Enhancing Concrete Quality: Strategies to Address Common Issues with Water-Reducing Admixtures
Water-reducing agents are mostly anionic surfactants, with polycarboxylate-based water-reducing agents and naphthalene-based water-reducing agents commonly used in the market. They play a significant role in adjusting the performance of concrete by significantly reducing the mixing water without changing the slump, enhancing concrete strength, and reducing the occurrence of cracks.
However, influenced by multiple factors such as the decline in raw material quality and the lack of professional expertise among workers, concrete mixes with water-reducing agents may exhibit various issues, which are analyzed and resolved as follows:
1. Adhesion Phenomenon
The cement mortar partially adheres to the drum wall of the mixer, resulting in uneven, less grey, and sticky concrete upon discharge. Reason: The stickiness of concrete often occurs after the addition of retarding water-reducing agents or in drum mixers with a similar axis-to-diameter ratio.
Solutions:
(1) Pay attention to cleaning and removing residual concrete in time;
(2) Add aggregate and part of the water to mix first, then add cement, remaining water, and water-reducing agents for mixing;
(3) Use mixers with a larger axis-to-diameter ratio or forced mixers.
2. False Setting Phenomenon
(1) Insufficient calcium sulfate or gypsum in the cement leads to rapid hydration of tricalcium aluminate;
(2) Poor adaptability of the water-reducing agent to this type of cement.
(3) The dosage of triethanolamine exceeds 0.05-0.1%, which results in quick initial setting but no final setting.
Solutions:
(1) Change the type of cement;
(2) Appropriately adjust the additives for reasonable compounding;
(3) Supplement with Na2SO4 in the additives.
3. Non-setting Phenomenon
Concrete does not set for a long time, even overnight, after the addition of water-reducing agents; or surface bleeding occurs with a yellow-brown color.
Reasons:
(1) Excessive dosage of water-reducing agents;
(2) Overuse of retarders.
Solutions:
(1) Do not exceed 2-3 times the recommended dosage, although strength may slightly decrease, but the reduction at 28 days is less, and the long-term strength reduction is even less;
(2) After final setting, appropriately increase the curing temperature and enhance watering;
(3) Remove the formed part and re-pour.
4. Low Strength Phenomenon
The strength is much lower than the results of the same age period test, or the concrete has set but has very low strength.
Reasons:
(1) Excessive dosage of air-entraining water-reducing agents, causing excessive air content in the concrete;
(2) Insufficient vibration after adding air-entraining water-reducing agents;
(3) Not reducing water or actually increasing the water-cement ratio.
(4) The quality of the water-reducing agent does not meet the requirements, such as too low effective component content.
Solutions:
(1) Adopt other reinforcement measures or re-pour;
(2) Strengthen vibration after pouring;
(3) Take measures for the aforementioned reasons;
(4) Identify the batch of water-reducing agents.
5. Rapid Slump Loss
Concrete quickly loses workability, with a reduction of 1-50mm in slump every 2-3 minutes after discharge, and there is a clear sedimentation phenomenon. This phenomenon is more likely to occur with high slump concrete.
Reasons:
(1) Poor adaptability of the water-reducing agent to the cement used;
(2) Bubbles introduced into the concrete continuously overflow, and water evaporates, which is particularly evident when using air-entraining water-reducing agents;
(3) High mixing temperature or ambient temperature of the concrete;
(4) The concrete has a large slump.
Solutions:
(1) Find the cause and take measures according to the cause;
(2) Use the late addition method, add the water-reducing agent 1-3 minutes after mixing the concrete or even before pouring, and re-mix;
(3) Be careful not to add water alone.
6. Settlement Cracks
After pouring, the concrete will have several short, straight, wide, and shallow cracks before and after the initial setting.
Reason: The concrete is more viscous after adding water-reducing agents, does not bleed water, and does not easy to fully settle flat, often appearing above the reinforcement. Solution: Press and smooth the cracks before and after the initial setting until they disappear.
