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Polycarboxylate superplasticizers have developed rapidly domestically and internationally in recent years because of their superior performance and pollution-free production. Polycarboxylate water reducers differ greatly from traditional water reducers in terms of molecular structure, principle of action, and performance behavior in concrete. Therefore, a correct understanding and rational use are important steps in promoting the application of polycarboxylate water reducers. This article mainly explains common problems for polycarboxylate superplasticizers in concrete application.
The composition of cement and cementitious materials is complex and varied. From the perspective of the adsorption-dispersion mechanism, it is impossible to find a water reducer that can adapt to everything. Although polycarboxylate water reducers are more adaptable than the naphthalene series, they may still perform poorly with some cement types. This adaptability is mostly reflected in a lower water reduction rate and greater slump loss. Even for the same cement type, the effect of water-reducing agents varies when ball-milled to different fineness levels.
Phenomenon: A certain mixing plant used P-042.5R cement from the area to supply C50 concrete to a construction site, using polycarboxylate superplasticizer. When preparing the concrete mix ratio, they found that the amount of superplasticizer used with this cement was slightly higher than with other cements. However, during actual mixing, the factory visually measured the slump of the concrete mixture at 210mm. When it was unloaded into the concrete pump truck at the construction site, the concrete could not be unloaded. The factory was notified to send a bucket of superplasticizer for mixing, and the slump was visually measured at 160mm, which met the pumping requirements. However, during the initial unloading process, the concrete could not be unloaded. The concrete truck was immediately returned to the factory, and a large amount of water and a small amount of water-reducing agent were added. The water-reducing agent was barely discharged and almost solidified in the mixer truck.
Reason analysis: Failure to consistently conduct adaptability tests with admixtures for each batch of cement before opening.
Prevention: Conduct a compounding test on each batch of cement using the construction mix ratio before opening. Choose appropriate admixtures. Cement adaptability with coal gangue as an admixture and polycarboxylate superplasticizer is poor, so avoid using them.
Because polycarboxylate superplasticizer is used, concrete water consumption is significantly reduced, with water consumption per cubic meter mostly ranging from 130-165 kg; the water-cement ratio is 0.3-0.4, or even less than 0.3. With low water usage, fluctuations in water addition can significantly change slump, leading to a sudden increase in slump and bleeding in concrete mixtures.
Phenomenon: A certain mixing plant used P-032.5R cement from a certain cement plant to prepare C30 concrete. The contract requires a slump of 150 mm ± 30 mm at the construction site. When the concrete left the factory, the measured slump was 180mm. After transport to the construction site, we measured the concrete slump at 210 mm. Two consecutive trucks of concrete were returned, and upon returning to the factory, we verified that the slump was still 210mm, with bleeding and delamination.
Reason analysis: The cement has good compatibility with this water-reducing agent, and the dosage of the water-reducing agent is slightly higher. The mixing time was insufficient, and the concrete slump at discharge did not reflect the true slump due to the short mixing time.
Prevention: For cement sensitive to polycarboxylate superplasticizer dosage, the additive dosage should be appropriate, and measurement accuracy should be high. Properly extend the mixing time; even with a double horizontal shaft forced mixer, the mixing time should be at least 40 seconds, preferably more than 60 seconds.
From a pumpability and durability perspective, increasing air-entrainment capacity appropriately is advantageous, as many polycarboxylate superplasticizers have relatively high air-entraining capacity. Polycarboxylate-based water reducers, like naphthalene-based water reducers, also have a saturation point. The saturation point of this additive in concrete varies with cement type and dosage. If the admixture dosage is close to its saturation point, the flowability of the concrete mixture can only be improved by adjusting the slurry content or using other methods.
Phenomenon: A certain mixing plant has been using polycarboxylate superplasticizer to prepare concrete for some time. Suddenly, one day, a construction site reported that after removing the shear wall formwork, they found too many bubbles on the wall surface, and the appearance was too poor.
Reason analysis: On the day of pouring concrete, the construction site repeatedly reported low slump and poor fluidity. The on-duty personnel at the concrete mixing plant laboratory increased the additive dosage. The construction site uses standardized steel formwork. During pouring, too much material is added at once, resulting in uneven vibration.
Prevention: Strengthen communication with the construction site and recommend strictly following the standard requirements for feeding height and vibration methods. Adjust the slurry content or use other methods to improve the fluidity of concrete mixtures.
Phenomenon: The concrete slump is high, and it finally takes 24 hours to set. A construction site reported to the mixing plant that a portion of the concrete had not yet solidified 15 hours after pouring the structural beam and slab concrete. The mixing plant sent engineers to inspect the situation, and after heat treatment, the concrete finally took 24 hours to set.
Reason analysis: The dosage of water-reducing agent is relatively high, and coupled with the low ambient temperature at night, the hydration reaction of concrete is slow. Unloading workers on the construction site illegally added water to the concrete, significantly increasing the water content.
Prevention: Use the appropriate additive dosage and measure precisely—reminder: When the construction site temperature drops, pay attention to insulation and maintenance. Polycarboxylate additives are sensitive to water usage, so do not add water arbitrarily.
Phenomenon Case 1: The concrete mixed with polycarboxylate superplasticizer and naphthalene superplasticizer was poured in the same area, resulting in severe mold sticking during the demoulding of shear walls. During the trial application of polycarboxylate superplasticizer, a mixing plant provided concrete with different mix ratios to two construction sites. Because of a power outage at one construction site, the dispatcher transferred a truck of concrete mixed with polycarboxylate superplasticizer to that site. However, severe mold sticking was found during demoulding at this construction site.
Reason analysis: Concrete prepared with two different types of additives has different setting times and shrinkage sizes. Adverse reactions occur when you mix two additives with different properties.
Prevention: Do not use concrete prepared with two different types of additives in the same area. After discovering the problem, communicate with the construction site in advance and delay the demoulding time appropriately. Use each additive type separately, and schedule production at designated stations, vehicles, and construction sites.
The strength of concrete prepared by mixing two types of water-reducing agents is extremely low. A worker at a certain mixing plant accidentally discharged a small amount of naphthalene-based water-reducing agent into a polycarboxylate-based admixture while unloading the admixture.
The concrete prepared with the mixed admixture not only has a high admixture dosage, but also poor flowability and rapid collapse loss. However, the setting time was delayed, and the concrete specimen strength was 30%-50% lower than that of concrete with the same cement dosage.
Phenomenon Case 2: Mixing residual concrete with a naphthalene-based water-reducing agent resulted in extremely poor workability. A production worker mixed a small amount of residual concrete prepared with a naphthalene-based water-reducing agent with concrete prepared with a polycarboxylate-based water-reducing agent. As a result, the slump was 190mm when it left the factory, 100mm when it was delivered to the construction site, and it had almost no fluidity. During vibration, it took a long time for the holes to heal after the vibrating rod was pulled out.
Reason analysis: Concrete prepared with two different types of additives has different setting times and shrinkage sizes.
Prevention: With current technological conditions, two types of additives cannot be mixed. During production, strengthen management, label clearly, and prevent such accidents.
Polycarboxylate superplasticizers is a high-performance water reducer with low dosage, high water reduction rate, and low slump loss. As research deepens and the engineering community’s understanding grows, the application scope of polycarboxylate water reducers will continue to expand. Based on the above cases, improper use of polycarboxylate superplasticizers can also cause rapid setting, slow setting, bleeding, delamination, and reduced strength. We need to continue conducting experimental studies to avoid these problems.
