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Selected Engineering Properties and Applications
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4. Design and Construction ConsiderationsAlthough geofoam has been utilized successfully in engineering applications failure is reported in very few of them. Fire, block flotation, differential icing of pavement are main causes of failure (Horvath, 1999a). Some potential items should be taken into account when utilizing geofoam in engineering applications. Engineers and technicians should be aware of such concerns when designing and during all con-struction phases. Design and construction considerations can be summarized in the following points:
4.1 Buoyancy 4.1 Buoyancy EPS geofoam is a super lightweight material, as was mentioned earlier with practical density ranges between 11 and 30 kg/m3. Uplift force due to buoyancy effect can be a reason of failure during or after construction. Enough surcharge can be pro-vided or the ground water table has to be controlled during and after construction to avoid instability against uplift. Utilizing uplift resisting anchors such as concrete slab anchored to the ground (Ninomiya and Makoto, 1996) or geotextiles (George, 2000) can be a useful alternative to solve the buoyancy problem. In all cases all design as-pects such as the strength limit and the limiting strain must be checked to avoid creep while solving the buoyancy problem. 4.2 Concentrated Loads EPS geofoam is a material that punctures easily. Direct application of concen-trated loads should be avoided. Placing of a concrete slab and providing an adequate thickness of the cover fill material are two solutions to avoid such problem. The bet-ter the load distribution is the less the strain will be. The cost and the time consumed while placing the concrete slab or the extra fill cover should be taken into considera-tion. Also, the heavier the dead load on the foam mass the more strain; hence the more creep will be faced. A good engineering judgment has to be taken. Test results reported by Nishi, et al., (1996) show that the concrete slab on the top of the EPS geofoam mass has a high effect in load distribution. Angles equal 50~70 degrees with the vertical presents the load distribution within a concrete slab. In the foam it self, the angle is reduced to 20 degrees for the same applied load. 4.3 Chemical Attack As was mentioned earlier, EPS geofoam show different resisting behavior to-wards organic and non-organic fluid and substances. Geofoam dissolves in gasoline and other fluids or their vapor that may exist during construction or while in service. Also the surrounding soils could be contaminated and contain EPS geofoam solvents. Utilizing a proper cover to the EPS geofoam mass will solve the problem. Solutions such as rapping the foam blocks in plastic sheets or covering with geotextiles or plac-ing a concrete slab on the top of geofoam are engineering wise feasible (Negussey and Elragi, 2000a). 4.4 Flammability As was mentioned earlier geofoam is a combustible material. Although fires is unlikely hazard in service it can be easily occur during construction or in storage. Since EPS geofoam is utilized in the construction site with other engineering materi-als such as steel, usage of open flame such as welding is likely to be occurred. Includ-ing the fire-retarding additive during production can be a solution against small igni-tion source (Huntsman, 1999h) but not complete fire prevention. In such case the cost of EPS geofoam block will be increased by 5 – 10%. If operations utilizing a source of ignition such as welding, fire watch procedures must be followed. In all cases all workers and incorporating persons in the site or using the storage area has to be aware of the flammability property of the substances they are dealing with (Negussey and Elragi, 2000a). 4.5 Ultraviolet Degradation Construction conditions may require long storage duration of geofoam uncov-ered or foam can be exposed to sunlight after completing one construction phase and waiting for the other phase in schedule. As ultraviolet degradation has effects on the interface friction between EPS geofoam blocks and between some other engineering material and EPS geofoam blocks, it can be avoided by covering the geofoam mass with opaque sheeting during extended periods of open-air storage. As the affected surface is eroded or washed away a non-affected surface can be exposed and the ad-hesive bond strength is improved (Negussey and Elragi, 2000a). 4.6 Differential Icing Having one portion of the road with icy condition while adjacent portions are ice-free is known as differential icing condition. Differential icing was first encoun-tered in practice in the 1960s (Horvath, 1995c). This phenomenon normally occurs between typical grades supported pavement and bridge decks. Similar situation oc-curs between bad designed insulated pavements and pavement portions that are not insulated. Differential icing is considered to be a safety issue. On roads vehicles driv-ers do not expect to encounter sudden pavement icing when a road is elsewhere ice-free. Differential icing is somewhat recognizable by motor vehicles drivers for the case of bridge-deck icing, as the visual change between the two different surfaces is normally clear especially if proper warning signs are utilized. On the contrary, the sudden change in the icy condition for the case of insulated portions are totally unex-pected by the drivers and can cause serious accidents. EPS geofoam below pavements has a tendency to hinder frost penetration and restrict the upward flow of ground heat during slightly sub freezing temperature days. This can result in different ice formation rates and melting between areas that do and do not have EPS geofoam. Increasing the capability of the base and or the subbase material to provide some ground heat flow to the pavement surface can minimize the extent of differential icing on traveled roadways. One way to do so is to provide an adequate thickness for the base and/or the subbase taking in consideration that in-creasing the thickness above a certain amount can result in undesirable immediate or creep settlement. 4.7 Insect Infestation EPS geofoam is non-nutritive to any living organism (Horvath 1999a). There-fore there is no potential that EPS placed on or in the ground will be consumed as a food source. On the other hand insects sometimes nest in foam. Carpenter ants and termites have been encountered around wood framed residential structures. EPS geo-foam used as thermal insulation is likely to be attacked by insects such as termites and while tunneling through or nesting on their way to wood structural elements. Pro-jects located far from the existence of wood structural elements such as road insula-tion or Embankment fills have no known evidence of insect damage. Appropriate pro-tection methods need to be determined based on project location and situation. Con-sultation with EPS geofoam manufacturers can result in solutions to avoid insect in-festation such as utilizing additives to geofoam blocks while processing to make them resistant to such attacks (R-Control, 1999b, 1999d). 4.8 Moisture Absorption As was mentioned earlier EPS geofoam tends to absorb a little amount of moisture over time. Practically less than 10% by volume is absorbed in the life of ser-vice for geofoam. A 10% value will increase the density of geofoam to approximately 100 kg/m3 for type VIII geofoam. In applications when geofoam is utilized as light-weight fill. It is important to take into account the increased density, as this will be on the conservative design side. It’s important to note that some degradation in thermal properties may occur with increased moisture absorption. (Negussey, 1997). 4.9 Gaps Between Blocks During construction and while placing EPS geofoam blocks strong attention should be made to accurately place the blocks with the minimum amount of gaps be-tween individual EPS geofoam blocks and successive EPS geofoam layers. The exis-tence of gaps will create stress concentration in the area of contacts. Excessive imme-diate strains and large creep strains will result from such gaps. The resulting overall settlement may result in unsatisfactory situations such as decreasing the lifetime of a pavement structure (Negussey and Elragi, 2000a). 4.10 Immediate Deformation The elastic modulus of EPS geofoam is small compared to the elastic modulus of some other engineering materials such as concrete, wood and steel. EPS geofoam working strains are normally between 0.4% and 1% strain. A 0.5% immediate strain was recorded for a 9.12 m EPS height backfill (Cho, et al., 1996). This 0.5% is con-sidered both elastic strain and gap closing. For high fill of EPS geofoam, the immedi-ate elastic deformation may be a considerable. Some projects have utilized more than 10m-fill height of EPS geofoam. In such cases, more than 0.1m of immediate defor-mation was measured. The designer has to take that in to account in calculations (Ne-gussey and Elragi, 2000a). 4.11 Connections with Structural and Architectural Elements According to the design of a project EPS geofoam may experience large set-tlements from either gap closure, immediate elastic deformations or creep deforma-tions as previously mentioned. Adjacent structural or architectural connections such as fascia walls, which may be attached to an EPS geofoam embankment, e.g. may be damaged due to differential movement. The designer has to be aware of such a prob-lem and detailed drawings have to provide solutions to such cases. Slotted or pinned connections may be a solution, where deformation can take place in the EPS geofoam structure without damaging the adjacent connections and elements (Negussey and El-ragi, 2000a). 4.12 Sliding The density of EPS geofoam is very low. Hence during construction the whole EPS geofoam mass can slide under the effect of any lateral force, if nothing is placed on top of it to increase the frictional normal force. This situation can happen in the case of backfilling while the foam layers are still uncovered. The construction se-quence should take this into consideration (Negussey and Elragi, 2000a). 4.13 Blocks Alignment To increase the integrity of the geofoam mass, blocks have to be aligned in such a way that the vertical and horizontal joints between the blocks must not be con-tinuous. Traditionally if multiple layers of geofoam are required, successive block layers are placed perpendicular to the previous layer. Also offset vertical block joints between layers (Negussey and Elragi, 2000a). 4.14 Transition Zones For designs that incorporate both expanded polystyrene fill and traditional fill materials, differential settlement can be a problem where the two types of construc-tions are transitioned (Thompsett, et. al., 1995). This can be overcome by reducing the number of EPS geofoam layers, layer by layer along the length of construction. The length of this transition zone depends on the structure and the calculated settle-ment rate. This type of construction detail is commonly used for geofoam approach ramps to bridges. |
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