Pervious concrete pavements act as a stormwater management tool. In recent times, several cities around the world have encountered frequent flooding due to the combined results of increased rainfall and high impermeable surface areas. As a result, the drainage system gets chocked leading to flash flooding, thus causing severe problems to the functioning and maintenance of road transport and flooding of basement parking of shopping centers. To avoid frequent flooding and manage stormwater runoff, an engineered solution is needed. Pervious concrete pavement with little or no-fines aggregate and a high voided content is a better option than the conventional concrete pavement. Pervious concrete can be used for the construction of parking lots, secondary roads, walkways, driveways, and sidewalks. The pervious concrete pavements also serve the purpose of reducing noise pollution to some extent and because of their ability to reduce the noise they are sometimes referred to as low-noise road surfaces. Worldwide, sustainability is a desperate need whenever there is an inclusion of the construction industry. Investigators believe that the use of construction and demolished waste support the cause of sustainable development in the field of highway and transportation industry. Concerning sustainable development, the use of recycled concrete aggregate is being increased rapidly. In a developing country like India, the primary requirement of any project after performance criteria is its economic feasibility and serviceability criteria. The use of Recycled concrete aggregate in pervious concrete is one such alternative to meet the economy of the project.
To form a pervious concrete with the desired compressive strength and optimum permeability, the amount of water, cement, type of aggregate, size of aggregate, and compaction must be kept in mind. Even a slight variation in compaction may lead to the failure of the pervious concrete mix. ACI committee reported that the pervious concrete is a special type o concrete with a high proportion of large-sized pores, typically 2-8 mm. The porosity of pervious concrete ranged from 15% – 30% & the presence of an interconnected large pores system allows the water to flow easily through the pervious concrete. ACI committee suggested that the minimum vertical force required for compaction is about 0.07 MPa. The strength parameters and structural performance vary too much for the previous concrete as compared to conventional concrete and it mainly depends on porosity. Greater porosity leads to higher permeability values but tends to decrease compressive strength too much. The good quality recycled concrete aggregate may have similar properties like good quality virgin aggregate. Since recycled concrete aggregate and cement paste, which is typically weaker than the original aggregate, hence it is desired to remove as much hardened cement paste possible. The part replacement of natural aggregate with recycled concrete aggregate in mid-strength concrete reduces compressive strength, tensile strength, and modulus of elasticity and increases the dry shrinkage of concrete. The changes in mix composition and supplementary cementitious additives like fly ash and silica fumes can easily be used to increase the compressive strength of pervious concrete. The 15% recycled concrete aggregate has strength parameters similar to the control mix and suggested that up to 30% replacement of natural aggregates with recycled aggregates is feasible without compromising strength and permeability too much. The typical pervious concrete mix consists of binder material 180-355 kg/m3, coarse aggregate 1420-1600 kg/m3 & water to cement ratio ranged from 0.27 to 0.43. The typical 28 days compressive strength ranges from 14% to 31% and the permeability coefficient varies from 0.25 to 6.1 mm/sec. The porosity for pervious concrete lies in the range of 15% to 25% and is dependent on water-cement ratio and compaction effort. The density of pervious concrete lies in the range of 1600 kg/m3 to 2000 kg/m3 which lies in the upper range of lightweight concrete. The compressive strength of pervious concrete can vary from 3.5 to 28 MPa which is applicable for a wide range of applications. Compressive strength mainly depends on the material properties, its proportioning, porosity, and compaction techniques.
The aim is to find out the optimum percentage of recycled aggregate for producing low-density pervious concrete. Six different types of concrete mix containing different percentages of recycled concrete aggregates are prepared to estimate the compressive strength, the flexural strength, and the permeability of pervious concrete or permeable concrete or permeable concrete at the age of 7 and 28 days. The methodology for preparing the pervious concrete is described below as there is no specific technique for the construction of pervious concrete.
Recycled concrete aggregates are generally more porous than the primary coarse aggregates. The main property of pervious concrete is its permeability, so the part of fine aggregate can be reduced and the part of natural aggregate can be replaced with recycled concrete aggregate to enhance permeability without much affecting the strength of the concrete. The materials used for making pervious concrete are cement, primary coarse aggregates, recycled coarse aggregate, and water. With a purpose to carry out the investigation sequentially, the work has been divided into the following phases:
Phase – I: Preparation of Recycled concrete aggregate.
Phase – II: Determination of physical properties of primary coarse aggregate and recycled coarse aggregate.
Phase – III: Partial replacement of primary coarse aggregate by recycled coarse aggregate.
Phase – IV: Establishing the mix proportion of pervious concrete.
Phase – V: Determination of properties of pervious concrete.
The physical properties of recycled concrete aggregate carried out are specific gravity, water absorption abrasion value, and impact value. The properties of pervious concrete estimated are the compressive strength, flexural strength, and permeability. Primary coarse aggregates are replaced by recycled concrete aggregates in different percentages to make the pervious concrete. The different mixes are then compared with the referral mix and an optimum percentage of recycled concrete aggregate is determined based on test results.
Preparation of Recycled Concrete Aggregate
Stepwise procedure for the preparation of recycled concrete aggregate is stated below:
Collect rejected/demolished concrete samples.
Collect concrete samples that can be crushed out into small pieces.
Crush more of the crushed pieces.
Sieve the crushed materials through a sieve of 20 mm to get an average size of 20 mm aggregate.
Materials retained on 20 mm sieve are again crushed into the small pieces to get a 20 mm size of aggregate.
Crush the retained material on a 20 mm sieve again in crusher so that 20 mm size aggregate can be collected.
Crush the retained materials retained on 12.5 mm sieve again in the crusher and then sieved to get 10 mm size of aggregate.
Material passing 6.3 mm sieve can be used to prepare a 4.3 mm size of aggregate.
Properties of Aggregates
Required properties of the recycled concrete aggregates are determined as per IS standards to compare them with the properties of primary aggregates and also to estimate their suitability for the partial replacement of primary aggregate in pervious concrete.
Lower specific gravity and higher water absorption values of recycled concrete aggregate refer to the fact that recycled aggregates are more porous than primary aggregates. Specific gravity is an indicator of the strength of the aggregates and hence it implicates that the recycled aggregates have lesser strength as compared to primary aggregate. The impact value of recycled aggregate is lower than that of the primary aggregate. The possible reason for this may be the difference in the composition of aggregates. The higher abrasion value of recycled aggregate can be understood from the fact that it contains a higher percentage of cement mortar which is removed in the process of abrasion.
Partial Replacement of Primary Coarse Aggregate with Recycled Concrete Aggregate
20%, 25%, 30%, 35%, and 40% of the primary coarse aggregates can replace with recycled concrete aggregates to prepare the different mixes of pervious concrete.
The mix containing 20% replaced recycled concrete aggregate is termed as RC20. Similarly, the term RC25, RC30, RC35, and RC40 can be assigned. Referral concrete with only primary coarse aggregate is termed as RC. The different mixes can be prepared to investigate the potential of recycled concrete aggregate and the effect of replacement of primary coarse aggregate by recycled concrete aggregate.
Mix design of Pervious Concrete
There is no such standard code of practice for the mix design of pervious concrete. So, it is imperative to establish trial mixes by the literature study on pervious concrete design. The testing of pervious concrete is carried out as per IS standards. The strength and permeability characteristics of the prepared mixes are carried out from the laboratory test. The mix design is finalized when the strengths and permeability of the mixes fulfill the minimum stipulated criteria.
Properties of Pervious Concrete
Concrete containing no recycled aggregate is considered as the reference and this referral concrete is then compared with the one having recycled aggregates. Based on the test results, an optimum percentage of recycled concrete aggregate can recommend. The characteristics of a different mix of pervious concrete at the age of seven days and twenty-eight days to be presented respectively. Replacement of primary coarse aggregates by recycled coarse aggregates leads to a decrease in the strength of the pervious concrete.
It is observed that with the increase of percentage replacement of primary coarse aggregate by recycled concrete aggregate the compressive strength decreases. Compressive strength at twenty-eight days is considered as the characteristic strength of concrete and it is assumed that the concrete attains about its 100% compressive strength in these twenty-eight days. To select the optimum percentage of recycled concrete aggregate, twenty-eight days characteristic compressive strength has been chosen as the prime deciding parameter.
The permeability of pervious concrete increases with the increases in the percentage of recycled concrete aggregate.
- The compressive strength and flexural strength of the pervious concrete decrease with the increase in the percentage of recycled aggregates.
- Permeability increases with the increase in the percentage of recycled aggregates.
- The strength of the previous concrete with optimum replacement of primary coarse aggregate by recycled concrete aggregate achieves a maximum of the characteristics strength of the concrete.