Frost resistant concrete refers to the structural design that requires concrete to have the durability to resist freeze-thaw cycles for a long period of time, i.e., to meet the frost resistance level specified in the structural design.
When frost-resistant concrete is placed in a winter environment, technical measures for winter construction must also be taken. Frost-resistant concrete, no matter what season construction, must be mixed with air-entraining agent to achieve the structural design of the frost-resistant level requirements. Improve the air content of concrete (4% to 6%) is the most effective technical measures to improve the frost resistance of concrete. The application of anti-freeze concrete projects mainly include waterworks, ports, bridges, and highways.
Anti-freeze grade and anti-freeze grade
Concrete frost resistance according to different test methods, divided into frost grade and frost mark. Frost resistance level is indicated by the symbol F, while the frost resistance mark is indicated by the symbol D. Both methods use the age of 28d specimens in the water absorption saturation, to detect its performance changes under repeated freeze-thaw cycles.
The frost resistance grade is determined by the maximum number of freeze-thaw cycles when the relative dynamic modulus of elasticity of the specimen decreases to not less than 60% or the mass loss rate does not exceed 5%. The frost-resistant grade is determined by the maximum number of freeze-thaw cycles when the rate of loss of compressive strength does not exceed 25% or the rate of loss of mass does not exceed 5%.
Commonly used concrete frost grade: F50, F100, F150, F200, F250, F300, etc., respectively, indicating that the concrete can withstand repeated freeze-thaw cycles of 50, 100, 150, 200, 250 and 300 times.
Factors affecting the frost resistant concrete
The main factors affecting the frost resistant concrete are the average bubble spacing, water-cement ratio, air content, aggregate and cementitious materials.
Average bubble spacing
Average bubble spacing is the most important factor affecting frost resistant concrete. The larger the average bubble spacing, the greater the hydrostatic pressure and infiltration pressure in the capillary pores during freezing and thawing, the lower the frost resistance of concrete. General average bubble spacing coefficient in 500μm or less to obtain high frost-resistant concrete.
Water-cement ratio
The larger the water-cement ratio, the more the content of freezable water in the concrete, the faster the concrete freezes. The worse the bubble structure, the larger the average bubble spacing. The lower the strength of concrete, the worse the ability to resist freezing and thawing.
Water-cement ratio in the range of 0.45 to 0.85 change, not mixed with air-entraining agent of concrete frost resistance changes little. Only after the water-cement ratio is less than 0.45, the frost resistance is significantly improved with the reduction of the water-cement ratio. Water-cement ratio of less than 0.35 concrete, even if not mixed with air-entraining agent, also has a high frost resistance.
Air content
In a certain range, the more air content, the better the frost resistance of concrete. However, the air content exceeds a certain range, the frost resistance of concrete is reduced, the reason is that the air content increases in reducing the average bubble spacing at the same time, reducing the strength of concrete (concrete air content per 1% increase in compressive strength decreased by 3% to 5%). Generally when the maximum size of the natural aggregate used is 10 ~ 40mm, so that the air content in the newly cast concrete to 4% ~ 7%, can obtain sufficient frost resistance.
Concrete strength
When the hydrostatic pressure and infiltration pressure exceeds the tensile strength of concrete, concrete is freeze-thaw damage. Therefore, as a characterization of the ability to resist freeze-thaw damage of concrete strength also has an impact on the frost resistance of concrete. When the air content or the average bubble spacing is the same, the strength of concrete with high freezing resistance is higher than that of concrete with low strength. However, relatively speaking, the strength of concrete frost resistance is not as great as the degree of influence of the bubble structure.
Aggregate
When the aggregate water absorption saturated by freezing in the aggregate pores and aggregate – cement paste interface to produce static pressure, more than the strength of the aggregate or interface to produce frost damage. Therefore, the main factors affecting the frost resistance of the aggregate are the aggregate water absorption rate and aggregate size.
The use of large water absorption of aggregate (such as light aggregate) to formulate frost-resistant concrete more dependent on the admixture of air-entraining agents; the larger the size of the aggregate, the easier it is to destroy after freezing, but the fine aggregate has little effect on the frost resistance of concrete. In addition, the solidity of the aggregate, the degree of weathering, clay content, impurity content, etc. on the frost resistance of concrete also has an impact.
Cement varieties and dosage
Cement with the amount of mixed materials into the increase in the amount of concrete frost resistance decreases. Therefore, anti-freeze concrete with silicate cement formulation to be better than with other varieties of cement. For non-air-entrained concrete, the cement variety and amount have a certain influence on the frost resistance of concrete. For air-entrained concrete, this effect is not significant.
Mixed materials
Fly ash in a certain range, and the strength and gas content of the same conditions, with and without fly ash concrete frost resistance is basically the same. However, when the amount of fly ash blending exceeds a certain range, it will reduce the frost resistance of concrete. When the amount of silica fume does not exceed 10%, the frost resistance of concrete is improved, and the frost resistance is significantly reduced when it exceeds 15%.
Conditioning
The early maintenance of concrete after placement has a significant impact on the strength of the concrete structure entity. The test was conducted with C30 pumped concrete, formed 150mm cubic specimens. From the test results, the watering curing 14 days of the specimen compressive strength on average 4.4MPa higher than the specimen without watering curing. watering curing 28 days of the specimen carbonation thickness of 1.5 to 2.0mm less. not watering specimens on the rebound presumed strength of greater impact.
This fully illustrates that the method of curing also has an effect on the frost resistance of concrete. Therefore, the concrete should be poured in time to take effective moisture conservation measures to both enhance and prevent cracking and improve the durability of concrete.
About air-entraining agents
The use of air-entraining agents prolongs the service life of concrete and increases the durability of concrete.
Definition of air-entraining agent
In the concrete mixing process can introduce a large number of uniformly distributed, stable and closed tiny air bubbles and can be retained in the hardened concrete admixture called air-entraining agent.
The contribution of air-entraining agent
Concrete mixed with air-entraining agent, improve the cohesiveness of concrete, reduce the mixture of segregation and water secretion, as bubbles isolated from each other, cut off the capillary channels, so that water is not easy to penetrate, but also buffer its water freezing expansion effect, and thus can significantly improve the frost resistance, frost resistance, seepage resistance and corrosion resistance of concrete. The degree of improvement is not tens of percent, but usually several times, or even a dozen times higher, thus greatly extending the service life of concrete in the case of freeze-thaw cycles, which plays an irreplaceable role in improving the overall durability of concrete. Therefore, air-entraining agent is an important component of anti-freeze concrete and anti-freeze concrete.
The selection and application of air-entraining agents
Different varieties of air-entraining agent not only affects the amount of gas but also the quality of bubbles, generally sodium dodecyl sulfate and sodium dodecyl benzene sulfonate strong foaming ability, but larger foam, poor stability; alkyl alcohol polyoxyethylene ether poor foaming ability; and rosin soap, rosin hot poly, three post saponin can produce a large number of uniform, stable micro bubbles, so they become the preferred air-entraining agent.
The amount of air-entraining agent is very little, generally a few hundred thousandths of the total amount of cementitious materials to one or two thousandths, should be appropriate to the air content of the concrete mix to 4% to 6%, excessive admixture, concrete workability instead of reducing, more will have a negative impact on the compressive strength of concrete, frost, seepage resistance, anti-carbonation properties. Air-entraining agent can be used with other additives compound, when mixed in the concrete mixing process alone, should first be prepared into a solution, and then diluted to a certain concentration after mixing.
Factors affecting the air content of concrete mixes
The size of the air-entraining agent gas content in addition to its own quality, cement and aggregate and concrete production, construction factors also have a significant impact on it.
Composition materials
Cement fineness, high alkali content, gas content are reduced. When the same mixture, the gas content of silicate cement is greater than that of ordinary cement, slag cement and volcanic ash cement in turn. The gas content of cement is reduced by about 1% for every 90kg/m3 increase.
Fly ash has a strong adsorption effect, will significantly reduce the air content of concrete. Therefore, when the requirement of a higher air content of concrete, the amount of fly ash should be controlled or appropriate to increase the amount of air-entraining agent. In addition, the greater the fineness of the mineral admixture, the smaller the air-entraining volume.
Coarse aggregate diameter is large, the gas content is small. Pebbles contain more gas than crushed stone aggregates.
Natural sand contains more gas than artificial sand. Sand particle size range in 0.3 to 0.6mm when the concrete contains the largest amount of gas. And less than 0.3 mm or more than 0.6 mm, the concrete gas content are significantly reduced. Sand rate is large, the air content is also large, but when the sand rate increases to a certain extent, the change in the air content is not obvious.
Water-cement ratio
The water-cement ratio affects the viscosity of the concrete mix. If the concrete water-cement ratio is low, the viscosity of the mix is large. The viscosity of the concrete mix will reduce the generation, quantity, and quality of air content. Therefore, the smaller the water-cement ratio means the increase in the amount of air-entraining agent.
Mixing
Mechanical mixing is greater than manual mixing air content. General mixing 5min gas content is the largest. The gas content is small if the mixing time is too long, and the gas content is small if the mixing time is not enough. Different mixing equipment has a certain impact on the gas content, the best mixing time should be determined by the actual production of mixing equipment combined with the test.
Concrete temperature
Concrete temperature high gas content loss fast, every 10 ℃ increase in gas content can be reduced by 20% to 30%. Therefore, in the hot summer construction should be appropriate to increase the amount of air-entraining agent.
Concrete slump
The concrete slump has a certain influence on the air content, the greater the slump, the higher the air content. In other words, the increase of mixing water will also increase the air content.
Placement and transportation
The longer the mix is placed and transported, the greater the loss of air content, but the loss of air content is different for different air-entraining agents.
Pumping process
By pumping concrete, the air content of concrete after pumping decreases, but the loss of air content varies with different air-entraining agents.
Vibrating method
All kinds of pounding methods will reduce the air content of concrete, the use of inserted vibrating rod pounding than flat, table vibration loss of air content is greater. High-frequency pounding, pounding time will be significantly reduced gas content.
In summary, the impact of the air content of concrete mixes more factors. The determination of the air content should be considered both to improve the internal structure of concrete, improve frost resistance, seepage resistance, etc., but also to take into account the adverse effects on the strength of concrete. Therefore, the formulation of air-entraining agent concrete should be cautious. It is necessary to determine the air-entraining agent species and dosing through simulation tests according to the material, process conditions, etc.