio of 30% (PC-30) compared to the reference mortar. However, in test pieces with a higher proportion of recycled ceramic, flexural strength decreases due to the increase in the amount of water used in the mixing process, as well as to the aggregate distribution in the mixtures, which is less homogeneous and has increased porosity.

Compression strength considerably increases in the mixtures with a proportion of 20 and 30% (PC-20, PC-30), in relation to the reference test piece (PC) (Figure 12). From that percentage onwards, no improvements in mechanical strength are achieved.

The results for mechanical resistance provide data that is very useful when selecting the best mortar for manufacturing prefabricated pieces in horizontal structures, the mortar with 30% recycled ceramic being the most appropriate for this purpose.

Mechanical resistance development during the curing process provides information on how the ceramic content influences the setting process and the endurance of the mortars. In the case of mortars with a maximum 30% recycled ceramic, compressive strength at 7 days amounts to around 60–70% of the resistance values obtained at 28 days. Therefore, ceramic as a substitute for aggregate does not affect the development of resistant qualities over time for mortars with these percentages. Above this amount of recycled additives, progress evolves in a different way especially in mixtures with 50% ceramic (Figure 11 and 12).

3.6. Modulus of elasticityTOP

The modulus of elasticity is determined by flexural rupture testing based on the stress-strain graphics (36). An increase in this parameter was observed in mortars with recycled ceramic additives up to 20% (PC-20) (Figure 13). This indicates greater compactness in the mortars and improvements in their mechanical properties. Nevertheless, mixtures with a high-recycled ceramic ratio (40–50%) experience a reduction in the modulus of elasticity, resulting in a less compact and homogeneous mortar since it is affected by an increase in the mixing water and a heterogeneous ceramic distribution within the pieces (Figure 14).

4. DEVELOPMENT OF A CONSTRUCTION APPLICATION: AN INFILL MADE OF MORTAR WITH CERAMIC WASTETOP

The experimental study conducted has allowed us to characterise mortars with different proportions of recycled ceramics, with the conclusion that the addition of ceramic at a ratio of 30% produces a lighter material with better physical and mechanical properties than traditional mortar. Subsequently, an infill, or joist-to-joist fill, was produced using this mortar, for use mainly in the rehabilitation of traditional structural floors with brick jack arch slabs and wood beams (Figure 15), as well as to recover traditional typologies. The production of this new piece can help resolve the problem entailed in rehabilitating buildings with wood structural floors, typical of 17^th–to-19^th century constructions, by replacing the original infill blocks with the recycled solution proposed.

4.1. Description of the pieceTOP

The infill, or joist-to-joist fill, has the following technical characteristics (Figure 16) (37):

4.2. Experimental procedureTOP

In order to carry out an experimental analysis of the infill blocks, three series of six test samples each made of mortars PC-10, PC-20 and PC-30 were prepared. They were then mechanically tested for flexural strength according to the procedure described in the UNE 67-042-88 standard (38) (Figure 17). Although this standard is specific for large-format baked clay ceramic pieces, it was applied here because of the formal similarity between the pieces used in this experiment and those referenced in that regulation, and because there is no existing legislation that corresponds to the pieces developed.

The results from this test are compiled in Table 5. In all cases, the rupture values corresponding to flexural strength are higher than the ones required by current legislation for this type of elements (≥125Kgf). On the other hand, it is observed that the greater the key thickness (f), the higher the value for the load breaking point, making this a crucial parameter for the flexural strength of the piece. Another relevant aspect is the difference between the theoretical value of the key (f) and the real mean value once the mould is removed from around the piece (f-real), with a difference of between 1 and 2 mm.

The insertion of this piece into a structural floor was carried out as detailed below. It is important to emphasise that the piece must be supported on wood strips nailed to the existing wood beams or to the new ones (Figure 18):

4.3. Applying the new piece to a real rehabilitation case: “Casa del Pumarejo”, in Seville (Spain).TOP

Building DescriptionTOP

To verify the construction and structural viability of the element developed, it was proposed to include the piece in the plans for the rehabilitation of a real building, the Casa del Pumarejo. This is a house with an internal patio of great historical, artistic and patrimonial value located in the old city centre of Seville (Figure 19).

The building is constructed around two arcaded patios that reflect the status of the original occupants: one patio has wooden columns that symbolize the noble space of the house that contains the most interesting decorative elements, and a second patio that is simpler in design, acting as a serving area. These patios influence the spatial organization of all other areas of the building (Figures 20 and 21).

Constructive characterizationTOP

The structural system of the building is based on load-bearing walls of solid clay brick measuring two-and-a-half feet (about 60 cm) laid as headers and stretchers. The horizontal structure has a finish that consists of wooden infill beam slabs (Figure 22) resolved by a two-threaded beam fill or vaulted partition walls of solid clay clinker brick with a lower-side finish of plaster cladding containing alcatifa and clay flooring.

Intervention proposalTOP

The highlighted zone in red in Figure 20 indicates the structural floors that have been analysed in detail for this study.

Two solutions are proposed for the rehabilitation of the slabs that will enable the original wooden beams to be conserved, as well as their dimensions and construction typology. The first option is to construct the brick partition vaults on site (Figure 23a) following the traditional construction procedure. The second option (Figure 23b) is to incorporate the piece developed using recycled clay aggregate.

In both cases, an upper reinforcement is put in place by means of a light reinforced concrete slab (mixed wood-concrete decking). This solution not only maintains the original aesthetic of the slabs on the lower face, it also improves the mechanical behaviour of the beams, achieving greater inertia and resistance, and simultaneously improving its anti-inflammatory characteristics and boosting thermal and/or acoustic insulation, among others. In addition, the use of light concrete considerably reduces the beam weight, a crucial factor in rehabilitation interventions.

Structural studyTOP

The structural assessment of the rehabilitation hypotheses takes into account the procedure in DAV-SE-M (39), in the development of Spanish regulations in CTE-DB-SE-M (38) and consists of a simplification of the calculation specified in Eurocode 5 (40), that refers to cases involving wood-concrete beams in residential buildings. Structural tests were carried out with reference to Limit State Design (LSD) to determine the flexural strength, shear load and shearing force; and to the Limit State Service specification relating to the slab arrow calculation. Also taken into consideration were the load data in CTE-DB-SE (41). The load-bearing capacity of the structural element for fire resistance was also measured based on the method proposed in Annex E of CTE-DB-SI (42).

The reuse of the original wooden beams (13 x 20 cm) was also considered in the calculation, by determining the mechanical behaviour of the beam for two lighting placements, at 3 and 5 m. The results for this calculation are presented in Table 6, which shows that the two hypotheses comply with the legal structural requirements.

In the case of lights at 3 m, the solution using a prefabricated piece complies with minimum mechanical resistance requirements with a compression layer of less than 5 cm. In fact, this thickness is the same for both the solutions proposed as it is the minimum legally permitted value. As a result, both solutions need the same number of connectors per beam (8 distributed at intervals of 40 cm), for the effective linkage of the wooden material and the concrete.

On the other hand, there is a significant difference in weight between the hypothesised recycled hollow vault compared to the solution consisting of clay floor tiles (2.74 kN/m² and 3.20 kN/m², respectively); this improves structural performance with lights at 5 m, reducing the compression layer for the proposed recycled hollow vault to a thickness of 3 cm. However, in this case the number of connectors needed is greater when using the prefabricated piece (33 every 15 cm, compared to 25 each 20 cm).

5. CONCLUSIONSTOP

The study carried out can be concluded as following:

ACKNOWLEDGEMENTS TOP

This research has been supported by the Ministry of Economy and Competitiveness of Spain (reference number BIA2013-43061-R). The author Mª Jesús Morales-Conde acknowledges the financial support of the V Research Plan of the University of Seville. Pedreño-Rojas, MA (author) wishes to acknowledge the financial support provided by the FPU Program of Spain’s Ministry of Education (FPU15/02939). The authors acknowledge the collaboration in this work of the architects Carmen Calama, Carlos Campos and Carlos Girón.

REFERENCESTOP