Please note that this PDF file replaces the first version of the manuscript e076 in which density (kg/m3) appeared with incorrect units.
Recycled aggregate (RA) from construction and demolition waste is traditionally used for the manufacture of concrete for different applications. Due primarily to high water content required by RA, the quality of the concrete is determined by the amount of replacement RA. The aim of this study is to determine if RA pre-soaking enhances the properties of pre-cast concrete for street furniture, with low mechanical and structural requirements, in which 100% of the coarse fraction is replaced. The results of physical and mechanical tests performed on concrete specimens in which the RA was pre-soaked using five different methods applied are compared with a reference concrete sample and a concrete sample made with non-pre-soaked RA. The results show that non-pre-soaked RA offers improved physical-mechanical properties for pre-cast concrete, except for the workability; problems arising from poorer workability could be improved with the use of plasticizers, which can be easily included in the production process.
The construction industry has several negative effects on the environment, including the consumption of raw materials and the generation of construction and demolition waste (C&DW). These activities contribute significantly to environmental degradation at different stages: extraction and manufacture of materials, design, construction and M&E services (Maintenance and Evaluation services). The construction industry is a major generator of large-volume waste with an important visual impact. The composition of this waste is mainly inert, although some C&DW is hazardous (
RA from concrete differs from NA in that it often contains impurities such as crushed clay brick, crushed ceramic material and gypsum as well as mortar adhered to the original NA (
The higher water absorption capacity of RA is the result of the presence of mortar adhered to the original aggregate (
The characteristics of RA, particularly its high water absorption, limit the use of this material in the manufacture of concrete. Studies such as that of Etxeberria et al. (
It is not found references to the use of RA as aggregate in the manufacture of non-structural concrete for street furniture such as concrete benches, fountains, planters, beach walkways, etc. This type of pre-cast concrete has lower mechanical and structural requirements, according to EN 13198 (
Pre-soaked RA could improve the behaviour of recycled concrete by preventing the transfer of water between the RA and the cement paste. This would reduce the amount of water required to achieve greater strength and lower water absorption. This study, therefore, focuses mainly on determining the benefits of pre-soaked recycled coarse aggregate (RCA) on the properties of fresh and hardened concrete after replacing natural coarse aggregate (NCA) with RCA. Five pre-soaking methods have been formulated, which differing mainly in terms of pre-soaking time, the amount of water added to the mix, and the mixing time, and then tested and compared these against an RC containing non-pre-soaked RCA and a concrete with NCA. The methods were analysed to determine the potential advantages of pre-soaking on the properties for pre-cast concrete for beach walkways made with RCA.
Concrete is composed of sand and coarse aggregate, cement and additives. In this study, two types of concrete were manufactured using NA and RA, respectively. The components used to manufacture both types of concrete are described below:
Cement. The cement used in this study was BL I 42.5R.
Additive. The water-reducing admixture (Chryso®fluid Optima 227) was used to improve the workability of concrete.
Aggregate. NA and RCA were used in this study. Their physical, mechanical and chemical properties and the standards applied are summarized in
Aggregate grading curves.
Aggregate properties according to EHE-08 (
Property | Standard | NA | RA | Limit value | ||
---|---|---|---|---|---|---|
NFA | NCA | RCA | ||||
- Fraction (d/D) | EN 933-1 ( |
0/4 | 4/16 | 4/16 | – | |
- Sieve distribution | EN 933-1, 2 ( |
Continuous | Continuous | Continuous | – | |
- Material fines than <0.063 mm (%) | Fine aggregate | EN 933-1 ( |
0.50 | – | – | 6–16 |
Coarse aggregate | – | 0.90 | 0.30 | 1.5 | ||
- Saturated-surface-dry particle density (kg/m3) | EN 1097-6 ( |
2635 | 2733 | 2595 | – | |
- Water absorption (%) | 24 h ( |
EN 1097-6 ( |
1.02 | 0.379 | 2.136 | ≤5 |
10 min | – | – | 1.935 | – | ||
- Moisture content (%) | EN 1097-5 ( |
0.43 | – | 0.30 | – | |
- RCA composition (%) | Constituent metal, glass, soft materials, bitumen | EN 933-11 ( |
– | – | 0.1 | <1 |
Constituent asphalt | – | – | 1.8 | <1 | ||
Constituent ceramic material | – | – | 0.2 | <5 |
(–) Test not performed or no restrictions for this parameter
Five pre-soaking methods were tested, 4 of them (Method 1 to 4) from the literature (
Making specimens process.
For the purpose of the study, a control series (Series C) using NA and 6 further series in which RCA totally replaced NCA (methods 0 to 5) were manufactured according to the manufacturer's recommended dosage and the concrete quality control data (
Concrete quality control according to manufacturer
Data | |
---|---|
Cement type | BL I 42.5R |
Superplasticizing admixture type | Chryso®fluid Optima 227 |
Sample type | Cylindrical 15×30 cm |
|
|
Consistency (Abrams cone in cm) | 8 |
Compressive strength-7 days (Mpa) | 42 |
Compressive strength-28 days (Mpa) | 50 |
Water content of mixtures
Component | Series C | Series 0 | Series 1 | Series 2 | Series 3 | Series 4 | Series 5 |
---|---|---|---|---|---|---|---|
Effective water (%) |
8 | 8 | 8 | 8 | 8 | 8 | 8 |
Added mixing water (%) |
0 | 0 | 0.9 | 1.1 | 1.1 | 0.9 | 0 |
Total water (%) |
8 | 8 | 8.9 | 9.1 | 9.1 | 8.9 | 8 |
Pre-soaking water (%) | 0 | 0 | 8.9 | 4.6 | 1.1 | 6.2 | 6.4 |
By mass of cement plus aggregate (%).
By mass of effective water (%).
Effective water plus added mixing water (%).
For the purpose of this study, the main properties of fresh and hardened concrete were tested according to the manufacturer's recommendations, European standard EN 13198 (
Tests and procedures used to determine properties of concrete and restrictions
Tests | Procedure | Manufacturer |
Classification |
Limit EN 13198 ( |
|
---|---|---|---|---|---|
|
|||||
without melting salts | with melting salts | ||||
Fresh state Fresh state | |||||
Consistency (Abrams cone in cm) | EN 12350-2:2009 ( |
soft ( |
dry (0–2) | – | – |
plastic (3–5) | |||||
soft (6–9) | |||||
fluid (10–15) | |||||
liquid (16–20) | |||||
Fresh density (kg/m3) | EN 12350-6:2009 ( |
– | – | – | – |
Hardened state | |||||
Water-saturated density (kg/m3) | EN 12390-7:2009 ( |
– | – | – | – |
Water absorption (%) | EN 13369:2004 Annex G ( |
– | – | <7 | <6 |
Compressive strength-7 days (Mpa) | EN 12390-3:2009 ( |
42 | – | – | – |
Compressive strength-28 days (Mpa) | EN 12390-3:2009 ( |
50 | – | 37 | 45 |
According to the manufacturer's concrete quality control data.
(–) No limitation for this parameter.
In the case of fresh concrete, workability was measured in terms of slump using the Abrams cone method immediately after complete concrete mixing. Density testing was performed according to the reference standard (
The properties of hardened concrete were measured in nine cubes measuring 150×150×150 mm, cast using plastic moulds and compacted on a vibrating table, according to EN 12390-2:2009 (
The pre-soaking methods were evaluated and compared with the control and non-pre-soaking methods (methods C and 0, respectively).
Slump values of manufactured concretes.
Relationship between pre-soaking time and slump cone values of pre-soaked series.
Fresh density of manufactured concretes.
Water-saturated density of manufactured concretes.
Water absorption of manufactured concretes.
Compressive strength of manufactured concretes at 7 and 28 days.
Strength of concretes from 7 to 28 days.
Tests, results of concrete series
Test and result | Series C | Series 0 | Series 1 | Series 2 | Series 3 | Series 4 | Series 5 |
---|---|---|---|---|---|---|---|
Fresh state | |||||||
Consistency type (EHE-08) ( |
Soft | Plastic | Fluid | Liquid | Plastic | Soft | Dry |
Abrams cone test (cm) ( |
6 | 3 | 10 | 22 | 4 | 7 | 1 |
Fresh density (kg/m3) ( |
2387 | 2384 | 2360 | 2380 | 2366 | 2357 | 2368 |
Hardened state | |||||||
Water-saturated density (kg/m3) ( |
2412 | 2386 | 2380 | 2374 | 2376 | 2378 | 2397 |
Water absorption (%) ( |
5.05 | 1.97 | 2.17 | 2.40 | 2.10 | 2.30 | 1.80 |
Compressive strength-7 days (Mpa) ( |
50.36 | 44.64 | 47.62 | 43.11 | 39.80 | 44.99 | 53.13 |
Compressive strength- 28 days (Mpa) ( |
55.42 | 55.54 | 51.75 | 46.67 | 50.39 | 46.09 | 54.65 |
EN 13198 (
Several authors have studied the importance of pre-soaking and mixing time, defining 10 minutes as the optimal pre-soaking time to obtain the best cone values (
Based on the results obtained, therefore, pre-soaking affect the consistency of the concrete with respect to the pre-soaking time, but not with respect to the total water content. Therefore, to obtain the desired consistency for the manufacture of pre-cast concrete for street furniture, the RCA would need to be pre-soaked for 10 minutes to achieve a stable moisture level of around 90% of absorption, thereby preventing it from absorbing water from the other components, which would make the pre-soaking effective.
Neither EN 13198 (
This parameter is included neither the manufacturer's recommendations nor in EN 13198 (
The hardened-state density values of the series tested ranged from 2374 kg/m3 (Series 2) to 2397 kg/m3 (Series 5) (
EN 13198 recommends water absorption values with and without melting salts of less than 6% and 7%, respectively. In fact, other studies consulted limited this parameter to similar values. Soutsos et al. (
The water absorption values of concrete made with RCA using different pre-soaking methods (Series 1–5) show that the values of all samples tested are within limits, and there are no significant differences between the different pre-soaking methods (
Contrary to what was expected, all RC absorption values observed in this study are below established limits, and are lower than that of the sample manufactured with NCA. The percentage reduction ranges from 52% to 64%, corresponding to Series 2 and Series 5, respectively. So, natural concrete absorption is more than twice that of the RC absorption, which could be attributed to the lower content of pores interconnected in recycled concretes. Most studies report a greater absorption of water in the case of concrete made with RCA as a result of the incorporation of a more porous material such as cement paste adhering to the aggregate (
This can be attributed to the discontinuous system of pores generated within the hardened concrete and better RCA bonding, which creates a continuous phase (aggregate particles enveloped by old cement paste). Therefore, the pores do not contribute to water absorption.
After analysing the results obtained from this study, it can be concluded that pre-soaking does not affect water absorption. However, the significant improvement in water absorption may be due more to the type of RCA used, mainly concrete, than the pre-soaking method.
EN 13198 (
An analysis of the benefits of pre-soaking (
At 28 days of age, the compressive strength of series manufactured with RCA (Series 0–5) does not follow the same trend observed in the preliminary curing ages (
The compressive strength at 7 days was not affected by the effect of total water content (
However, if the series manufactured with NCA (Series C) is compared to those manufactured with RCA (Series 0–5), replacing NCA with RCA reduces resistance provided the w/c ratio is maintained. The resistance results obtained in this study (
Several factors could explain the loss of strength (
Therefore, the results showed that pre-soaking adversely affected the mechanical strength of the concrete manufactured in this study, since without pre-soaking the RCA (Series 0) had a similar resistance to the series manufactured with NCA (Series C).
This study evaluates potential benefits of pre-soaking on the properties of pre-cast concrete for beach walkways made with RCA, following EN 13198 (
In general, the workability of concrete made with RCA improved with pre-soaking. This improvement was conditional on a 10-minute pre-soaking time, but total water content has no effect.
The benefits of pre-soaking on the compressive strength of the samples tested showed that the initial resistance was enhanced due to the curing effect, which would be of special interest in the manufacture of pre-cast concrete products. However, resistance at 28 days is lower than that of non-pre-soaked RCA, although certain methods guarantee that even such lower values are within the limits set by the manufacturer.
It can therefore be concluded that pre-soaked RCA does not enhance the physical-mechanical properties of pre-cast concrete, except in terms of workability. Due to the complexity and higher costs involved in the implementation of pre-soaking techniques on an industrial scale, the results suggest the use of non-pre-soaked recycled coarse aggregate as a replacement for NCA in the manufacture of pre-cast concrete products for street furniture. Similar workability can be achieved by the addition of plasticizers, capable of providing the desired consistency to such concrete.
We would like to thank the staff of Prefabricados López y Mochón, S.L. in Atarfe in the Spanish province of Granada and Inertes Guhilar S.L. Alhendín in the Spanish province of Granada for their help in this study, and also the Departments of Building Construction and Civil Engineering of Granada University.