Precast beams are a kind of precast component widely used in bridge engineering. Still, due to various factors, a large number of surface bubbles will appear, which seriously affects the apparent quality. This article discusses and summarizes the influencing factors and control points of pores on the surface of precast beams from the aspects of concrete mix ratio, mold, release agent, etc.
01 Selection Of Concrete Raw Materials And Mix Ratio Design:
Physical Transformation
1. Excessive concrete consistency or poor concrete workability will make it difficult to discharge bubbles during concrete vibration. Therefore, unreasonable aggregate grading, needle-like and improper particle size, and low sand ratio in production ratio, easy to cause the concrete mixture to be not dense, easy to introduce bubbles during mixing, and the bubbles generated do not easily disappear, resulting in pores on the surface of the concrete.
2. The influence of cement. At present, to improve the grinding efficiency and strength of cement, cement manufacturers often add grinding aids, such as triethanolamine, triisopropanolamine, etc., when grinding cement. Due to the addition of some grinding aids, the produced cement has an air-entraining effect, which is also easy to cause pores to form on the surface of concrete.
3. The influence of water reducer. When anionic surfactants are used or used in excess, the surface tension of free water in the mixed concrete will decrease unevenly, thus forming bubbles. At the same time, large bubbles will be introduced during the concrete mixing process due to mechanical force. We can reduce the number and size of bubbles by adding an appropriate amount of defoamer to the admixture.
Concrete Mix Design Requirements
The performance requirements of concrete for precast beams should be stricter than those of commonly used concrete. Through a large number of tests, it is found that to reduce the pores on the surface of precast beams, the following performance requirements should be met:
1. Slump control range: 180±20mm, expansion range: 400±50mm;
2. The concrete does not separate and bleed, the slurry has appropriate cohesiveness, and a small amount of stone is allowed to be exposed;
3. It has an appropriate air content, generally controlled at 2.0-3.0%;
4. The particle size, morphology, and gradation of the stone affect the number of holes, and the impact crusher is better than the ordinary stone;
5. According to the construction requirements, the operating time is not less than 1.5 hours;
6. The sand ratio is as low as possible under reasonable circumstances, which can effectively reduce the floating slurry;
7. The final setting time of the concrete is controlled at about 8h, which is subject to the specific construction requirements. If the setting time is short, it needs to be cured as soon as possible.
Raw Material Selection
Raw materials used in the test and production: P·O42.5 cement, Songneng II grade fly ash, Sangang S95 grade mineral powder, granite crushed stone, river sand. Its performance requirements can meet the use of ordinary commercial concrete. Special attention should be paid to the fact that compared with conventional water reducers, the polycarboxylic acid water reducer for precast beams should appropriately reduce the amount of air-entraining agent and reduce the air content of concrete. At the same time, according to the rhythm of the construction process, the final setting time should be controlled within 8 hours.
Mix Ratio Design
The concrete mix ratio used for precast beams should first meet the design strength requirements and have good construction performance. At the same time, to minimize the probability of bubbles on the surface, the mixture needs to have good fluidity and appropriate cohesion, which can reduce the resistance of bubbles floating up, and at the same time, the concrete has good anti-delamination ability after vibration. The main parameter selection basis and detailed proportions are as follows:
1. Aggregate grading and sand ratio: According to the type and performance of aggregate, the most compact packing density test is used to find the optimal stone matching ratio. At the same time, the sand ratio is selected as low as possible under the premise of ensuring the encapsulation of the concrete mixture to avoid the generation of floating slurry and improve the rebound strength. This experiment uses a combination of 50% 5-20mm impact crushed stone + 50% 5-20mm ordinary crushed stone, and the sand ratio is 37%.
2. Cementitious material: According to the "micro-aggregate effect" and "volcanic ash effect", the binder uses silicate cement and adopts a double blending method of fly ash and mineral powder. The relationship between the total amount of binder and the number of bubbles on the surface of the test block is as follows: When the binder is too little, the amount of cement slurry is small, and the encapsulation of the concrete mixture is poor. As the total amount of binder increases, the number of bubbles first decreases and then shows a stable trend.
3. Water consumption: After fixing the water-binder ratio, the water consumption of concrete should be controlled by the slump of concrete entering the mold. According to the relationship curve between slump and the number of bubbles, the slump of precast beams entering the mold is controlled at 180±20mm, and the expansion is 400±50mm, which is the best.
02 Selection Of Molds And Release Agents
The material of the mold and the type of release agent will directly affect the apparent quality of the component. This experiment selected three types of molds: wooden molds, steel molds, and aluminum molds. Oily-release agents and water-based release agents were used respectively. Concrete was molded under the same conditions. The number of bubbles per unit area was measured to judge the influence of molds and release agents on bubbles. The following conclusions were drawn through a large number of tests:
1. The influence of mold material, shape, and surface finish on pores. The surface finish of the mold has an impact on the existence of holes. It is recommended to polish the mold before installation, and manually clean the mold with a steel brush until there is no concrete slag or other residue on the mold to ensure the surface finish of the mold;
2. Release agent is a key factor affecting the existence of holes. In terms of appearance, water-based release agents are generally better than oil-based release agents; the effects of water-based release agents vary greatly, and a suitable release agent requires a lot of tests to verify; the brushing process of the release agent is also an influencing factor. Uneven brushing, thin ones will cause concrete sticking film, and too thick ones will more easily produce sand lines when vibrated; apply the release agent after the mold is installed, and use a suitable water-based release agent and water in a certain proportion before spraying or brushing. Requirements for brushing release agent: brush evenly without missing, no obvious wall hanging, not too thick and prevent contamination.
