The paper focuses on the reuse of crushed asphalt (GA) as a partial replacement (up to 20%) of natural aggregates for concrete manufacture. Addition of GA aggregates produced a positive effect on workability loss. The GA mixes, however, showed a significant tendency to bleed and segregate at the highest replacement percentage applied. GA led to a decrease of compressive strength in concrete (with respect to that of the reference concrete) up to 50% due to the weakness of the cement paste / recycled aggregate interface. To compensate for this negative effect, a reduction of w/c for the GA concretes was necessary. A decrease of w/c allowed the GA concretes to show drying shrinkage values substantially similar to those of reference concrete with the same cement factor. The experimental results confirmed the possibility of partial substitution (max. 15%) of natural aggregates with crushed asphalt for making concrete.
In the last 15 years, sustainability has become an important research topic in the field of construction and building materials. The quantity of waste produced each year in the European Economic Area (EEA) is estimated at 3.0 billion tons, of which 40% arises as construction and demolition waste (C&DW) (
Crushed asphalt (GA) is a typical by-product in the field of asphalt road rehabilitation. Reuse of this material is described in the EN 13108 standard, including material classification and testing. Generally, crushed asphalt is considered re-usable in the field of road construction. Indeed, a common practice of utilisation is based on re-heating the recycled aggregate to recover bitumen and mixing it with a specified amount of new bitumen to obtain a material with adequate physical properties for road applications. However, the asphalt recovery industry in some countries faces the problem of excessive material storage due to the limited quantities of GA that can be reused in new mixes. The storage costs in Italy are also high due to the classification of GA as waste rather than by-product. Hot reuse is, in fact, limited to 30% maximum substitution level in the case of new roads, and lower (less than 10% in the top layer) in the case of road rehabilitation. The annual report of the European Association of Asphalt Manufacturers (
Cold reuse is suitable for bituminous sub-base and base layers where GA replaces natural raw materials. Cold recycling is an economical and environmentally friendly operation. No heating of GA is necessary, which means energy savings and reduced emissions. However, this technique is not very widespread compared to hot-recycling, yet it remains a good solution for temporary restoration such as road patches.
In this paper an alternative cold-recycling method is presented for using crushed asphalt in Portland cement concrete production. Feasibility of partial substitution of natural aggregates with crushed asphalt was evaluated in terms of rheological and physical properties of such concrete. This process was considered to help increase the percentage of recycling and solve the problem of excessive material immobilization in waste disposal before reuse in road construction.
The reuse or recycling of by-products for concrete manufacture is a key-factor for sustainable development in the building industry. This research is devoted to the reuse of crushed asphalt for concrete manufacture - both to reduce the use of natural aggregates and to dispose of GA waste. Crushed asphalt in Italy is mainly used for new road construction, especially in the sub-base and base layer. However, in the case of road maintenance and restoration, only a small fraction of crushed asphalt can be reused and the demand for waste storage dramatically increases. This research proposes an alternative cold recycling process for GA.
Concrete mixes (GAC) were manufactured with GA as partial replacement for natural aggregates at levels of 5%, 10% and 20%. Three series of concretes were manufactured; three samples were cast for each test. Rheological and mechanical behaviour were compared to those of the reference concrete (RC) manufactured only with natural aggregates.
Cement CE II/B-LL 32.5R (
Chemical composition of cement, % by mass
SiO2 | Al2O3 | Fe2O3 | TiO2 | CaO | MgO | SO3 | Na2O | K2O | Cl |
---|---|---|---|---|---|---|---|---|---|
16.25 | 3.86 | 1.59 | 0.2 | 60.34 | 2.34 | 2.58 | 0.15 | 0.67 | 0.054 |
A superplasticizer (SP, ester of acrylic or methacrylic acid monomer) having 1000 g/mol side chain length and an acid/ester ratio equal to 3.5 was used. Crushed asphalt was supplied by a local recycling company; it was not cleaned before turning it to aggregate, only air dried. The experiments aimed to reuse crushed asphalt on site, so no pre-soaking or conditioning was carried out to simulate real in-situ manufacturing. Three natural aggregates and GA were used ( CG: coarse gravel (1 ÷ 30 mm) FS: fine sand (0 ÷ 5 mm) CS: coarse sand (8 ÷ 12 mm) GA: crushed asphalt (10 ÷ 20 mm)
Density and water absorption of natural and waste aggregates
Aggregate | Density (kg/m3) | Water absorption (%) |
---|---|---|
2620 | 1.0 | |
2660 | 0.9 | |
2690 | 1.9 | |
2430 | 2.5 |
The grading of natural and GA aggregates was evaluated by sieve analysis according to EN 933-1 (
Grading of natural and GA aggregates.
Water absorption and density of natural aggregates and GA were evaluated according to EN 1097-6 (
The concrete mixes were optimized (
All concrete mixes were prepared in a power-driven rotary mixer with a moving base (but without blades or paddles). The mixing procedure was as follows: first the dry components were mixed for 2 minutes, then 70% of the total water was added and mixed for 1 minute, then the rest of the water was added and mixed for one more minute. Superplasticizer was added to meet the target workability.
Three series of concrete mixes were prepared: the first (a) with a cement content of 330 kg/m3 and a fixed w/c ratio of 0.53 (
Concrete mixes for series (a)
Component | unit | RC | GA 5% | GA 10% | GA 20% |
---|---|---|---|---|---|
kg/m3 | 330 | 330 | 330 | 330 | |
kg/m3 | 175 | 175 | 175 | 175 | |
0.53 | 0.53 | 0.53 | 0.53 | ||
kg/m3 | 811 | 811 | 811 | 682 | |
kg/m3 | 869 | 775 | 681 | 625 | |
kg/m3 | 185 | 185 | 185 | 185 | |
kg/m3 | 0 | 85 | 171 | 342 |
Concrete mixes for series (b)
Component | unit | RC | GA 5% | GA 10% | GA 15% | GA 20% |
---|---|---|---|---|---|---|
kg/m3 | 330 | 330 | 330 | 330 | 330 | |
kg/m3 | 175 | 149 | 149 | 149 | 149 | |
0.53 | 0.45 | 0.45 | 0.45 | 0.45 | ||
kg/m3 | 811 | 811 | 811 | 811 | 682 | |
kg/m3 | 869 | 775 | 681 | 587 | 625 | |
kg/m3 | 185 | 185 | 185 | 185 | 185 | |
kg/m3 | 0 | 85 | 171 | 256 | 342 |
Concrete mixes for series (c)
Component | unit | RC | GA 5% | GA 10% | GA 20% |
---|---|---|---|---|---|
kg/m3 | 330 | 350 | 350 | 350 | |
kg/m3 | 175 | 180 | 180 | 180 | |
0.53 | 0.45 | 0.45 | 0.45 | ||
kg/m3 | 811 | 792 | 792 | 666 | |
kg/m3 | 869 | 756 | 665 | 610 | |
kg/m3 | 185 | 180 | 180 | 180 | |
kg/m3 | 0 | 83 | 167 | 334 |
Samples were cast in 150 mm cube steel moulds, which conformed to EN 12390-1. The moulds were cleaned and lightly coated with oil before the casting procedure. Concrete was compacted on a vibrating table. After that, the samples were covered with polyethylene film and left to set for 48 hours. Then they were removed from the moulds and cured in water (at a temperature of 20±2 °C) for 7 days and in a curing chamber (at an air temperature of 20±2 °C and relative humidity ≥95%) for another 21 days or until testing, thus conforming to EN 12390-2.
Dosage of the polycarboxylate-based superplasticizer was adjusted for all mixes to attain the same 200 mm slump at the end of mixing (S4 class, according to EN 206-1) as the reference concrete. Slump was measured immediately at the end of mixing and 30 and 60 minutes thereafter according to EN 12350-2. Entrapped air and density were also evaluated on the fresh concrete according to EN 12350-6 and EN 12350-7, respectively. The density and compressive strength of the hardened concrete at 1, 7, 14 and 28 days were measured in accordance with EN 12390-2. Finally, elastic modulus at 28 days and drying shrinkage up to 120 days were determined on the hardened concrete according to EN 12390-13 and UNI 11307 respectively.
The addition of increasing percentages of GA in the concrete mixes reduced the superplasticizer demand as evidenced in
SP dosage for all concrete mixtures.
Higher GA addition led to higher workability at 60 minutes (
Slump loss of GA and reference concretes with w/c=0.53 and 330 kg/m3 cement content.
Flow value of GA and reference concretes as a function of time with w/c=0.53 and 330 kg/m3 cement content.
Slump loss of GA concretes (w/c=0.45) and reference concrete (w/c=0.53) at 330 kg/m3 cement content.
Slump loss of GA concretes (w/c=0.45 and 350 kg/m3 cement content) and reference concrete (w/c=0.53 and 330 kg/m3 cement content).
No bleeding or segregation tendency was detected for any of the concretes except for the mix with 20% of GA. For this reason, the maximum percentage of GA to avoid bleeding must be limited to 15%. No anomalous air entrapment was noticed for any of the mixes. The density of GA concretes was substantially similar to that of RC with only natural aggregates.
Compressive strength tests were carried out to evaluate the effect of GA addition on the mechanical properties of concrete. An increase in GA addition produced a decrease in compressive strength of concrete at early ages (
Compressive strength vs. age of GA and reference concretes with w/c=0.53 and 330 kg/m3 cement content.
The compressive strength decrease is evident for the highest substitution of natural aggregates; at 20% substitution, the compressive strength decrease of GA concrete was about 50% with respect to RC. This could probably be ascribed both to the presence of oils in bitumen that adversely affect the kinetics of cement hydration and to a poor aggregate-cement paste interface, as evidenced by the failure paths during the compressive strength tests. The preferential crack propagation detected by visual observation, in fact, was through the cement paste-GA interfaces, in the zones where the presence of hardened bitumen was high. However, the reduction of the w/c ratio (second series: b), produced a positive effect on the compressive strength of concrete prepared with GA aggregates (
Compressive strength vs. age of GA concretes (w/c=0.45 and 330 kg/m3 cement content) and reference mixture (w/c=0.53 and 330 kg/m3 cement content).
Compressive strength vs. age of GA concretes (w/c=0.45 and 350 kg/m3 cement content) and reference mixture (w/c=0.53 and 330 kg/m3 cement content).
Drying shrinkage was also measured on samples made with a cement content of 330 kg/m3, w/c=0.45 and 5–15% GA replacing natural aggregates (
Drying shrinkage of GA concretes (w/c=0.45 and 330 kg/m3 cement content) and reference mixture (w/c=0.53 and 330 kg/m3 cement content).
Young's modulus of hardened concrete was measured on cylindrical samples at 28 days (UNI-EN 6556). The higher the GA replacement the lower the Young's modulus, see
Young's modulus of GA concretes (w/c=0.45 and 330 kg/m3 cement content) and reference mixture (w/c=0.53 and 330 kg/m3 cement content).
Results of an experimental study are presented to evaluate the substitution of natural aggregates by crushed asphalt in concrete. This addition produced positive effects on both initial workability and workability retention. The effect could probably be ascribed to the presence of oil traces that can play a synergistic role with the superplasticizer. However, the substitution percentage of crushed asphalt must be limited to 15% in order to avoid bleeding and segregation. The higher the crushed asphalt substitution is the lower the compressive strength. A 50% strength decrease was noticed when GA was used at 20% replacement, which can be ascribed both to the weakness of the cement paste / recycled aggregate interface and to the presence of oils in bitumen that adversely affect the kinetics of cement hydration, although reduction of w/c ratio mitigated this effect. The maximum substitution rate of GA is best limited to 15% to avoid both rheological and mechanical underperformance. Reusing GA in concrete also led to increased drying shrinkage. However, the improvement of cement paste quality, i.e. the reduction of w/c ratio, should limit this effect. The higher the GA replacement is the lower the Young's modulus. However, reduction in w/c ratio (0.45 vs 0.53) allowed achieving a similar Young's modulus for the 15% substitution of natural aggregates than for RC. In all, the experimental work evidences the feasibility of partial substitution of natural aggregates by crushed asphalt in concrete.