BITUMEN EMULSION AND FOAM BITUMEN FOR COLD RECYCLED AND BITUMEN STABILIZED MATERIALS: A COMPARISON BASED ON PERFORMANCES, COSTS AND SAFETY

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1 Paper ICMPA068 8th International Conference on Managing Pavement Assets BITUMEN EMULSION AND FOAM BITUMEN FOR COLD RECYCLED AND BITUMEN STABILIZED MATERIALS: A COMPARISON BASED ON PERFORMANCES, COSTS AND SAFETY Maurizio Crispino 1 and Claudio Brovelli 1* 1 Faculty of Civil Engineering, Politecnico di Milano, Italy * Corresponding Author s claudio.brovelli@mail.polimi.it ABSTRACT The strong demand for environmental sustainable techniques in road pavements has increased during the years due to environmentally-friendly worldwide policies of the major industrialized countries. One of the consequences is the adoption of cold, in place recycling and bitumen stabilization as two of the preferred technologies for addressing rehabilitation by reusing material of existing pavements. Stabilization with bitumen enhances the performances of subgrade, providing durability equal or exceeding that achievable through virgin materials, even at lower costs. Cold recycling and bitumen stabilized materials for road pavements, using either bitumen emulsion or foamed bitumen, are internationally considered environmentally sustainable and cost effective; when correct construction techniques are used, these pavements perform very well. Cold recycling has long been used in the past, but little research is aimed at comparing the two binders (bitumen emulsion and foamed bitumen). In this paper the authors analyze performance, cost and outline risks related to bitumen emulsion and foam bitumen focusing on an Italian context. In particular, cost, equipment and environment sustainability are compared based on results from laboratory and real scale applications. Finally, a simplified analysis on major causes of risk is reported. KEY WORDS: foam, emulsion, recycling, stabilization. INTRODUCTION Worldwide, the state of road pavements is deteriorating, and the demand for their rehabilitation far exceeds the demand for new roads. In-place recycling and bitumen stabilization are the most used techniques by reusing material in the existing pavement (1). The addition of bitumen enhances the properties of recycled materials, providing durability equal or exceeding those achievable through virgin materials, all at lower costs. Cold recycled and bitumen stabilized materials, using either bitumen emulsion or foam bitumen, are generally considered environmentally sustainable and cost effective if adequate construction techniques are used. Cold recycled materials are suited to both construction of new pavements and to pavement rehabilitation using onsite recyclers or even more conventional construction equipment (1). No significant distinction can be made between the mechanical results obtained with bitumen emulsion and foam bitumen in case of bitumen stabilization treatments (1) while the same cannot be stated for recycled pavements. Cold material mix design procedure does not require optimization only in terms of volumetric-mechanical characteristics and performance analysis, but also an optimization of the results in term of economical profit, safety and duration of works (22). Recently these concepts are becoming familiar to most designers due to the global politics of environmental sustainable. Therefore today, the main purposes of a pavement mix design are oriented to the realization of a pavement that absolves all the functional and structural requirements and guarantees a life-cycle cost in term of reasonable sustainability to the collectivity. The building yards are often considered particular work places because of the high rate of risk to the operators. Road yards are considered less dangerous with respect to safety but, because of some particular processes they can also be considered equally dangerous. High temperatures and traffic are the principal parameters that affect risk in road treatments. The objective of the research was to investigate the advantages of both technologies making a comparison of technical and economical points of view and including a simplified risk analysis. TECHNOLOGIES A bitumen emulsion is a solution of water bitumen and a chemical agent (emulsifiers, rejuvenates) (2). It is usually mixed in plant by colloidal milling which splits hot bitumen into droplets (µm) and allows it to

2 be mixed in the water phase. Emulsions can be used in most road applications due to their versatility which depends on their characteristics as a solution of water and bitumen and their workability both at room temperature (20-30 C, cold recycling) versus being quite heated (60-80 C, warm and half warm applications) (2).They can belong to almost all categories of commercialized bitumen, standard or modified. The water phase guarantees a quite uniform distribution of the emulsion in the mix and allows the bitumen droplets to cover the aggregate surfaces, independently from the size. Nevertheless a bitumen emulsion is usually selected considering the type of aggregate to be treated (in particular for anionic emulsions, it is strongly recommended to avoid acidic rocks (1)). In particular the addition of bitumen emulsion should be carefully evaluated to prevent delays due to the breaking phenomena or ionic incompatibility between ph level of water and emulsion if the moisture content is high. Foam bitumen is mainly influenced by the bitumen (ordinary or modified, soft or hard) and the dosage of water added during the expansion phase. The foaming process requires hot bitumen (at variable temperature depending on bitumen used) and a small percentage of cold water 2-5% (3). The bitumen must be at C to evaporate water sprayed through the injectors into the expansion chamber. In terms of environmental savings, the addition of heat could mean a disadvantage which could induce the exclusion of foam as an environmentally friendly solution. The process is also strongly influenced by aggregate grading (fine percentage on aggregate weight) and external temperature (under 10 C) due to the coating behavior of foam in the mixture. In fact foam distributes exclusively to the finer particles, producing cohesive links between bitumen droplets and smallest particles. Nevertheless, under a technical point of view neither could be considered better in absolute terms. MECHANICAL AND FUNCTIONAL PERFORMANCES Cold recycled materials are similar both to bound or unbound granular materials depending on percentage and type of binder, size and class of aggregate and percentage and role of filler (5). If compared to traditional HMA, they have higher voids and less cohesion, due to the lack of coating of binder added and presence of old bitumen on aggregate (percentage of RAP) but could achieve similar performances if all the mix compounds are well calibrated (6-7). Therefore the behavior of a cold mix can be influenced by several factors such as: grading, ionic charge of virgin aggregate but also the type and percentage of aged bitumen on RAP (Reclaimed Asphalt Pavement). Aggregate A significant improvement of cohesive strength and reduced moisture sensitivity can be observed by testing materials bounded with emulsion and foam (6). In the presence of bitumen emulsion, both coarse and fine aggregate are completely coated by bitumen. The mix appears as a solution in water in which aggregates can be mixed depending on the total water content (5) because the bitumen emulsion traditionally acts as a lubricant during the compaction process and breaks only after the material has been compacted. In case of foam bitumen, hot bubbles coat the fine particles but not the surfaces of larger aggregates (1). In fact, unlike HMA, cold mixes are not black at all. The nature of dispersion of the bitumen is different from emulsion to foam. When bitumen emulsion is added to the material, the charged bitumen droplets are attracted to the smaller particles with an opposite charge while the larger aggregate particles are partially coated by the mastic of binder and filler. The tiny bitumen particles that are produced when the foamed bitumen bubbles burst have only enough energy to warm the smaller aggregate particles sufficiently to permit adhesion and partially coat the larger. The purpose of fine aggregate, especially filler, serves the function of binder (1). That s why during the characterization of grading curve of a cold mix, the percentage of fine (on the total weight of aggregate) must be considered as one of the key parameters. The gradation analysis depends on natural material and the quantity of RAP added. Moreover the exact content of fine particles on the total weight must be calibrated to guarantee the binder dispersion during the mix phase (1, 5). Using foam bitumen, approximately 5% filler is required to produce a treated material that performs well. Where lower bitumen content is used, this value can be reduced to 4% (1). Active filler The presence of high filler content in the pavement layers is usually considered undesirable in pavement engineering practice. Therefore in case of cold mixtures the binder disperses mainly among the finest particles, resulting in a bitumen-rich mortar between the coarser particles (1, 8) causing a slight darkening in the color of the material after treatment. A small amount of active filler (cement or hydrated lime) is

3 traditionally added to the mix in conjunction with bitumen emulsion or foam bitumen in order to improve the mechanical performances on saturated conditions. In addition filler assists the extraction of the water phase from a bitumen emulsion causing breaking and it acts as a catalyst in dispersing the bitumen particles using foam bitumen. However there are lots of experiences (9) in which active filler is not used. To this end the addition of cement or lime must be related to the specific case. Some authors (1, 6) suggest limiting the maximum content of active filler, especially for cement. High filler content induces strong bitumen demands (due to the increment of the specific area of particles) increasing stiffness and reducing the mechanical properties on long term (10). The negative effects of high filler content on foamed bitumen stiffness behavior was evaluated (11) but also (8) confirmed that a continuous mineral filler phase, especially when soaked, provides pathways for easy fracture propagation through the specimen during testing, thus yielding lower strengths. Introducing excessive small particles, especially to filler content higher than 12% (8) or 5% or 2% (1) (foam and emulsion respectively) may be uneconomic and strongly discouraged for both practical and mechanical issues. Observations at a molecular level (12, 13) confirmed that the bituminous emulsion and cement does not generate a new binder. The stiffener effects are due to two simultaneous mechanisms: the emulsion breaking and the cement hydrating during the watery phase of the bituminous emulsion (13). To this end the cement content is usually limited to 2% on mix weight reaching a good compromise for both mechanical performances and water adsorbing phenomena (1). Finally a more throughout investigation on the effects of cement on performances and curing on foam and emulsion treated materials is available (12). Curing time The curing conditions strongly affect the evolution of performances and have a relevant effect on the stiffness development (6, 14). Chemistry plays a significant role on curing of cold mixtures. In fact, layers treated with bitumen emulsion continue to evolve, changing their mechanical behavior significantly (also due to the presence of hydraulic binders such as cement) and also after several months from being laid. Some authors estimate the continued effects of aging on the mechanical characteristics of cold recycled over time (7, 15, 23). Curing is the displacement of water and resultant increase in stiffness and tensile strength of the mixture. In case of bitumen emulsion, water is an intrinsic component of the system and it regulates the breaking phenomena that could influence compaction effectiveness via migration and evaporation rate. In case of foam bitumen it mainly influences the expansion ratio and half-life. Even if the half life time (the time by which the foam bitumen reduced its maximum volume by 50%) could not be extended over more than a few seconds, foam mixes remain workable for hours (in wet conditions) (1). Otherwise in the presence of bitumen emulsion, it is impossible to extend workability over the breaking time. Some recent studies investigated the evaluation of the measurement and influence of moisture conditions on cold mixture to ensure how water could modify the behavior before and during the curing period (16). Two curing temperatures are adopted in most of the studies reported in the literature: 60ºC, 40ºC, and the ambient temperature (17) but many other curing procedures are adopted throughout the world (18). The developments towards a unified curing protocol to date have been mainly pursued in South Africa (23) but nowadays no unique procedure can be considered representative. Temperature The behavior of cold materials evolves with time as shown by various studies conducted (6, 9). Higher curing temperatures (in laboratory or on site) reduce the minimum time to reach the required mechanical performances. In particular for bitumen emulsion an increase in curing temperature during the first hour (after compaction) guarantees a decrease in the voids percentage and reduces breaking time. During summertime the pavement surfaces can easily surpass 40 C. To this end most authors suggest to curing laboratory samples for 72h at 40 C to reproduce the real conditions. However, cold materials treated with bitumen emulsion or foam bitumen show a considerable lower thermal sensitivity than HMA (6). In fact, because of their lower bitumen content, they have a partial coating of large aggregate with cement bitumen mastic (as discussed above), while HMA has a coating of large aggregate with controlled bitumen film thickness (3). In addition, the presence of strong bonds guarantee high stiffness properties at high temperatures (19) due to the cement (or lime) and the residual internal friction between aggregate particles. Indirect Tensile Strength

4 To highlight the equivalence between foam and emulsion on mechanical points of view, some numerical results (ITS dry at 25 C) from collected Italian experiences, referring to different jobs, are plotted in Figure 1. They refer to recycled base layers with foam bitumen and bitumen emulsion at 72h (3 days) curing and 40 C (please note that there is no correspondence between the number of sites). The results presented in Figure 1 show that there isn t a considerable influence of the binder type on the ITS value of all the sites monitored. Both results from emulsion and foam had similar averages (450 kpa) and little variance. ITS [kpa] ITS dry - Foam Bitumen ITS [kpa] ITS dry - Bitumen Emulsion A 0 Site 1 Site 2 Site 3 Site 4 Site 5 Site 6 Site 7 Site 8 Site 9 Site 10 0 Site 1 Site 2 Site 3 Site 4 Site 5 Site 6 Site 7 Site 8 Site 9 Site 10 FIGURE 1 Dry ITS of cold recycled materials treated with foam bitumen (A) and bitumen emulsion (B) It could indicate that emulsion and foam cold mixes can achieve the same tensile strength (7) independently from external conditions. Due to this the Italian Standards for recycled materials impose the same limits for bitumen emulsion and foam bitumen in cases of base and sub base layers. All national specifications are related to short periods ( h) because the curing phenomenon of emulsion could reduce values of dry ITS compared to those mixed with foam (12) by changing curing time. In particular, this behavior evolves in time depending on water adsorbtion and the setting of active filler (cement or lime), and reduces until all significant differences between foam and emulsion performances is covered. Some authors demonstrated the relationship between M r (Resilient Modulus) and curing time for cold materials (20) and ITS on percentage of cement (21). Therefore no significant differences were found between technologies. Also ionic charge of emulsion doesn t induce any variation in performances. As demonstrated (12), the mechanical behavior of anionic emulsion treated materials is similar to those with cationic or foam bitumen both for dry and soaked ITS. Fatigue Past researches show that the mechanical behavior of bitumen emulsion treated materials positively evolves during time. (15). Dispersed bitumen particles change the shear properties of the material by significantly increasing the cohesion value whilst effecting little change to the internal angle of friction. It was demonstrated (21) (7) that by varying the test conditions of temperature, frequency and load, results could be different. In particular adding cement up to 3% there are no considerable effects on temperature susceptibility of recycled mixes. The fatigue life of foamed mixes seems to be higher than emulsion mixes at lower stress level (in indirect tensile fatigue configuration). A possible reason could be that foam mixes are stiffer at lower stress levels. In fact, according to higher stress levels longer fatigue life is usually registered for emulsion. According to some studies (7), fatigue failure of emulsion cold mixes exhibited plastic fracture, while foam cold mixes exhibit brittle fracture. This could be due to the viscosity of emulsion instead of the stiffer behavior of foam treated materials. Therefore further studies are needed to define the fatigue behavior of cold materials. COST ANALYSIS A simplified cost analysis was carried out in order to compare two road treatments: one with bitumen emulsion (70% of residual bitumen) and the other with foam bitumen (ordinary bitumen). Two mixes at 2% of total bitumen (in weight on aggregate) were compared but one more point percentage of bitumen emulsion was added to obtain the exact equivalence of bitumen content. Water and cement contents were selected using national requirements and according to literature review (7% and 1% respectively). The unit cost of emulsion and foam was initially compared based on results of a national investigation. Some B

5 of the most important refineries were interviewed regarding petroleum, bitumen and bitumen emulsion cost. National results demonstrated that binder cost is over 5 times the cost of other components so even one more point percentage of binder had a strong impact on final result (Figure 2). Before evaluating all costs related to the road treatment, some considerations on material cost were conducted. Figure 2 shows that the cost of the mix with emulsion was higher than the equivalent with foam due to the different content. In fact, the total costs were: /mc (emulsion) and /mc (foam). Cost [ ] Emulsion Foam Cem II Water Binder Percentage on total cost [%] 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% 6.3% 9.0% 0.5% 0.7% 93.2% 90.3% Emulsion Foam Cem II Water Binder FIGURE 2 Simplified Analysis. Incidence of elements on total cost per cubic meter of mix: A) costs in euros B) costs by percentage The results shown in Figure 2A refers to the average unit cost per ton of binder added: ordinary bitumen for foam and a solution of water, ordinary bitumen and a chemical additive for emulsion. From obtained results the choice of binder had a major influence on project cost (Figure 2B). In a second analysis, the costs related to the laying and compaction phases were added including transportation and manpower. For transportation, it was hypothesized to use heated tanks in cases of foam bitumen and traditional (not heated) tanks for bitumen emulsion by assuming an increment of 3 times for the heated. This hypothesis was based on the need for hot bitumen in the foaming process. The daily productivity was based on national values and set up on 1,350 mc/day for both treatments by considering one-lane-treatment of 0.2 m deep and a work shift of 6 h/day. Figure 3 A-B shows the weight of each element on the total cost per cubic meter of mix, in euro (left) and in percentage (right). Cost [ ] A A Emulsion Foam Material Manpower Equipments Transportation Percentage on total cost [%] 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% 7.3% 9.3% 11.9% 3.6% 77.1% Emulsion 17.6% 4.6% 68.5% Foam Material Manpower Equipments Transportation FIGURE 3 Detailed Analysis. Incidence of elements on total cost per cubic meter of mix: A) costs in euros B) costs by percentage As shown in these figures, material had the strongest impact on total cost (more than 50% over both technologies) inducing the following costs: (77%) and (68%) per cubic meter. Since the difference on total cost between foam and emulsion was reduced by the other elements (such as equipment that considers heated or non-heated tanks) the gap was relevant. The difference was lower than the first analysis but still significant. These analyses resulted an order of costs because the real cases are influenced by several other parameters that were not included in the analysis. Some of these variables could be: distance from quarry, percentage of RAP, exact amount of binder, binder availability (i.e. distance from refineries), work delays, time before reopening, weather conditions, etc. In particular delays could be considered as one of the main disadvantages for foam bitumen due to the presence of heated tanks. These tanks burn fuel to feed the heaters to maintain the bitumen. This means extra costs must be added. On the other hand, time may be critical for bitumen emulsion because it requires a minimum of curing to achieve required performances. B B

6 SAFETY Table 1 reported some of the factors of risk linked to a road infrastructure when cold materials are used. Each line refers to a specific subject involved in a potential hazardous situation, while each column refers to bitumen emulsion or foam bitumen. TABLE 1 Main Risks for Cold Recycling treatments SAFETY URBAN RURAL EMULSION FOAM EMULSION FOAM WORKERS & PEDESTRIAN Burns - VERY HIGH - VERY HIGH Scalds - HIGH - HIGH Shocks - HIGH - HIGH Inhalations LOW HIGH LOW HIGH ENVIRONMENT & TRAFFIC Discharge LOW HIGH LOW HIGH Fire and ignitions LOW MEDIUM LOW MEDIUM Ground pollution LOW - MEDIUM LOW Air pollution - MEDIUM - MEDIUM All reported values were quantified in an Italian survey conducted on companies dealing with pavements and infrastructures. LOW corresponds to a situation with no (or low) risk, while VERY HIGH means that the hazardous event has heavy consequences and the probability of an accident is high. Air and ground pollution were considered in terms of fumes released during the execution (for foam) and contaminated water released during curing period (for emulsion). Ground pollution was related to the geology of subgrade that could be generally found onsite. Due to this the water released could be contaminated by chemical additives or emulsifiers increasing pollutant levels in the soil. In an urban context the soil is usually paved and deep layers are already used for other purposes (e.g. presence of undergrounds, pipelines, etc.) so the filtration is minimal. Instead in other non-urban contexts most surfaces are unused or partially paved. In this sense, the effect of releases of polluted water must be considered during the breaking phase by bitumen emulsion. Hot temperatures mean serious consequences for workers and heavy damages for the environment and equipment in general. In particular if the temperature is extremely high (230 C F) the exposure for workers to volatile oils becomes dangerous. To this end the specific exposure condition would relate to the type and the chemical behaviour of the binder or the conditions of the working area itself. This means that by starting the foaming process, the area directly near the machine (reclaimer e.g.) may be saturated by toxic fumes (especially if the bitumen is modified). For that reason, some agencies measure the temperature just behind the reclaimer (or the paver) with infrared instruments. This practice is frequent in the plant to measure the thermal pollution product with the new hot mix asphalt. When the foam bitumen is used, the operations between tanks (containing hot bitumen) and the reclaimer are fundamental for safety. In particular it is helpful during transfer operations, to keep tank hatch covers closed, or at least lowered, to preserve a vapour-rich atmosphere above the binder. As a precaution against any sudden pressure build-up (from a trace of water), these hatches must be latched shut. In case of long transfers it is necessary to keep the tank of bitumen at a temperature not lower than the workability value (depending on class of bitumen), which means that during all the travel the tank must be set up in order to prevent all possible damage or discharge of bitumen. The low working temperature of emulsion induces less risk than the case of foam bitumen, in particular the use of the emulsion at room temperature (up to C) being synonymous with higher safety standards for workers. CONCLUSION Cold recycled materials are a good opportunity to save money and resources. Bitumen emulsion and foam bitumen are widely considered the best products to carry out asphalt mixes at room temperature but neither can be considered better in absolute terms. Based on the research conducted, the following conclusions may be drawn:

7 Bitumen emulsion and foam bitumen induced very different behavior in the mix. In presence of emulsion, both coarse and fine aggregate are completely coated by asphalt and the mix appears as a solution of water and bitumen. The tiny bitumen particles that are produced when the foamed bitumen bubbles burst have only enough energy to warm the smaller aggregate particles sufficiently to permit adhesion. The dispersed bitumen bubbles only partially coat the larger aggregate. High cement or lime content could induce a concrete-type behavior in the cold mix (high stiffness, low deformability) reducing the mechanical properties for long periods so a fine balance between the cement and bitumen must be considered by the addition of an active filler. The optimal strength and flexibility must be taken into account on the structural design. No unique curing procedure could be found for both emulsion and foam materials in the laboratory. The most used procedures suggest 72h at 40 C or 60 C (23). Temperature strongly influences curing time. By keeping the performance constant, an increase in temperature reduces time required for curing. Cured materials are less influenced by temperature in respect to traditional HMA due to the lower asphalt content and the presence of cement. The dosage for a traditional bitumen emulsion is higher the corresponding of foam due to the high water content of the first. From an economic point of view, a simplified analysis was not sufficient to evaluate which technology allows for more savings. In fact, too many parameters influence the real cost per cubic meter in a road treatment. A detailed economic analysis of each single case is required to identify the best solution. Binder choice is one of the key parameters to evaluate the total cost of cold treatments. Dosage is a direct consequence of binder choice. Foam bitumen seems to be more dangerous for workers due to the high temperature. In addition to risks related to discharges, equipment failures and fumes released are more dangerous if compared to the case of bitumen emulsion. Bitumen emulsion could pollute soil if the water released during the breaking phase is not controlled. REFERENCES (1) Asphalt Academy, Technical Guideline: Bitumen Stabilized Materials, TG2 Second Edition, May (2) AEMA, Asphalt Institute, Basic Asphalt Emulsion Manual, MS-19 Fourth Edition, January (3) Jenkins K. J., Mix design considerations for cold and half-warm bituminous mixes with emphasis on foamed bitumen, PhD Thesis, University of Stellenbosch, South Africa, September (4) Lee H. D., Y. Kim, Validation of the mix design process for cold in-place rehabilitation using foamed asphalt, Iowa Highway Research Board, Iowa, USA, June (5) Jenkins K.J., S. Robroch, M.G. Henderson, J. Wilkinson, A.A.A. Molenaar, Advanced testing for cold recycling treatment selection on N7 near Cape Town, 8 th Conference on Asphalt Pavements for Southern Africa (CAPSA'04), Sun City South Africa, September (6) Bocci M., A. Grilli, F. Cardore, A. Graziani, A study on the mechanical behaviour of cement-bitumen treated materials. In Construction and Building Materials, v. 25, n 2, p , February (7) Yan J., F. Ni, M. Yang, J. Li, An experimental study on fatigue properties of emulsion and foam cold recycled mixes. In Construction and Building Materials, v 24, n 11, p , November (8) P. Fu, D. Jones, J. T. Harvey, The effects of asphalt binder and granular material characteristics on foamed asphalt mix strength. In Construction and Building materials, doi: /j.conbuildmat , (9) Lee H. D., Y. Kim, S. Hwang, K. Jeong, Use of warm mix asphalt additives for cold in-place recycling using foamed asphalt, MAIREPAV 5, Park City, Utah, USA, 8-10, August (10) Liebenberg J. J. E.. A structural design procedure for emulsion treated pavement layers, Masters dissertation, Faculty of Engineering, University of Pretoria, April (11) Fu P., D. Jones, J.T. Harvey, S.A. Bukhari. Laboratory testing methods for foamed asphalt mix resilient modulus. In Road Materials & Pavement Design, 10(1), p , 2009.

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