Points to Note During the Use of Polycarboxylate Superplasticizer
Release Time:
2015-07-28
Source:
Points to Note During the Use of Polycarboxylate Superplasticizer
During the production of polycarboxylate superplasticizers, it is essential to have a thorough understanding of the various factors that influence product performance and to make timely adjustments to the synthesis process parameters in order to ensure consistent product quality. Furthermore, by leveraging collaborative efforts or independent R&D to strengthen technological reserves and better meet market demands, we can effectively safeguard the stability of polycarboxylate superplasticizer performance. Once product quality is assured, what key considerations should we keep in mind during the application of polycarboxylate superplasticizers? Let’s explore this together below:
Carefully read the product specification, and conduct trial mixes of concrete incorporating admixtures in accordance with the test results and the specific mix design for the project’s concrete materials. When conducting trial mixes, use the raw materials actually employed in the project; the test items shall be determined based on the design and construction requirements, and the testing conditions shall be identical to the actual construction conditions. If there are any changes in the raw materials used in the project or in the performance requirements for the concrete, additional trial mixes shall be carried out.
Avoid contact between polycarboxylate superplasticizers and ferrous materials: The conveyance and storage of polycarboxylate superplasticizers shall not use ferrous materials (except stainless steel); instead, materials such as plastic and glass should be employed. Contact with ferrous materials can sometimes lead to changes in the performance of polycarboxylate superplasticizers.
The admixture of any other water-reducing agents or other types of chemical admixtures is strictly prohibited, and this prohibition carries two important implications. First, the formulation of polycarboxylate-based water-reducing agents—such as blending with lignosulfonates, air-entraining agents, defoamers, or set-retarding additives—must be carried out exclusively by the admixture manufacturer or supplier. The user of the water-reducing agent, i.e., the concrete producer, is only responsible for conducting performance tests and accepting the material into storage; under no circumstances may any additional components be blended into the product, nor should any unintended admixtures be introduced through oversight. Furthermore, the use of pumping and metering equipment without thorough cleaning after previously handling other admixtures is absolutely forbidden. Second, concrete mixing equipment, transport vehicles, and pumping equipment should ideally be dedicated to producing concrete containing polycarboxylate-based water-reducing agents. When such equipment must be shared among different types of concrete, it must be thoroughly cleaned before being used for concrete incorporating other admixtures, and vice versa.
Strictly measure water-reducing admixture and mixing water: When preparing concrete mixes incorporating polycarboxylate-based water-reducing admixtures, the optimal dosage of the admixture and the amount of mixing water determined by laboratory trials must be measured with exacting precision. It is imperative to avoid arbitrarily increasing the dosage of the admixture or the amount of mixing water, as such practices can lead to undesirable phenomena such as segregation, bleeding, hardening, and increased air content, thereby compromising the normal pumpability and placement quality of the concrete. Furthermore, the moisture content inherent in the raw materials—namely, sand and coarse aggregate—must be accurately measured and deducted from the total water requirement to prevent adverse consequences arising from inaccurate assessment of this intrinsic moisture.
Properly addressing the compatibility issues between polycarboxylate-based water-reducing admixtures and cement/admixtures: The issue of admixture–cement/admixture compatibility has been a long-standing concern. Over the years, extensive research on the compatibility of naphthalene-based high-efficiency water-reducing admixtures and their formulated blends has yielded significant results. First, through the concerted efforts of researchers in raising awareness, stakeholders—including concrete producers, admixture manufacturers, cement and admixture suppliers, concrete contractors, and supervising engineers—have come to recognize this issue, thereby shifting away from the previous situation in which admixture manufacturers and suppliers were solely held responsible. Second, the incompatibility between admixtures and cement/admixtures has been attributed to numerous influencing factors, and in-depth studies have been conducted to elucidate the underlying mechanisms and patterns of these effects. Third, for specific instances of incompatibility between admixtures and cement/admixtures, a range of effective mitigation measures have been developed and refined.
The secondary addition of polycarboxylate-based water-reducing admixtures shall be carried out under the guidance of experts and following rigorous testing: According to GB 50119–2003 “Technical Specification for the Application of Concrete Admixtures,” when concrete containing a pumping admixture is transported from a ready-mixed concrete plant to the placement site and, due to long distances, traffic congestion, or excessive waiting time before placement, experiences excessive slump loss to the point where it is no longer suitable for pumping or placing, a secondary addition of the pumping admixture may be employed. In this approach, a specified amount of the pumping admixture is added directly into the transit mixer during transport, the mixer is run at high speed until the admixture is uniformly distributed, and the slump is measured to ensure it meets the required specifications before pumping and placement commence. In principle, for concrete containing polycarboxylate-based water-reducing admixtures, if unpredictable factors result in excessive slump loss, a secondary addition of the water-reducing admixture may also be used to restore the concrete’s workability and thereby avoid waste of the concrete mix. However, since the performance of the concrete mix is highly sensitive to the dosage of polycarboxylate-based water-reducing admixtures, in actual construction practice the decision whether to perform a secondary addition, as well as the quantity to be added, must be based on expert advice and supported by rigorous testing to fully verify feasibility before implementation.
Strict control of vibration radius and vibration duration is essential: Because concrete prepared with polycarboxylate-based water-reducing admixtures typically exhibits a high slump and low mix viscosity, the vibration radius and vibration time after placement must be determined through testing or in accordance with expert guidance. An excessively small vibration radius or an overly prolonged vibration duration can readily lead to serious structural defects, such as a substantial reduction in air content and severe segregation between the aggregate phase and the paste phase.
Strengthen early-age curing to strictly prevent cracking: In all circumstances and for any concrete mix, both early- and late-age curing after placement are of paramount importance. Polycarboxylate-based water-reducing admixtures have only a minor impact on concrete’s drying shrinkage; in other words, their addition does not substantially increase drying shrinkage. However, this does not mean that concrete containing such admixtures can be allowed to forego or even eliminate curing. As with concrete incorporating other admixtures, post-placement secondary troweling of the surface, film covering, or spray curing—following thorough compaction by vibration—are highly effective in preventing plastic-shrinkage cracks. Moreover, continuous moist curing for 7 or 14 days is not only essential for the proper development of concrete strength but also serves as a critical safeguard against drying-shrinkage cracking. To further enhance the crack resistance of polycarboxylate-containing concrete, it is equally important to incorporate a specified amount of fibers; yet even such concrete still requires adequate wet curing to ensure that the structure remains crack-free.
