Every year thousands of structures fail resulting in injuries, property damages etc. The causes of these damages are different and can be minimized by proper care and caution. One of the causes for damages is the freeze-thaw of concrete. Concrete is a porous material with capillary voids and pores. When these pores are filled with water in cold weather, the water in the voids start to form ice. It is very well understood that ice occupies larger space then water, so the voids that were once filled with water are now filled with ice creates stresses in concrete. The formation of ice in the pores also creates hydraulic pressures in the concrete units and hence concrete starts to deteriorate with a constant freeze and thaw cycles over the whole winter season. In the cold areas where the temperatures drop from -0Â°C to -20Â°C and lower the effects of freeze and thaw deterioration is a major issue. Freeze and thaw in a concrete or concrete masonry units and especially retaining wall units is of great importance because for a structure like retaining walls it is critical to be able to predict and prevent the deterioration.
2-2) Freeze and thaw cycle:
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Concrete in cold weather is exposed to numerous freeze and thaw cycles. Freeze cycle for concrete is when concrete is exposed to low temperatures of below 0Â°C which causes freezing of water in the voids of the concrete. Thawing cycle in concrete is when the frozen concrete or in other words the frozen water in concrete pores are slowly melted turning solid ice to liquid water. In areas where winters are extreme, concrete structures which are exposed to the weather experience number of freeze and thaw cycles. Due to these freeze thaw cycles the strength and durability of concrete structures in the area are effected greatly. Therefore it is very important to be able to analyze and have an understanding of the effects of freeze and thaw cycles on concrete.
2-3) Durability and Strength of Concrete:
Strength and Durability of concrete are one of the few key elements which determine the stability and lifecycle of a concrete structure. It is therefore very important to have a thorough knowledge of the factors that affect strength and durability of concrete. For this paper, we are focusing our attention towards the effects freeze and thaw has on concrete's strength and durability. As mentioned earlier, the freeze and thaw cycles do play a vital role in determining the strength and durability of concrete in cold regions. The freezing and thaw effect in concrete creates stresses in concrete by the freezing and thawing of water in the concrete pores and hence the intended strength and life for the concrete structure is compromised. As the water in the pores or air pockets of the concrete expands to form ice, the water in the pores does not find empty space to move and thus it creates stress in concrete. Due to these stresses, cracking and splitting of concrete takes place and consequently resulting in loss of strength and durability in concrete.
2-4) Water and Chloride Permeability:
Freeze and thaw in concrete is due to the permeability of water in concrete. The stresses are created in concrete by the water in the pores and air pockets when it expands. The concrete itself does not experience freeze and thaw, but instead it is due to the water permeability in the concrete that makes it susceptible to freeze and thaw. When the initial cracks are formed in concrete it actually allows more water to penetrate in the concrete which causes great damage to strength and durability of concrete and may also result in failing of structures.
3) Testing of Concrete:
3-1) Testing concrete for resistance towards freeze-thaw:
There are many standards and tests available for insuring that concrete resists freeze and thaw. One of the standards is the ASTM: C 666-92 "Standard test method for Resistance of Concrete to Rapid Freezing and Thawing". In this standard a test method is proposed to measure the resistance of freeze and thaw cycles from a concrete. In this testing method there are two test procedures which are: A) Rapid freezing and thawing in water, B) Rapid Freezing in air and Thawing in water. Both the procedures just measure the resistance of concrete towards the freeze and thaw cycles and does not measure the life of structure exposed to the freezing and thawing cycles. It is to give an idea about how well the concrete would resist the freeze thaw cycle but not how long it would do so.
3-2) Testing of concrete for durability against freeze-thaw:
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There are many standards and tests available for testing the durability of concrete against the effect of freeze and thaw. ASTM standards are mostly used to test and predict the durability of concrete when subjected to freezing and thawing. The famous ASTM standards are: ASTM C 666-92 "Standard test method for Resistance of Concrete to Rapid Freezing and Thawing", ASTM C 1262-94, "Standard Test Method for Evaluating the Freeze-Thaw Durability of Manufactured Concrete Masonry Units and Related Concrete Units". The above mentioned and many more ASTM Standards are used to determine the durability of concrete. The tests for concrete durability are of great importance because more and more use of concrete units is found in applications like retaining walls, paving units and also along the highways.
4) Preventions and Recommendations:
Admixtures are an important part of concrete. Different applications of concrete require different types of admixtures. Some of the famous admixtures used are; silica fume, fly ash, rich husk ash, plasticizers and super-plasticizers, etc. The admixtures in concrete do play a role in the resistance against the freeze-thaw but from the results in the research paper "Freeze-Thaw Deterioration of Concrete Masonry Units", shows that different types of admixtures only contribute to a small change in durability of the concrete. According to Steven A. Scott from the test results, it was apparent that mixes with dry block and Krete-HQS were the mixes that failed within the 500 cycles of the test. It is not apparent from the test results as to why this odd behavior occurred in the blocks which contained these two admixtures. The results also showed relatively better durability in the mixes that did not have the above two mentioned admixtures. From the test results it is also interesting to note that mixes with Rx-190, Rx-910, decca pave and plasticizer ultramix were among the ones that were the most durable.
4-2) Concrete Mix:
Concrete mix places a vital role in the durability of concrete which is subjected to the freeze and thaw cycles. Concrete is a heterogeneous materials and hence to produce and mix concrete with same properties is very difficult, but slight difference in concrete mixes can be neglected as the overall strength and durability does not change dramatically. Concrete mix composes of water, Portland cement, aggregates and admixtures. Concrete exposed to cold weather is highly susceptible to deterioration as the water filled pores of the concrete freezes to ice. The formation of ice and melting of ice actually is due to the voids in the concrete. Therefore it is important to have a controlled water-to-cement ratio so as to have the water just enough for the cement paste to hydrate. It is a common construction practice to add more water then that specified to have better workability with the concrete, but the extra water in the concrete creates voids and capillary pores. In wet weather these pores and voids are filled with water as concrete is a permeable material and the same water in the pores freezes to deteriorate the concrete. It is of great importance to have controlled water-to-cement ratio so air voids and capillaries can be avoided.
Concrete mix also has aggregates that play a vital role in the durability of the concrete. Aggregates in concrete mix are good fillers and although aggregates do not provide strength to the concrete, it provides durability to the concrete. It is then of great importance to concrete mix to have aggregates that can withstand the freeze and thaw cycles during the winter months. Aggregates in the concrete mix should be well graded as results have shown that concrete mix with well graded aggregates are more durable then the concrete mixes with gap graded or poorly graded aggregates. Aggregates are also very important factor against freeze and thaw as some aggregates tend to absorb more water than the others, therefore to reduce the effects of freezing and thawing it is recommended to use aggregates that do not absorb water as much.
4-3) Surface Treatments:
Surface treatment is also recommended in areas where it is critical to apply to the new existing structure or to preserve an old structure. It is a simple theory; place some kind of sealant to prevent water from getting into the concrete pores and freezing. Surface treatment is also of great importance because in area where snow and ice are issues, the salt and attack can also deteriorate the reinforcement inside the concrete. surface treatment is also used not only to prevent/restrict from entering in the concrete pores but also helps in preventing the salt from penetrating into the units or structures and causing damage. Research on the type of material used for sealing the surface of the concrete has been done and is also on going, but there is a number of sealant available today to apply as a surface treatment on the concrete units or structures.
4-4) Air Voids:
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The freeze and thaw action in concrete is primarily due to the presence of air voids in the concrete. The air voids when saturated and subjected to temperatures below 0Â°C freezes to form Ice and hence concrete deteriorates. Air voids or air pocket in concrete can be due to several reason; one of them being high w/c ratio. Another factor which plays a vital role in creating air pockets is poor handling of concrete. Proper vibration of concrete is important as it eliminates air voids and air pockets in concrete. These air voids are later filled with water and consequently causes freezing and thawing deterioration.
Air entraining system in concrete is a Primary factor in determining the durability of concrete against freeze and thaw. Air entraining system acts like a cushion for the unexpected expansion due to freezing and thawing. The air entraining air bubbles are not to be confused with the air voids and entrapped air. Air entrained bubbles are smaller in size and are evenly distributed across the mix, they are also closed bubbles. The water from the voids and pores when expand can infiltrate into the air entrained bubbles providing them space to expand and eliminating stresses as a result. According to tests and result analysis by Steven A. Scott in his research, two types of mixes are evaluated. The first mix has almost 16% air entrained bubbles whereas the second mix had only 8% and showed better results.
The intention for this research paper was to evaluate the problem of freezing and thawing in the region and its effects on the strength and durability of concrete. There are number factors that play a vital role in determining the overall strength and durability of concrete. The most important factors that affect concrete are Compressive strength, use of admixtures, concrete mix and air voids. High strength concrete is more likely to withstand the freeze and thaw deterioration. Although the use of admixtures in concrete against freeze and thaw is not as critical as some of the others but it is also understood that admixtures do play a vital role towards the compressive strength of concrete. One of the most important factors to be considered is the concrete mix itself as the proper selection of aggregates and aggregate gradation is of great importance and contributes to concrete durability. It is also stressed that proper handling of concrete during placing and curing also can contribute to freeze and thaw durability of concrete. Air entraining in concrete mix is also a significant factor as discussed above. It is hard to select from the above factors and draw a conclusion about the most dominant factor which effects the freeze and thaw deterioration in concrete. It is also appreciated that there are other factors and methods by which the freeze and thaw in concrete can be evaluated and therefore also opens up new fields to do research and study on this topic.