Pervious Concrete Technology

Pervious Concrete Pavement

Pervious Concrete Technology

Sustainable Solution to Mitigate Waterlogging and Flashfloods in Urban Conglomerates Avishreshth, Doctoral Research Scholar, Department of Civil Engineering, Indian Institute of Technology Tirupati, Andhra Pradesh, India, Anush K. Chandrappa, Engineer, Design and Technical Audit Department, Dilip Buildcon Limited, Bhopal, India, Prasanna Venkatesh Sampath, Assistant Professor, Krishna Prapoorna Biligiri, Associate Professor, Department of Civil Engineering, Indian Institute of Technology Tirupati, Andhra Pradesh, India

Introduction In recent years, the rise in employment and higher standard of living, amongst numerous other factors, has led to a gradual shift of people from rural to urban conglomerates, consequential of increased urbanization. Since then, the construction of houses, buildings, and road pavements has increased exponentially, covering the natural pervious ground and converting it into impervious fabric [1]. It is expected that more than 53% of the emerging world and 83% of the developed world will be urbanized by 2050 [2]. Although, this is a healthy sign for socio-economic development, it manifests several facets of environmental degradation.

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Environmental Impacts of Impervious Pavement Surfaces Road pavements constitute about 2040% of impervious surfaces in an urban conglomerate [3]. Pavement surfaces are fundamentally built to ensure better connectivity of men and materials to various destinations. However, this results in a multitude of detrimental effects on the environment, which can be broadly classified as variations in hydrological facets and change in the ambient temperature [4].

natural ecosystem and causing problems such as increased runoff and groundwater level depletion. In addition, the runoff generated in the event of precipitation contains pollutants, which need expensive treatment processes, in order to prevent contamination of the receiving water bodies such as lakes, streams, rivers, etc. Further, the situation of waterlogging leads to breeding of mosquitoes, causing health hazard to humans, and disrupting the functional performance of roads as well. In addition, skidding and hydroplaning are some other issues pertaining to wet pavement surfaces causing road accidents.

The stormwater flowing on the impervious pavement surface is not infiltrated into the ground, thus, creating an imbalance in the

In terms of surrounding temperature variations, the conventional impervious pavements have tendency to absorb and

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Figure 1: Pervious Concrete Specimens

store heat before reflecting it back to the atmosphere. This results in a phenomenon referred to as Urban Heat Islands (UHI), which causes an increase in the ambient temperature by about 2-6oC, causing thermal discomfort to urban dwellers [4].

Application of Pervious Pavement Surfaces: A Sustainable Approach In order to sustain the impact of impervious pavement surfaces on urban habitat, sustainable roadway materials are needed that can serve the purpose of groundwater recharge as well as preserve the environment. One such material is pervious concrete (PC), where coarse aggregates are coated and bound with sufficient cement paste to hold the composites together. PC differs from conventional concrete in which it uses a little or no fine aggregates, thus, creating interconnected macro porous channels within the system, and is also referred to as porous concrete, gap-graded concrete, no-fines concrete, or enhanced porosity concrete [5, 6]. Figure 1 presents typical PC cylindrical specimens manufactured in the laboratory or in-situ.

voids, PC acts as a filtering medium and removes pollutants such as zinc, copper, and other fine particles [8, 9]. Moreover, it has been observed that PC has the ability to reduce UHI [10].

General Engineering Properties of Pervious Concrete The engineering properties of PC are functions of shape, size and density of aggregates, aggregate-water-cement ratio, and the adopted compaction procedures. A binary gradation with smaller sized aggregates occupying the void spaces between large sized aggregates leads to the development of a mix that has higher density [11]. An increase in the cement-to-aggregate (c/a) ratio tends

to lower the total void volume leading to a decrease in permeability. Similarly, an increase in water-to-cement (w/c) ratio results in densification of the PC mix due to enhanced lubrication [12]. Furthermore, higher compaction efforts can produce a PC mix with clogged voids and reduced permeability [13]. Hence, formulating a balance between strength and porosity by multiple test iterations is deemed essential for the design of PC. The general range of properties for PC material is tabulated in Table 1.

Mix Proportions for Pervious Concrete Continuous research is underway across different parts of the world to develop mix proportions and compaction procedures to achieve a balance between structural and hydrological parameters of PC. Some typical mix proportions are given in Table 2.

Construction of Pervious Concrete Pavements The construction of pervious concrete pavements (PCPs) is usually accomplished in the following sequence: • Site reconnaissance • Preliminary soil investigations • Selection of soil stabilizers (if required) for weak subgrade • Excavation

The porosity of PC varies in the range of 15-35%. PC, by virtue of its high porosity, allows water to percolate through it at a considerably high rate, thus, eliminating the problems of flash flooding, surface runoff, and waterlogging. The intergranular porous matrix of PC lets the infiltration of water through it and contributes to groundwater recharge [6]. Also, the characteristic rough surface texture provides good resistance to skidding [7]. As the water is allowed to pass through a number of interconnected

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monitored as absorption or supply of excessive moisture by aggregates may significantly affect the strength and pore properties. A nearly zero slump should be maintained. The equipment used for mixing of PC is similar to that of conventional concrete. The mixing time should be such that a homogeneous PC mix is developed. Due to open-graded nature of PC, it loses moisture content very rapidly; hence, the ideal situation is to perform mixing in-situ in a drum mixer. However, if the place of mixing is away from the construction site, the mix may be transported via transit trucks (for faster rate of discharge) and fully dispensed within an hour after initial mixing. Admixtures or retarders can be used to account for increased haul time [5, 6, 15]. Figure 3 shows the mixing (in drum type mixer) and transportation of PC at site.

Placement, Consolidation, Joint Preparation and Finishing

Figure 2: Steps involved in construction of subgrade: (a) Excavation (b) Compaction of subgrade (c) Spreading of granular sub-base material (d) Compaction of sub-base

• Compaction of subgrade • Laying and compaction of granular subbase (GSB) or alternative base course using sand • Preparation of formwork to cast PC slabs • Batching, mixing, and transportation • Laying and spreading of fresh PC mixture • Compaction, joint preparation, and finishing • Curing

Construction of subgrade and sub-base The first and most important step in the construction of subgrade for PCP is to ensure that the soil is free from plastic fines and that it possesses a uniform character, preferably sandy-gravel in nature. The subgrade is compacted with a static steel wheel roller to achieve a compaction level of 90-95% of theoretical density. Nonwoven geotextile filter fabric can be used to impart additional strength to PCP and further prevent the fines from rising up in the pavement.

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The sub-base material must be clean and free from dust and fines. The thickness of sub-base usually varies between 75 and 250 mm depending upon the nature of traffic flow and water detention requirements. The gradation given in IRC:58 can be adopted as a reference to meet the drainage requirements of subbase layer. However, the most commonly used aggregate size is 19.5 mm. The porosity of granular sub-base generally varies from 20-40% [5, 6, 15]. The different steps involved in the construction of subgrade and sub-base are depicted in Figure 2.

Before placing the PC mix, water should be sprinkled over the sub-base to prevent loss of moisture. It should be ensured that the formwork is free from irregularities or misalignment. Since PC dries at a very fast rate, water should be sprinkled continuously over the surface during spreading and before consolidation. The PC mix should be stroked off approximately one inch above the formwork to allow for

Batching, blending and hauling of PC This is the stage for the preparation of PC mix. The aggregate gradation should be carefully designed as aggregates are major contributors for the development of design strength and permeability, if blended with right proportions of cement and water. The water content should be cautiously

Figure 3: Mixing and hauling using dolly at site



joints can also be provided in PCPs. A roller joint tool as shown in Figure 5, also referred to as pizza tool cutter, is commonly used to construct joints in PCP [5, 6].

Curing

Figure 4: Placement and Vibration of PC Field Slabs (a) Placement and Spreading of PC (b) Vibratory Consolidation of PCP

consolidation. Traditionally, PC mix is compacted by single or multiple passes with rollers. The drawbacks associated with roller compaction are surface tears and heaping of material in front of the roller. However, with recent advancements in construction technologies, vibratory rollers are used to consolidate PC, which results in a rough and flat surface, which is ready for use after curing. Vibrations for a period of 60 to 90 s are enough to consolidate the PC mix. Excessive vibrations may result in choking of voids, thus, vibratory compaction should be carefully monitored. The PC mix near the edges of the formwork should be compacted with steel tamps to

prevent raveling. Longitudinal rolling gives better compaction than cross rolling. The consolidation process should be completed within 15-30 minutes after placement of the PC mix. It is not desirable to provide curves in PCP, therefore, triangular slabs should be constructed at junctions/ intersections where change of horizontal alignment is required [5, 6, 15]. Figure 4 depicts steps involved in the placement and consolidation of PC. Joints should be installed in PCP slab soon after consolidation. As controlled crack development is not a consideration in PC, there are no specific recommendations on joint spacing and depth. Full depth isolation

Figure 5: Joint Preparation in PC using Pervious Joint Cutter (Bunyan Industries) [16]

Due to open graded nature of PC, the loss of moisture is very fast, so, water is sprinkled continuously over the PC mix while laying, spreading, and compacting. After consolidation, curing compounds can be used to control the rate of evaporation and maintain adequate moisture levels within the PC. Further, the surface is covered with plastic sheets and water should be sprayed continuously, preferably by fog misting. The use of fine matter such as sand and dust to hold the plastic sheets in place should be avoided, as they may clog the voids, if spilled over the surface. Bricks and stakes can be used to hold the plastic sheet in position. Curing should begin within 2030 minutes after slab placement and joint construction. The minimum recommended period for curing before opening the pavement to traffic is seven days. However, 28 days curing is still a better choice [5, 6, 15]. Figure 6 shows a PCP system covered with plastic sheets and wet gunny bags for curing purposes. The layout of a typical PCP system where low to moderate traffic is expected is shown in Figure 7.

Figure 6: Plastic sheets and wet gunny bags over a PCP, Tirupati Municipal Corporation

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Development, 32(1), pp.23-51 3. Qin, Y., and Hiller, J.E. (2014). Understanding pavement surface energy balance and its implications on cool pavement development. Energy and Buildings, 85, pp.389-399 4. Chandrappa, A. K., and Biligiri, K. P. (2016). Characterization of pervious concrete fundamental properties, 12th Transportation Planning and Implementation Methodologies for Developing Countries, Indian Institute of Technology Bombay, India, 19-21 December 2016 (Conference Proceedings) 5. ACI 522R-10, Report on Pervious Concrete, ACI Committee 522, American Concrete Institute, Farmington Hills, USA

Figure 7: Schematic of multi-layered PCP system

PCP Demonstration Test Sections: Smart City Projects in India A recent advancement in the construction of PCP is the successful completion of two pilot demonstration projects in India. The first initiative (March-June 2017) was the construction of PCP test sections on-campus IIT Kharagpur. Six PC mixtures with three slabs per mixture type were constructed with each having dimensions of 3m (length) x 2m (width) x 0.15m (thickness). Since the PC slab was expected to serve only two-wheelers or pedestrians, a 75mm sand layer was adopted as the base course material. The second study (March-May 2018) involved construction of a 125m long PCP parking lot on the campus of the Municipal Corporation of Tirupati. The pilot study was part of the Smart City initiative under the infrastructure thematic area proposed by Government of India. Over 20 PC slabs of dimensions 4m (length) × 4m (width) x 0.15m (thickness) were designed and constructed using a single PC mixture. As the parking lot was expected to carry light to moderate traffic loads with occasional heavy vehicular loads, a 0.25m thick granular sub-base layer was adopted as the base course layer. It is noteworthy that both the pilot sections have been performing well till date with no maintenance whatsoever. Since construction, several field experiments were performed on the sections, including, infiltration rates that have recorded permeability values anywhere between 0.12 and 2cm/s. Since the sections have withstood at least one monsoon season,

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it is anticipated that these sections would sustain the adverse climatic conditions through the future.

Limitations of PCP • Due to its open graded matrix, it suffers from low strength, and is thus applicable in low-medium volume roads, parking lots, sideways, medians, and shoulders • Special skill is required during construction of this special pavement system • Periodic maintenance is essential to ensure that the voids are not clogged due to dust and/or debris, and to ensure the desired rate of percolation of stormwater through the multi-layered PCP system.

Discussion

PCP is a sustainable roadway system that contributes to green infrastructure by allowing water to percolate through it and enhancing groundwater recharge. It further eliminates problems of flash flooding, surface runoff, and waterlogging. PCP has higher sound absorption capacity than conventional pavements, and it minimizes UHI phenomenon. All of these factors make PCP a promising alternative to conventional impervious pavements as it addresses the various challenges of urbanization related to environmental sustainability.

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