Frost resistence of cement based materials Henan polytechnic university School of Materials science and engineering Frost resistence of cement based materials

Durability of Concrete Main contents Introduction Freezing and thawing Chemical attack Alkali-aggregate reaction Corrosion of metals The synergistic effect of multiple factors

Durability of Concrete Introduction What is durability? In short, durability is quality! High durability concrete is good quality concrete！ So, to have a more sustainable concrete structure, durability is the key!

Durability of Concrete Introduction Concrete is an artificial stone, so it is inherently with good durability, and its life is very long. Normally, it is, but no absolutely.

Durability of Concrete Introduction How to define it? The durability of concrete is defined by the ability of concrete to resist weathering action and chemical attack, while maintaining its desired engineering properties.

Durability of Concrete Introduction The importance of durability for us: Due to the use of inappropriate materials, poor construction practices, insufficient curing and/or improper mix designs, many structures show serious premature deterioration.

Durability of Concrete Introduction The importance of durability for us: This is a global problem that costs both the public and private sectors worldwide billions of dollars annually.

Durability of Concrete Introduction The importance of durability for us: About 5 billion cubic yards of concrete is produced globally every year, with The New Civil Engineer (NCE)/Institution of Civil Engineers (ICE) reporting that approximately 40% of construction work in the UK goes toward repair and maintenance work rather than creating new infrastructure.

Durability of Concrete Introduction The importance of durability for us: The average life of a construction with concrete is 200 years in UK, 120 years in USA, and 60 years in China, and it is the shortest in China!

Durability of Concrete Introduction What should we do? Optimizing the durability of concrete ultimately leads to fewer repairs or premature replacements of deteriorating structures.

Durability of Concrete Introduction What should we do? Measuring the durability factor of concrete to determine its longevity, and ability to withstand local weathering and use, is difficult to predetermine. It is, however, an important factor in identifying how sustainable the structure will ultimately be.

Durability of Concrete Introduction Is the durability absolute? No, it is relative. The durability of a concrete mix is very contingent on environmental factors. The concrete mix design must be customized for the environment, and high end materials do not necessarily make for a durable concrete.

Durability of Concrete Introduction Is the durability absolute? No, it is relative. A durable mix design in one location may not be durable in another. For example, in dry, desert-like environments, air entrainment is not necessary for concrete to be durable, whereas it is a critical factor in anywhere that experiences freeze/thaw issues, as seen in most of North America.

Durability of Concrete Introduction Is the durability absolute? No, it is relative. Another factor in defining durability depends on how long a building is actually designed to last. If a building is designed to last fifty years, and lasts sixty before major repairs or replacements are necessary, then that building by definition is durable.

Durability of Concrete Introduction Is the durability absolute? No, it is relative. If a building is designed to last 100 years, and only lasts 70, it cannot be considered a durable structure.

Durability of Concrete Introduction Questions to consider when determining concrete durability: What is the project to be used for ( current and future use)? What type of environment will this structure be exposed to? How many years must this structure remain?

Durability of Concrete Why is concrete destroyed? Introduction Why is concrete destroyed? Because it is exposed and easily attacked in a variety of environments, for example, freezing and thawing, sulfate attack, alkali-aggregate reaction, and so on.

Freezing and thawing Introduction Freezing and thawing is a physical attack. Because the damage to concrete from freezing and thawing arises from the expansion of pore water during freezing.

Freezing and thawing Introduction Where does the damage usually happen? The frost attack happens in the winter, so it should pay much more attention in the northern of my country or some cold countries, for example, Canada, UK, Iceland, etc.

Iceland-The port's road is badly frozen. Freezing and thawing Iceland-The port's road is badly frozen. Introduction

Freezing and thawing Mechanisms 1.Deterioration is caused by the interior expansion. Due to frozen water in the concrete, cement tissue expansion, there are initial crack occurred, to continue the freezing and thawing, when the expansion exceeds limitation, part of the concrete structure will crack. For this kind of cold damage, to add an appropriate amount of air-entraining agent is quite effective.

Freezing and thawing Mechanisms 2. The spalling and cracking on the surface Concrete surface is wetted by water, and wet part will be destroyed due to expansion, namely surface spalling. In this case, only to add air-entraining agent can't deal with, the most important is to reduce water cement ratio and cure fully, obtain the sound concrete structures.

Freezing and thawing Definition Concrete in the condition of saturated water, can withstand repeated freezing-thawing cycle without damage, nor severely degrades the strength. It is one of the main indicators to assess the durability of concrete.

Grade of frost resistance Freezing and thawing Grade of frost resistance According to the number of concrete can withstand repeated freezing and thawing cycles, grade is divided into F10, F15, F25, F50, F100, F150, F200, F250, F300 etc., in all nine grades.

Freezing and thawing Influence factors 1. Aggregate Water absorption of aggregates: large water absorption is disadvantage for frost resistance. Frost resistance of the aggregate in itself: poor solid aggregate, such as weathered rock, affects the frost resistance of concrete.

Freezing and thawing Influencing factors 1. Aggregate Frost resistance of lightweight aggregate concrete is better: lightweight aggregate concrete perforated, and these pores can not easily be saturated by water. When the concrete is frozen, no frozen water can be transported into the pores of aggregates by frozen expansion pressure, thus reducing the swelling pressure and stresses in the concrete.

Freezing and thawing Influencing factors 2.Cements Cement activity and type have an impact on frost resistance of concrete, mainly because the clinker has different relative volume and changing hardening speed. The frost resistance of concrete increases with increased activity of cement.

Freezing and thawing Influencing factors 2.Cements The order of frost resistance of domestic various cement from high to low is: Portland cement > ordinary Portland cement > slag Portland cement > volcanic ash Portland cement ( such as fly ash Portland cement).

Freezing and thawing Influencing factors 3. Strength Strength is the ability to resist damage, the favorable factors of frost resistance. Of course, in the same air content or under the condition of the average bubble spacing, the higher the strength is, the better frost resistance is.

Freezing and thawing Influencing factors 3. Strength But on the other hand, the influence of the bubble structure of concrete is much greater than the influence of intensity for frost resistance of concrete. Some people think that frost resistance must be good for high strength concrete, and this view is not comprehensive.

The influence of water-cement ratio on frost resistance Freezing and thawing Influencing factors 4. W/C ratio The influence of water-cement ratio on frost resistance

Freezing and thawing Influencing factors 4.Water-to-cement ratio It can be seen from the figure that: frost resistance is obviously decreased with the water cement ratio increasing, and the air-entraining admixture can improve the frost resistance capacity. For the air-entraining concrete, the decrease of frost resistance is not obvious with the increase of water cement ratio.

Freezing and thawing Influencing factors 4.Water-to-cement ratio Water cement ratio directly affects the porosity and pore structure of concrete.With the increase of water cement ratio, not only the opening of saturated water volume increases, but also does average pore diameter increase. Hydrostatic pressure and osmotic pressure is big in the process of freezing and thawing , so the frost resistance will be reduced.

Freezing and thawing Influencing factors 5.Space of bubble Generally, for high frost resistance of concrete, the average bubble spacing shall be less than 0.25 mm, because when it is greater than 0.25 ~ 0.30 mm, frost resistance falls sharply. The concrete mixed with silica fume can obviously improve bubble structure.

Freezing and thawing Influencing factors 5.Space of bubble Average bubble radius decreases, and the average bubble spacing is reduced. Frost resistance is the function of water cement ratio and average spacing among bubbles, which are the two main factors of the frost resistance.

The key factors to improve the frost resistance Freezing and thawing The key factors to improve the frost resistance To choose good aggregate (gravel, pebbles); As far as possible the use of ordinary portland cement, such as fly ash and other mineral admixture, air-entraining admixture should be suitably increased;

The key factors to improve the frost resistance Freezing and thawing The key factors to improve the frost resistance After selecting the raw materials, the key parameters are the air content and water cement ratio; After the water cement ratio is determined, according to frost resistance requirement the air content should be determined at 3%~6%, and then the dosage of air entraining admixture will be determined;

The key factors to improve the frost resistance Freezing and thawing The key factors to improve the frost resistance The decrease of compressive strength is caused by the introduction of air bubbles, so the water cement ratio must be adjusted to compensate for the loss of strength.

Testing methods of frost resistance Freezing and thawing Testing methods of frost resistance Generally, the level of frost resistance is determined according to freezing-thawing cycles at 28 days and it is tested with quick-freezing or slow-freezing method. The maximum number of freeze-thaw cycles is recorded when concrete is destroyed.

Testing methods of frost resistance Freezing and thawing Testing methods of frost resistance Testing method for quick frost resistance There are two kinds of quick-freezing methods in ASTM, one kind is full of water, and concrete is frozen and thawed in the water, which is suitable for automation testing.

Testing methods of frost resistance Freezing and thawing Testing methods of frost resistance Testing method for quick frost resistance The other method is to freeze in the air with the freezer , and then moved to the indoor pool to thaw. Quick frozen method is widely used at present.

Testing methods of frost resistance Freezing and thawing Testing methods of frost resistance Testing method for quick frost resistance After freezing and thawing cycle has been done for 2h~4h, the samples should be taken out to test fundamental frequency every 25 times as a cycle , and the relative elastic modulus should be calculated and the loss of mass should be recorded.

Testing methods of frost resistance Freezing and thawing Testing methods of frost resistance Testing method for quick frost resistance According to the experiment the number of freezing-thawing cycle will be determined.The evaluation law of the fast freezing experiment is: the loss of its quality does not exceed 5%, and relative dynamic modulus of elasticity is not less than 60%.

Testing methods of frost resistance Freezing and thawing Testing methods of frost resistance Testing method for slow frost resistance Slow test evaluation index is: the loss of mass is no more than 5%, and the loss of strength is less than 25%. The specimen of freezing-thawing cycles is its level of frost resistance.

Chemical attack Introduction When concrete is exposed to a chemical environment or medium, it may be subject to chemical attack, such as chemical production, chemical waste, ground water with higher sulfate concentrations, sea water, sewage and fresh water flows under pressure, and so on.

Durability of Concrete Thank You!