by At present, in the construction of expressway, the requirements for the internal and external quality of high-performance concrete components are higher and higher, and the detection methods are more and more advanced and standardized.
As a simple and fast detection method, the rebound strength test plays an important role in the detection of the strength of concrete components.
In the construction quality control, the rebound strength is not only the primary basis for judging the strength of concrete structures, but also the main basis for tensioning conditions in the construction of prestressed components.
Only when the rebound strength and the strength and age of synchronous concrete specimens meet the requirements at the same time, can tensioning be carried out.
Therefore, the rebound strength not only determines the quality of components, but also affects the construction progress of prestressed components in the construction of structures.
The puzzle often encountered in construction is that the strength of the standard concrete specimen and the strength of the co-curing specimen meet the design requirements at a certain age, but the rebound strength of the concrete component cannot meet the requirements.
Therefore, it is preliminarily determined that the component is unqualified or fails to meet the conditions for the tensioning of the prestressed component.
In view of this problem, the factors affecting the rebound strength of concrete members are classified and summarized in combination with the actual construction.
1 Quality control of concrete 1.1 Quality of raw materials 1.1.1 Cement, fly ash and mineral powder.
Cement strength has the greatest impact, and the instability of cement strength will directly affect the strength of concrete.
For high-performance concrete, it is required to use P.O42.5 cement from a large manufacturer with stable quality, and according to the reaction principle, early strength cement cannot be used due to the addition of admixtures; If the quality of fly ash fails to meet the requirements, the water demand ratio will increase.
When the concrete reaches the original design workability, the water consumption will increase, and the water-cement ratio will increase, thus directly reducing the concrete strength; The activity of mineral powder is insufficient.
On the one hand, there are few active substances, and its strength will naturally decrease.
On the other hand, the same fluidity of concrete will increase the water consumption, increase the water-cement ratio, and reduce the strength.
1.1.2 The change of sand modulus can ensure the workability and influence on the strength by adjusting the sand rate, but the size of sand silt content greatly affects the concrete strength, and the influence mode is also to increase the water consumption per cubic meter to achieve the designed workability.
In the process of trial mixing concrete, the slump of the sand with 1.6% mud content is 22cm, and that of the sand with 3.0% mud content with the same other materials and the same mix ratio is 17cm.
If the workability is still 22cm, it is necessary to add a water-cement ratio and 0.1% additive.
However, if the mix ratio and workability remain unchanged during the construction, and the original workability is only guaranteed by the adjustment of water consumption, the water-cement ratio will be increased by at least 0.01, so the strength will be reduced.
In addition, in a rebound test area, most of the rebound points are on cement mortar, so the quality of cement, admixture and sand has a more prominent impact on the rebound strength than the compressive strength of the test piece.
1.2 The quality of the concrete mixture with slurry ratio is the main factor that determines the strength of the concrete component.
For the rebound strength of the component, in addition to the influence of raw materials, slump and air content, the influence of the slurry ratio on the rebound strength of the component is greater than that on the strength of the test piece.
The rebound strength is measured by the hardness of the surface of the concrete component.
The slurry ratio is too large.
After construction vibration, the cement mortar on the surface of the component will be too thick, and the hardness of the cement mortar is far less than the hardness of the gravel, which will reduce the rebound strength.
In view of this influence factor, first of all, in the design of concrete mix proportion, especially the design of concrete mix proportion above C50, it is assumed that the theoretical density of concrete per cubic meter should be 2450~2500kg/m3, and the sand rate should be the best state to meet the requirements of mixture construction, and high sand rate should not be pursued; Secondly, in the construction process, the water content test of sand and gravel should be accurate and representative, and the control of construction mix proportion can be accurate.
Otherwise, the actual sand consumption ratio is too large, resulting in a large slurry aggregate ratio.
1.3 Due to the influence of water-cement ratio on the stability of the mixing station, high-performance concrete has greater influence than water-cement ratio on ordinary concrete.
The stability of the metering system of the concrete mixing station has greater influence on the quality of concrete mixture and concrete strength.
The most important impact indicators are water cement ratio and sand ratio.
The unstable concrete quality, the uneven slump size and the grout aggregate ratio result in a large standard deviation of rebound results in the ten test areas of the box girder.
Even if the average strength is qualified, the presumed strength is not easy to reach the design strength.
The standard curing test piece and synchronous curing test piece can only represent the concrete quality of a certain plate and a certain vehicle, while the rebound concrete component can detect the quality of the whole batch of concrete.
Therefore, the stability of the mixing station has a prominent impact on the rebound index.
2.
In the construction process of construction vibration, the most important factor affecting rebound is vibration.
Under the same conditions, the concrete is vibrated compactly, with small voids and high rebound strength.
This can intuitively show that the strength of the rebound zone with dense and smooth bubbles on the same structure is higher than that of the rebound zone with more bubbles.
High performance concrete has high viscosity and is difficult to discharge bubbles, so the requirements for vibration are far greater than those for ordinary concrete.
In addition, the rigidity of the formwork itself and the stability of installation are also factors related to vibration that affect the compactness of concrete.
3.
The formwork removal time of components should not be too early, and the external formwork removal time should not be less than 15h when the average temperature in summer is above 25 ℃, and the dry weather at noon should be avoided.
.
