Reinforcement engineering is one of the cores of concrete structure engineering. In the current frame structure construction, the practices of raw materials, anchorage, joints, stirrups and substitution of steel bars often cause some quality problems due to negligence or irregularity, leaving engineering hidden dangers to some extent. According to my own construction experience in recent years, I summarized the following problems that have been existing in the construction of frame structure reinforcement project, and put them forward for discussion, hoping to attract enough attention from my peers. s

First, the misuse of raw materials.

Nowadays, the scale of buildings is getting bigger and bigger, and the building functions are diversified. A community of tens of thousands or even hundreds of thousands of square meters consists of houses, shopping malls, office buildings and apartments, and is under construction at the same time. According to the requirements of use function and structural form, the design unit designs secondary steel and tertiary steel with the same specifications in the same project. However, in the construction process, it is difficult to distinguish them because they are all rebar. In particular, the raw materials of secondary steel in some steel mills are marked with three words, so it is easy for construction workers to regard secondary steel as tertiary steel. Others misread or write C as φ, which also leads to misuse. The standard tensile strength of secondary reinforcement is 335Mpa, and that of tertiary reinforcement is 400Mpa. The reinforcement ratio is different with different reinforcement strength values. Once improperly used, it will seriously affect the bearing capacity of the structure, leaving quality hidden dangers.

Control measures: firstly, check the samples. The foreman and quality inspector must look at the pictures carefully, and the technicians can only process the bill of materials after reviewing them. After the reinforcement comes into play, the material engineer and the operator should check the signs with quality certificates and take samples for a second interview under the witness of the supervisor. Unmarked or unqualified rebars will be considered as exits and shall not be used in the project. Re-inspection of qualified steel bars should be classified and stacked and marked.

Second, the reinforcement anchorage problem

In the actual construction process, some construction workers did not fully understand the drawings, specifications and atlas, and did not pay enough attention to the anchorage of beam reinforcement of civil air defense roof and roof frame. The beam reinforcement was not embedded in the column before tamping the top column concrete, resulting in insufficient anchorage length of beam reinforcement of some frames. Similarly, when processing column rebar, the melting amount of rebar during welding is not considered. When the reinforcement reaches the top floor, the main reinforcement can't reach the top of the beam, which shortens the anchorage length of the reinforcement. Sometimes the amount of melting is too much, and the main reinforcement is higher than the top of the beam when it reaches the top floor. Construction workers often cut off the anchorage and cannot anchor it into the beam slab as required. It is also a common problem that the top anchorage of seismic KZ side columns and corner columns is anchored according to the middle column and non-seismic KZ. These problems weaken the integrity and seismic capacity of the structure.

Control measures: On the basis of being fully familiar with the drawings, combined with the construction specifications and related atlas, the on-site construction management personnel shall make technical disclosure to the steel bar team before work, and keep enough burning quantity when allocating column steel bars. When the construction reaches the top floor, the main reinforcement of the floor slab shall be cut flat at one time according to ≥Hn (column clear height) /6 and ≥hc (long side of section) ≥500, and then according to the actual height and anchorage requirements. Before reinforcement is concealed, self-inspection, mutual inspection and special inspection shall be strictly carried out.

Third, the reinforcement connection problem

1, electroslag pressure welding connection: At present, electroslag pressure welding is used for the main reinforcement of most engineering frame columns, but the qualified rate of welding quality of steel bars with diameters of 28 and 32 is low. Although the Code for Welding and Acceptance of Steel Bars (JJ18-2003) stipulates that the application scope of electroslag pressure welding is 14 ~ 32, the quality problems of large-diameter steel bars such as undercut, slag inclusion, porosity and incomplete penetration are often caused by external factors such as the quality of operators, current and voltage. In addition, the management personnel did not carefully check the welded joints of steel bars one by one, leaving a major quality hazard.

Solution: In view of the current construction situation, it is recommended to use mechanical sleeve connection for steel bars with a diameter greater than 25. The quality is more assured.

2. Straight thread steel bar connection: In recent years, the construction technology of straight thread steel bar connection has been relatively mature and the quality is relatively guaranteed. At present, the main problem that is easy to appear is that the quality of reinforced sleeve can not be guaranteed. It is seen that the sleeves supplied by many manufacturers are not long enough, the inner diameter is too large, and the length and tightness of the joints can not meet the requirements. In the process of construction, workers don't need measuring tools and calipers when tapping, and the tapping parts of steel bars are different in length, which leads to that the steel bars in the sleeve can't be pressed against each other when connecting the steel bars, or there are many exposed threads. All these weaken the tensile and bending strength of reinforced joints.

Control measures: After the reinforcing steel sleeves enter the site, the material engineer and quality inspector shall inspect the sleeves one by one according to the requirements of the type inspection report, and those that do not meet the requirements must leave the site. In the process of construction, strengthen the education of operators' quality awareness, correctly use measuring tools and calipers, supervise and inspect regularly, and cut off unqualified thread heads for redoing.

Fourthly, the problem of stirrups.

1. External processing of stirrups: At present, most construction units entrust external processing of stirrups, and the external processing units do not charge processing fees, and earn profits entirely by the elongation of steel bars. Therefore, the stirrup processed outside is not as large as the specification requirements. The cold-drawn rates of HPB235 steel bars are all greater than 4%, and the cold-drawn rates of HRB335 steel bars and HRB400 steel bars are all greater than 65438 0%.

2. Stirrup encryption: seismic KZ needs encryption area. Except for columns with full-height stirrup in concrete engineering design, columns with seismic grade of 1-4 should be encrypted according to the bottom column root ≥Hn (clear height of story column) /3, the middle layer should be encrypted according to the length and side length ≥Hn/6, and the floor is ≥500, which is the maximum value, including beam-column joints.

3. Insufficient stirrup size: Some construction workers mistakenly think that the thickness of the protective layer is calculated from the edge of the stirrup (it should be calculated from the edge of the main reinforcement) due to insufficient understanding of the protective layer of the steel bar, so that the actual cutting size of the stirrup is reduced and the effective section of the component is reduced.

On the one hand, the above three stirrup problems can not meet the requirements of stirrup and resist shear, on the other hand, they restrain the concrete in the core area of the section of the member and improve the overall bearing capacity of the member.

Control measures: In 2007, Nanjing issued "Several Provisions on Strengthening the Construction Quality Management of Building Structures (Trial)" No.32 document: "Cold drawing, cold extrusion and external processing are not allowed when processing round steel bars. Due to the difficulty of the site, the off-site reinforcement processing approved by the construction unit should be accepted and rechecked in batches when entering the site, and the appearance and physical and mechanical properties should meet the requirements of the specification. " It should be implemented seriously according to this requirement. Strengthen the process control of steel bar processing and steel bar binding. Before steel binding, stirrups must be checked and accepted. All stirrups with small size and insufficient straight length of hook should be reworked. In the process of steel binding, we should focus on checking the encryption area, especially the beam-column joints.

Five, steel substitution problem

During the construction, because the model and specification of steel bars do not conform to the design, it is necessary to obtain the consent of the design unit when replacing steel bars and meet the requirements of relevant codes. Some construction units only meet the design requirements in terms of strength when replacing steel bars, but ignore the relevant provisions of the ratio of tensile strength to yield strength of steel bars. Article 5.2.2 of Code for Acceptance of Construction Quality of Concrete Structures (GB 50204-2002) stipulates that the longitudinal reinforcement strength of frame structures with seismic fortification requirements should meet the design requirements; When there are no specific requirements in the design, the measured strength values obtained by inspection shall meet the following requirements of the first and second seismic grades:

1, the ratio between the measured tensile strength and the measured yield strength of reinforcement shall not be less than1.25;

2. The ratio of the measured yield strength of reinforcement to the standard strength value should not be greater than 1.3.

Control measures: for the replacement of steel bars, after obtaining the consent of the design unit, it should be replaced according to the following principles. The strength of reinforcement after equal strength replacement should not be less than the original strength of reinforcement, and the ratio of tensile strength to yield strength of reinforcement should be guaranteed; If the members are reinforced according to the minimum reinforcement ratio, the same level of reinforcement can be replaced according to the principle of equal area; When the structural members are controlled by the requirements of crack width or crack resistance, the replacement of steel bars needs to be checked for cracks and crack resistance. It can be seen that the substitution of steel bars in actual construction needs to be calculated and tested, and careful handling can ensure safety.

Concluding remarks

According to the characteristics of frame structure and long-term engineering practice, the author summarizes several problems that are easily overlooked in the current steel bar construction. As the reinforcement project is the key process in the main construction of the building project, the above problems are the key to structural safety. Therefore, as long as we attach importance to the construction process, strengthen the system and process inspection in advance, strengthen the acceptance, and accurately use the inspection means and methods, we can completely solve the problems in construction and ensure the engineering quality.