How to Safely Use Polycarboxylate Admixtures

Important Considerations for the Safe and Effective Application of Polycarboxylate Water-Reducing Agents:

According to surveys, only a few manufacturers of polycarboxylate water-reducing agents in China have independently developed their products. Most companies either introduce technology from universities and research institutions or directly adopt technologies from other enterprises. Due to the limited technical capabilities of these manufacturers, a deep understanding of the technology behind polycarboxylate water-reducing agents is lacking, making it difficult to adapt to variations in raw materials and processes, and thus undermining product quality stability.

Therefore, it is recommended that producers of polycarboxylate water-reducing agents actively collaborate with universities and research institutions to fully understand the various factors affecting the performance of polycarboxylate products. They should adjust synthesis process parameters in a timely manner to stabilize product quality and strengthen technical reserves through joint or independent research and development to meet market demands. Additionally, the prompt establishment of raw material standards for polycarboxylate water-reducing agents and the stability of raw material prices are critical for ensuring the performance stability of these products.。

Some product specifications for water-reducing agents may appear impressive, or even show excellent test results, yet may not be suitable for specific projects. Conversely, some products with less outstanding test results may precisely meet the requirements for concrete material formulation in a particular project. Additionally, due to variations in raw materials, the test results at the time of bidding may not reflect the actual situation during project implementation. According to GB 50119-2003 "Technical Specification for the Application of Concrete Admixtures," Section 2.1.4 states that all raw materials used for concrete with admixtures, such as cement, sand, stones, supplementary materials, and admixtures, must comply with current national standards. When formulating concrete with admixtures, the raw materials used in the project must be employed, and the testing items should be determined based on design and construction requirements, with testing conditions matching construction conditions. If the raw materials or performance requirements change, retesting should be conducted.

For manufacturers and suppliers of polycarboxylate water-reducing agents, it is essential to use dedicated production equipment and lines for synthesizing and compounding these agents, without sharing production lines with other water-reducing agents. Furthermore, the transportation and storage of polycarboxylate water-reducing agents should not involve iron materials (except for stainless steel), but rather use plastics, glass, or other materials. Contact with iron can sometimes alter the performance of polycarboxylate agents.

The mixing of other water-reducing agents or types of admixtures is strictly prohibited. This has two implications. First, the compounding of polycarboxylate water-reducing agents (such as with lignosulfonates, foaming agents, defoamers, retarders, etc.) should only be conducted by the admixture manufacturer or supplier. Users of water-reducing agents, i.e., concrete producers, should only test and receive related performance data without adding any other components or carelessly mixing in other components. Using pumps and measuring equipment without proper cleaning after using other admixtures is absolutely forbidden. Second, concrete mixing equipment, transport vehicles, and pumping equipment should preferably be dedicated to concrete containing polycarboxylate water-reducing agents. When sharing mixing equipment, transport vehicles, and pumping devices, these must be thoroughly cleaned before being used for concrete containing other types of admixtures, and vice versa.

When preparing concrete mixtures with polycarboxylate water-reducing agents, it is crucial to adhere strictly to the optimal dosage of water-reducing agents and water determined in the laboratory. Avoid arbitrarily increasing the amounts of these components, as this may lead to adverse phenomena such as segregation, bleeding, hardening, and increased air content, which can affect the normal pumping and quality of the concrete.

The moisture content of the sand and aggregate must be accurately measured and deducted from the total water volume to prevent negative consequences from inaccurate moisture detection.

The compatibility issues between admixtures and cement/supplementary materials have been longstanding. Over the years, significant progress has been made in the research on the compatibility of naphthalene-based high-efficiency water-reducing agents and their composite products. First, through the efforts of researchers, producers and users of concrete, as well as suppliers of cement and supplementary materials, have collectively recognized this issue, moving away from the previous practice of placing all responsibility on the admixture suppliers. Second, the incompatibility problems between admixtures and cement/supplementary materials are attributed to many influencing factors, with in-depth research conducted on their impact patterns. Additionally, effective solutions have been explored for specific incompatibility phenomena.

According to GB 50119-2003 "Technical Specification for the Application of Concrete Admixtures," if concrete containing a pumping agent is transported from a ready-mixed concrete plant to the pouring site, factors such as long distances, traffic jams, or prolonged wait times may result in excessive slump loss, making it unsuitable for pumping or pouring. In such cases, a secondary addition of the pumping agent can be employed. A certain amount of pumping agent can be added to the concrete transport mixer and mixed quickly until uniform, after which the slump is measured to ensure it meets the requirements before pumping and pouring. In principle, when concrete containing polycarboxylate water-reducing agents experiences excessive slump loss due to unpredictable reasons, a secondary addition of the water-reducing agent can also be used to restore fluidity and prevent waste. However, since the performance of concrete mixtures is highly sensitive to the dosage of polycarboxylate water-reducing agents, excessive amounts can easily lead to segregation, bleeding, or even layering. There has been little research on the effects of secondary addition on concrete performance, so during actual construction, decisions on whether to add polycarboxylate water-reducing agents again and in what quantity should be made based on expert opinions and strict testing to fully verify feasibility.

Due to the generally high slump of concrete prepared with polycarboxylate water-reducing agents and the low viscosity of the mixture, the vibration radius and time after pouring should be determined through testing or expert guidance. Too small a vibration radius or too long a vibration time can easily lead to significant decreases in air content and severe layering between the aggregate and paste phases.

In all cases, both early and later curing of any concrete mixture is extremely important. The influence of polycarboxylate water-reducing agents on the drying shrinkage of concrete is relatively small, meaning that their addition does not drastically increase drying shrinkage, but this does not imply that curing can be relaxed or eliminated.

Like concrete with other admixtures, surface treatment such as secondary troweling, film covering, or spraying after pouring and vibrating concrete containing polycarboxylate water-reducing agents is very effective in preventing plastic shrinkage cracks. Continuous moist curing for 7 or 14 days is not only necessary for the normal strength growth of concrete but also serves as a safeguard against drying shrinkage cracks.

To enhance the crack resistance of concrete with polycarboxylate additives, it is also very necessary to incorporate a certain amount of fibers, and this concrete still requires good moist curing to ensure that the structure does not crack.

In projects, various technical challenges, technical disputes, and even conflicts arising from project accidents may occur. It is believed that all parties involved in the project ultimately aim for smooth project execution and quality assurance. The history of polycarboxylate water-reducing agents in production and application in actual projects is relatively short, and the accumulated engineering experience is quite limited, which can lead to application issues and even project accidents. In such cases, it is particularly important for construction or management units to collaborate closely with concrete producers, suppliers of polycarboxylate water-reducing agents, and suppliers of cement/supplementary materials. Through technical cooperation and in-depth analysis, effective solutions can be proposed.