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The main reasons and countermeasures for the unsuitability of concrete admixtures
Blog The main reasons and
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With the rapid development of infrastructure projects in China, large volume concrete is becoming increasingly widespread. Because cement-based materials undergo volume shrinkage, such as drying shrinkage, self-shrinkage, and thermal shrinkage, cracking can occur, negatively affecting the durability of concrete structures.
However, large volume concrete releases a large amount of heat during hydration, and its internal heat dissipation rate is slow, increasing internal temperature and the temperature difference between the concrete’s interior and exterior, thus creating a large temperature gradient. As the ambient temperature decreases, surface cracks continue to expand, making it easy for dangerous cracks to develop.
Because of external factors, these cracks are likely to induce corrosion of internal steel, causing structural damage and, in severe cases, reducing the structure’s service life. Magnesium oxide expansion agent is a “slow expansion” type expansion agent that converts the chemical energy of magnesium oxide hydration into mechanical energy, forming internal prestress that can compensate for shrinkage and tension during cooling, thereby indirectly reducing the equivalent comprehensive cooling temperature.
At the same time, because of its unique slow expansion characteristics, its expansion rate matches the concrete’s shrinkage rate, compensating for volume shrinkage during cooling and helping reduce cracking.
Research has found that magnesite heated at high temperatures undergoes slow swelling during hydration reactions, meaning that volume expansion occurs relatively late when magnesium hydroxide is formed. The hydration of magnesium oxide to produce magnesium hydroxide crystals is the main reason for its slow expansion.
The expansion energy of magnesia sand is determined by its absorption force and the crystallization growth pressure of magnesia sand. In the early stage of hydration, the particle size of magnesium hydroxide crystals is extremely small, and the swelling force of the crystals is the main driving force for their expansion. Over time, magnesium hydroxide crystals gradually grow, and crystal expansion becomes the main driving force.
The degree of expansion in magnesium oxide cement stone depends on the location, morphology, and size of magnesium hydroxide. In addition, the type and dosage of the magnesium oxide expansion agent, environmental temperature, mineral composition, and free calcium oxide content all significantly affect the expansion performance of magnesium oxide cement. The type and dosage of magnesium oxide, as well as the free calcium oxide content in cement clinker, also significantly affect its expansion performance. Together, these factors affect the final expansion characteristics and engineering quality of concrete.
This project aims to study the effect of different dosages of magnesium oxide expansion agents on concrete shrinkage performance through indoor simulation.
The raw materials used in the experiment include:
To investigate the effect of different expansion agents amounts in C25P8F150 concrete, the following preparation methods were proposed: C25P8F150 ordinary concrete with expansion-agent amounts of 0, 2%, 3%, and 4%, and the concrete mix proportions are detailed in Table 1. The concrete has a slump of 160-180 mm and an air content of 3.5%-5.5%. This plan aims to explore the influence of expansion agents on concrete performance and provide a reference for practical engineering applications.
Based on existing research, the raw materials were calculated according to the given mix design parameters, and their performance was experimentally evaluated.
A self-made volume deformation test chamber is a closed specimen made of galvanized steel plate, with a diameter of 200 mm and a height of 500-600 mm. When conducting volume deformation testing on C25P8F150 secondary ordinary concrete, the following procedure should be followed: before testing, the sample bucket should be inspected to confirm that there is no leakage or air tightness, and then a rubber lining with a thickness of 1- 2 mm should be applied to the inner wall of the bucket to isolate it from the concrete.
When forming the sample, place the calibrated differential resistance strain gauge at the center of the sample, ensuring the measurement baseline is centered and aligned with the sample’s longitudinal axis. After molding, tightly seal the cylinder cover to ensure close contact with the sample end to be tested, and seal the strain gauge cable outlet and outer circumference with paraffin.
After packaging, place it at (20 ± 2) ℃ to prevent vibration, then immediately conduct resistance and resistivity tests, using the strain gauge values measured 24 hours after molding as reference values. Perform resistance and resistance tests at 2, 6, 12, and 24 hours after the specimen is formed. After that, tests were performed once a day, every two weeks, and every six months. All tests lasted for one year (365 days). Finally, the test result is the average of the measured data from the two samples.
Based on the experiment, the following conclusions can be drawn:
(1) Without the addition of an expansion agent, the volume deformation of concrete is mainly manifested as shrinkage. At around 60 days, the self-weight deformation of concrete reaches a critical value of -16 × 10^-6. Compared with 300d, the self-weight deformation of concrete increases by 5.2% at 365d. Overall, shrinkage in concrete changes little in the later stage.
(2) The expansion agent significantly affects the shrinkage of concrete, indicating that the expansion agent is very effective in improving the crack resistance of concrete. The shrinkage rate of concrete decreases with the increase in the amount of expansion agent used; Specifically, when the dosage of expansion agent is 2%, it can cause a 2% self-weight deformation of concrete; In the case of adding 3%, a gravity deformation of 61.0 × 10-6 can be obtained; But when the addition amount is 4%, a gravity deformation of 146.4 × 10-6 can be obtained.
In addition, the more expansion agent used, the higher the expansion rate. When the amount of expansion agent added is 3%, the self-generated volume deformation increases by 175% compared to when only 2% is added; At a dosage of 4%, the self-generated volume deformation is 137% higher than that of cement mortar with a dosage of 3%; However, the growth rate of expansion rate decreases with the increase of expansion agent dosage. Therefore, when selecting expansion agents, full consideration should be given to improving concrete crack resistance and the influence of the expansion rate.
(3) At room temperature, place the constrained expansion sample in a steam curing oven at 40 ℃ for curing to obtain the constrained expansion rate in water. The results showed that samples without a magnesium oxide expansion agent shrank as age increased. In contrast, adding 2%, 3%, or 4% magnesium oxide expansion agent produced sufficient expansion at 40 ℃ to compensate for concrete shrinkage. Among these, the 4% magnesium oxide expansion agent fully developed its expansion performance after 20 days, reaching 0.031%.
After fully utilizing its expansion performance, the specimen showed almost no rebound, and its volume stability was very good. In comparison, the magnesium oxide expansion agent at 3% dosage showed only a slight improvement in expansion performance, with a limited expansion rate of 0.048% after 60 days. The initial expansion performance of a 2% magnesium oxide expansion agent is poor, with only 0.007% in the first 20 days.
To quickly compensate for early-age concrete shrinkage without causing excessive expansion after hardening, it is advisable to use as muchas much magnesium oxide expansion agent as possible.
A water conservancy hub consists of a right-bank power plant, fishway, drift hole, 18-hole spillway gate in the river, left-bank ship lock, left- and right-bank concrete gravity dams, substation, etc. The net width of the spillway tunnel is 14m, with 19 holes, a gate pier thickness of 3.5m, and a total width of 336m. The top of the spillway gate is roughly the same height as the riverbed, and the dam body is a crushing structure. The downstream buildings are 53.0m long downstream, with a maximum dam height of 50.0m and a total length of 133.5m. The connected buildings on both sides are gravity roller-compacted dams.
Because this project involves large-volume concrete, special attention must be paid to shrinkage and cracking. Large-volume concrete undergoes significant shrinkage during hardening. If effective measures are not taken, it may lead to concrete cracking, seriously affecting the project’s safety and durability. It is necessary to choose a higher dosage of expansion agent to compensate for the shrinkage of large-volume concrete and prevent cracking. Therefore, based on the region’s temperature gradient characteristics, the project selected 4.5% and 5.5% dosage schemes and conducted simulation analyses in combination with the no-dosage scheme.
Based on the calculations, considering the 43m long side and 18.6m high foundation of the spillway dam section, the results are basically within the foundation limits. After adding magnesium oxide, transverse stress in the confined foundation area decreases, especially in the strongly confined area, where the reduction is greater. In contrast, the reduction in the weakly confined area is very small. Magnesium oxide content is positively correlated with its stress-compensation effect; as the content increases, the stress-compensation effect gradually strengthens.
For example, when 5.5% is added, the stress-compensation effect of steel bars is greater than that of 4.5% steel bars, and the corresponding horizontal stress in the dam constraint area is reduced. From the results of stress analysis, it can be seen that the horizontal stress of most concrete in dams without magnesium oxide treatment exceeds the allowable value and does not meet the design requirements.
After adding 4.5% magnesium oxide, both the maximum stress before 28 days and the maximum stress over the entire age period decreased to some extent, and the range that did not meet the stress control requirements also decreased accordingly. As magnesium oxide content increases, the dam’s stress level also increases, generally meeting the design requirements. Adding 5.5% magnesium oxide effectively suppresses temperature cracking in dam concrete.
Adding magnesium oxide can significantly improve stress within the strong confinement zone, but it has little effect away from the bedrock. For the inner roller compacted concrete, the amount of magnesium oxide can be increased to ensure the stability of the concrete after adding magnesium oxide, while further improving its stress compensation degree, so that the tensile stress of the inner roller compacted concrete will not exceed the allowable range, achieving temperature control and preventing cracking.
(1) By optimizing the mix design of compensating shrinkage concrete and using different dosages of magnesium oxide expansion agent, the compensating shrinkage effect of magnesium oxide expansion agent can be better achieved, which maximizes the crack resistance measures of large volume concrete.
(2) For large-volume engineering, magnesium oxide expansion agent can effectively compensate for cracks caused by rapid cooling in the early stage, which is of great significance for controlling early temperature shrinkage cracks in concrete.
(3) Magnesium oxide expansion agent not only has the function of compensating for shrinkage, but also has the function of filling and improving the internal pore structure of concrete and enhancing its density, thereby improving the crack resistance, impermeability, mechanical properties, and long-term durability of concrete.
This study provides useful references for addressing temperature gradients and self-constrained stress issues in large-volume concrete engineering, and offers feasible solutions for engineering practice. In the future, further research can be conducted on the mechanism of action of magnesium oxide expansion agents, as well as optimized application schemes in different engineering environments.
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