混凝土外文翻译---力学性能和热性能评价超高纤维钢筋混凝土的工程应用.docx

外文原文二mechanical and thermal evaluation of ultra high performance fiber reinforced concretes for engineering applicationsvaleria corinaldesi , giacomo moriconidepartment of materials and environment engineering and physics, universit politecnica delle marche, via brecce bianche, 60131 ancona, italyabstract：ultra high performance fiber reinforced concrete (uhpfrc) is a cement-based material, which behaves like a low-porosity ceramic material with excellent mechanical performance. this work was aimed to study soft cast (flowable at casting time) uhpfrc s and, in particular, the time development of compressive strength, flexural strength and elastic modulus was monitored for uhpfrc prepared by varying the water to cement ratio from 0.20 to 0.32. silica fume, steel fibers and acrylic-based superplasticizer were employed to prepare the uhpfrc mixtures. optimum workability and mechanical performance were obtained with a water to cement ratio of 0.24. thermal conductivity was determined for the same uhpfrc, in the presence and in the absence of steel fibers. the scope was to evaluate the effect of steel fibers on the thermal conductivity coefficient, in order to predict the uhpfrc capacity for heat loss. this information as well as its drilling characteristics, in order to test its suitability to be machined, could be essential for possible fields of application such as in mechanical engineering, where uhpfrc materials can be employed as high abrasion-resistant dies in the molding process of metal and polymer products. keywords：machine workability；mechanical performance；uhpfrc silica fume；thermal conductivity1.introductionultra high performance fiber reinforced concrete is a special cement-based material which behaves like a low-porosity ceramic material with excellent mechanical performance. in particular, it is a superplasticized silica fume concrete, often reinforced with fibers, with improved homogeneity because traditional aggregates are replaced with very fine sand up to 400 lm 1. according to richard and cheyrezy 1, if soft cast and cured at room temperature, its compressive strength can achieve 200 mpa. in fact, uhpfrc represents the highest development of high performance concrete (hpc) and its ultimate compressive strength depends on the curing conditions (either standard curing or steam curing or autoclave curing 2,3), on possible thermal treatments 4,5 as well as on the manufacturing technique adopted, and its value could rise up to 800 mpa in the case of compressive molding 6.just to give an idea of the excellent mechanical performance of uhpfrc, the stressstrain curves of an ordinary performance concrete (opc), of a high performance concrete (hpc) and of an ultra high performance fiber reinforced concrete (uhpfrc) are shown for comparison in fig. 1 7. in most industrial countries hpc materials are currently employed in infrastructural engineering works where heavy static and dynamic stresses or severe environmental aggression have to be counteracted, such as in sea platforms for oil extraction, long span bridges, undersea tunnels, and skyscrapers in seismic areas.however, uhpfrc materials show, although in an experimental phase with some field tests, much higher performance than hpc. particularly, besides extraordinary compressive and flexural strength (see fig. 1), very high ductility as well as toughness and fracture energy 24 encourages new applications for these materials, competing with innovative ceramics and structural metals in the field of mechanical and environmental engineering, as well as civil and building engineering 7.the following applications are promising in relation to uhpfrc materials utilization 8. in environmental and chemical engineering, highly reliable containers can be advantageously produced to store hazardous (toxic, inflammable, etc.) fluids or solids, since the use of uhpfrc materials, in which negligible diffusion of ionic and molecular species occurs, can allow to make insignificant the release of toxic or radioactive wastes from the container to the environment.in civil engineering, construction of extraordinary buildings, whose sizes or location require very high performance in terms of both mechanical strength and ductility as well as toughness, may be realized with uhpfrc materials. as a matter of fact, in tokyo, owing to a lack of available areas, plans of buildings as high as 1000 m are being reliably studied. in mechanical engineering, high impact-resistant products, against burst or shot, or high abrasion-resistant dies in the molding process of metal products, such as steel sheets, can be successfully developed. actual dies are characterized by very high unit cost which, for their economic amortisation, need high volume production lines. this fact, for instance, prevents supply flexibility in relation to coachwork changes in the car industry. the use of uhpfrc materials, depending on the strength level requirements, can allow the production of cheaper prototypes, and mediumlow volume dies. remarkable interest is also emerging in the plastics industry for the production of dies, whose requirements could be easily met by adequately adjusting the mixture composition and proportioning.this work was aimed to study soft cast uhpfrcs, in particular their mechanical performance, their thermal conductivity, in order to predict uhpfrc capacity for heat loss, as well as their adaptability to machining processes. in fact, aim of this paper is also to investigate some drilling characteristics of uhpfrc in order to assess the capability of this material to be machined by conventional tools.in terms of uhpfrc mixture proportion optimization, the attention was focused on the effect of the type of superplasticizer used and of the water to cement ratio (ranging from 0.20 to 0.32) on uhpfrc performances. in order to reduce the price of producing uhpfrc, local natural sand was used as replacement material for the more expensive silica sand normally used to produce uhpfrc, similarly with the attempt made by yang et al. 9. generally, due to the limited available resource and the high cost of silica fume, many authors also tried to reduce uhpfrc cost by searching for the substitution of silica fume by other materials with similar functions such as ground granulated blast furnace slag (ggbfs) 24,10, ultra fine fly ash 2,4,10, rice husk ash 11. however, in this work the only mineral addition tried was silica fume, besides to cement. concerning the kind of curing, standard curing at 20 _c was chosen (without thermal treatment), which represents the cheapest way of producin g soft cast uhpfrc.2. materials and methodscommercial portland-limestone blended cement type cem ii/a-l 42.5 r according to the european standards en-197/1 12 was used. the blaine fineness of cement was 0.42 m2/g and its relative specific gravity was 3.05. its chemical composition is shown in table 1. silica fume powder with a specific surface area of about 18 m2/g, evaluated by means of bet surface method, and a relative specific gravity of 2.20 was used. the chemical composition of silica fume is also shown in table 1. as aggregate, well-graded very fine natural sand was used with particle size up to 100 lm. the steel fibers used in this work were 13 mm long and 0.18 mm thick with an aspect ratio of 72, just like those employed by richard and cheyrezy 1. two acrylic-based superplasticizers (labeled spa and spb) were employed in order to compare their effectiveness for producing uhpfrc. they were both constituted of a carboxylic acrylic ester polymer in the form of 30% aqueous solution, but type b is a new formulation promising to be more effective in reducing water dosage.2.1. uhpfrc mixture proportionsseveral uhpfrc mixtures were prepared by varying the water to cement ratio from 0.20 (rpc-20) to 0.32 (rpc-32) by using an acrylic-based superplasticizer at a very high dosage of about 5% by weight of cement in order that adequate workability could be achieved. the materials with a water to cement ratio of 0.24 (rpc-24) and 0.26 (rpc-26) were prepared by alternatively using the two types of acrylic-based superplasticizer, in order to compare their effectiveness in improving workability at a constant water dosage. uhpfrcs with all other water to cement ratios were prepared by using only the superplasticizer labeled spb, which appeared to be more effective on the basis of preliminary results. the cement to sand ratio was 1:1 (by mass) in every case. for each mixture, the dosages of steel fibers and silica fume were maintained equal to 20% and 25% by weight of cement, respectively. uhpfrc mixture proportions are reported in table 2. workability of uhpfrcs at the fresh state was monitored by means of the flow table according to the procedure described in en 1015-3 13 and results obtained in terms of consistency of fresh mortars are also reported in table 2. the rpc-20 was not at all flowable; however, the uhpfrc workability gradually increased with higher water to cement ratio, up to rpc-32 characterized by very good flowability. this attempt of combining excellent mechanical performance and high workability was conducted also by other authors 14 with good results. the expected higher effectiveness of spb with respect of spa is confirmed by data given in table 2.2.2. preparation and curing of specimensthree prismatic specimens (40 by 40 by 160 mm) were manufactured for each mixture and for each curing time in order to evaluate mechanical behavior of the seven uhpfrc mixtures (table 2): they were soft cast in steel forms (vibrated for 30 seconds after casting), then wet cured at 20 _c (standard curing) for flexural and compressive strength measurements.moreover, flat circular specimens (diameter of 200 mm, 20 mm thick) were cast and cured in the same way as previous ones for drilling tests (see below).finally, flat circular specimens (diameter of 200 mm, 30 mm thick) were manufactured, cast and cured in the same way as previous ones and subsequently suitably polished (by sanding the bases) for thermal conductivity tests.3. results and discussion3.1. compression testcompressive strength was evaluated according to en 1015-11 15 after 1, 3, 7 and 28 days of curing.as a first step, the compressive strength of uhpfrcs prepared with different types of superplasticizer was compared, in order to determine the most effective admixture. the results obtained for uhpfrc with w/c of 0.24 and 0.26 by using either spa or spb superplasticizers are given in fig. 2. a slightly better result, with concern to homogeneity and strength development, was detected when the spb type was employed from an early age.then the uhpfrcs containing the spb admixture were prepared by varying water/cement from 0.20 to 0.32. the time evolution of their compressive strength is shown in fig. 3. it can be noticed that after 1 day of curing the compressive strength was always higher than 30 mpa. it is quite evident that the positive effect obtained by lowering the water to cement ratio was not valid for uhpfrcs with water to cement ratio lower than 0.24. in fact, the concrete prepared with w/c of 0.20 showed the lowest compressive strength. the reason lies in the low compaction capacity of the material due to the poor workability of the fresh concrete (see table 2). an even higher amount of superplasticizing admixture would be necessary in this case.3.2. bending testflexural strength was evaluated according to en 1015-11 15 after 1, 3, 7 and 28 days of curing.the modulus of rupture (mor, in mpa) was obtained as follows: here l is the maximum load applied (n), d is the distance between the supports (100 mm) and e is the edge of the specimen (40 mm), in order to obtain average information on the elastic behavior before first cracking.as can be observed in fig. 4, the results obtained by flexural tests confirm the higher effectiveness of the spb superplasticizer with respect to that labeled spa when a water to cement ratio of 0.24 is adopted. on the other hand, by using the spa type, higher strength was obtained both in compression and bending, with a w/c of 0.26 instead of 0.24. the reason can be ascribed to the values of fresh mortar workability (see table 2). in fact, the best mixtures, whatever the kind of superplasticizer employed, were characterized by slump flow values in the range 2122%, which is likely the best to achieve high compaction rate when vibration is applied for 30 s, as in the case of this experimental work.the time evolution of modulus of rupture is shown in fig. 5 for uhpfrcs prepared with spb admixture and by varying the water to cement ratio. it can be noticed that after 1 day of curing the tensile strength was always higher than 10 mpa except for the uhpfrc with water to cement ratio of 0.20. also in this case, the best mechanical performance was obtained for the concrete prepared with w/c equal to . tangent elastic modulustangent modulus of elasticity was measured from the stressstrain curve obtained in compression, at the point of interest, which corresponds to one third of the compressive strength of the material.time development of the tangent modulus is given in fig. 6 for uhpfrcs prepared by varying the water to cement ratio and by adding the spb superplasticizer to the mixture. in agreement with the results obtained in terms of compressive and flexural strength, as is easily apparent the stiffest uhpfrc turns out to be that prepared with w/c equal to . machine workabilityin order to produce abrasion-resistant dies made of uhpfrc some final machining operations are usually needed such as, typically, drilling, contouring and cutting. the analysis of these operaoperations is then of primary importance, because any damage induced by this final step of the production cycle might cause the discard of the component as scrap, thus frustrating the whole manufacturing process.in this work the attention was focussed on uhpfrc drilling. the scope was to evaluate the propensity of uhpfrc to be damaged when drilled or cut and, in particular, the presence of delamination, which can occur both at entry and exit side of the hole. this information could be essential for possible fields of application like mechanical engineering, where uhpfrc materials can be employed as high abrasion-resistant dies in the molding process of metal and polymer products, such as steel sheets or plastic shapes as shown in fig. 7.within the factors which can affect the quality of drilling, the thrust force plays the most important role, leading to off-axis movement of the tool, hole eccentricity, poor surface roughness and tolerances and, eventually, breakage of the tool 16. another important issue in drilling is the tool wear.experiments were carried out with two diameters (4 and 6 mm) and two feeds-motor (0.13 mm rev_1 and 0.34 mm rev_1). the spindle rotational speed was kept constant at 930 rpm.machine workability was investigated for uhpfrc prepared with type spb superplasticizer and water to cement ratio of 0.24, in the presence of steel fibers.an example of the results obtained with 4-mm diameter and0.13-mm rev_1 feed is shown in figs. 8a and 8b where two pictures are reported of the entrance and the exit zone of the tool, respectively.the presence of steel fibers outgoing from the exit zone, that points out the scarce quality of the machining, is evident. in the entrancem zone this phenomenon is nearly absent and exclusively due to some off-axis forces. for holes carried out with low feed the exitzone shows a smaller number of outgoing fibers, and, contemporarily, the extension of the delaminated area is smaller, thus indicating a better quality of the machining. results obtained showed that the extent of the delaminated zone is nearly insensitive to the tool diameter, even though the axial thrust forces were higher. this fact shows that this material is very sensitive to delamination, even under low forces.it is finally noticeable that the diameter of the entrance is always greater than that of the exit. this is due to a bad centering and to an insufficient guide of the tool during drilling. in order to compensate for such a drawback, either a pre-hole or a chasing, or even a mask of guide, can be provided for. obviously, both these msolutions will involve an increase of the costs of the machining.however, in order to appreciate the quality of the machine workability, it should be remembered that uhpfrc is a cementitious material. in this light, the result obtained was very encouraging also because it allows for the possibility for easy re-profiling of the diameter flange.3.5. thermal conductivity testthermal conductivity measurements were carried out by using the guarded hot plate method, according to uni 7745 17. specimens were previously dried in the oven at 375390 k to constant weight (mass loss under 1%). the experimental device is made of a heating element (central element for measuring plus an external guarded ring), a cooling element and external thermal insulation. the temperatures of the heating and cooling elements as well as those of the frontal surfaces of the specimen were measured by means of thermocouples.measurements were carried out on specimens prepared with rpc-24 mixture (see table 2), with and without steel fibers, in order to ascertain their contribution to the increase in thermal conductivity, since stainless steel has a very high thermal conductivity with respect to concrete, as is well known.experimental results obtained for uhpfrc are reported in table 3, in comparison with the thermal conductivity coefficient of steel and ordinary concrete. no significant influence of steel fibers was apparent; however, the very