Clog-Resistant Permeable Pavement: Methods of Performance Measurement

Permeable pavements are considered to be one of the effective measures for LID (i.e. Low Impact Development) and SUDS (i.e. Sustainable Urban Drainage System) globally. The major three types of permeable pavements are (a) pervious concrete (PC), (b) previous asphalt (PA), and (c) permeable interlocked concrete paver (PICP). The performance of all types of permeable is satisfactory at the start of the service life but greatly reduced if not maintained on time. Considering the long lengths of the pavements, measurement of permeability at different locations is a cumbersome and tedious task. Therefore, ease of performance and realistic results are of vital importance. In this study, ASTM C1701/C1701M—17a single ring infiltrometer, Stormwater Infiltration Field Test (SWIFT), and NCAT permeameter Methods are discussed in detail and examined for their relative ease of setting-up, versatility and performance. It is concluded that ASTM C1701/C1701M—17a single ring infiltrometer is the most versatile, easy to set up and produces accurate results.


Introduction
A properly designed road will ensure effective drainage of rainwater ensuring prolonged service life and effective service to road users. 1 Such roads allow quick removal of water from pavement surface minimizing loss of bitumen from bituminous roads.3] To achieve effective and low-cost sustainable urban drainage system enabling quick removal of storm water which is economical also is a very important question and need of the hour.Permeable pavements are considered to be one of the effective measures for LID (i.e.Low Impact Development) and SUDS (i.e.5] A Permeable pavement temporarily stores runoff water into its porous layers and later releases it at a controlled rate thus reducing and delaying the peak of runoff discharge.5][6] There are three types of permeable pavement systems categorised based on the construct of its top layer namely (a) pervious concrete (PC), (b) previous asphalt (PA), and (c) permeable interlocked concrete paver (PICP). 6A sectional profile of PC, PA and PICP pavements are shown in Figure 1(a), 1(b) and 1(c).The Wet Mix Macadam (WMM) layer is acting as a load dispersion layer and Granular Sub Base (GSB) layer is acting as water storage media due to its high porosity. 6MoRTH (Ministry of Road Transport and Highways), Government of India recommends for use of GSB Grade-III and in the USA use of AASHTO, No 2 aggregate is recommended for the Sub-base layer for permeable pavements.
All permeable pavements (i.e.PC, PA and PICP) exhibit high permeability in their first year of service and rapid reduction is observed in the following years. 6The prime reason for the reduction in permeability is clogging pores of the topmost layer of the pavement due to the accumulation of fines such as road dust etc.The rate of permeability reduction is very high and in a few cases reached near zero in as early as 3 years of service. 7esearchers had worked to control the clogging phenomenon and suggested regular cleaning to rejuvenate the permeable pavement performance.Several cleaning techniques were assessed, and pressure water-washing with vacuuming was found to be the most effective alternative but warrants high maintenance costs.Measurement of the rate of infiltration of the permeable pavement surface is the most reliable monitoring technique for the assessment of permeable pavement performance.
To maintain permeable pavement effectively, periodic assessment of pavement performance is essential.Due to the longer lengths and variable usage conditions, several observations are required to assess actual pavement performance and identify such sections of the road which require maintenance.The fact that the road is continuously being used and temporarily shut down for testing causes problems for the users.Therefore, it is essential to choose a testing method which is quick to set up & execute and requires minimal equipment for easy transportation.There are three most prominent road permeability measurement methods (single ring infiltrometer method, NCAT method and SWIFT method) available and this study aims to explore the relative effectiveness and versatility of existing methods of permeability measurement to avoid/reduce the chances of clogging permeable pavement by taking timely corrective actions.
Clog-Resistant Permeable Pavement Technology Porous Concrete (PC) Permeable Pavement Porous concrete has three types of pours for the effective permeability of the concrete (i) Effective pours, (ii) Semi effective pours and (iii) non-effective pours.The effective pours are those having considerable interconnected pore areas with low tortuosity.9] It is generally considered that the high porosity of concrete results in better permeability but this is not entirely true.Only interconnected pores contribute to the permeability of concrete.This is obvious that the higher the porosity better the chances of having (i) & (ii) type of pours.Moreover, especially type (ii) pours tend to choke much faster due to the high torture path10.The total porosity is inversely proportional to the compressive strength of concrete; therefore, several efforts have been made by researchers by analyzing the combination of different aggregate sizes, w/c ratios and cement content to reduce type (iii) pours as compared to type (i) and (ii), but there is no established method to achieve it. 11rous Asphalt (PA) Permeable Pavement Similar to PC, Porous Asphalt (PA) have an effective, semi-effective and non-effective pour system.The significant difference is due to the binder (asphalt) and particle size distribution.12 Since asphalt has adhesion properties and a relatively low softening point, it tends to adhere to fine particles and fill the pours with 600 µm and 1.18 mm aperture.Studies established that in all the PA and PC mixes with different porosity, the PA mix always exhibits lesser permeability than the PC mix of the same porosity showing that the ratio of non-effective to total pours is higher than in PC mixes.The clogging behaviour of PA is similar to PC and permeability reduction with an increase in the age of pavement for both the mixes follows similar trends.[13][14] The Maintenance and rejuvenation techniques for permeability of permeable pavements like regular vacuuming, pressure washing and vacuuming with pressure washing are the most acceptable.15 But methods for designing clog-resistance Porous Asphalt permeable pavement are not available.

Permeable Interlocking Concrete Pavers (PICP) for Permeable Pavement
A PICP pavement is a positive step towards developing clog-resistance permeable pavement.A Permeable pavement consisting of a top layer made up of interlocking concrete blocks utilizes the joint gap to transmit the water.This gap is filled with fine aggregate to lock the concrete blocks in their place and allow water passage. 16But clogging of joint filler material causes a reduction in permeability and ultimately leads to complete choking with fines.The PICP with vertical perforations is tested for its permeability by subjecting the test specimen to several dry and wet clogging cycles.The test results show that low tortuosity (=1) perforations of 5-6mm diameter did not clog and maintained their permeability for pavement samples with 5% and above porosity. 17However, the performance of such PICP with bedding layers has not been tested.The large aperture of paver blocks (Joints gaps) helps in prolonging the complete choking of interlocking concrete block pavements but a reduction in permeability is inevitable.Furthermore, a high-pressurized air and vacuum system are found to be the most effective maintenance method for permeability restoration. 18

Performance Assessment Methods of Permeable Pavement
The runoff water accumulating on the top layer of permeable pavements must be allowed to pass through and infiltrate into WMM and GSB layers to temporarily store in the pours.This stored water eventually exfiltrates into neighbouring natural soil strata.It is essential to assess that permeable pavement is working and efficiently infiltrates the surface water, the surface infiltration test has to be performed periodically at a different location on the site.The results of the tests govern the maintenance requirements of permeable pavement.This method is developed to keep simplicity in mind and requires very little instrumentation.The test can be performed using a 20L bucket with a 40mm diameter hole in the centre of the bottom.This hole is fitted with a plug attached to a string to prevent water leakage when plug-in and easy release of water when needed.The test setup is described in Figure 3.

The Infiltration Rate Measurement of Pervious
The S.W.I.F.T. method 6 works on the basic principle of 'higher the filtration rate, better the condition of the pavement, lesser the number of wetted blocks with the same amount of water'.SWIFT method realizes on counting the wetted blocks thus no prewetting is required.If repeat tests are needed at the exact same section of the pavement tested earlier a minimum of 24hr gap should be provided to let it dry.The test steps are as follows Step (i) Locate any paver block from the test surface and place a 20L bucket in such a way that the drainage hole centres the chosen block.
Step (ii) Plug-in the drainage hole tightly and pour 6L of water into the bucket.
Step (iii) By pulling the string, remove plug and let all water drain.
Step (iv) Remove the device and count only that blocks which have a completely wet surface.Record the numbers.A photograph of the wetted surface may be recorded for further analysis.v) Using Table 1, the condition of the PICP pavement may be inferred

NCAT Permeameter
In the year 1999, Allen L. Cooley, 19 NCAT (The National Center for Asphalt Technology), USA developed a field test method to determine the permeability of porous pavements.The working principle of this permeameter is falling-head  Only 5.75-inch diameter tube may be utilized to measure permeability of highly permeable surfaces and smaller diameter tubes are useful in case of lower permeability surface measurements.
Base on the apparent permeability, appropriate tube combinations were selected and using plumber's putty instrument is mounted on test surface in such a way that no lateral leakage is observed.Water is poured in the graduated tube/s from the top till tubes are completely filled.Elapsed time is recorded using a stopwatch for the level of water to fall from each tier of permeameter.
The recorded time to drop one tier is to be used to measure the permeability coefficient (K), which is also described as the hydraulic conductivity of the material in the saturated state.Saturated Permeability (K s ) is calculated using following equation where: K s → Hydraulic conductivity (Saturated) in cm/sec a → Internal area of the cylinder, cm-square l → Thickness of pavement, cm A → Sectional area of the teat pavement, cm 2 t → Time elapsed (average) by water to fall (t 1 -t 2 ), sec h 1 → Head measured at time t 1 (in cm) h 2 → Head measured at time t 2 (in cm) While performing a permeability test using an NCAT permeameter, one of the critical factors is to develop a no-leak seal between the base plate and the test surface.Although, plumber's putty is widely used, but new silicon rubber-based sealant products are available in the market which ensures an even better seal and prevent any leakages. 20A sufficient quantity of silicon rubber-based sealant is placed along the periphery of the vertical tube at the base in an inverted position and leave it for 2-3min.Turn the permeameter upright and carefully mount it on the surface under test.Secure the base plate with weights provided by placing them on the base plate.Before the commencement of the test, ensure that there is no lateral movement in the apparatus by pushing it with a little lateral force at the base plate.

Methodology and Test Results
To achieve the objectives of the current study the following methodology was adopted.
To perform the test 10 sections on each type of road is identified.All the three apparatus were installed, perform the test and removal with disassembling had been done.Table 2 contained the data collected.

Conclusion
Based on the effectiveness and versatility of the permeability measurement methods described above it may be concluded as i.ASTM C1701/C1701M-17a single ring infiltrometer method may be used for porous concrete, porous asphalt and PIC pavements' performance assessment by measuring its' permeability.ii.

Stormwater Infiltration Field Test (SWIFT)
Method is only suitable for ICB pavements' performance assessment.iii.
NCAT permeameter Method may be used for porous concrete and porous asphalt pavements' performance assessment by measuring its' permeability.The use of this method on PCIP pavement requires trained personnel since achieving a proper seal poses a challenge.iv.Time required to run the test with SRI and SWIFT is almost identical.v.
Permeability measurement test using NCAT permeameter requires almost triple the time as compared to SRI or SWIFT.

Fig. 1 :
Fig. 1: A sectional profile of PC, PA and PICP pavement types with different layers.

Fig. 2 :
Fig. 2: Single ring infiltrometer method (a) animated 3D view (b) Plan and (c) sectional elevation of the setup

Table 1 : SWFIT test results and Condition of PICP pavement
of water column and is widely used for permeability assessment of OGFC (Open Graded Friction Course) and pervious asphalt (PA) pavement.An animated 3D view of NCAT permeameter is presented below in Figure-4.