Sampietrini Stone Pavements: Distress Analysis Using Pavement Condition Index Method

Abstract

In several Italian cities, it is possible to find historical pavements such as the Sampietrini pavements, which are mainly located in the center of the city of Rome. The Sampietrini pavement is a particular road surface paved in natural stone with irregular sharp elements that are assembled by hand with the evident not plan effect. Because of their peculiarities, they are not suitable for streets where high speed is allowed. In many cases, high vibration and noise levels due to road traffic traveling on Sampietrini pavements are caused by inadequate maintenance, which is also affected by the absence of specific evaluation criteria regarding surface conditions and performances of Sampietrini pavements. It is not possible, in fact, to adopt common approaches developed to be used for flexible and rigid pavements, because they present completely different features and distresses. In this paper, to overpass this problem, a new evaluation criterion based on Pavement Condition Index (PCI) method established for block pavements is proposed. Furthermore, to fully characterize this kind of pavements, other analyses, i.e., International Roughness Index (IRI) and comfort level evaluation based on ISO 2631 standard, were also carried out. The results showed a good correlation between PCI and IRI approaches (R² = 0.82), also highlighting that new or reconstructed Sampietrini pavements present not negligible roughness level. This aspect was also confirmed estimating the comfort level perceived by users traveling at several speeds (≤50 km/h). Finally, speed related threshold values to be adopted for PCI and IRI methods are proposed. The proposed method can be implemented by pavement managers in a PMS ad hoc for stone block paving and thus, it can be integrated with other equivalents methods of visual inspection based on PCI.

1. Introduction

In order to guarantee the proper maintenance of a road network, it is very important to monitor the conditions of road pavements along their service life. In this sense, the development of an adequate Pavement Management System (PMS) is a very useful tool for road agencies, identifying appropriate intervention thresholds and maintenance strategies for the restoration of the optimal performances, in terms of grip, bearing and roughness levels. [1,2,3,4].

Most PMSs, actually adopted, are related to major roads and airport infrastructures [5] but, nowadays, considering the lack of budget available to administrations, a general attention to the applications of PMS to urban areas is paid [5,6,7,8]. Many PMSs include both visual and automatic surveys in their procedure, adopting several indices for the evaluation of road pavement performances. Among these, two of the most commonly used ones are the Pavement Condition Index (PCI) elaborated by Shahin [2] and the International Roughness Index (IRI), which was elaborated from a World Bank study in the 1980s [9]. The first one provides a global assessment of pavements condition surveying several distress categories. In particular, it was designed for the assessment of distress related to rigid and flexible pavements [10], which are the most common types that can be found in a generic road network. IRI, instead, is used worldwide as road roughness evaluation criterion and its calculation requires the measurement of road profiles using specific devices, also known as profilometers [11].

In recent studies [12,13], the correlation between IRI and PCI was investigated, with particular attention to urban roads. In addition, based on IRI and distresses relation, some IRI threshold values to be used in urban areas are proposed in [14].

In urban networks, however, different kinds of pavement can be found, such as block or modular pavements, which are defined as pavements composed of pre-formed modular pavers of brick and concrete, which, generally, have been successfully used worldwide for low volume roads and pedestrian areas [15,16]. Considering the peculiar characteristics of these types of pavements, it is not possible to extend to them distresses and evenness evaluation methods designed for rigid and flexible pavements. In particular, starting from the PCI method described in [2,10], some authors developed specific guidelines to survey and evaluate distresses affecting block pavements [17,18,19].

In several Italian cities, it is possible to find historical pavements such as the Sampietrini pavements [20], which share similar features with modular block pavements. This kind of pavement is especially present in the center of Rome and, because of its peculiar characteristics, it is not possible to apply consolidated survey methods or threshold values commonly adopted for flexible and rigid pavements.

For this reason, a specific case of study related to the Sampietrini block pavement was performed in this paper, modifying the PCI method related to block pavement in order to be suitable for this kind of pavement. Furthermore, for each section the sample surface evenness evaluation was carried out calculating the corresponding IRI value, investigating also possible correlations between the proposed PCI method and IRI. In addition, some considerations concerning users’ comfort perception traveling at different speeds (<50 km/h) along this type of pavement were performed using the frequency-weighted vertical acceleration (a_wz) approach described in [21,22,23].

2. Methodology and Data

2.1. Sampietrini Pavements

In several Italian cities, it is possible to find historical pavements such as the Sampietrini pavements, which are especially present in the center of the city of Rome. The first documented use of Sampietrini stones in Rome was during the reign of Pope Pius V (1566–1572). The name derives from their first use in St. Peter’s Square and later, over the next centuries, the stones were used to pave all the main streets of Rome, providing a smoother and stronger surface for carriages than bricks. This kind of pavement is made of beveled stones of black basalt, also simply called Sampietrini, placed one next to the other. The shape of each block (also called sampietrino) can be approximate by a cubic or a square-based truncated pyramid solid. Sampietrini blocks of different dimensions can be used. The largest ones are characterized by a square head of 12 cm × 12 cm and height of approximately 18 cm; however, the common height is equal to 6 cm. The smallest and rare ones, which are located in specific areas of Rome such as Navona Square, present a square head of 6 cm × 6 cm. In Figure 1, a typical cross section of this type of pavements is represented, while Figure 2 presents the two main geometric patterns which can generally be realized using Sampietrini blocks. The mechanical properties of Sampietrini material are reported in

Table 1. Mechanical properties of basalt used for Sampietrini.

Material Property	Value
Young’s modulus (GPa)	37–60
Poisson ratio (-)	0.15–0.38
Compressive resistance (MPa)	241–320
Flexural resistance (MPa)	32–75
Tensile resistance (MPa)	18–20

and, as shown by some studies performed by Penta in the 1950s, the performance of the Sampietrini pavements depends on the resistance of each block but also on the tile pattern realized, which meaningfully affects the global behavior of this kind of pavement [20].

Sampietrini pavement presents some advantages; in fact, it does not completely cover the ground, leaving small spaces for the water to pass through and can easily adapt to the irregularities of the ground. Furthermore, being of volcanic basalt, Sampietrini are very strong and resistant.

The main problem associated with Sampietrini blocks concerns that they get very slippery when they are wet, being a hazard above all for two-wheeled vehicles moving in the city. In addition, due to their irregular shape, traveling on this kind of pavement is very uncomfortable and noisy; moreover, heavy vehicles passing on it may cause wide vibrations that can damage the surrounding buildings, with particular attention to historical ones.

Because of their peculiarities, Sampietrini pavements are not suitable for streets where high speed is allowed, thus, nowadays, Sampietrini were replaced in many roads of Rome but they are still used in slow traffic areas (speed ≤ 50 km/h), such as the center of Rome such as Trastevere (Figure 3).

Although the percentage of the urban road network characterized by Sampietrini pavement is pretty low (about 2%), it presents a whole extension equal to about 100 km, which cannot be neglected in an appropriate and optimized PMS. Furthermore, the aforementioned pavements not only constitute an important historic heritage of the city of Rome, but they are also located in the most visited and prestigious areas of Rome, as can be seen in Figure 3.

In addition to the urban road network of 100 km, several other roads in Sampietrini are located within the Lazio region and in other towns in Italy, whose extension and the accurate location is difficult to know with sufficient precision.

The construction of this kind of pavement is very complex (Figure 4) and it requires very skilled and specialized workers, which are very rare nowadays. In the past, in fact, the knowledge and the know-how were handed down from father to son.

For this reason, the costs to construct Sampietrini pavements are significantly greater than the costs required for asphalt concrete pavements. The estimated costs provided by the city of Rome [24], in fact, indicate a cost equal to about 200 €/m² for Sampietrini pavement and about 50 €/m² for asphalt concrete ones. This meaningful gap (ratio of 4:1) further motivates the need to pay particular attention to identifying distresses’ type and location and selecting the appropriate maintenance actions to fix them.

Sampietrini pavements need a lot of maintenance since the blocks are not fixed into the ground with cement or any other bonding agent, but are simply hammered into the sandbed.

In many cases, high vibration and noise levels due to road traffic traveling on Sampietrini pavements are caused by inadequate maintenance [25], which is also affected by the absence of specific evaluation criteria regarding surface conditions and performances of Sampietrini pavements. It is not possible, in fact, to adopt common approaches developed to be used for flexible and rigid pavements, because they present completely different features and distresses.

In this paper, in order to overpass this problem, a new evaluation criterion based on PCI method established for block pavements is proposed. In particular, 14 sections belonging to different low volume roads and located in the center of the city of Rome, were analyzed according to three different approaches: PCI, IRI and a_wz. In Figure 5 and Figure 6, two examples of the examined road sections are depicted.

For all sections, having a length of 80 m and width equal to 3 m (for a total pavement area of 240 m²), the two alignments (right and left) were measured using a contact profilometer (i.e., Dipstick).

2.2. Performed Analyses

In order to fully characterized Sampietrini stones pavements, three different approaches were considered: PCI for global distresses assessment, IRI for evenness evaluation and a_wz for ride quality estimation. The aim of the analyses performed using the two latter indices, is trying to define (or to estimate) the roughness level and the ride comfort related to new or reconstructed Sampietrini pavements. In the following sections, a brief description of each road evaluation method applied to the Sampietrini pavements is provided.

2.2.1. Pavement Condition Index (PCI)

The PCI is a numerical indicator, based on the inspection of different distress types affecting road pavement surface. For each distress, the corresponding severity and quantity are identified and recorded on specific survey data sheet reported in [10]. PCI rating scale (

Table 2. Standard PCI rating scale.

PCI	Verbal Rating
86–100	Good
71–85	Satisfactory
56–70	Fair
41–55	Poor
26–40	Very Poor
11–25	Serious
0–10	Failed

) varies from 0 (failed pavement) to 100 (perfect condition).

As already stated, this method was developed to be used for rigid and flexible pavements [2,10], even though some authors have proposed guidelines in order to apply PCI approach to concrete block pavements [18,19].

Considering the similarities between Sampietrini and concrete block pavements (i.e., they are both modular pavement, and each block presents homogeneous mechanical characteristics), a specific distresses catalogue for Sampietrini pavements was established starting from the one reported in [18]. In particular, threshold values related to the three severities (low, medium and high) were modified, taking into account the specific shape size of a single block of Sampietrini pavements.

Therefore, the distress catalogue proposed for Sampietrini pavements includes all the distresses (

Table 3. List of distresses proposed for Sampietrini pavements.

Distress ID.	Description
101	Damaged Sampietrini
102	Depressions
103	Edge restraint
104	Excessive joint width
105	Faulting
106	Heave
108	Joint sand loss/pumping
109	Missing Sampietrini
110	Patching
111	Rutting

) described in Appendix A, whose descriptions are mostly taken from [18]. Compared to distress catalogue presented in [18], horizontal creep distress (distress identified by 107) can be considered negligible for Sampietrini pavements.

2.2.2. International Roughness Index (IRI)

The International Roughness Index (IRI) is the worldwide used road roughness evaluation indicator, elaborated from a World Bank study in the 1980s. It is based on a mathematical model called quarter-car and developed in order to assess the ride quality on road pavements. The assessment is performed by a simulation model, calculating the suspension motion on a single profile and dividing the sum by the distance traveled according to Equation (1):

$$IRI=\frac{1}{l}\underset{0}{\overset{\frac{l}{v}}{{\displaystyle \int }}}\left|{\dot{z}}_s-{\dot{z}}_u\right|dt$$

(1)

where l is the length of the profile in km, v is the simulated speed equal to 80 km/h, ${\dot{z}}_s$ is the time derivative of vertical displacement of the sprung mass in meters and ${\dot{z}}_u$ is the time derivative of vertical displacement of the unsprung mass in meters. The result is the IRI value and it is expressed in slope units (e.g., m/km or mm/m).

In the present work, the IRI calculation was performed by means of a Matlab© code, where the algorithm provided by ASTM E1926 [26] standard is implemented. For each analyzed section, two alignments (right and left) were measured, calculating the corresponding IRI value for both of them. Finally, the mean value characterizing the whole section was obtained using Equation (2):

$$IR{I}_{section}=\frac{IR{I}_{left}+IR{I}_{right}}{2}$$

(2)

2.2.3. Frequency-Weighted Vertical Acceleration (a_wz)

In addition to IRI and PCI methods, a further analysis performed to fully characterize Sampietrini pavements was the ride quality evaluation carried out adopting the frequency-weighted vertical acceleration (a_wz) parameter, described in [27]. To determine the frequency-weighted vertical acceleration on users due to road roughness [28], several simulations at different speeds (10–50 km/h) were performed using the eight degrees of freedom (d.o.f.) full car model developed by Cantisani and Loprencipe [21] and calibrated in order to represent the behavior of a common passengers’ car.

Starting from the vertical accelerations calculated by this model traveling along the road profiles, it is possible to determine the root mean square (RMS) accelerations through the evaluation of the power spectral density (PSD). Acceleration PSD is then calculated in correspondence of the 23 one-third octaves bands, representative of the frequency range of interest for the human response to vibrations (0.5–80 Hz), as specified by ISO 2631 standard. The final value for a_wz index is then obtained using the following Equation (3):

$$a_{wz}=\sqrt{{\displaystyle \sum }_{i=1}^{23}{\left(W_{k,i}\times a_{iz}\right)}^2}$$

(3)

where W_k,i are the frequency weightings in one-third octaves bands for seated position, provided by the standard, and a_iz is the vertical root mean square (RMS) acceleration for the i-th one-third octave band.

Then, a_wz values can be compared with the threshold values proposed by ISO 2631 for public transport (

Table 4. Comfort levels related to a_wz threshold values proposed by ISO 2631 for public transport.

awz Values (m/s2)	Comfort Level
<0.315	Not uncomfortable
0.315–0.63	A little uncomfortable
0.5–1	Fairly uncomfortable
0.8–1.6	Uncomfortable
1.25–2.5	Very uncomfortable
>2	Extremely uncomfortable

) to estimate the corresponding comfort level perceived by users traveling along the examined road sections.

By means of this analysis, it is possible to estimate the maximum speed at which drivers can transit on new or reconstructed Sampietrini pavements that cannot present a perfectly smoothed surface. In this way, the chance of using this type of pavement for certain road categories in the urban network is also assessed.

3. Results and Discussions

PCI values calculated for the sections examined in this paper, present a good variability going from 12 to 94, although just one section has PCI <40, corresponding to very poor pavement condition or worst. More details about the distresses presented in each inspected section are reported in Appendix B. As can be seen in Figure 7, a good correlation between PCI and IRI was found (R² = 0.82). In particular, it can be noted that for good Sampietrini pavements (PCI > 86), IRI value varies between 6 and 8. These values, in case of rigid or flexible pavements, might be commonly associated to damaged pavement [11].

The peculiarity of IRI range values found for Sampietrini pavements is better underlined looking at Figure 8, where the relation between PCI and IRI for different types of pavement is depicted.

In particular, it can be seen that, according to the relations found by Arhin et al. [12], regardless of the type of pavements, IRI values are always lower than 10 m/km, even for PCI close to 0. Considering the relation found by Park et al. [29] for asphalt pavements, instead, for PCI values lower than 40 (corresponding to pavements condition from very poor to failed, see Table 2) very high values of IRI are found.

In any case, it is clear that different IRI values must be associated to new or reconstructed Sampietrini pavements, compared to the most common types of pavement (flexible and rigid).

Starting from this result, it was decided to evaluate the ride quality perceived by road users traveling along Sampietrini pavements at different speeds. As already stated, taking into account that this type of pavement is present just in urban areas, velocity range between 10 and 50 km/h was considered. As can be seen in Figure 9, very weak correlations (R² = 0.44–0.70) were found between IRI and a_wz values at different speeds.

Similar results, in terms of correlation coefficient R², were obtained comparing PCI and a_wz approaches (Figure 10). In fact, as depicted in Figure 11, R² values at different speeds for IRI-a_wz and PCI-a_wz correlations are very close to each other.

Considering for example two sections having PCI >85 (corresponding to a pavement in good condition), it is possible to evaluate the comfort level induced on road users traveling at different velocities on new or reconstructed Sampietrini pavements. As can be seen in Figure 12, also moving at 50 km/h (maximum legal speed allowed on Italian urban roads) the worst comfort level perceived by drivers is equal to “fairly uncomfortable”. In the same figure, two sections characterized by PCI values between 41 and 55 (corresponding to poor pavement condition) are also reported. In this case, at the maximum allowable speed (i.e., 50 km/h), the comfort level was found to be very uncomfortable. Furthermore, levels equal to fairly uncomfortable/uncomfortable are already reached at 30 km/h.

The aforementioned results show the chance of using Sampietrini pavements on the urban road network, allowing a little level of discomfort on users. In order to prevent an excessive decay of level of service (in terms of comfort), it would be appropriate to adopt comfort and speed related threshold values for IRI and PCI approaches, as also proposed in [30].

Using the correlations previously found, for example, it is possible to determine IRI and PCI limits, respectively, depicted in Figure 13 and Figure 14.

As already stated, weak correlations were found between the two aforementioned indices and a_wz. For this reason, it would be necessary to increase the sample size to better investigate these correlations, eventually improving them.

4. Conclusions

In this paper, several road pavement evaluation methods were applied to 14 sections of historical Sampietrini pavements: PCI, IRI, and a_wz. In particular, starting from the PCI method established for block pavements, specific severity threshold values for all possible distress types characterizing Sampietrini pavements were proposed.

A good correlation factor (R² = 0.82) was found between PCI and IRI values calculated for the aforementioned sections. In addition, considering that PCI values greater than 85 correspond to pavements in excellent conditions, it was confirmed that new or reconstructed Sampietrini pavements are characterized by not negligible roughness level (IRI = 6–8 m/km). In fact, they cannot be used for high-speed roads (>50 km/h).

For a Sampietrini pavement in good condition (PCI = 88–92, good), the values of a_wz calculated at velocities within the range from 10 to 50 km/h showed that, for speed equal to 40 and 50 km/h, ride quality might be little or fairly uncomfortable. When the pavement conditions get worse (PCI = 46–54, poor), ride comfort also decreases, arriving at a very uncomfortable level at 50 km/h.

Although weak correlations were found between PCI-a_wz and IRI-a_wz, to develop an appropriate Sampietrini management system based on the ride quality perceived by road users, speed related PCI and IRI thresholds are also proposed.

In this sense, wider and extensive in situ measurements should be performed to improve the accuracy and the efficiency of the proposed approach for the surface conditions assessment of Sampietrini pavements.