| China is a great country with a vast territory. The colder northern part is our country's heavy industrial base and transportation centre. There are a large number of bridges and workshops which are easy suffering from fatigue. So there exist great potential chances of applying strengthening fatigue structure, especially CFRP strengthening. This paper deals with the experimental results of the fatigue capacity of RC beams strengthened with CFRP sheets at low temperatures. Then theoretical analysis was carried out based on the experimental results. The major studies and conclusions are as follows:1. First put forward the force transferring of CFRP strengthened RC beams under the fatigue load circumstance, which causes the improvement of loaded capacity of RC beams. This leads to great improvement of the beams' fatigue life.2. The flexural capacity of RC beams strengthened with CFRP sheets is higher than that of RC beams. The flexural capacity with one layer of CFRP is increased by 27%, and 35% with two layers. The deflection of RC beams strengthened with CFRP sheets is lower than that of RC beams. The deflection strengthened with one layer of CFRP is decreased by 52%, and 35% with two layers of CFRP. This indicates that with the increase of layers, the RC beams strengthened with CFRP sheets gradually show its quality of ductility damage.3. Through analyzing the data of fatigue experiments on the RC beams strengthened with CFRP sheets, the concrete fatigue-strain damage process can be divided into 3 stages:1). The early cracks occur in the weak parts and the strain increases quickly, making up 32.8% of the total;2). The cracks expand steadily and the strain increases slowly, making up 14.4% of the total;3). The cracks expand rapidly and the strain spreads, making up 52.8% of the total.4. Macroscopically, the fatigue numbers increase with the rising of temperatures, but the increase extents are different. When the temperatures change as follows:-10℃→-5℃→0℃→+5℃→+10℃, the fatigue numbers of CFRP strengthened RC beams with one layer increase as follows: 9～18%→-2～4%-3～15%→8～28%. The fatigue numbers with two layers increase as follows: 8～13%→9～15%→9～18%→10～25%. It is recommended that when considering strengthening the RC beams, the CFRP sheets should be used at least two layers. Then the data will be rather steady.5. With the increase of the CFRP layers, the fatigue numbers increase obviously. But the increase extents vary. Adding 'U' hoops can obviously increases the fatigue numbers. The average data indicate that the fatigue numbers with two layers are 1.66 that with one layer. The fatigue numbers with three layers are 1.24 that with two layers. The fatigue numbers with 'U' hoops are 2.86 that without 'U' hoops.6. Due to anti-tense of CFRP sheets, the force distribution at the bottom of the strengthened beams is more uniform. Compared with the un-strengthened beams, the crack numbers are more. Most of them are short and tiny. This greatly improves the anti-fatigue capacity of RC beams.7. The experimental results indicate that the low temperatures don't affect the damage characters of CFRP strengthening system itself. It is possible to paste CFRP sheets to improve the fatigue at low temperatures. Adding 'U' hoops at the two ends and the midspan can avoid peeling the CFRP sheets from the RC beams. It also can obviously improve the fatigue capacity and finally lead to ductility damage.8. It is recommended that using the deflection change to predict the fatigue numbers of beams. This reflects the essential characteristics of CFRP strengthened RC beams and the integrated influences of temperatures, strengthening layers, fatigue amplitude and the maximal fatigue stress, etc. Through numerical calculation, the paper gives the simulated equation of deflection rates and fatigue number rates of beams. The results from calculation fit well with the experimental results. |
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