GAS-FIRED ('INFRA-RED') HEATER FOR USE ON THE JOINTS

Transcription

1 ROAD RESEARCH LABORATORY Ministry of Transport RRL Report LR123 GAS-FIRED ('INFRA-RED') HEATER FOR USE ON THE JOINTS IN WEARING-COURSE ROLLED ASPHALT by B.F. Buglass CROWTHORNE ROAD RESEARCH LABORATORY 1967

2 CONTENTS Page Abstract 1 l Introduction Description of heater Tests and observations 3. 1 Site observations 3. 2 Laboratory measurements Discussion Conclusions Acknowledgements Q CROWN COPYRIGHT 1967 Extracts from the text may be reproduced provided the source is acknowledged

3 Ownership of the Transport Research Laboratory was transferred from the Department of Transport to a subsidiary of the Transport Research Foundation on I st April This report has been reproduced by permission of the Controller of HMSO. Extracts from the text may be reproduced, except for commercial purposes, provided the source is acknowledged.

4 GAS-FIRED ("INFRA-RED") HEATER FOR USE ON THE JOINTS IN WEARING-COURSE ROLLED ASPHALT ABSTRACT This Report describes observations and measurements made at sites where a gas-fired heater, attached to a paving machine, was used to heat the adjacent cold wearing-course material so that, wfthout cutting back and "painting", a sound waterproof joint would result. Further measurements were made under Laboratory conditions to assess the effectiveness of this heater. The results indicate that the heater attachment can produce a satisfactory joint without damage to the asphalt in normal working conditions. Tests were not carried out at low ambient temperatures but the results obtained suggest that the technique can be employed to make satisfactory joints in cold weather. 1. INTRODUCTION Wearing-course rolled asphalt material of low stone content, as required by the M.O.T. Specification Clause 901, is intended to be impervious towater and every effort is made to preserve this characteristic. When the material is laid by paving machine, a few passes of a roller, while the material is still hot, are sufficient to compact it into a dense impervious layer but the roller cannot venture too near the unsupported edge of the layer without spreading the hot plastic material laterally. A strip a few inches wide of relatively uncompacted material is therefore left at the unsupported edge. If the adjacent lane of asphalt is laid very shortly afterwards (for example using another paving machine in echelon), before the initial material has cooled significantly, the roller can now roll over the longitudinal joint between the two lanes and adequate compaction of the edge strip is achieved, as well as a knitting together of the material in the two fanes at the joint. In this way an impervious layer can be ensured right across the two lanes. This technique of laying lanes in echelon is practicable on new roads, but is unsuitable for resurfacing existing roads, where traffic

5 considerations normally prevent the use of two laying machines in parallel lanes. Normally, therefore, the paving contractor has to lay the second lane of asphalt alongside a cold layer, the outer strip of which is inadequately compacted. The heat provided by the new asphalt is generally inadequate to warm up this strip sufficiently for it to be compacted by the roller, and, if suitable steps are not taken, a band of pervious material will remain alongside the longitudinal joint. The joint itself may be poor, as hot asphalt does not always adhere readily to the cold material, particularly if the latter has been contaminated in any way. The relevant British Standard (B.S. 59) therefore requires all unsupported longitudinal (and transverse) edges, in the wearing course to be cut back to fully compacted material and the vertical face so obtained is then painted with asphaltic cement so that the new hot asphalt will adhere to give a permanent waterproof bond. Such a procedure is wasteful of material and manpower, and causes delay. Much economy could be made if the inadequately compacted strip at the unsupported edge of the material already laid could be reheated while the adjacent asphalt is being laid, so that the roller could then compact it together with the new asphalt, giving an impervious layer right across the joint. A gas-fired heater has been developed for this purpose, this Report describes some preliminary investigations into the efficiency of the device. and The Report includes comments on four site-vis'its, the examination of cores cut from the area of the joint and also measurements rhade in the Laboratory of the temperature increase in asphalt wearing-course material subjected to the heater. 2. DESCRIPTION OF HEATER The particular type of heater investigated in the Report is illustrated in Plate l, which shows an 8ft long heater attached to the side of a paving machine. Fig. i shows diagrammatically the principle of the heater. Such heaters are currently being made in Great Britain under licence from America, by Millars' Machinery Co.'Ltd. The units suitable for attachment to paving machines are made with the burners mounted in pairs, each pair being 2ft long. The complete heater units are 6 or 8 feet long and are suspended by adjustable chains from brackets on the side of the paving machine. The most recent models have a single lever

6 ~Adjustfarbal.e sufp;an vs~orn to, bguarsn-~irrt d ~ ~ V I q~"~-joi nt heat er 11.), L ~, / 2\~./\../'~.,/~../~/\/'\,/~,/'\ ~,y" "~/'~/~/\/"~," y"\/,,/,",,/\ A./~ ~/,/ -,/ -~.-.,.. "~-- ~-Radiat[on., WEARING-COURSE. \, " ~/~./'\,/\'~ ~ \, ~/"x ~, \., \, \ ", BASE-COURSE Fig.1. DIAGRAMMATIC CROSS SECTION OF JOINT HEATER IN OPERATING POSITION Road Research Laboratory M o.t. R PLATE Joint heater mounted on a bituminous paving machine I 3

7 controlling the height. Continuously firing sparking-plugs ignite the gas, carried as liquid propane. An automatic cut-out device can be fitted so that the gas supply is turned on only when the main clutch of the paver is engaged and the paver is moving. These heaters are usually termed 'infra-red' heaters Site observations 3. TESTS AND OBSERVATIONS Four sites were visited where a joint heater was being used. Site A Weather conditions - warm and sunny. At this site the laying gang were not experienced with rolled asphalt. Two heaters were being used, an 8ft long unit attached to the side of the paver and an independent mobile unit for the transverse joints. Cutting back of the transverse joint was still necessary to preserve good riding quality. All the joints appeared quite satisfactory. A thermistor probe and a thermocouple probe were used to measure the temperature increase of the cold material and the results are given in Table i. Temperatures of wearing-course asphalt, taken in the vicinity of the longitudinal joint when an 'infra-red' joint heater was being used on normal site work Cross Section Penetration of probe (inches) Temperature in.~c (mean of three) I Cold material heated New hot material Joint by 'infra-red' heater l O O i00 8O 6 Distance I from joint joint I (in) l Shade temperature 17 C Road temperature 28~C Temperatures were measured close behind the paver.

8 Site B Weather conditions - cool and damp. The work was inspected after completion; the joints appeared satisfactory. Site C Weather conditions - cool and bright. The 'infra-red' jointheater was on trial at this site but th'e laying gang did not cooperate and after a short run the contractor's agent discontinued the use of the heater. The subsequent joints made by traditional methods were of a poor standard. Site D Weather conditions - mild and dull. At this site the position of the longitudinal joint could not be detected after rolling but, from measurements to the kerb, cores were cut over the joint. The cores showed no sign of a joint and the material was impervious to water Laboratory measurements Two sample blocks of fully compacted wearing-course asphalt were made up in the Laboratory, each 2in thick and 12 in square. Holes were drilled from the underside with a tungsten-carbide-tipped drill and thermocouples inserted at precise positions. The holes were grouted with hot bitumen. A standard, 6ft long, 'infra-red' heater was used in a static position to heat the edge of each sample and the temperature changes were recorded on a Kent multi-channel recorder. The heater was operated for 36 secs in the first test and for 5 secs in the second test; these heating times correspond to paving speeds of 13½ft/min and 10½ft/min with a heater 8 ft long and 10ft/min and 8ft/min and a 6ft heater. In this test, however, no hot material was placed against the jointing edge and there was no transfer of heat from adjacent material. The positions of the sensing thermocouples records are shown in Fig. 2. and the temperature THese temperatures cannot be compared directly with those given in the Table in Section The precise shape of the laboratory test samples probably influences the effect of the heater. On the road, theredge of the cold material would not be vertical and would present a greater area to the heater.. DISCUSSION Asphalt has a low thermal conductivity (of the order of 0. 5 Btu/ft.h. deg F) and its thermal capacity is about 0.2Btu/lb/deg F, so that it is not to be expected that a.moving heater can raise the temperature of the whole thickness of the asphalt layer by many degrees if severe burning of the surface is to be avoided. The laboratory tests made with thermocouples

9 embedded in the asphalt samples (Fig. 2) suggest that with a surface temperature of 170 C to 180 C (which is about as high as practical i without risking burning), the rise in temperature about X in below the i surface is 25 to 30, and that 15 in below the surface is only 10 e to 15 C. The heat supplied by the adjacent new asphalt will of course augment the temperature still further. The results in Fig. 2 may be used to calculate approximately the 3 rise in temperature, due to the heater, to be expected about ~ ~n below 1.- i the asphalt surface namely 25 C xin from the joint and 20 ~ C lxin from the joint. Using the approximations it can then be deduced from the

10 200 0 ~5o / ~.100 E {g.50 No 1 'No2 ',No3 No No6 No?.No5 200 "A-mbient temp.26 C 0 [ 1 I I I I I I: I I Time (s) Test 1 Time of heating 36s Corresponds toa6ft heater moving at 10ftlmin or an 8ft heater moving at 131/2ft/ rain ~ll3////llliilllllilijji H e ate r No, x Position of thermocouptes ~ NoS x No x No3X No2x ~V2 _. (cross section of sample). Sample I/ill/ No8 x No7 X No6 X ----[1/2" C,I 01 P 150 ~ 100,1 E 50 I I l~ I 3" "s_ I- 5" I No 1 ~ /'~/No2 /., No 3 /No ~No5 o7 I I I I IAmbie~ t = I I O Time (s), Test 2 Time of heating 5s Corresponds to afft heater, moving at eft/rain or an 8ft heater moving at 101/2 ftlmin Fig,2. RATE OF INCREASE OF TEMPERATURE IN SAMPLES OF ROLLED ASPHALT UNDER AN INFRA- RED HEATER Rood Research Laborotory M, o.t. R

11 ! results in Table l that, on the road, at a distance of ~ in from the joint, the heater is responsible for about one-third of the rise in temperature i at this level and that at a distance of 13 in from the joint the heater is responsible for about one half of the rise in temperature at this level. These calculations probably underestimate somewhat the effect of the heater because on the road the unsupported edge of the existing asphalt presents a greater area to the heater than the laboratory specimen. The temperatures measured during the site trials show that the combined effects of the fresh hot asphalt laid alongside and the joint heater are sufficient for good compaction. This is confirmed by the other observations made on the road. It must, however, be pointed out that the ambient temperatures were not as low as can be expected in winter work and further observations are required to make sure that: the heater is effectiveness of the heater is of course dependent on its length and the speed of the paver and there seems little doubt that if these parameters are adjusted, adequate heating can be achieved even under adverse conditions. 5. CONCLUSIONS l. The 'infra-red' heater can contribute substantially to the heating of the cold material on one side of a longitudinal joint in asphalt; up to about one half of the rise in temperature in this material being caused by the heater, the remainder of the rise being contributed by the freshly laid hot material.. In practice, when the heater was used, the finished joints were of good appearance and were waterproof.. The heater did not burn or overheat the asphalt when moving at normal paving speeds.. Observations were not made during this investigation in cold weather but it is expected that suitable adjustments to the length of the heater and/or the speed of the paving machine will enable good joints to be obtained even in these conditions. 6. ACKNOWLEDGEMENTS These observations and measurements were carried out with the co- operations of Millars' Machinery Co. Ltd., who also supplied the heater unit for the laboratory tests. 8