Underwater non dispersible concrete is a kind of concrete technology developed rapidly under the construction of underwater concrete in the world. Since the non dispersible concrete admixture since 1974 the German SIBO company developed under water. Internationally, it has been widely used in offshore oil platforms, submarine pipelines, artificial islands, ports, Zhanqiao, docks, dams, hydropower stations, bridges and other projects. Akashi bridge typical engineering makes the reliability of the Japanese NDC is more reliable. In recent years, this method has created many high-quality underwater projects, realized the underwater construction “land oriented”, and made the new structure, new design and new construction of the hydropower project difficult to imagine realized.
Hydroxypropyl methyl cellulose (HPMC) has excellent thickening, can be used as a good concrete anti dispersion agent, the material is the past domestic shortage of fine chemical products, high cost, due to various reasons limit its application in the construction industry in our country. In recent years, with the continuous development of external wall insulation technology, as well as the continuous progress of cellulose production technology, as well as the excellent characteristics of HPMC itself, HPMC has been widely used in the construction industry.
Setting time test
The setting time of the concrete is mainly related to the setting time of cement, aggregate has little effect, so can be used to replace the setting time of mortar HPMC on non dispersible underwater concrete setting time, the setting time of mortar by water cement ratio, cement sand ratio, so in order to evaluate the effect of HPMC on setting time mortar, mortar fixed water cement ratio and cement sand ratio.
The experimental results show that the addition of HPMC has obvious retarding effect on mortar mixture, and with the HPMC content increase the setting time of mortar have been extended, the same HPMC content under the condition of water forming mortar than the setting time longer molding in the air. When the water is measured, the setting time of the mortar mixed with HPMC is delayed 6~18 h and the final setting is delayed by 6~22 h compared with the blank specimen. So HPMC to use a composite early strength agent.
HPMC is a macromolecular polymer, which is a macromolecular linear structure, with hydroxyl groups on functional groups, and can form hydrogen bonds with mixing water molecules, so that the viscosity of the mixing water increases. The long molecular chains of HPMC attract each other, so that HPMC molecules are entangled with each other to form a network structure, and the cement and mixing water are wrapped up. Because of the formation of network like structure and wrapping of cement, HPMC can effectively prevent the volatilization of moisture in mortar and hinder or retard the hydration speed of cement.
Bleeding of mortar and concrete are similar, will cause the aggregate settlement is serious, leading to the top of slurry water cement ratio increases, the slurry and plastic shrinkage cracking in the early, large, and slurry surface strength is relatively weak. The experimental results show that when the content is above 0.5%, the basic no longer bleeding phenomenon. This is because when the HPMC mortar, HPMC film and network structure, and the large molecule long chain hydroxyl adsorption, so that the formation of flocculent and mixing water cement mortar, ensure the stable structure of mortar. Join the HPMC again after the mortar, it forms many independent tiny bubbles. These bubbles will be evenly distributed in the mortar and aggregate hinder deposition. This technology has a great influence on the properties of HPMC cement based materials, such as dry mortar, often used to prepare polymer mortar cement based composite material, which has good water retention, water resistance.
Water requirement test of mortar
In the content of HPMC is very small, it has a great influence on the water demand of mortar. Under the condition of maintaining the same extension of fresh mortar, the amount of HPMC and mortar water demand change linearly in a certain period of time, and the water demand of mortar decreases first and then increases obviously. The HPMC content is less than 0.025%, with the increase of dosage, the same degree of expansion conditions, reduce the water requirement of mortar, which indicates that the HPMC content is small water reducing effect on mortar, air entraining effect with HPMC, make the mortar there are lots of tiny independent bubble, these bubbles lubricates the. Mortar fluidity improved. When the content is greater than 0.025%, mortar water demand increases with the content, this is because the network structure of HPMC to complete the flocculation, clearance of the long chain molecules has attracted shortened, and the cohesion function, reduces the mobility of mortar. So in the spread of the same case, the slurry showed the water demand increases.