Precast products made of recycled concrete
Up to now, only a few papers investigated the use of precast elements produced with recycled aggregates. Nor is there any publication that pertains to concrete that has been compacted by a vibrating press. This article reports on such investigation and the related findings.
A concrete was envisaged that would allow for recycled aggregates as an input material, rather than just an additive or material put into the process at a later stage. The project generally aimed to achieve a high ratio of recycled materials. This concrete should include a higher percentage of recycled materials than established in previous investigations into the behavior of other types of concrete. During the research, a no-slump concrete was “sought” that would tolerate the properties of the recycled aggregates from Majorca.
The production of precast elements from vibrated concrete was examined with the aim to make use of the concrete with recycled aggregates that are produced at crushing plants or similar facilities on Majorca. In this endeavor, the investigation concentrated on research into recycled concrete. The corresponding idea emerged from the intention to put a reasonable end to the life cycle of construction and demolition waste. The range of elements consisting of vibrated concrete includes all small precast products that are produced on block machines or concrete compaction units (such as blocks, pavers, cement tiles, slabs, curbs, water pipes, round edges, hollow blocks or permanent formwork). Due to its heterogeneous nature and its ratio of unwanted particles and contaminations, the use of recycled aggregates from Majorca (Mac Insular) is associated with some disadvantages. For this reason, the research focused on small non-reinforced precast elements made of no-slump concrete.
The investigation concentrated on paving blocks due to their geometry and mechanical characteristics. As a company with in-house expertise with respect to precast elements to be used for street furniture, Pastor is now also using concrete with recycled materials for this type of concrete product, and is currently initiating activities to develop the corresponding production equipment. Initial pilot studies have been completed already, which is referred to in more detail in the last section of this article.
The approach taken in this case is to put concrete with recycled aggregates to uses with less demanding mechanical requirements since this concrete cannot be used for structural purposes due to its performance characteristics.
In an urban environment, there is always a wide range of elements or products that are suitable for this purpose: these include, for instance, marking stones in pedestrian areas, decorative wall copings, as well as facework in gardening and landscaping and many other applications.
Issues brought about by recycled aggregates from Majorca and their possible use in concrete
The higher degree of heterogeneity of mixed recycled aggregates, such as those being produced on Majorca, results in a share of unwanted particles and contaminations that has an adverse effect on flowable concrete types. Therefore their use in concrete is generally not recommended. For this reason, the project tried to identify the allowable aggregate ratio in dry vibrated concrete.
The service life of the concrete products is the most problematic factor since it is significantly compromised by the existing cracking potential. The share of contaminations in recycled aggregates that can trigger unwanted reactions that, in turn, cause normal concrete to crack differs, albeit to a relatively minor extent. Such a cracking behavior is problematic, in particular if these types of concrete are used for structural purposes or if they are subjected to heavy loads during their period of use. A minimum amount of cracking in paving blocks (and other products made of vibrated concrete) that are used for public areas exclusively designed for pedestrians or for ornamental applications does not constitute a deficiency in use.
Up to now – two years after the first pilot project –, no cracking occurred on the test surfaces. It should be noted that cracking occurs very often in such pavements. However, these pavements still fulfil their intended purpose.
The use of recycled aggregates for precast products made of vibrated concrete results in the following advantages:
» The products are not reinforced.
» The low degree of flow does not compromise the characteristics of the concrete. It is even desirable to ensure the workability of this type of concrete. For this reason, the addition of fine recycled aggregates does not pose the workability problems known from flowable concrete.
» Precast products made of vibrated concrete offer a wide range of possible applications for general purposes.
» They can be used for precast products with lower mechanical and structural requirements.
» Molds with a higher degree of precision can be used.
» Variable production parameters can be controlled more effectively.
Production and testing of prototypes
The first prototypes produced and tested were paving blocks (12.5 x 25 x 8 cm, 10 x 20 x 8 cm) and blocks (40 x 20 x 20 cm) (Fig. 1). All of them were manufactured with 100% recycled aggregates. The tests complied with applicable standards [4, 5]: the parameters tested included compressive strength for the blocks, tensile splitting strength for the pavers and dimensional accuracy for both types of products. No significant shrinkage was detected when checking the dimensions. Rather, dimensional tolerances that conformed to the applicable standards were found. The recycled aggregates could be compacted effectively during vibratory compaction. The blocks were produced and tested as specimens in order to arrive at an actual compressive strength value for a factory-produced concrete with recycled materials. Both the compressive strength values for the blocks and the strength values for the pavers (both products were manufactured with 100% recycled aggregates) were acceptable and satisfactory.
Pilot pavement with RC pavers and life cycle study
On the basis of the findings, it was decided to lay a pilot pavement (consisting of RC pavers) at the factory in order to monitor their behavior. After two years during which the road pavement had been exposed to use and weathering, it still exhibits good strength parameters (load-bearing capacity and resistance). No cracking was found. In some areas, a minimum amount of spalling is visible, which is due to internal factory traffic (Fig. 2).
First pilot project subjecting a pavement to real-life conditions
Starting from the favorable behavior of the pilot pavement, it was decided to test a sample pavement in service under real-life conditions. When the pavement is put to public use, its behavior can be analyzed under actual use conditions.
The alterations of the pedestrian area that forms part of the Iglesia de Alcudia (Majorca) enabled the construction of a pedestrian strip covering an area of 300 m². Two-layer RC paving blocks were used for this project. The 8 cm thick base layer consists exclusively of recycled aggregate. The second bedding layer (with 2 mm grooves) has been made of natural mortar, i.e. white cement with fine aggregates containing lime and silica. The paving blocks were designed in two layers in order to color them as desired by the construction management (Juan José Lemm, road engineer). On the basis of the experience gained with the pilot pavement laid at the factory premises, it was decided to add silica to the pavement in order to improve the wear resistance of its surface.
Street furniture consisting of recycled paving blocks
The last phase that Pastor initiated as part of its R&D&I project (research/development/innovation) to investigate concrete with recycled aggregates aims to continue the sustainable design of street furniture in line with the approach taken at the initial stage of constructing the pavement. Since the beginning of 2009, benches (prototypes) have been produced from recycled concrete to provide appropriate street furniture. For this concrete, fine 0–8 mm aggregate from the crushing plant is used (i.e. the same aggregate as used for paver production). The coarse aggregate added is taken from crushed production waste from the Pastor S.A. marble workshop. This ensures that a major part of the waste generated by the plant is recycled. These aggregates recycled by Pastor S.A. are in the 8–16 mm size range. This approach ensures that the continuity of the grading curve is retained at a 50% share of each of the two aggregate components. The resulting concrete, which consists of virtually 100% of recycled mineral material, has excellent mechanical properties. This is due to the fact that aggregates from the marble factory (i.e. granite and metamorphic rock) provide a much higher compressive strength than the natural lime aggregates available on Majorca. The general approach is to rely on simple designs that are less sophisticated in structural terms, although some performance testing is carried out for more complex designs. This new project aims to identify a range of products and elements suitable for street furniture that can be easily manufactured from recycled concrete.
The recycled aggregates were identified, and the amounts of recycled aggregate to be added were determined. Specimens consisting of vibrated concrete were produced with the respective amounts of material, and tested. Concrete blocks and paving stones were produced from 100% recycled aggregates and tested for tensile strength. In a pilot project, a road pavement was constructed using paving blocks that consist of 100% recycled materials. It is currently being investigated how the performance of this pavement is likely to develop under the impact of weather and traffic. The results of this series of tests are highly satisfactory. Following the ongoing monitoring of the quality of vibrated concrete produced with recycled aggregates, a first pavement was laid in a genuine construction project as part of a pilot study in the community of Alcudia (Majorca). Recently, the development of products and elements for street furniture has begun –these are also produced using recycled aggregates.