Column Concrete Reinforcement

Construction quality overview

Reinforced concrete frame structure, 6 floors above ground, 0 floors underground, the first floor is 3.2 meters high, and the cross-section size of the structural column is 700mmx700mm, 600mmx600mm. On August 2, 2018, the first-story column structure concrete pouring construction was completed, and the second-story beam slab-column structure concrete pouring construction was completed on the 7th. After the column formwork was dismantled on the 9th, the construction unit and the supervision unit carried out an appearance inspection of the concrete quality of the first-story column and the second-story beam slab surface, and found that the following problems existed mainly

The appearance of the column concrete poured first by the shaft is normal, and there is no obvious concrete quality problem. However, in the second beam-slab-column concrete, the concrete at the junction of the new and old concrete beams and columns was suspected of not vibrating, the appearance of the concrete showed roughness, and the poor junctions of the frame columns were mainly concentrated around the interface. In the range of about 0mm~300mm from the horizontal joint surface, the top of the column (bottom of the plate) is in a better position. At the same time, the upper concrete density of the joint surface is lower than the lower concrete poured in advance

The concrete strength of the above two columns is rebounded, and the concrete strength of the 9/E axis column head part is determined to be C22.2, and the concrete strength of the 11/E axis column head part is C21.6, which is lower than the design value C35.

Cause Analysis

After research and analysis, the main reasons for insufficient concrete strength are as follows 

1 , poor quality of raw materials:   

(1) Poor cement quality: 1. The actual activity of the cement is low; 2. The cement stability is unqualified;   

(2) Poor quality of aggregate (sand, stone):

①The strength of the stone is low;

② Poor volume stability of stones;

③The shape and surface condition of the stone is poor;

④The sand and stone contain high organic impurities, high dust content, and high mica content in sand;   

(3) The quality of the mixed water is unqualified;   (

4) The quality of the admixture is unqualified or the composition ratio is improper   

2 , Improper concrete mix ratio:   

The mix ratio of concrete is one of the important factors that determine the strength. The water-cement ratio directly affects the strength of the concrete. Others such as water consumption, sand ratio, and bone-ash ratio also affect the various properties of concrete, resulting in insufficient strength accidents.

(1) Increase in water consumption: the more common ones are the inaccurate measurement of the water adding device on the mixer; the water content in the sand and stone is not deducted; even water can be added at the irrigation site;  

(2) Use mix ratio at will; 

(3) The admixture content is inaccurate;  

(4) Improper use of additives;   

(5) Inaccurate sand and gravel measurement and insufficient cement consumption.  

3 , Incorrect construction process (main reason) 

(1) After the concrete was transported to the construction site, there was no mixing again or poor mixing. The time was too short or too long to cause unevenness, and the water content of the first pumped concrete was too large (the pump pipe has moisturizing water);  

(2) The formwork is not tight, and the cement slurry leaks seriously during pouring;  

(3) Improper pouring method, no chute or tandem tube for one-time pouring super high, resulting in segregation of concrete during pouring, and insufficient vibrating during concrete pouring, resulting in separation of gravel and mortar in concrete;  

(4) Forming vibration is not compact and not in place or there is no vibration (leakage);  

(5)  Improper maintenance, early water shortage maintenance caused concrete drying.

Special reinforcement treatment plan for the first floor frame column

Structural columns are the main force-bearing members of reinforced concrete structures, which are compression-bending members and are subject to great axial forces. The destruction of the structural column will cause serious consequences, not easy to repair and even cause the structure to collapse. Therefore, structural columns with quality defects should be treated with caution, carefully analyzed and effective reinforcement measures should be taken to meet the requirements of bearing capacity and durability.

According to the actual situation and data of the site, combined with the characteristics of the reinforced parts, after research and analysis, our company plans to adopt the reinforcement method of enlarged cross-section or the reinforcement method of replacement concrete.

The increasing section reinforcement method has the advantages of simple principle, rich experience, reliable force, and low reinforcement cost. However, it also has some shortcomings, such as heavy workload of wet work, long maintenance period, increased structural weight, occupation of building space and impact on use functions, etc., so that its application is limited. This is the traditional reinforcement method. It is the most stupid but often the most reliable method. It is a method that is often used in engineering practice.

The concrete component replacement reinforcement method refers to the use of high-strength concrete or grouting material to replace the concrete with quality defects in the original structural component to restore or improve the bearing capacity, stiffness and durability of the reinforced structural component, and to ensure that the component meets the bearing capacity requirements.  The advantages of this method are similar to those of the enlarged section method, and the clearance of the building will not be affected after reinforcement, but it also has the disadvantage of long wet working time for construction. It is suitable for the reinforcement of concrete load-bearing members such as beams and columns with low concrete strength or serious defects in the compression zone.