International Journal of Architecture, Engineering and Construction
Vol 1, No 3, September 2012, pp. 174-182
Environmental Evaluation of Abrasive Blasting with Sand, Water, and Dry Ice
Lauren R. Millman**Corresponding author. Email: email@example.com ,
James W. Giancaspro
Department of Civil, Architectural, and Environmental Engineering, University of Miami,
Coral Gables, FL 33146, United States
Abstract: The objective of this case study was to perform an evaluation of the environmental effects of three surface preparation methods used in civil infrastructure: sand blasting, water jetting, and dry ice blasting. The study was based upon a bridge rehabilitation project in which surface preparation of the reinforced concrete pier caps was undertaken. The assessment considered four response variables: carbon dioxide (CO2) emissions, fuel consumption, energy consumption, and project duration. The results indicated that for sand blasting and water jetting, CO2 emissions stemming from vehicular traffic near the construction site was the primary factor contributing to environmental detriment. However, the CO2 contribution from sublimation of the dry ice translated into 80% and 64% more CO2 than sand blasting and water jetting, respectively. Compared to sand blasting and water jetting, dry ice blasting yielded the shortest project duration and reduced fuel consumption by 7.6% and 13%, respectively.
Keywords: Carbon dioxide, case studies, concrete construction, construction materials, emissions, environmental issues, life cycle, rehabilitation, sustainable development, surface preparation, blasting
Surface preparation of concrete and other structural materials has been a vital task in many engineering applications and has been typically accomplished using abrasive grit media, such as sand, or water propelled to high speeds. Sand blasting has been routinely used in industrial settings to remove corrosion and other chemical residues from intricate machinery components (Stratford 2000), while water jetting or hydro-blasting has been used to remove paint and coatings (Teimourian et al. 2010). In civil infrastructure, sand blasting and water jetting have been utilized to remove accumulated organic matter and other debris from substrates such as concrete, steel, and wood. Achieving a clean and uniform substrate is a vital prerequisite to the application of specialized coatings and repair materials (Mailvagnam et al. 1998).
Sand blasting produces vibrations, significant noise, and an effluent cloud consisting of dust, the waste sand, and the organic matter shed from the substrate. Moreover, the sand particles can act as a carrier to transport contaminants such as lead-based paint residue (Stout 1996). While sand blasting has remained a staple practice due to its simplicity, the byproducts are considered safety risks to on-site personnel and hazardous to the surrounding environment. Water jetting has become a principal practice and is advantageous in the reduction of dust exposure, especially for surfaces that contain lead-based paint or silica-based coatings. However, the contaminants could become aerosolized and inhaled (Rosenberg et al. 2006). To prevent environmental pollution and to protect the health of on-site workers, both sand blasting and water jetting must include careful containment, collection, treatment, and disposal of the effluent under strict regulations (ASTM E 1857 2004; Weston et al. 2005; Code of Federal Regulations 2006).
To circumvent many of the problems associated with sand blasting and water jetting, frozen carbon dioxide (dry ice) pellets can be utilized as the blasting media (Figure 1). This method is commonly known as dry ice blasting or cryoblasting and was developed by the Lockheed Corporation (Stratford 2000). Dry ice blasting was to serve as an alternative method of debris removal and stripping paint to overcome those issues associated with sand blasting. The bond between the debris and the substrate is broken by three unique events: the kinetic energy of the dry ice pellets, the micro-thermal shock induced by the low temperature of -79∘C (-174∘F), and the accompanying pressure wave (Stratford 2000; Spur et al. 1999). The primary advantage of using dry ice in lieu of sand or water is that the pellets sublimate (change from a solid to gas) immediately after impact, producing little waste other than the contaminate being removed. Hence, containment measures (like those associated with sand blasting and water jetting) are no longer essential unless environmental conditions warrant it. Sublimation of the pellets also permits surface preparation in complicated cavities and structural joints that would typically trap grit blast media.
From a sustainability viewpoint, one obvious drawback of dry ice blasting is the intentional release of CO2 into the atmosphere (Figure 2). The typical coverage rate of dry ice blasting (0.28 m2/min) is about one-third of that for sand blasting (0.79 m2/min) (Cold Jet 2009; Foster 2006) and similar to that of water jetting (0.14 m2/min) (Schmid 2005) as reported by equipment manufactures and literature. However, the overall duration of a surface preparation project using dry ice may be shorter than using sand or water because the need for waste containment is potentially eliminated in most circumstances. This reduction in project duration also reduces the CO2 emissions associated with on-site construction equipment and traffic congestion. Therefore, it was hypothesized that dry ice blasting may offer a higher degree of sustainability than sand blasting or water jetting.
To test the hypothesis, this case study compared the environmental effects of dry ice blasting, sand blasting, and water jetting by quantifying the environmental impact of two key indicators, namely, CO2 emissions and fuel consumption. The assessment considered the commitment of environmental resources originating from the construction staging area until the final disposal (end of construction activities) for a bridge repair project undertaken in Rhode Island (USA). The analysis consisted of (1) a baseline study to identify the variables that had the most significant effect on the environment, and (2) a sensitivity analysis to examine the variation of those critical variables identified in the baseline study.
To compare the processes, this case study analyzed a bridge repair project undertaken in South Kingstown, Rhode Island (USA) (Giancaspro et al. 2009). The rehabilitation included the application of fiber-reinforced polymer (FRP) overlays to the reinforced concrete pier caps supporting the road deck of the Silver Spring Cove Bridge. The substrate of the pier caps required appropriate cleaning and surface abrasion (with water jetting) to allow for intimate contact between the parent substrate and composite overlay (ACI 2008). The dry ice and sand blasting were conducted off-site for comparison.
The environmental assessment included three main steps: goal definition and scoping, inventorying, and interpretation. Goal definition and scoping were comprised of defining the goals of the project, the type of information that was needed, use/reuse/maintenance, the recycle/waste management protocol, and identifying the materials. The inventory required the development of a data collection plan, collecting data, and evaluating and documenting the results. The interpretation step included the identification of issues of interest, evaluating the completeness, sensitivity, and consistency of the data, and drawing conclusions and recommendations.
This environmental assessment is not a Life Cycle Assessment (LCA), a Life Cycle Inventory (LCI) nor a Life Cycle Impact Assessment (LCIA). A LCA encompasses project start from the extraction of raw materials from the earth continuing through the period of use up to the return of the materials back to the earth (end) (Curran 2006). The LCI is used to quantify the energy and other discharges over the course of the life cycle (Curran 2006). The LCIA is applied to evaluate the potential impact on humans’ health and the environment (Curran 2006).
The goal of the case study was to determine under which conditions each surface preparation method (dry ice blasting, sand blasting, or water jetting) performed better in terms of CO2 emissions, fuel and energy consumption, and project duration.
The scope was to determine the most environmentally conscious (with an equivalent mechanical effectiveness) substrate preparation and cleaning system. The functional unit was the Salt Pond Road Bridge rehabilitation project.
The study covered the implementation processes of the three systems and measured the energy required and CO2 emission from project start to end, and from construction staging area to the final disposal of the blasting media at the landfill or wastewater treatment plant.
The assumptions of the baseline study were based on equipment specifications, collected field data, and approximations based on the authors’ experience. These included:
The assumptions specific to dry ice blasting included:
The assumptions relevant to sand blasting included:
Assumptions specific to water jetting included:
The system flow for the dry ice blasting process included the sublimation of pellets upon impact and the CO2 gas released into the atmosphere. The contaminant shed from the substrate was transported to its end location (landfill).
The system flow for the sand blasting process began with the sand propelled at the substrate using compressed air, then the mixture of waste and sand were collected after use. The waste was transported to the end location for disposal.
The system flow for the water jetting process began with the water propelled at the substrate using compressed air, and the mixture of shed contaminant and water were collected after use. The waste was transported to the end location for disposal.
For each system, the environmental effects were categorized into several project stages (or sources): site setup and deconstruction, equipment operation (usage), vehicular traffic, media sublimation, and disposal of process wastes. More specifically:
The impact factors included project duration, CO2 emissions, and fuel and energy consumption. The blasting process found to have the lesser values was considered the process with the least environmental impact. Tables 1 and 2 show the CO2 emissions and energy consumed by project stage, respectively.
Table 1 facilitates an equivalent comparison of the durations of the projects for the baseline study. The setup and removal of scaffolding required a total of 9.4 days regardless of the blasting media employed. The actual surface preparation (blasting) time using dry ice was 10.8 hours, while that for sand blasting was 5.42 hours and 8.66 hours for water jetting. Because the dry ice blasting was nearly twice as long as sand blasting, it may seem that sand blasting was the faster method. However, this misconception can be disproven by comparing the overall project durations of the methods. Sand blasting required 12.2 days, dry ice blasting required 11.0 days, and water jetting required 12.8 days. Comparing the dry ice and sand, the difference of 1.2 days was a 10.4% reduction in time for the scenario employing the dry ice. This significant savings in time stems from the 1.6 days needed for the combined setup and removal of containment measures, which was unnecessary for the dry ice blasting. Because the dry ice sublimates, the overall project duration was reduced by 10.4% as compared to sand blasting, and by 14.3% as compared to water jetting. This reduction in time translated into reduced traffic delays and, hence, less CO2 emissions from vehicular traffic.
As shown in Table 1, the dry ice blasting scenario produced 57.7 kg of CO2 from the setup and equipment stages, 969 kg of CO2 from vehicular traffic, and 2,077 kg of CO2 directly from blasting with dry ice (sublimation) for a total of 3,104 kg. The sand blasting project produced 28.9 kg of CO2 during the setup and equipment stage, and 1,082 kg of CO2 from vehicular traffic, for a total of 1,111 kg. The water jetting project produced 46.2 kg of CO2 during the setup and equipment stage, and 1,129 kg of CO2 from vehicular traffic, for a total of 1,176 kg. Thus, the sand blasting released 1,993 kg less than that of dry ice blasting and 65 kg less than water jetting; this corresponded to reductions of 64.2% and 5.5%, respectively. The majority of these reductions in CO2 for sand blasting stemmed from the blasting duration component. The sand blasting cleaning rate was about double the rates for dry ice blasting or water jetting which allowed for a faster cleaning time and shorter blasting duration. For the sand blasting and water jetting scenarios, vehicular traffic contributed the largest amount of the total CO2 emissions as shown in Table 1 (31.2% for dry ice blasting, 97.4% for sand blasting, and 96.0% for water jetting). Because the blasting duration overwhelmingly governed the total CO2 emissions, the release of CO2 via dry ice sublimation was equitably significant.
The energy consumption analyses in Table 2 considered the energy consumed on a system-wide perspective, which included contributions from vehicular traffic. Energy consumption for dry ice blasting was 20.7 MW ⋅h during blasting, and 0.22 MW ⋅ h for disposal (total of 21.0 MW ⋅ h). For sand blasting and water jetting, energy consumption was comparable to that of dry ice for the disposal stage (0.22 MW ⋅ h). However, the blasting stage (for sand) consumed 22.9 MW ⋅ h, which was 10.5% more energy than that consumed for the dry ice. For water jetting, the blasting stage consumed 25.6 MW ⋅h, which was 23.6% more energy than that for dry ice. This difference in energy consumption was due to the longer project durations, which increased vehicular traffic for both the sand blasting and water jetting systems. Additionally, the blasting apparatus of the equipment stage increased the total energy consumption for the water jetting system. In terms of the total energy consumed, the dry ice scenario used 21.0 MW ⋅ h whereas the sand blasting scenario used 23.1 MW ⋅ h and the water jetting scenario used 25.8 MW ⋅ h. These differences equated to 10.0% and 23.2% less energy when using the dry ice blasting process.
As expected, the reduction in vehicle speeds due to congestion increased the fuel consumption per vehicle. For all systems, the assumed increase was 0.0133 L of fuel per vehicle, as shown in Table 1. Vehicles in the dry ice blasting system consumed an additional 437 L of fuel during the project, while traffic for the sand blasting system consumed 8.2% more (total of 473 liters) and the water jetting scenario consumed 14.4% more (total of 500 liters).
The results of the baseline study indicated that the variable with the most significant environmental impact stemmed from traffic disruptions. To further examine the sensitivity of this variable, two scenarios were evaluated:
The number of traffic lanes was varied from 0 to 8 lanes, though the average daily flow remained constant at 4,250 vehicles/lane. Zero lanes of traffic corresponded to bridges that do not have roads passing beneath them; thus, vehicular traffic would not be affected. For these bridges, the environmental effects from traffic disruptions were removed and the effect of dry ice sublimation became more pronounced and significant. To demonstrate this effect, consider the CO2 emissions for zero lanes of traffic in Figure 3. Dry ice blasting released 2,140 kg, while that for sand blasting and water jetting were negligible. When the number of lanes was increased to 1, both the sand blasting and water jetting methods contributed nearly equal amounts of CO2 (540 kg). This trend continued, although diverging slightly with an increasing number of lanes (about 4% for 8 lanes of traffic), and with water jetting contributing the greater amount of CO2. Thus, sand blasting and water jetting released less CO2 if traffic was unaffected by the construction or if the number of traffic lanes was limited to 1 lane. Dry ice blasting consistently contributed more CO2 than sand blasting or water jetting. This was attributed to the additional release of CO2 via sublimation. Regardless of the number of traffic lanes, fuel consumption for water jetting and sand blasting was always greater than that for dry ice blasting. However, the difference was marginal (only about 10% for 8 lanes of traffic). This was expected since the construction equipment consumed fuel at a constant rate; the blasting method with the longer overall project duration would consume more fuel.
To examine the sensitivity of the environmental effects to variations in traffic volume, the average daily traffic flow was varied from 0 to 12,000 vehicle/lane (Figure 4). The minimum value of 0 vehicle/lane corresponded to bridges that did not span other roadways. In this scenario, the traffic flows were broadly categorized as either 4-lane, low volume/rural arterials or 8-lane, high volume/urban arterials or interstates (FHWA 2007). Figure 4 demonstrates that sand blasting and water jetting released less CO2 than dry ice blasting at daily flow rates below 6,000 vehicle/lane.
The analysis presented in this study compared the environmental effects of sand blasting, water jetting, and dry ice blasting based on an actual bridge rehabilitation project. For the specific project conditions and assumptions, the following conclusions were drawn for the baseline study:
Since traffic-related CO2 emissions and fuel consumption comprised the majority of the environmental detriment, both response variables were most sensitive to variations in traffic volume. When explored in the sensitivity analyses, the results led to the following conclusions:
Considering the amount of CO2 emitted, the surface preparation method using sand blasting may be the most
environmentally conscious option. If project duration, fuel and energy consumption, and volume of debris are of
importance, then the dry ice blasting method may be a better alternative. In urban projects where significant traffic
disruptions are anticipated, surface preparation using sand blasting may be the better option. Although
dry ice blasting does yield more carbon dioxide emissions, it may be a preferred alternative for use on
bridges located near delicate ecosystems, or where the use of containment measures would be difficult to
implement. Water jetting appears to be a suitable intermediate option between dry ice and sand blasting. Of
course, this assumes that the structural rehabilitation requirements can be achieved using the specified
blasting media. Further analyses and studies may encompass economic considerations and statistical
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