- INTRODUCTION
Recycled aggregate is the result of processing appropriate construction and demolition waste. Thereby it is to distinguish between concrete rubble and mineral building material rubble. The processing leads to crushed sand, crushed stone and crushed gravel, derived from concrete rubble and mineral building material rubble respectively. Figure 1 shows the denomination of the different types of recycled aggregate.
| Figure 1 | Denomination of recycled aggregate | |
Aggregate composed of recycled concrete generally has a lower specific gravity and a higher absorption than conventional gravel aggregate. New concrete made with recycled concrete aggregate typically has good workability, durability and resistance to saturated freeze-thaw action. The compressive strength varies with the compressive strength of the original concrete and the water-cement ratio of the new concrete. It has been found that concrete made with recycled concrete aggregate has at least two-thirds the compressive strength and modulus of elasticity of natural aggregate concrete.
2.EXPERIMENTAL STUDY
An experimental study was conducted to assess the properties of concrete made from recycled aggregates. The experimental programme and the results of the experiments are briefed below.
2.1 MATERIALS USED:
- Cement: Ordinary Portland cement of 53 grade.
- Fine Aggregate: Locally available sand having fineness modulus of 3.37.
- Coarse Aggregate: Conventional coarse aggregate conforming to IS 383:1999.
- Recycled Aggregate: Obtained by crushing concrete cubes having mean compressive strength of 23.35 MPa.
- Fly ash: Having fineness of 3437cm2/gm and silica content of 59.09%.
Four types of mixes were used for experimentation. They were:
- Natural Aggregate Concrete (NAC): Cement, Fine aggregate and Coarse aggregate in the ratio 1:1.6:3.3.
- Recycled Aggregate Concrete (RAC): Natural coarse aggregate was completely replaced by recycled aggregate and the mix was in the ratio 1: 1.6: 3.3.
- NAC1: 10% fly ash (by weight of cement) was added to NAC replacing cement.
- RAC1: 10% fly ash (by weight of cement) was added to RAC replacing cement.
- Water –cement ratio: 0.4 by weight.
- Water reducing admixture: 125ml/50 kg of binder.
The following specimens were cast using the four different mixes, namely, NAC, RAC, NAC1, RAC1, for various tests. For each mix, the following specimens were cast:
1. twenty four cubes of size 150 mm
2. eleven prisms of size 100 mm X 100mm X 500mm.
All specimens except those for drying shrinkage test were demoulded 24 hours after casting and were subsequently water cured for 28 days.
2.4 RESULTS AND DISCUSSION
A number of tests were conducted and the following properties of recycled concrete were obtained from the results.
2.4.1 COMPRESSIVE STRENGTH
In general, RAC showed lower compressive strength at all ages compared to NAC. An average of 15% reduction in compressive strength was noted compared to NAC. The modulus of rupture of the RAC mix at 28days was also reduced by 6% compared to NAC mix. The inferior properties of recycled aggregates and the presence of weaker bond areas between recycled aggregate particles and old or new mortar in RAC may lead to such lower strength of RAC. Also, it was observed that10% fly ash addition substantially improved the compressive strength of RAC at all ages.
Mix Designation Compressive Strength, MPa
7-day 28-dayNAC 35.26 46.13
RAC 29.33 38.11
NAC1 41.70 48.52
RAC1 36.70 53.55
2.4.2 DRYING SHRINKAGE
The typical development of drying shrinkage with time for the four mixes is shown in the figure For the RAC mix, the observed increase in drying shrinkage compared to NAC may be due to the combined effects of lower aggregate modulus and higher amount of shrinkage mortar with many pores. Also it is regarded that the cement, which adhered on the aggregate surface, was not fully hydrated and thus causes larger shrinkage at later stages.
2.4.3 PERMEABILITY AND WATER ABSORPTION
For permeability test, standard cube specimen of size 150mm X150mm X150 mm was installed in the apparatus. Water pressure of 0.1 MPa was applied for48hours, and then pressure of 0.3 MPa and 0.7 MPa, each for 24 hours, was applied. After this, the specimen was split vertically in the middle applying compressive forces. The greatest penetration depth was measured from the split surfaces. The test indicated that the permeability as well as water absorption of RAC is more compared to that of NAC. But the addition of fly ash showed a remarkable improvement in the performances of the permeability and water absorption of both RAC and NAC.
Depth of penetration
2.4.4 DURABILITY
To evaluate the degree of deterioration of all the concrete mixes against accelerated sulphate and acid attack, standard prisms of size 100mm X 100mm X 500mm were immersed in testing baths. The first testing bath contained 7.5% MgSO4 and 7.5% Na2SO4 by weight of water and the second contained H2SO4 of pH value 2. The durability of RAC was found to be slightly lesser than that of NAC. The addition of fly ash showed an improved resistance to deterioration.
2.5 SUMMARY
Studies show that the strength of RAC is lesser than that of NAC. But with the use of fly ash, it may be possible to produce RAC with an improvement in strength. The results of this investigation also show that drying shrinkage strain, permeability and water absorption of RAC is more compared to that of NAC. But it is possible to produce RAC with improved qualities by the addition of fly ash. Therefore, the results of this study provide a strong support for the feasibility of using recycled aggregates instead of natural aggregates for the production of concrete.
3.PROPERTIES OF RECYCLED AGGREGATE CONCRETE
3.1 WORKABILITY
Workability is the property of fresh concrete that determines the amount of useful internal work necessary to produce full compaction. The results of slump test and compacting factor test conducted on fresh concrete indicates that, with an increase in the percentage of recycled aggregates, slump and compacting factor show a decreasing trend. This reduction in workability may be attributed to higher water absorption of the recycled aggregates, which may increase the harshness of the mix. Pre-soaking or pre-wetting of the recycled aggregates can reduce the rapid loss of workability.
3.2COMPRESSIVE STRENGTH
The compressive strength of recycled aggregate concrete is lower compared to that of natural aggregate concrete. The compressive strength of recycled concrete is about two-third as that of conventional concrete. The reduction in compressive strength may be due to
1. the relatively higher water requirement of recycled aggregate concrete than that of conventional concrete
2. lower resistance of recycled aggregate to mechanical action than that of natural aggregate and
3.the weaker bond between the fresh mortar and the old mortar adhering to the recycled concrete aggregate.
3.3 MODULUS OF RUPTUREThe modulus of rupture of recycled aggregate concrete is less than that of conventional concrete. The modulus of rupture of recycled aggregates is about 6 times lesser than that of natural aggregate.
3.4 PERMEABILITY
The permeability of recycled aggregate concrete is more compared to that of natural aggregate concrete. The addition of fly ash can improve the permeability characteristics of recycled aggregate concrete.
3.5 FREEZE-THAW ACTION
The concrete made from recycled aggregates has higher resistance to saturated freeze-thaw action.
3.6 DURABILITY
The durability of recycled aggregate concrete under sulphate and acid action is equal to or slightly inferior to that of natural aggregate concrete. But the durability of recycled concrete can be improved by the addition of fly ash.
4.APPLICATIONS
Recycled aggregate has two mainly types of applications. They are
- Structural applications
- Non structural applications
For structural applications, the complete replacement of natural aggregate with recycled aggregates is not desirable. This is due to the fact that the creep and shrinkage of recycled aggregate concrete is lower compared to that of natural aggregate concrete. Inspite of this, some building projects such as the Vilbeler Weg and the Waldspirale in Darmstadt have replaced a part of natural aggregates with recycled aggregates. During the Waldspirale project, after conducting a series of experiments, it was observed that concrete with recycled aggregate shows no relevant difference to concrete made from natural dense aggregate and can be casted or pumped just like a standard concrete mixture. This observation is a welcome note as far as the use of recycled aggregate concrete for structural purposes is concerned.
4.2 Non structural applications
Recycled aggregates have a wide range of non structural applications. It is primarily used in pavement reconstruction. It has been satisfactorily used as an aggregate in granular subbases, lean-concrete subbases, soil-cement, and in new concrete as the only source of aggregate or as a partial replacement of new aggregate. It can be used as backfill for filling depressions or trenches. In road pavement construction, it can be used as aggregate for bituminous concrete.
5. BENEFITS
Use of any recycled material helps to keep that material out of landfills. Recycling practices also can decrease the environmental impact of obtaining / manufacturing the material from virgin resources.New concrete made from recycled concrete aggregate generally has the same properties as stone or gravel aggregate. Recycled concrete aggregate sells in the approximate range of $3.50 to $7 per cubic yard, depending on the specifications (size limitations) for the aggregate and local availability. This is about one half the cost of non-recycled aggregate used for construction purposes.
Glass aggregate typically acts as a crack arrestor, benefiting concrete durability, though this depends on the specific glass aggregate properties, the concrete and its end-use. The allowable size of the aggregate largely determines its prices, which are in the vicinity of $15 to $20 per cubic yard. Smaller aggregate (e.g. “glass sand”) is the more expensive. Glass aggregate can allow a greater range of aesthetic/decorative options for concrete.
Some of the additional benefits of using glass as an aggregate material in concrete include:
- concrete unit cost decrease
- lowering of freight cost
- avoided landfill costs
- boosting or creating secondary markets around recycling and selling additional common types of glass
6. LIMITATIONS
Lack of widespread reliable data on aggregate substitutes can hinder its use. To design consistent, durable recycled aggregate concrete, more testing is required to account for variations in the aggregate properties. Also, recycled aggregate generally has a higher absorption and a lower specific gravity than conventional aggregate.Research has revealed that the 7-day and 28-day compressive strengths of recycled aggregate concrete are generally lower than values for conventional concrete. Moreover, recycled aggregates may be contaminated with residual quantities of sulfate from contact with sulfate rich soil and chloride ions from marine exposure.
Glass aggregate in concrete can be problematic due to the alkali silica reaction between the cement paste and the glass aggregate, which over time can lead to weakened concrete and decreased long-term durability. Research has been done on types of glass and other additives to stop or decrease the alkali silica reaction and thereby maintain finished concrete strength. However, further research is still needed before glass cullet can be used in structural concrete applications.
7. CONCLUSION
Recycling of materials in construction field is a welcome note towards the conservation of natural resources. But the complete replacement of natural resources by recycled materials has not yet been possible. Even today only upto 30% of the total aggregates can be replaced by recycled aggregates. If the quantity of recycled aggregates is more than this value, the quality of concrete can be affected. The quality of recycled concrete can be improved by the addition of fly ash.
There are no standard regulations currently addressing the use of alternative concrete aggregate for engineered use or structural applications. Some state and local codes specifically address the use of alternative aggregate, for example the Washington State Department of Transportation. However, this should be verified on a project-by-project basis. Research should be carried out and a standard code for the use of recycled aggregates should be evolved. This will promote the use of recycled aggregates as an alternative for natural aggregates.
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