Concrete Recycling
Concrete Recycling
While at present mostly recycled into road subbase, the amount of demolished concrete in Japan is expected to increase rapidly and exceed the demand for road subbase in the near future. To promote the recycling of concrete, a technology to produce high-quality recycled aggregate has been developed. This technology employs the heating and rubbing method. In order to investigate a future concrete recycling system, first of all, a specific model considering indices of economic activity is established to forecast the amount of demolished concrete in the future.
Introduction
When structures made of concrete are to be demolished, concrete recycling is an increasingly common method of disposing of the rubble. Concrete debris was once routinely shipped to landfills for disposal, but recycling has a number of benefits that have made it a more attractive option in this age of greater environmental awareness, more environmental laws, and the desire to keep construction costs down.
Concrete aggregate collected from demolition sites is put through a crushing machine, often along with asphalt, bricks, dirt, and rocks. Crushing facilities accept only uncontaminated concrete, which must be free of trash, wood, paper and other such materials. Metals such as rebar are accepted, since they can be removed with magnets and other sorting devices and melted down for recycling elsewhere. The remaining aggregate chunks are sorted by size. Larger chunks may go through the crusher again. Smaller pieces of concrete are used as gravel for new construction projects. Sub-base gravel is laid down as the lowest layer in a road, with fresh concrete or asphalt poured over it. Crushed recycled concrete can also be used as the dry aggregate for brand new concrete if it is free of contaminants.
There are a variety of benefits in recycling concrete rather than dumping it or burying it in a landfill. Keeping concrete debris out of landfills saves space there.
Using recycled material as gravel reduces the need for gravel mining. There are also economic benefits. Recycled concrete is a construction material that the community does not need to pay for; those who generated the concrete waste pay a fee to have it recycled.
Concrete is widely used as a basic material for construction and infrastructure. Approximately 500 million tons of concrete were produced in Japan around 1990. In recent years, approximately 35 million tons of demolished concrete are being generated every year. Actually, this figure might be an underestimate partly because some concrete is illegally dumped or mixed with construction soil that is not treated as waste. 95% of the concrete is recycled using cascade recycling and subsequently reused as low-quality road sub-base.
High-quality level recycling, in which recycled aggregate is made from concrete, is not carried out at present. In the near future, a substantial amount of concrete from construction undertaken during the economic growth of the 1960s and 1970s will reach its end of life, and the generation of demolished concrete is expected to rapidly increase.
Method of Recycling
We have developed a technology to produce high-quality aggregate from demolished concrete using a ‘heating and rubbing method’. Using this technology, aggregate can be recycled as raw material for ready-mixed concrete, while fine powder (HRM powder) from cement paste can be recycled as raw material for cement, cement admixture, or soil stabilizer. We propose that this technology be used in a level concrete recycling system (Fig. 1).
While the production of recycled aggregate using the HRM consumes much fuel for heating and electricity for rubbing, our life-cycle analysis showed that the use of the HRM can reduce CO2 emissions through the utilization of the HRM powder as cement-related inputs. Many studies dealing with technologies for producing recycled aggregate and their life-cycle analysis have been carried out in world. However, there are currently no studies that attempt to forecast future trends in the amount of demolished concrete, or to analyse the future role of advanced concrete recycling systems. This study aims at filling this gap.
HRM: A Novel Technology for Producing High-Quality Recycled Aggregate
- High-Quality Recycled Aggregate Producing Process
In the HRM, when demolished concrete is heated to approximately 3000C, the cement paste is made brittle by dehydration. To remove the cement paste from the surface of aggregate, the heated concrete is rubbed in a mill by media. Figure 2 shows the effect of the heating temperature on the quality of the coarse aggregate treated using this method. The effect is almost saturated at approximately 3000C. Therefore, 3000C is chosen as the heating temperature. Koga et al. confirmed that the qualities of aggregate heated up to 5000C do not deteriorate by examining the changes in density, absorption, etc. Figure 3 shows the oven-dry density and absorption of the recycled coarse aggregate and the recovery ratio to the original aggregate at each quality level as the result of an experiment using laboratory scale apparatus. The degree of rubbing treatment varied according to the change in amount of media or rotational velocity of the mill. There is a linear relationship between the oven-dry density and absorption. The recovery ratio decreases as the oven-dry density increases and absorption decreases. This implies that the amount of adhesive mortar or cement, which determines the quality of aggregate, is changed by the degree of rubbing treatment. Therefore, the quality of aggregate can be controlled by changing the amount of media or the rotational velocity of the mill. The Results of aggregate recovering tests using laboratory scale apparatus and a 300 kg/hr pilot plant showed that both coarse and fine aggregates meeting the standard of normal aggregates prescribed by the Japanese Industrial Standard ‘Ready-mixed concrete JIS A 5308’ could be recovered at a high ratio. Based on these achievements, a portable test plant for recovering high-quality aggregate with a capacity of 5t/hr was developed. This plant is composed of approximately 20 portable units. These units can be separated to allow easy transportation on a trailer and reassembled at the collection site of the demolished concrete. Figure 4 shows the whole view of the plant.
Figure 5 shows the process flow of the plant. The concrete rubble crushed to a size under 50 mm is heated to 300°C in a vertical kerosene-fuelled furnace. The heated concrete is sent to primary rubbing equipment. The equipment is a tube type mill with inner and outer cylinders. In the equipment, the heated concrete is rubbed by steel balls as media and the generated mortar portion is discharged through the screen provided on the inner cylinder to increase the efficiency of rubbing treatment. Then, the coarse aggregate and the removed mortar are sent into the secondary rubbing equipment in which the cement adhering to the fine aggregate is removed by the coarse aggregate as media. All the aggregate from the secondary equipment is fed into a 5mm vibrating screen and separated into coarse and fine aggregate. The fine powder generated in the secondary equipment is swept by air and collected in a bag filter. The average ratios of aggregate recovery and fine powder generation to the original concrete in terms of weight are 35% of coarse aggregate, 30% of fine aggregate, and 35% of fine powder, respectively. Based on the field test using the portable plant, a commercial portable plant with capacity of 10 ton/hr was designed and constructed, and used for two large-scale building reconstructions.
- Quality of recycled aggregate and its concrete
The quality of the aggregate recycled using the HRM meets the JIS A 5308 standard. The performance of the HRM-recycled aggregate concrete is comparable to that of ordinary concrete as demonstrated by a series of tests. Furthermore, the aggregate has been applied in several concrete buildings. In these cases, the properties of strength and durability were the same as those of normal concrete, and pumpability and castability were also satisfactory.
- Usage of HRM powder
We have examined the properties of the HRM powder and proved that its ability to absorb water makes it suitable as a soil stabiliser. We also verified using a concrete performance test that blast furnace slag cement (BFSC) containing 10% of HRM powder could be used as ordinary BFSC.
Results
(1) Basic scenario
The breakdown of the treatment of demolished concrete in the basic scenario is shown in Fig. 13. The sum represented by each bar corresponds to the generation of demolished concrete, reaching 200 million tons a year in 2030. The road sub-base is the main treatment of the concrete in every year. The HRM is introduced in 2020, and its contribution is 2.5 million tons a year. In 2030, the HRM and Mechanical Grinding Method (MGM) contribute 17 and 60 million tons a year, respectively. The reason for the introduction of the HRM and MGM is their ability to avoid the final disposal of the demolished concrete when its amount exceeds the demand for road sub-base, especially when the latter decreases with increasing cutting overlay usage in road construction.
The breakdown of the ready-mixed concrete with LQ-recycled aggregate begins to be produced, and concrete with BFSC increases due to increased demand in construction, as given in the scenario in 2010. Concrete with HRM powder cement begins to be produced with the introduction of the HRM in 2020. In 2030, all production of concrete with BFSC has shifted to that with HRM powder cement. HRM powder is mainly used for HRM powder cement, but not for cement raw material, when cost are minimised. This indicates that the operation of the HRM method reaches a maximum and the utilization of the powder is the key to increasing use of the HRM.
(2) Influence of a carbon tax
Next, the influence of a carbon tax is analyzed. At rates of 2,500 and 1,500 yen/ton-CO2 in 2010 and 2020, respectively, the contribution of the HRM is increased from 0 to 4 million tons and from 2.5 to 7 million tons in 2010 and 2020, respectively, as shown in Fig. 15. If the rates are lower than those shown above, no effect is observed. Since the MGM does not reduce CO2 emissions, its contribution is not increased by the tax. Thus, the carbon tax accelerates the introduction of the HRM.
(3) Influence of a subsidy
The breakdown of contributions is shown in Fig. 16, when the subsidy for high-quality recycled aggregate production is set to 750 yen/ton-concrete. The contribution of the HRM is increased from 0 to 4 million tons and from 2.5 to 7 million tons in 2010 and 2020, respectively. The MGM is introduced from 2010, and its contribution increases from 60 to 135 million tons in 2030. The subsidy has no effect, and can be stopped in 2030. Thus, a subsidy at a reasonable rate is very effective for the introduction of both the HRM and MGM.
(4) Influence of spread of cutting overlay method in road construction
Many parametric studies have been carried out to analyse the influence of the variation of spread ratio of mixed cement, low-quality aggregate, FA as cement admixture, etc. In this paper, the case in which the cutting overlay method does not become popular in road construction is analysed. Assuming that the upper ratio of the cutting overlay is kept at 16% (estimated value in1995), neither HRM nor MGM are introduced, even by 2030. This is because the conventional method in road construction requires a lot of road subbase and the demolished concrete can still be used for road subbase. Even in this case, the subsidy explained above is effective at the same price. With a subsidy of 750 yen/ton-concrete, the extent of use of the HRM and MGM is almost same, as shown in Fig. 16.
Conclusion
To promote the recycling of concrete, a technology to produce high-quality recycled aggregate by the heating and rubbing method has been developed. In this paper, after a description of this technology, a future scenario of concrete recycling system is analysed in order to assess the applicability of the technology using an optimization model. The model is connected to an input- output table that has been extended by a detailed description of concrete-related industries as well as some concrete recycling processes. For this scenario, a specific model considering indices of economic activity is established to estimate the amount of demolished concrete in the future. The following conclusions are drawn:
(1) The demolition rate of buildings can be regressed using a proportional hazard model with covariates such as the GDP growth rate and official bank rate. The average lifetime of non-wooden buildings built in later years is longer at approximately 48 years than that of buildings built in 1950, for the average of 13 cities in Japan.
(2) This model can be widely used for forecasting the lifetime of buildings and the generation of waste from their demolition in other countries by choosing the proper covariates.
(3) Under the estimated generation of demolished concrete, concrete recycling using the heating and rubbing method (HRM) is introduced in 2020. The amount of the treatment by the HRM is increased to 17 million tons a year in 2030, when the generation of demolished concrete reaches 200 million ton. In order to increase the amount of HRM treatment, the HRM powder generated at the aggregate recovery must be used more efficiently as a cement admixture or otherwise.
(4) A carbon tax of 2,000 to 3,000yen/ton-CO2 would accelerate the introduction of the HRM because it would reduce CO2 emission through the use of the HRM powder as a cement admixture.
(5) A subsidy for the production of high-quality recycled aggregate is very effective for the introduction of both the HRM and the mechanical grinding method.
(6) Where Does Concrete Recycling Occur?
1. At The Jobsite
2. Contractor’s Yard
3. Municipal or Regional Depots
(7) Benefits of Recycling Concrete
1. Local Product – Local Sources
i. Reduces Truck Traffic
ii. Alternative to a Non-Renewable Resource
iii. Cost Savings
iv. No Disposal Fees
v. Better Trucking Utilization (Reduced Costs)
vi. Other Benefits
vii. Allow up to 10% Deleterious Materials
(8) Future Opportunities
1. Products Availability
i. Current Research and Development
ii. Recycled Concrete Introduction into Ready Mix
iii. Potential Use in Lower Specified Strength Mixes
iv. Studies in United States Currently Underway
v. Little Work On Going in Canada Currently
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Gopal – Great article. RecycleMatch is working hard to reduce the barriers you noted. We are finding ways to use the internet to significantly improve the efficiency of the ‘business ecosystem’. We are really focused on helping move things from the ‘waste’ category (where you note they have no value) to the ‘reusable’ or ‘recyclable’ category by finding matches for previously undervalued materials.
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