Evaluating Life-Cycle Unit Costs of Traditional Cement Concrete and New Polymer Concrete Manholes in Wastewater Systems

Abstract

Wastewater manholes are crucial infrastructure components in sewage systems. They provide necessary access points for inspection and maintenance. However, limited studies were conducted on the life-cycle cost analysis of manholes. The primary objective of this study is to compute and compare the Life-Cycle Unit Cost (LCUC) of cement concrete and polymer concrete manholes to identify a cost-effective alternative for public agencies. To achieve the objective, this study analyzed commonly used 1.83 m diameter manhole data; 343 cement concrete manhole and 88 polymer concrete manhole cost data were collected from the Clark County Water Reclamation District (CCWRD), Las Vegas, Nevada, United States. The results show that the initial costs of polymer concrete are higher than those of traditional cement concrete. Statistical tests were conducted to determine the group differences. The findings show that the LCUCs of polymer concrete manholes are significantly cost-effective when compared to traditional cement concrete manholes. Public agencies can utilize polymer concrete manholes to save costs in future water and wastewater pipeline manhole construction projects.

1. Introduction

Manholes are an important component of wastewater sewer systems since they provide direct access to the sewer system [1]. While performing a condition assessment, professionals identify the system’s current state and whether it is time for maintenance and repairs or replacement to keep the system safe. A manhole is often located where two or more sewer lines converge or where the inclination or direction of a sewer line changes. There are about 20 million manholes in the United States. Out of that, about four million are older than 50 years, and another five million are 30 to 50 years old [2]. According to the 2021 Infrastructure Report Card [3], the average grade for America’s wastewater system is D+, which is considered a “Poor” condition.

Cement concrete is the most widely used material to construct manholes and sewer pipes [4]. Studies show that cement concrete manholes are highly susceptible to chemical corrosion due to Hydrogen Sulfide (H₂S) generated in the sewer systems [5,6,7]. Due to such corrosion issues, concrete structures are expensive to repair and have a history of unpredictable maintenance and rehabilitation needs [8]. The condition of the manholes is regularly evaluated through inspection. A lower inspection frequency of 1–2 years is advised for manholes with corrosion and other maintenance issues, while a maximum inspection frequency of 10–15 years is utilized for typical manhole conditions [5]. Deteriorated manhole structures are maintained using a variety of rehabilitation techniques, including cementitious restoration, cast-in-place concrete restoration, polymer coatings/linings, epoxy, polyurethane, cured-in-place composites, chemical grouting, various spot repairs, cast-in-place structural relining, corrosion protection from hydrogen sulfide, cured-in-place lining, panel liners, and mechanical seals [5,9]. The cement concrete manholes are also protected against sulfide corrosion by various methods, such as chemical sulfide generation control, designed to prevent sulfide formation; calcareous aggregate; protective coating; and a variety of polymeric materials [10]. Due to corrosion issues, rehabilitation works are carried out frequently, but such works are expensive and increase the life-cycle cost of manholes. Studies indicate that various attempts were made to protect concrete from direct acid (sulfuric acid) corrosion, but they have been unsuccessful for the last 70 years [11,12,13,14]. Over the next 20 years, a study finding has shown that much more investment in manholes will be required [15]. Maintenance and rehabilitation works will increase manhole maintenance costs and cause public inconvenience due to road closures [9].

Due to chemical impacts, the cement concrete manholes have a shorter lifespan. Polymer concrete uses a combination of aggregates as a mineral filler and resins as a binding agent, such as saturated polyester, vinyl ester, epoxies, and methacrylates [16]. Another study conducted in 2013 shows that polymer concrete is more resistant to chemicals and more durable compared to cement concrete. However, the type of resins employed affects their properties [16,17].

Although polymer concrete is expensive [18], it has a high level of chemical resistance [19], making it a preferred material for underground facilities [19,20]. In recent decades, polymer concrete manholes have replaced some of the cement concrete manholes in a few southern cities in the United States, such as downtown Austin, TX [21,22,23]. The polymer concrete manhole companies provide 50 years of warranty on their products [19]. Unlike cement concrete manholes, polymer concrete manholes do not require maintenance activities [19,24,25].

Polymer concrete manholes have higher initial costs, limiting their use despite their durability. Some studies have compared the life-cycle cost of different alternatives, such as Jung et al. [26], who have computed and compared the life-cycle costs of concrete and asphalt pavement overlays in Korea. Another study comparing life-cycle costs of long-span and cable-stayed bridges in China [27,28] has also used life-cycle cost analysis to determine cost-effective options in coastal concrete bridge maintenance. The primary goal of this paper is to compare the cost-effectiveness of utilizing cement concrete and polymer concrete manholes. It intends to help public agencies choose the most cost-effective manhole for their future water and wastewater projects.

2. Review Extant Publications on the Topic

The review of the existing literature has been divided into four major sections. The first covers the literature on corrosion resistance studies. The second covers the cost-effectiveness of studies related to sewage and drainage pipes, followed by the comparison of cement concrete and polymer concrete manholes’ specifications. The final section highlights the innovative materials and construction techniques used to install manholes.

2.1. Corrosion Resistance Studies

The American Society of Civil Engineers (ASCE) conducted a study on sulfide in wastewater collection and treatment systems [10]. The primary components of cement concrete manholes are Portland cement and steel reinforcing, and both are susceptible to chemical corrosion brought on by sulfuric acid. There are two types of aggregates: granitic aggregate (from granite rock) and calcareous aggregate (commonly from sedimentary rock). A concrete manhole structure’s rate of deterioration is substantially impacted by the aggregate used during construction. Due to the inert nature of granitic aggregates, they are not affected by sulfuric acid, while the calcareous aggregates are susceptible to chemical attacks. Only the cement bonding components are affected by acid when using granitic aggregates. This causes the cement material to corrode deeper into the structure wall, speeding up the corrosion process and separating the aggregate from the manhole structure wall, which exposes the fresh cementitious material. However, when utilizing calcareous aggregate, both the cement and aggregate experience acid damage. As a result, the corrosion resistance of the cement concrete manholes increases.

The quantity of sulfide produced in the sewer has an impact on the corrosion rate as well [5]. The USEPA study [6] shows that factors contributing to an increase in sulfide production are high sewage temperatures, high biochemical oxygen demand sewage, a release of dissolved oxygen, flat sewer slopes with lower velocities, prolonged detention times, increasing settlement of organic solids and grit in the sewer invert, and surcharging gravity sewers. Other studies show that acidic industrial waste causes corrosion in traditional concrete sewer systems [5,6,7,10]. The development of sulfuric acid, which also influences the rate of corrosion, is prevented by both the absence of moisture on the structure’s walls and the presence of oxygen in the sewer atmosphere [10]. Additionally, one of the reasons contributing to manhole structural deterioration is the weather. Due to the freezing conditions in colder regions, concrete structures develop cracks that allow sulfuric acid to penetrate deeply and speed up deterioration.

Some studies were carried out to evaluate the condition of concrete manholes [1,5,29], [30]. A similar study forecasts the state of manhole conditions based on prediction models [31,32] assesses manhole conditions based on Hierarchal Evidential Reasoning (HER), fuzzy expert system, and Analytic Network Process (ANP). To determine the overall state of the manhole, this study considers five distinct scales: excellent, good, fair, bad, and critical.

A study was conducted to enhance the mortar and concrete that use polymers to increase sewer pipe durability [33]. Eight types of polymer emulsion were used to modify a polymer concrete, including the curing circumstances, polymer-cement ratio, and the various types of polymer emulsion used. For most modification alternatives, when the porosity decreases, the flexural strength of the polymer-modified mortar increases. The standard water curing for 2-days and 5-days showed to have higher compressive strengths. Additionally, research on the effects of 0.5 percent sulfuric acid solution on the samples revealed that polymer-modified concrete exhibits significantly lower rates of sulfate corrosion than unmodified concrete.

Another study has investigated whether the physical characteristics of polymer concrete are suitable for manholes [34]. The physical characteristics examined were specific gravity, absorption capacity, workability, strength, elastic modulus, and Poisson’s ratio. The mean absorption capacity, found to be 0.39 percent, is considerably less than that of cement concrete. This suggests that the material’s waterproofing capabilities have significantly improved. Since polymer concrete does not leak like cement concrete does, its reduced absorption capacity makes it beneficial for usage in manhole constructions near subsurface water. The study findings also demonstrate that the flexural strength of polymer concrete is higher than that of traditional cement concrete. Additionally, the comparison of the stresses and strains of polymer concrete and cement concrete reveals that polymer concrete has a larger strain, indicating a higher degree of toughness, making it ideal for manhole structures. The researchers concluded that high-strength polymer concrete may be developed as needed for use in improved manholes.

In a study, 20 traditional manholes that had been corroded by hydrogen sulfide were replaced with polymer concrete manholes [25]. The polymer concrete manholes’ design allowed for connections between manholes and pipes of various sizes, shapes, and orientations. To fit inside the footprint of the existing manholes, the replaced manholes were built with a reduced outside dimension. Polymer concrete manholes provide long-term chemical resistance without the need for linings, coatings, or cathodic protection. The study also found that the installation was faster than cement concrete manholes. The replaced polymer concrete manholes are anticipated to last at least 50 years.

Studies show that cement concrete manholes in Austin, Texas, had serious corrosion issues [21,22,23]. Since rehabilitating the corroded manholes is difficult and expensive, Polyvinyl chloride liner and other protective coatings were utilized inside the cement concrete manholes. However, it was difficult to fix post-installation issues in the coating or lining. Thus, seven deep tunnel access manholes, which range in depth from 21.33 to 24.43 m, were chosen to be replaced with polymer concrete manholes along the 6276-m gravity flow wastewater tunnel in the Austin Downtown Wastewater Tunnel project. The use of polymer concrete manholes significantly decreased the amount of construction work and sped up the project timeline without increasing construction costs.

2.2. Cost Effectiveness Study Related to Pipes

Life-cycle cost analysis is used to investigate the long-term expenses of an asset across its full life cycle [35]. There has only been one limited study that has used life-cycle cost analysis to explore the benefits of various roadway asset al. alternatives [36]. Bozkurt and Islamolu have compared the manufacturing costs of cement concrete pipes and polymer concrete in Turkey [18]. Their manufacturing expenses included the price of raw materials, labor, and overhead. Cement- and polymer-based pipes were thought to have service lives of 15 and 50 years, respectively. The researchers chose six sizes of cement-based pipes with diameters ranging from 150 mm to 600 mm and nine sizes of polymer-based pipes with diameters ranging from 415 mm to 3974 mm for the cost comparison. Based on the market pricing, the unit costs of the raw materials used to make both types of pipes were calculated. In addition to the cost of the raw materials, it was expected that both types of pipes would be manufactured under identical conditions in terms of starting inventory costs, labor costs, and manufacturing overhead. The cost comparison considered the 1000-m pipe length and the corresponding service lives of the two pipe types. The annual cost of polymer-based pipes was found to be roughly 120 percent higher than that of cement-based pipes when only material costs were considered. While considering the whole cost of the manufacturing process over the service life, it was found that the annual cost of the polymer-based pipe was 2.5 times more economical than that of cement-based pipes. For the duration of the service life, using polymer-based pipes costs less in repair and maintenance. Based on the findings, this study recommends utilizing polymer-based pipes to increase strength, extend service life, lower maintenance costs, and provide corrosion resistance.

2.3. Comparison of Technical Specifications

Several studies have provided the technical specifications of traditional cement concrete and newer polymer concrete manholes.

Table 1. Technical Specification Comparison of Cement Concrete and Polymer Concrete Manholes [37,38,39,40].

No	Areas	Cement Concrete Specification	Polymer Concrete Specification
1.	Material	Portland cement, normal sand, crushed stone, rebar	Thermosetting resins, high-strength silica, inert fillers, Fiberglass reinforcement
2.	Maintenance	Need regular inspection and maintenance	Need minimum/no maintenance
3.	Costs	Lower upfront cost but costly over the lifespan	Higher upfront cost, but economical over the lifespan
4.	Durability	Moderate corrosion resistance, prone to degradation in harsh environments 50–75 years if no chemical exposure	High corrosion resistance, resistant to harsh environments 75–100 years if no chemical exposure
5.	Environmental Impact	Moderate impact due to frequent repairs or replacements	Lower impact due to longer lifespan and reduced repairs
6.	Compliance with Standards	ASTM C478: Precast Reinforced Concrete Manhole Sections [37]	ASTM C581: Chemical Resistance of Thermosetting Resins [38]

presents a comparison of these two manhole types. The comparison was made regarding material composition, the need for maintenance, costs, durability, environmental impact, and compliance with standards.

2.4. Manhole Materials and Installation Techniques

It was found that concrete is the primary material used for constructing manholes in the world [41]. The cement concrete manholes are produced using sand and aggregate. Based on the manhole types, the mix design of the concrete can vary. To reduce the life-cycle cost and the impact on the environment, the use of copper slag as a partial cement substitute in cement concrete is now common [42]. The main goal of using copper slag in cement concrete is to reduce its life-cycle cost. Using copper slag as a partial cement substitute has also been found to reduce about 12.8% of carbon emissions and will not have any negative impact on the strength of cement concrete.

Brick or masonry was also used in old manhole construction [41]. Bricks are generally used to build the walls of manholes so that they will provide structural support and stability. However, these types of brick masonry manholes are rarely used in the U.S. currently due to construction and durability problems. High-density polyethylene (HDPE) plastics are also used in certain manholes. As these materials are highly resistant to corrosion and chemical reactions, the life of the manholes can be increased if they are used. These manholes are lightweight and easy to handle and install.

Concrete grout and mortar are also used to build manholes [41]. The benefits of these types of manholes are that they provide stability, prevent water infiltration, and ensure a tight seal between various components of the manholes. Sometimes, bituminous materials are used for the base of manholes or floors to make the surfaces smooth, prevent groundwater infiltration, and offer resistance to chemical attacks.

In the last couple of decades, various composite and innovative materials have been used to construct manholes. Some of these materials are fiberglass-reinforced plastic (FRP) or other composite materials like polymers [41]. These materials are harder and stronger than concrete and lighter to handle. They also have high upfront costs but can last for longer periods of time, reducing the life-cycle cost of manhole construction, maintenance, and rehabilitation.

Generally, manholes are installed or replaced using the open-cut techniques in which the ground is excavated wide enough to install the manholes. However, trenchless technology has recently been gaining traction in the construction industry, so open excavation can be omitted, and manholes can be installed without opening the ground [43]. This will reduce public disruptions during construction periods. Various studies have found that trenchless technology used to install or replace the manholes reduces the overall costs, disturbance in urban areas, and environmental impacts.

2.5. Gaps in the Literature

A review of the literature reveals that chemical corrosion is a significant issue in sewer networks that leads to the structural degradation of cement concrete manholes. Most studies, therefore, focus on corrosion and the characteristics of cement concrete manholes. However, they have not highlighted the structural degradation in concrete manholes and the extra cost these manholes incur for maintenance and rehabilitation. It would have been valuable if the manhole construction life-cycle cost analysis and maintenance had been conducted using these structural degradation data. Some studies focus on the benefits and suitability of polymer concrete for sewer system manholes. These studies have not calculated the benefits and suitability of polymer concrete in terms of monetary value for the entire life cycle because the initial cost of these types of manholes will always be high due to the innovative materials and techniques used to produce them. However, when the life-cycle cost analysis of polymer concrete is calculated, owners will see the benefits of these manholes.

A study investigates manhole rehabilitation strategies and discusses and compares the open-cut versus trenchless rehabilitation of concrete manholes [9]. This study does not include a construction cost analysis of various manhole construction techniques. Another study compares the manufacturing costs of cement concrete and polymer concrete sewer pipes [18]; however, this study does not focus on the life-cycle installation and replacement cost of cement concrete manholes. No study compares the installation and replacement cost of cement concrete and polymer concrete manholes while considering their life cycles. The authors’ previous study investigates the LCUC of these two product types based only on initial installation cost. The findings demonstrate that polymer concrete manholes have much lower life-cycle installation costs than cement concrete manholes [44,45]. Therefore, a thorough analysis of the life-cycle costs associated with these two manhole types is necessary, and this paper fills that gap.

3. Materials and Methods

Figure 1 presents the overall research methodology of this paper. There are five major steps. First, the scope and objectives of this study are defined, and then extant works of the literature about cement concrete and polymer concrete manholes are reviewed. Data was collected to achieve the objectives; then, data analysis was carried out. Finally, conclusions are made. The following sub-sections detail the research methodology adopted in this study.

3.1. Collect Cost Data of Manholes

The authors have used a robust data collection method in this study. At first, they defined the objective of this study and planned the type of data to be collected to achieve it. For this, the authors had a series of meetings with the CCWRD engineers regarding the available data. Then, a detailed data collection plan was developed, including the timeframe for completing the study on time. The authors also engaged subject matter experts from the CCWRD office to ensure no data is missed and overlooked.

This study collected traditional cement concrete and polymer concrete (Armorock, Boulder City, NV, USA) manhole data from the CCWRD of Las Vegas, Nevada, in the United States. The data includes both primary and secondary data. The primary data includes the specifications of polymer concrete, such as the polymer concrete warranty information and product material details, while the secondary data consists of installation and replacement costs of cement concrete and polymer concrete manholes used for the sewer networks in the CCWRD, Las Vegas from 1977 to 2017. The data also consists of the physical properties of manholes, such as manhole identification (ID) number, depth, type of manhole, etc. For the manhole projects completed after 2007, data were available electronically, so the data were exported from their database (named “ProjectView”). This ensures the accuracy and reliability of the collected data. For the projects completed before 2007, data were collected from the hard copy documents (contract documents, technical specifications, and as-built drawings) archive. This study has collected cost information for all the new installation and replacement works separately. The installation and replacement costs consist of the cost of labor, material, and equipment, as well as the costs for other activities required to complete the manhole project.

A Microsoft 365 Excel spreadsheet was then created with all the data gathered from CCWRD. The information showed that 1.83 m diameter manholes were most frequently used in the Las Vegas sewer systems. Manholes with 1.22 m, 1.52 m, and 2.44 m diameters were also found, but they were not included in the analysis because the life-cycle cost analysis seems more reasonable for manholes of one size. There were 343 cement concrete manholes (1.83 m diameter) and 88 polymer concrete manholes (1.83 m diameter) installed in Las Vegas during those years. After the data had been recorded into the spreadsheet, the data regarding cement concrete and polymer concrete were separated. Further separate spreadsheets were prepared for installation and replacement.

3.2. Determine Life-Cycle Unit Costs of Manholes

To determine the LCUCs of cement concrete and polymer concrete manholes, it is necessary to calculate how long they last. As the authors collected cement concrete manholes dating back to 1977, there was enough data to determine when those cement manholes were replaced. Since cement concrete manholes were installed in 1977, they have been replaced as needed. Based on the data, this study computed the life of the cement concrete manholes, which is the duration between installation and replacement. For this, the authors matched the unique manhole identification numbers to identify which manhole was replaced and when. Then, the average duration for replacing the cement concrete manholes was calculated from the individual durations of replacement. The same procedure was utilized to determine the average duration before other pipes required replacement.

One of the major cost components of data required to calculate the life-cycle cost of cement concrete manholes is maintenance and replacement. This data is necessary to determine the total life-cycle cost of these manholes when comparing them with polymer concrete manholes. As polymer concrete manholes last longer than cement concrete manholes, it is necessary to collect the maintenance and replacement costs of the cement concrete manholes used by CCWRD. However, the authors could not retrieve their maintenance costs. There are 26 cement concrete manholes whose replacement cost data was found during the data collection phase. Therefore, the authors used the 26 cement concrete manholes’ initial installation and replacement costs to develop a regression model to predict the replacement costs of the remaining cement concrete manholes. Using the regression equation, the replacement cost was determined for all the cement concrete manholes. These replacement costs were used to determine their LCUCs.

For polymer concrete, since CCWRD began utilizing this type in 2015, none of this product has been replaced. Thus, this study considered the manufacturer’s warranty information as the service life of polymer concrete manholes at 50 years. Two studies also consider this information [18,24] in their condition evaluation and manufacturing cost comparison calculations.

Then, the LCUC of cement concrete and polymer concrete manholes were calculated. For cement concrete manholes, the LCUC was computed based on the unit cost of installation per meter and average replacement costs. The LCUC of a manhole is computed by dividing its unit cost of installation and replacement per meter by the total average amount of time that the cement concrete manholes lasted after the first replacement. The authors calculated the LCUC in two scenarios: considering only installation costs and considering both installation and replacement costs. In the first scenario, the initial installation cost of the cement concrete manholes was divided by the life of the manholes to calculate the unit cost of installation per year. In the second scenario, the initial installation and replacement cost were combined, and then that value was divided by the installation and replacement durations. For the polymer concrete manholes, because this product had not been replaced, the product’s warranty period was used to calculate the LCUC. The LCUC of polymer concrete manholes was computed by dividing the unit cost of installation per meter by the 50-year warranty period. In both scenarios, the unit costs of the polymer concrete manholes are the same.

3.3. Formulate Research Hypotheses

Two research hypotheses were developed to achieve the study objectives. They were related to the cost efficiency of two types of manholes. The first research hypotheses state that the LCUC of installing the polymer concrete manholes was significantly less than that of cement concrete manholes. Other hypotheses state that the LCUC of combined installation and replacement of the polymer concrete manholes was significantly less than that of cement concrete manholes.

3.4. Develop Null Hypotheses

Null hypotheses were created from the research hypotheses. The null hypotheses state that there were no significant differences between the LCUCs of installation only and combined installation and replacement costs of polymer concrete and cement concrete manholes. They can be written mathematically as:

μ (LCUC of installing polymer concrete manholes)
= μ_(LCUC of installing cement concrete manholes)

(1)

μ (LCUC of installing and replacing polymer concrete manholes)
= μ_(LCUC of installing and replacing cement concrete manholes)

(2)

3.5. Conduct Statistical Tests

To prove the research hypotheses, the LCUCs of cement concrete and polymer concrete manholes must be compared either using the t-test or the Mann-Whitney U test. Five assumptions need to be fulfilled to use the t-test. They are: (1) the dependent variable should be continuous; (2) the observations are independent of each other; (3) the samples should be randomly selected; (4) the data should be normally distributed in each group; and (5) the variances of both samples should be approximately the same. If one of these assumptions is not fulfilled, a non-parametric test, such as the Mann-Whitney U test, needs to be conducted to determine whether there is a significant difference in sample means between these groups.

There are three assumption criteria to be verified before using the non-parametric Whitney U test. They are: (1) the dependent variable should be continuous; (2) the observations are independent of each other; and (3) there should be one independent variable that consists of two categorical independent groups.

The Kolmogorov-Sminov and Shapiro-Wilk tests are performed to determine if the data are normally distributed. This null hypothesis is that the sample’s population is normally distributed. This test rejects the null hypothesis if the p-value is less than 0.05, indicating that the population of the sample is not normally distributed. If the dataset were normally distributed, parametric tests would be conducted to determine the group differences. Otherwise, non-parametric tests will be conducted to determine the group differences between cement concrete and polymer concrete manhole LCUCs. With these tests, the null hypothesis is that there is no significant difference between the median values of LCUCs for polymer concrete and cement concrete manholes. If the p-value is less than 0.05, it indicates that there is a significant difference between the median values of these two groups. The difference between these two groups is highly significant if the p-value is less than 0.01.

4. Results

This study collected dollar cost per manhole data from CCWRD, Las Vegas. From those costs, the first unit cost per linear meter was computed by dividing the cost by the depth of the manholes. The depth of cement concrete and polymer concrete manholes ranged from 1.89 m to 8.93 m (median depth of 3.93 m) and 2.04 m to 8.00 m (median depth of 3.32 m), respectively. The unit costs of the manhole per linear meter were then adjusted for inflation and time using the ENR Construction Cost Index (ENR 2022). The costs were adjusted to the 2022 base costs.

Before analyzing the LCUCs of the cement concrete and polymer concrete manholes, it is necessary to calculate the replacement costs of the cement concrete manholes. Using the available 26 replacement costs of the cement concrete manholes, a regression analysis was conducted using a simple regression model. Two important predictors for the regression analysis were the installation cost and the replacement year.

Table 2. Regression Analysis Results for Predicting Cement Concrete Manholes’ Missing Replacement Costs.

Predictor	Coefficients	Standard Error	T-Stat	p-Value
Intercept	560,512.99	207,882.29	2.70	0.01 *
Installation Cost	1.74	0.70	2.49	0.02 *
Year of Replacement	−278.37	103.90	−2.68	0.01 *

* Statistically significant at alpha level 0.05. Multiple R = 0.51. R-Square = 0.27.

shows the results of the regression analysis. There is a moderate correlation (R = 0.51) between the replacement cost and predictors (the initial installation costs and replacement year). With this, a simple interpretability of the model was developed. The R-square value of the equation is found to be 0.27. The result also shows that both predictors are significant (p-value less than 0.05). Based on this data, a linear regression equation is created. Equation (3) shows the regression equation to calculate the replacement costs of the remaining cement concrete manholes. These replacement costs were added to the cement concrete manhole installation costs to calculate their total LCUCs. The following section shows the descriptive statistics of the cement concrete manholes and polymer concrete manholes’ LCUCs.

Predicted replacement cost = 560,512.99 + 1.74 × Installation cost-278.37
× Replacement year

(3)

4.1. Descriptive Statistics

The provided box plot in Figure 2 displays the four types of LCUC data associated with two types of manholes: Cement Concrete Installation, Cement Concrete Installation with Replacement, Polymer Concrete Installation, and Polymer Concrete Installation with Replacement. The graphic displays the cost distribution for each category, including the median (thick horizontal line in the box), interquartile range (box), and a few outliers. This visual comparison emphasizes the cost differences and variations between typical cement concrete and polymer concrete manholes, providing a better understanding of the data’s patterns and trends.

Figure 3 and Figure 4 illustrate mean and medium LCUCs for installation and combined installation and replacement costs for cement concrete and polymer concrete, respectively.

Table 3. Descriptive Statistics of Installation Only and Combined Installation and Replacement Life-Cycle Unit Costs of Cement Concrete and Polymer Concrete Manholes.

Life-Cycle Unit Costs	Types of Manholes	Sample Size	Mean ($)	Median ($)	Std. Dev. ($)
Installation Only	Cement Concrete	343	274.47	231.35	142.69
	Polymer Concrete	88	262.34	188.61	182.97
Combined Installation and Replacement	Cement Concrete	33	227.88	217.90	74.88
	Polymer Concrete	88	262.34	188.61	182.97

presents a more detailed result of descriptive statistics with sample size, LCUCs, and standard deviation. Results show that in both scenarios (considering unit costs of installation only and unit costs of combined installation and replacement), the polymer concrete type costs less than cement concrete. When only the installation costs are considered, the median costs of polymer concrete manholes are 18.44 percent less when compared to the counterpart. Similarly, when combined installation and replacement LCUCs are considered, polymer concrete costs at least 13.42 percent less when compared to cement concrete manholes. When looking at the variation, the standard deviation in the cement concrete manholes is less when compared to the polymer concrete manholes.

4.2. Statistical Test Results

First, the data of each group were checked to determine whether the data satisfies the t-test assumptions. The assumption states that the dependent variable should be continuous. This assumption is satisfied because the LCUC of cement concrete and polymer concrete manholes are both continuous variables. The second assumption about the independence of the samples is also true because the LCUC of each manhole’s data is independent of each other. The data were randomly selected, so the third assumption also holds true. To check whether the data in each group are normally distributed, the normality test was conducted in the Statistical Packet for Social Science (SPSS). Two normality tests, Kolmogorov-Smirnov and Shapiro–Wilk, were conducted to check the normality of the data in each group. The results shown in

Table 4. Kolmogorov-Smirnov and Shapiro–Wilk Test Results of Installation Only and Combined Installation and Replacement Life-Cycle Unit Costs of Cement Concrete and Polymer Concrete Manholes.

Life-Cycle Unit Costs	Types of Manhole	Sample Size	Kolmogorov-Smirnov		Shapiro-Wilk
			Statistic	Sig.	Statistic	Sig.
Installation Only	Cement Concrete	343	0.14	<0.001 *	0.92	<0.001 *
	Polymer Concrete	88	0.32	<0.001 *	0.69	<0.001 *
Combined Installation and Replacement	Cement Concrete	33	0.08	0.20	0.98	0.80
	Polymer Concrete	88	0.32	<0.001 *	0.69	<0.001 *

* Statistically significant at alpha level 0.05.

indicate that combined installation and replacement LCUC of cement concrete is normally distributed as a p-value greater than 0.05. However, the p-values of the remaining dataset were less than 0.05, indicating that LCUCs of cement concrete and polymer concrete were not normally distributed.

The final assumption test regarding the homogeneity of each group was also tested using Levene’s test in SPSS. The results of this test are shown in

Table 5. Levene’s Test Results of Installation Only and Combined Installation and Replacement Life-Cycle Unit Costs of Cement Concrete and Polymer Concrete Manholes.

No	Life-Cycle Unit Costs	Levene Statistic	Sig.
1.	Installation Only	4.56	0.030 *
2.	Combined Installation and Replacement	13.82	<0.001 *

* Statistically significant at alpha level 0.05.

. The test results show that, in both scenarios, the homogeneity of the variance in both groups is significantly different, as the p-value is found to be below 0.05. As the normality and homogeneity of variance assumption tests are rejected, the Mann-Whitney U test must be conducted to determine the significant difference in LCUCs between these two groups.

Before conducting the Mann-Whitney U test, the assumptions of this test must be checked. The first two assumptions already hold true. The third assumption is also true in this case because the independent variable is the type of the manhole and is divided into two categorical variables, namely cement concrete manholes and polymer concrete manholes.

All the datasets were not normally distributed. Therefore, the authors conducted non-parametric Mann-Whitney U tests. The purpose of these tests was to compare the median values of LCUC of cement concrete and polymer concrete manholes.

Table 6. Results of Mann-Whitney U Tests for Lif-Cycle Unit Costs of Cement Concrete and Polymer Concrete Manholes under two Scenarios’ Installation Only and Combined Installation and Replacement.

Life-Cycle Unit Costs	Types of Manholes	N	Mean Rank	Sig.
Installation Only	Cement Concrete	343	221.61	0.03 *
	Polymer Concrete	88	189.37
Combined Installation and Replacement	Cement Concrete	33	67.58	0.04 *
	Polymer Concrete	88	58.53

* Statistically significant at alpha level 0.05.

presents the summary of the Mann-Whitney U test results. There were 343 installation cost data for cement concrete manholes and 88 cost data for polymer concrete manholes. There are also 33 cement concrete manholes that were replaced in 2017. The findings also show that the LCUC of polymer concrete manholes is significantly less when compared to the cement concrete types in both cases when only the installation cost is considered and the combined installation and replacement costs are considered. Since the p-values are below 0.05 in both scenarios, the differences between the two types of manholes are significant.

5. Discussion

There are over 20 million manholes in the United States, and they are one of the primary components of sewer network systems. Out of these 20 million, about four million are in poor condition and need replacement [2]. Studies show that two types of manholes are common: cement concrete and polymer concrete. Cement concrete manholes have been used for many decades, while polymer concrete is relatively new. This study calculates the LCUCs of 431 cement and polymer concrete manholes (343 cement concrete manholes and 88 polymer concrete manholes) based on two scenarios considering only installation costs and combined installation and replacement costs.

The cement concrete manholes were replaced when their condition became poor. One of the key factors is replacement cost. The study’s findings show that the average 2022 LCUC of installation and combined installation and replacement of a manhole (cement concrete) were $274.47 and $227.88, respectively. The average duration of a new replacement was found to be 23 years. A similar process was utilized in a study conducted by authors to determine the average frequency of chip seals and striping works. That means every 23 years, the CCWRD replaces the cement concrete manholes to keep their sewage systems in good operational condition. However, since the CCWRD started using the polymer concrete manholes in 2015, they have not needed to replace them. Thus, this study utilizes a 50-year service life for the polymer concrete manholes since the manufacturing company has a 50-year warranty for that product, and other studies [19,24,25] also use the same lifespan of the polymer concrete manholes. This value was used to calculate the LCUCs of polymer concrete manholes.

First, the new installation cost and replacement cost of cement concrete manholes were compared. The average replacement cost per meter (6870.37 $) was higher than the average installation cost per meter (6307.19 $). One of the key reasons for higher replacement costs of cement concrete manholes when compared to the installation costs may be that contractors need to deconstruct the existing concrete manholes before replacing them, such as cleaning the pipe surfaces, removing debris from pipes, and managing any existing sewage. Additional tests are also necessary for quality control before replacing the sewer manholes. Additional safety precautions are necessary for the protection of workers in the wastewater sewer environment. The scale of work could also be a factor in contractor bid prices [46,47]. The higher replacement cost of cement concrete manholes may, therefore, occur due to these factors.

For cement concrete manholes, the LCUCs were calculated in two scenarios—considering only installation cost and combined installation and replacement costs. When considering only the installation cost, using the installation cost and the average duration for replacement, the LCUC of the cement concrete manholes was calculated by dividing the unit installation cost by the average duration. The same procedure was also used for polymer concrete. This study observed that both the installation cost of cement concrete and the average duration are less when compared to polymer concrete. In other words, the unit installation cost of polymer concrete and the durability of this product are high compared to traditional cement concrete manholes. That is why the upfront cost of cement concrete manholes is significantly less, but their service life is much shorter.

Results show that the installation cost of polymer concrete manholes is higher than the installation cost of cement concrete manholes. The mean and median costs of polymer concrete manholes are also high compared to their counterpart. This is because the manufacturing unit cost of polymer concrete is higher than that of traditional cement concrete. Bozkurt and Islamoglu’s study showed that the manufacturing cost of polymer concrete was 120 percent more expensive than traditional cement concrete [18]. The limited use of polymer concrete or the scale factor may be one of the major reasons for higher overhead costs in their production. However, despite the fact that the manufacturing costs of polymer concrete are more expensive than traditional cement concrete, polymer concrete is more economical when comparing life-cycle costs [18].

When LCUCs were calculated for cement concrete and polymer concrete manholes, the results showed that the LCUC of polymer concrete is significantly less compared to traditional cement concrete manholes, even though the unit installation of cement concrete manholes costs less. Although the average upfront cost of polymer concrete is 108 percent more expensive than cement concrete, this product lasts 2.17 times longer than cement concrete manholes. The average life of traditional cement concrete and polymer concrete manholes is 23 and 50 years, respectively. The life of cement concrete manholes is much less compared to other cement concrete structures. This may be due to chemical degradation in a sewer environment. A study showed that, due to chemical impacts, the cement concrete manholes were more expensive and had erratic and disruptive maintenance histories [8].

The agency engineers also indicated that the cement concrete manholes need regular maintenance. However, the agency did not keep databases of the maintenance costs to allocate them to the individual manholes because they outsourced their maintenance works to contractors. If the maintenance costs were added up with the LCUCs of cement concrete manholes, this would increase the cost of cement concrete manholes even more. When the replacement is considered, the total service life of cement concrete becomes 46 years. Then, the average LCUC of cement concrete manholes was calculated considering both installation and replacement. Since the polymer concrete manholes have not yet been replaced, the average LCUC of polymer concrete manholes was used to compare the average LCUC of cement concrete manholes, considering both installation and replacement. When these values were compared, the median LCUC of polymer concrete manholes was significantly less (13 percent) when compared to traditional cement concrete manholes. The standard deviation of the cement concrete manhole’s LCUC is also much less compared to the polymer concrete manholes. This may be because the cement concrete manholes have been used for a very long time compared to the polymer concrete type; thus, the contractors might have more confidence in the work, and their bid costs were consistent. Other studies also show that, with new contracting or new types of construction, the unit cost increased compared to the traditional type [48].

In addition to the cost advantage of using polymer concrete manholes, this product has schedule advantages as well [26]. As opposed to cast-in-place manholes, polymer concrete manholes did not require any field curing time or the application of a corrosion protection coating, which considerably reduced the installation time. Additionally, dolly pulls and spark tests were not necessary. The time spent on traffic control was also greatly reduced. Furthermore, there are other advantages, such as minimum social and environmental impacts [2]. However, this study did not focus on the duration of installing polymer concrete and cement concrete manholes.

US Public agencies spend about $16 billion each year to maintain over 20 million manholes, with 4 million in poor condition. The nationwide adoption of polymer concrete manholes for these 4 million could significantly reduce expenses, resulting in an estimated yearly savings of 550$ million due to its 13% greater cost-effectiveness than traditional cement manholes. These savings might be used to fund other critical infrastructure projects and improve overall urban infrastructure. Similar economic benefits could be obtained around the world, helping to promote more sustainable infrastructure management.

Until now, no study has been conducted examining the life-cycle cost comparison of cement concrete and polymer concrete manholes. This is the first study to investigate the life-cycle cost of these types of manholes with real project data in detail. However, two of the authors of this study have conducted a preliminary investigation of cement concrete and polymer manhole life-cycle costs with a smaller sample size and without cement concrete rehabilitation costs [45]. The findings of these studies were similar.

The major practical implication of this study’s findings is that the public and private owners need to consider infrastructure projects’ life-cycle costs while deciding which methods, materials, and processes of construction should be used during construction. Most public agencies consider only upfront construction costs to show that they are using taxpayers’ money efficiently, which is required of them. However, conducting these types of life-cycle cost analyses of the infrastructure projects using various means and methods will provide data to make informed decisions. Another practical implication of this study’s findings is that the public agencies who work for the betterment of public life must seek information that will provide them with ways to reduce the overall life-cycle costs of infrastructure projects. The agencies should also change the procurement laws and contract documents to allow the contractors to come up with innovative ideas while bidding. In most public projects in the US, low-bid procurement methods are used that only take price into consideration while selecting the bidder. However, some public agencies have started using best-value procurement methods in which price is not the only factor while selecting a qualified bidder [49].

Limitations

Manholes are one of the primary components of sewer systems; however, the studies conducted on manholes are limited. This study has a limitation in the number of polymer concrete manhole data samples. This is because polymer concrete is a comparatively newer material used in manholes, and due to its higher upfront cost, public agencies are not using it. This made it impossible to increase the sample size of polymer concrete manholes. Since polymer concrete manholes have been in use in Las Vegas since 2015, a record of the product’s replacement frequency does not yet exist. This study thus relies upon the 50-year warranty period, and its findings are also limited to 1.83 m diameter manholes because other sizes were not considered due to insufficient data.

6. Conclusions

To achieve the study objective, 40 years (1977 to 2017) of cement concrete manhole data were collected from the Clark County Water Reclamation District (CCWRD) in Las Vegas, Nevada. However, polymer concrete manhole data were only available from 2015 to 2017. A total of 343 cement concrete manholes (1.83 m diameter) and 88 polymer concrete manholes (1.83 m diameter) data were analyzed. The data collected includes installation and replacement dates, physical characteristics of manholes (diameter, depth, type of manhole, manhole ID), and cost information (original installation costs and original replacement costs).

From the cement concrete manhole data, both the average life span of initial installation and replacement were computed as 23 years. The average service life of the cement concrete manholes was, therefore, 23 years. Since the polymer concrete manholes were recently introduced in 2015 in Las Vegas, the newly installed manholes have not been replaced because they have a service life of 50 years.

When the average installation and replacement costs of cement concrete manholes were compared, the findings showed that the replacement costs were a little higher compared to new installation costs. When the average installation costs of cement concrete manholes are compared to the same cost of polymer concrete manholes, the polymer concrete ones are significantly higher (108 percent). This implies that the upfront costs of polymer concrete are higher than those of traditional cement concrete. However, since the service life of polymer concrete is much longer than that of traditional cement concrete, the LCUC of polymer concrete is less when compared to the cement concrete manholes in both cases, considering only the installation cost and both the installation and replacement costs. To compare these groups, a non-parametric test, the Mann-Whitney U test, was conducted. The tests show that the LCUC of polymer concrete manholes is more cost-effective than that of traditional cement concrete products. This study analyzes only the 1.83 m diameter manhole data, so its findings cannot be generalized, and public agencies should take caution while applying its findings to other manhole sizes.

The authors recommend that future studies involve a comparative life-cycle impact assessment to fully evaluate both manhole products. This analysis should consider carbon footprint, resource usage, and the potential for environmental impacts. Future studies can establish a better understanding of the environmental effects of typical cement manholes versus polymer concrete manholes, driving more sustainable infrastructure decisions. This study also suggests carrying out a longitudinal study that evaluates polymer concrete manholes’ performance in terms of durability and maintenance needs over time in different climatic conditions. This will show how the polymer concrete manhole ages and wears in a real-world context.