An Experimental Investigation of Laser Scabbling of Concrete
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An Experimental Investigation of Laser Scabbling of Concrete B. Peacha,1,∗∗, M. Petkovskia,2,∗, J. Blackburnb, D.L. Engelbergc aDepartment of Civil and Structural Engineering, Sir Frederick Mappin Building, Mappin Street, University of Sheffield, Sheffield, S1 3JD, UK bTWI Ltd, Granta park, Great Abington, Cambridge, CB21 6AL, UK cResearch Centre for Radwaste & Decommissioning, School of Materials, The University of Manchester, Manchester, M13 9PL, UK Abstract Laser scabbling of concrete is the process of removal of surface material using a high power laser beam. The main aim of this investigation was to establish an experimental procedure for assessing the effects of various parameters that may be critical for the effectiveness of the process, such as material composition and initial moisture content. The study shows that the key characteristics of the process can be detected by monitoring surface temperature variations. This experimental procedure is used to provide data on the effects of each parameter to explain the mechanisms that drive the process. The results suggest that scabbling is mainly driven by pore pressures in the cement paste, but strongly affected by other factors. Reducing permeability by adding PFA to the cement paste resulted in significant increase in volume removal; but reducing moisture content by air-drying of the material did not result in the expected reduction in volume removal. Keywords: Concrete, Cement, Scabbling, Spalling, Thermal Analysis 1. Introduction eration equipment to be stationed away from contami- nated areas. As no reaction forces are developed during Concrete is widely used in the nuclear industry, of- laser scabbling, relatively lightweight deployment sys- ten in direct contact with radioactive materials. Con- tems can be used, making remote handling a real pos- crete contamination tends to extend 8-12mm from the sibility. Laser scabbling and surface glazing, with sub- surface [1]. Removal of the contaminated surface re- sequent surface removal, was used successfully in the gion can reduce the volume of waste requiring con- decommissioning of the Japan Atomic Energy Research trolled disposal. Mechanical scabbling and high pres- Reprocessing Test Facility [2]. sure water jetting have been used for surface removal Laser scabbling is comparable to explosive spalling in nuclear decommissioning. Water jetting produces exhibited in concrete exposed to fire conditions, but large volumes of secondary waste, whereas mechanical the large difference in heating rates would be expected scabbling involves large reaction forces, requiring ex- to produce different mechanisms driving the two pro- tensive deployment systems unsuitable for remote han- cesses. dling. Application of a high power laser on a concrete surface causes concrete fragments to explosively spall from the surface. Lasers of wavelengths around 1 µm 1.1. Laser Scabbling can be transmitted down optical delivery fibres several Laser scabbling was discovered in the early 1990’s hundred metres in length, allowing expensive laser gen- during concrete glazing investigations [3]. The phe- nomenon was attributed to the rapid expansion of wa- ter from cement paste dehydration. Blair [4], however, ∗Corresponding author ∗∗Principal corresponding author found scabbling to take place only in locations where Email addresses: [email protected] (B. Peach), limestone aggregate was close to the surface. A thicker [email protected] (M. Petkovski), mortar cover created a glazed layer preventing scab- [email protected] (J. Blackburn), bling. Blair concluded that the scabbling was driven by [email protected] (D.L. Engelberg) 1Tel: +44 (0) 114 222 5729 the pressure of gas produced by decarbonation of cal- 2Tel: +44 (0) 114 222 5759 cite. Preprint submitted to Construction and Building Materials April 28, 2015 MacCallum and Norris[5] suggested that the mech- several studies [5, 7, 8, 9]. Trials conducted using iden- anism responsible for laser scabbling was the build tical laser parameters on concretes of identical composi- up of strain in the interfacial transition zone between tion yielded vastly different outcomes, ranging between the aggregate and hardened cement paste. Multiple scabbling, vitrification and faint heat marks. passes were ineffective suggesting a critical strain may Laser beam parameters are critical for laser scab- be reached on the initial pass. The authors concluded bling of concrete. A number of authors have identified that scabbling was caused by expansion of aggregates, threshold power densities at constant scan speeds, de- and hence the beam diameter had to be sufficiently large fined as laser power per unit area, below which con- to heat whole aggregate pieces. crete scabbles and above which the surface vitrifies Dowden et al [6] investigated, through modelling, a [2, 4, 5, 10, 11]. Vitrification relieves strains prevent- mechanism of laser scabbling based on the hypothesis ing scabbling [5]. Blair [4] suggested that beam diam- that the laser beam causes expansion of a thin surface eter had to be large enough to heat entire pieces of ag- layer of concrete which induces internal stresses. Ten- gregate. MacCallum and Norris [5] went further, sug- sile stresses are formed beneath this layer, which are gesting that laser scabbling was not a scalable function relieved through fracture and buckling of the surface, with power density and heat input, but also depended resulting in ejection. on the ratio between beam diameter and aggregate size. The effect of type of coarse aggregate on scabbling Blair [4] also found that trials with high power density has been highlighted in several studies. Nguyen [7] and high scan speed resulted in the same concrete be- found that granite concrete consistently scabbled to a haviour as low power density and low scan speed tri- depth of 1.5 mm, whereas whinstone (basalt) concrete, als; suggesting that the rate of energy emitted by the cored from a nuclear facility currently being decommis- laser, to a unit area of concrete, was the key factor deter- sioned, failed to scabble for any set of laser parame- mining the scabbling of concrete. Experiments carried ters. A vitrification threshold power density was identi- out by MacCallum and Norris [5] using laser powers in fied for basalt concrete at 103 W=cm2, at higher power the range 200-1000 W and beam diameters of 0.05 mm densities the basalt melted, creating a glazed layer that and 26 mm also showed that the process was depen- prevented scabbling. It is not clear, however, why lower dant on heat input and diameter size. For a single pass power densities were insufficient to initiate scabbling. A on limestone concrete (with laser beam diameter of 60 number of factors could be responsible, such as damage mm, power 5 kW, scan speed 200mm/min, and power caused during coring (leading to increased permeabil- density 177W=cm2) Hilton [9] achieved removal rates ity and reduction of pore pressures) or the laser being of up to 217 cm3/min. Johnston and Spencer [12] com- applied to a cut rather than a cast face (different sur- pared the scabbling performance of a CO2 laser and a face conditions). Hilton [8] reported similar results for Nd : YAG laser, with wavelengths of 10.6 µm and 1.06 basalt concretes, however limestone concrete was seen µm respectively, and found very little difference: iden- to scabble successfully for a large range of laser param- tical laser parameters yielded similar removal rates with eters (120-960 W/cm3). slight variations in trough profiles. The effect of degree of saturation on laser scabbling has been investigated in several experiments. Blair 1.2. Explosive Spalling [4] found that specimens of limestone concrete soaked overnight experienced a greater extent of scabbling. Explosive spalling is a phenomenon observed in con- Hilton [8] also found that a wet surface increased vol- crete structures exposed to fire conditions, where the ume removal of limestone concrete specimens but had heating rates are much lower than those in laser scab- no noticeable effect on basalt or granite concretes. John- bling. Connolly [13] found the temperature at the on- ston found that laser scabbling was ineffective on spec- set of spalling to be higher for a lower heat rate (vary- imens that had free water removed by heating prior to ing from 130◦C for 75◦C=min to 340◦C for 25◦C=min), testing (reported in [6]). Rather than claiming that this whereas other authors reported that surface tempera- was due to a reduction in pore pressure spalling, Dow- tures at the onset of spalling are between 300 − 425◦C den suggested that the ineffectiveness was due to a re- [14] dependant upon material composition rather than duction in the coefficient of expansion, a reduction in heat rate. For laser scabbling heat rates are typically the Youngs modulus, an increase in density, an increase around 200◦C=s. The fragment sizes in the two pro- in specific heat capacity or a reduction in strength, as a cesses are also very different: 10 mm depth and 30 mm result of drying. width [8] for scabbling, and 100 mm depth and 200-300 Stochastic scabbling behaviour has been identified in mm width [14] for spalling. 2 It is generally accepted that high strength concrete a high heat rate may result in formation of microcracks (water/cement<0.4) has a tendency to spall more than which release pore pressures, as shown by Phan [18], normal strength concrete (water/cement>0.4) [15, 16, who found that specimens heated at 25◦C=min devel- 17, 18]. This is attributed to the higher permeability oped smaller pore pressures than specimens heated at in normal strength concrete, which allows dissipation rates of 5◦C=min. of water pressures within the pore network. Many au- It should be noted that spalling studies have identi- thors have highlighted the relationship between mois- fied stochastic behaviour, in that ‘identical’ specimens ture content (degree of saturation) and spalling, with exposed to ‘identical’ conditions did or did not spall for a higher moisture content leading to a greater degree unexplained reasons [14].