北京琦正德科技有限责任公司-ag凯发k8国际

 the research about frozen damaged concrete

long liang, xiuling lei,wei sui 

long liang: graduated from school of civil engineering, harbin institute of technology. having does research on concrete antifreeze for 15 years. ceo of beijing kingdom of science and technology ltd co. material engineer and ipmp, telephone: 13552190597.

xiuling lei: material engineer, northeast water conservancy and hydropower survey design co. ltd.

wei sui:material engineer, northeast water conservancy and hydropower survey design co. ltd.

abstract

compared with new concrete, the frozen damaged concrete will be degraded more quickly. the porosity and water absorption of concrete surface structure increase as a result of the erosion of surface, the water can be more easily transported to the deep concrete with not damaged by freezing, leading to a deeper damage step by step. in order to prevent this accelerated degradation, it is actually to improve . different from the ordinary concrete, analysis of frost resistance of frozen damaged concrete is based on the harden concrete. mixing air-entraining agent or mineral admixture to satisfy the required frost resistance level is just suit for fresh concrete rather than harden concrete. therefore, when the concrete surface erosion appears but not enough to reach the level of repair reinforcement, how to prevent the degradation from accelerating development, slow the freezing injury, extend the service life of buildings and avoid large-scale demolition and construction to save resources and reduce the operation cost. the key points of secondary freeze-thaw are choose of materials and technology, it is possible to reach even surpass the frost resistance level of fresh concrete modified by a low cost and simple technology. based on experiment results, the level of frost resistance reaches f550 in the second freeze-thaw test after simple treatment for frozen concrete with mass loss rate and dynamic modulus of elasticity exceeding standard. this value of level is even higher than that of fresh concrete, which has an outstanding meaning in frost resistance study for harden concrete.

keywords: ; single side freeze-thaw, rapid freeze-thaw salt solution; harden concrete; fresh concrete

1. introduction

tfrost resistance of frozen damaged concrete is different from fresh concrete. the required level of frost resistance for fresh concrete can be satisfied by means of adding air entraining agent, mineral mixture and choosing the type of used cement. however, these ways are not useful for harden concrete as a result of the design for frost resistance is based on harden concrete. thus, the design ideas are the key differences. the design of frost resistance for frozen damaged concrete is based on harden concrete, and the concrete had been damaged. therefore, how to prevent the deterioration of freezing resistance, enhance the ability of frost resistance and prolong the service time for structure are the purposes of this article. 

2. problem list of frozen damaged concrete

2.1. frost resistance level did not reach the design requirement

frost resistance is one of the most important indexes for durability design of concrete. research results showed that some concrete can satisfy the frost resistance level in lab, however, samples prepared by situ core examination at the same mix proportion can not satisfy the level in construction site. the reasons are so many that we can not discuss in this paper one by one. the most important question is, how to deal with those concrete which are not satisfy the required level? there is no doubt that the durability and lifetime for structure will be significantly influenced if we do not take any measures for improvement. if we do not have related technical scheme or just take down and then rebuild it, the cost is undoubtedly very large.

2.2. the decline of frost resistance for damaged concrete

the second situation is that the erosion problem of concrete building structure has come into being. the ability of frost resistance of concrete is related to the porosity and water absorption. the surfaces of frozen damaged concrete structures spall, the aggregates are exposed and the surface structure becomes porous, leading to a rapid increase of water absorption and porosity. if measures are not taken in time to prevent further development of deterioration, the strength of concrete will lose. for those concrete which has been frozen and a lightly spalling on the surface, there are not effective technical methods to prevent or slow the development of frost damage. the usual method is to repair and strengthen the damaged concrete until a severe erosion has happened on the surface of concrete. on the one hand, the cost of repair is very large. on the other hand, the micro cracks in the deep concrete structures can not be repaired by surface reinforcement after the severe erosion appears. moreover, the frost resistance of denudation concrete strengthened by this kind of repair material lack of a whole research, thus, it is uncertain that the level of frost resistance whether satisfy the required freezing resistance or not.

2.3. the freezing-thawing condition changes: fast freezing design can not meet the freezing-thawing requirements in the salt solution environment

since gb/t 50082-2009 (standard for test methods of long-term performance and durability of ordinary concrete) is implemented, the single surface freezing-thawing is adopted into thestandard of freezing and thawing experiment for concrete. in the past, quick freezing-thawing was the main index for detecting the frost resistance of concrete. the measure results are more according with salt frozen environment when the freezing and thawing damage in the salt solution is included in the single surface freezing-thawing. however, for concrete meeting the rapid freezing-thawing experiment does not necessarily meeting the freezing-thawing damage of the salt solution environment because of single freezing-thawing experimental condition is much different from that of rapid freezing-thawing. therefore, the ability of frost resistance measured by rapid freezing-thawing in salt solution condition will reduce and even damage when the environment changes.

3. application and experimental data of frozen concrete

3.1. project phase ii of huaneng power plant in yingkou, liaoning

measures of secondary design of frozen concrete vary from project to project. however, one design philosophy is always the same: the resistance to frost should be improved after renovation. the project is located in yingkou, liaoning province. the project was built along the coast. the second phase is the expansion and reconstruction on the basis of the first phase project. some concrete structures in the first phase, especially the concrete contacting with seawater, have severe chloride corrosion and freezing-thawing damage, as shown in fig. 1 and fig. 2.

this project is a typical case of unconsidered freezing-thawing environment. because at that time, there was no introduction of single-face freezing-thawing experiment method. therefore, its design of frost resistance is based on the rapid freezing-thawing method. less than two years, the concrete structure has a severe denudation. therefore, the damaged concrete should be repaired and strengthened, at the same time, expansion and the capacity increment are also required.

the solution design revolves around the concrete anti - salt freeze，the selected material was the ganada sps-801 provided by beijing kingdom of science and technology co. ltd. the selection is based on the experimental data of the salt freezing in harbin institute of technology. the experiment was based on the (prenv 1239-9 freeze-thaw resistance-scaling) due to that there was no experimental method of single freezing-thawing at that time. the experimental method is the earliest method of freezing-thawing in a single-side salt solution in china. the experimental data is shown in table 1. fig. 3 and 4 are experiment pictures. the first image in fig. 4 is the freezing-thawing erosion situation of 28 times for sample without sps - 801 coated, others are freezing-thawing erosion of 56 times for samples with sps - 801 coated.

the project began in 2005, and so far, has experienced nearly 12 years of operation, it is proved that its anti-salt freeze and corrosion resistance can achieve even more than the design anticipate, as shown in fig. 5 and fig. 6.

3.2. a power plant project in liaoning province

the plant was first put into use in 2016, only one winter operation, the concrete structure has a severe denudation, as shown in fig. 7, 8, and 9. the level of frost resistance for this project is f200, and it is coated with mn-j55b waterproof as a measure of anti-freeze and anti-corrosion. the paint was almost all peeling off based on the field observation. the project is a typical case that the frost resistance level does not satisfy the standard. in addition, chemical salts such as various scale inhibitors were added in order to improve the concentration ratio of the cooling tower circulating water, thus, the freezing-thawing environment becomes a salt solution. these two are the main reasons which result in rapid failure of the frost resistance.

the project does not have the drilling samples for freezing-thawing test. samples were sent to the second engineering quality inspection station of harbin institute of technology for testing. the testing was according to the single-side freezing-thawing method in gb/t 50082-2009 (standard for test methods of long-term performance and durability of ordinary concrete). the test results are shown in table 2 and table 3.

as can be seen from table 2 and table 3, even under the water freezing condition, samples failed in the 12^th freezing-thawing cycle. and under the salt solution condition, the mass loss exceeded the standard in the 8^th cycle. the concrete sample still does not reach 20 freezing-thawing cycles even under the condition of mn-j55b waterproof coating. however, sps-801 has the best improvement. this provides an idea for secondary freezing-thawing design. the concrete structure can satisfy the requirement of frost resistance through additional measures even the harden concrete is unqualified in level of frost resistance.

3.3. frost damage of high speed railway concrete structure in cold regions

according to the introduction of the designer, the snow on the concrete surface in high speed railway can generates a remarkable freezing-thawing damage because of the temperature difference in day and night in northeast china. due to the snow in winter, there have been signs of erosion. to prevent the freezing-thawing damage from further development, it is urgent to find a material, not only can solve the problem of freezing-thawing, but also can meet the construction requirements of high speed railway at special time.

in this case, the designer uses sps-801 to do the rapid freezing-thawing experiment. some experimental results are as follows:

after 500 times freezing-thawing cycles, the relative dynamic elastic modulus and mass loss rate of the control sample are 85% and 2.9% respectively. for the sample coated by sps-801, the relative dynamic modulus of elastic modulus and mass loss rate are 97% and 0.5% respectively. when the freezing-thawing cycles are 800 times, the relative dynamic elastic modulus and mass loss rate for samples with sps - 801 protected are 51% and 0.8%.

the designer experimented with sps-801 anti-freeze paint on the high-speed railway line in sujiatun, shenyang and liaoyang, liaoning province. the project will provide new ideas for repair of the frost damaged concrete.

4. conclusions

with the additional measures coated, the problem of the attenuation of the secondary frost resistance can be solved very well, which provides a solution for the anti-freezing design of concrete after curing.

tables and figures

list of tables:

table 1 ?data of resistance to chloride salt freezing test

table 2 ?/span> desquamation of surface

table 3 ?loss rate of relative dynamic modulus measured by ultrasonic

list of figures:

fig. 1 ?/span> salt freezing of concrete structure

fig. 2 ?salt freezing of concrete structure

fig. 3 ?/span> test image

fig. 4 ?/span> test image

fig. 5 ?/span> before repair

fig. 6 ?/span> after repair

fig. 7 ?salt freezing of concrete structure of cooling tower

fig. 8 ?salt freezing of concrete structure of cooling tower

fig. 9 ?salt freezing of concrete structure of cooling tower

table 1 data of resistance to chloride salt freezing test

notation a, b and c correspond to water to cement ratio of 0.40、0.36、0.32；d, e and f represent a, b and c coated with sps-801.

table 2 desquamation of surface

table 3 loss rate of relative dynamic modulus measured by ultrasonic

notation w means in water for the tested samples, s means in salt solution, sps means samples painted by sps-801, t means samples painted by mn-j55b waterproof coating.

fig. 1 salt freezing of concrete structure      fig. 2 salt freezing of concrete structure

                fig. 3 test image                         fig. 4 test image

notation the first image in fig. 4 is the freezing-thawing erosion situation of 28 times for sample without sps - 801 coated, others are freezing-thawing erosion of 56 times for samples with sps - 801 coated.

                       fig. 5 before repair              fig. 6 after repair

fig. 8 salt freezing of concrete structure of cooling tower 

     fig. 7 salt freezing of concrete structure of cooling tower

     fig. 9 salt freezing of concrete structure of cooling tower