Introduction to compilation

Lin sunweiqiang

Zhuhai Construction Engineering Quality Supervision and Detection Station 5 190 15

I. Introduction

The Measures for the Prevention and Control of Common Quality Defects in Construction Projects in Zhuhai and the Measures for the Prevention and Control of Common Quality Defects in Construction Projects in Zhuhai compiled by our station were implemented in the whole city in 2000 [1][2] with the approval of Zhuhai Construction Bureau. Since the implementation of the two measures for the prevention and control of common diseases, it has played an obvious guiding role in improving the quality of construction projects in our city, and a number of outstanding national, provincial and municipal model projects have emerged one after another; Among them, the teaching building of Sun Yat-sen University Zhuhai Campus won the "Luban Award". Since this period, public complaints about the quality of construction projects have also decreased significantly. Relevant leaders of the Provincial Construction Department and the Provincial General Station fully affirmed the measures taken by our city to control the common quality defects of construction projects, and the construction quality management department of twin cities also spoke highly of them. From the early 1990s to the present, among the quality complaints and quality appraisal cases accepted by our station, except for the damage caused by new wall materials such as foundation, hollow brick and secondary decoration and the damage caused by adjacent engineering construction to existing buildings, almost all other quality complaints and quality disputes can be found in two common disease treatment measures. The new generation design code and construction quality acceptance code of national building engineering have been released one after another since last year. According to the relevant management regulations of the state on local standards, the original two common disease prevention and control measures need to be revised, and the revision work has been completed, and it has been implemented within the city with the approval of the Municipal Construction Bureau [3]. The following introduces the prevention and control measures for common quality problems of construction projects in Zhuhai (hereinafter referred to as prevention and control measures):

Second, the civil part.

1, "preventive measures" are stricter than the current design specifications, including 14.2.5 and 14.2.6. Main considerations:

(1) The common quality problems of cracking in the external corner slab and beam slab with abrupt stiffness are everywhere. This kind of problem is the first hot issue of quality complaints about construction projects, especially residential projects in our city, and it is the starting point of various disputes between construction units and construction units, residential units and sales units. People in the industry have formed a * * * knowledge that the main causes of cracking are concrete shrinkage and temperature difference stress.

⑵ With the development of construction industry, super-long buildings often appear. High-rise buildings and long-span and heavy-load industrial buildings require higher and higher concrete strength. The widespread use of pumping concrete construction technology also makes the consumption of cement and admixture surge, and the influence of concrete shrinkage and temperature difference stress on construction projects is on the rise.

(3) Statistics of the measured data of 548 beam and slab members in 24 projects recently evaluated by our station show that the average measured strength of concrete beams and slabs is 0.0 ~ 15.0MPa higher than the design requirements after three years of construction, accounting for 66% of the total, and the number of members greater than 15.0MPa accounts for1%of the total. Code for Design of Concrete Structures (GB500 10-2002) takes HRB400 as the leading reinforcement, which requires higher durability of concrete structures and higher strength of concrete than GBJ 10-89 to meet the engineering needs. At present, in the projects designed by our city according to GB500 10-2002, the strength grade of beam and slab concrete is mainly in the range of C25~C35. Based on this, it is speculated that the average strength of beam and slab concrete will reach C25~C40 after three years of construction of these projects, and some even higher.

(4) According to the principle that the bending moment of steel bars configured according to the minimum reinforcement ratio of flexural members is equal to the cracking bending moment of plain concrete, the thickness h= 120mm and the width 1000mm, and C35 concrete bars can be calculated:

When HPB235 is used, As=420mm2 and ρ=0.35% (calculated by B and H).

When HRB335 is adopted, As=296mm2 and ρ=0.25% (calculated by B and H).

5] According to the principle that the tension of steel bars configured by the minimum reinforcement ratio is equal to that of plain concrete, the thickness = 120mm and the width = 1000mm, and C35 concrete can be calculated:

When using HBB235 steel bar:

ρ 1 = 1.05% (bidirectional)

ρ 2 = 0.52% (unidirectional)

When HRB335 rebar is used:

ρ 1 = 0.74% (bidirectional)

ρ 2 = 0.37% (unidirectional)

(6) The national structural design code is subject to the national economic situation, and China has a vast territory and a weak economic foundation; However, the economic development in different regions is extremely uneven, and it is unreasonable to adopt a unified standard throughout the country [4]. Zhuhai's national economic situation is much higher than the national average, and it is acceptable to increase investment appropriately for local structural quality problems with a large quantity and a wide range. Wang Tiemeng suggested that the reinforcement ratio of 0. 15% ~ 0.25% should be adopted for crack control of mass concrete [5].

(7) The slab at the external corner of the building and the beam-slab with abrupt stiffness change belong to tension-bending members under the action of concrete shrinkage and temperature difference stress, and the slab at the external corner of the building often has the second-order effect of indirect support for local slabs and overall bending. Clause 10. 1.9 of GB 500/kloc-0-2002 stipulates that the reinforcement ratio of "mending skylight" is 0. 1%. When the concrete slab is not cracked, the stress of steel bars is very small, but once the concrete slab is cracked, the steel bars yield immediately, and the crack width is bound to be very large. The external corner slab and the beam slab with sudden stiffness change are local components, so it is not expensive to take appropriate strengthening measures. Based on the above factors, HRB335 steel bar is adopted in the prevention measures, and ρ≥0.3%.

2. There are 3. 1.3, 5.2 and other clauses in the prevention measures for the blind spots in the design specifications. China's architectural engineering design code is a high summary of theoretical research results and mature engineering experience, and some immature practices will not be included in the code; In the design code, there are also cases where the railway police are in charge of one section, and blind spots often appear between sections; Design specifications are compiled for the use of specially trained professional and technical personnel, and there is no need to make provisions for design specifications that belong to the basic knowledge that professional and technical personnel have mastered. But designers ignore these problems, which will leave many common quality problems. For example, when a building adopts a long casement window, the structure often adopts the methods shown in Figure 1.a and Figure 1.b, and the masonry and concrete structure are only connected by mortar, so there are double differences in linear expansion coefficient, relative deformation and lack of necessary constraints. After a period of use, long horizontal seams often appear at the joint of the two materials, and some will cause leakage. If a constructional column is set every 3m or so, a reinforced concrete coping is set at the window sill, and a metal net is hung at the junction of the two materials, the above common quality defects can be prevented. However, this structural measure is not stipulated in the design code of concrete structure or masonry structure.

Figure 1 Common practices of long transverse windows

The external wall of a 20-story building in our city is in the form of figure 1.b, and the connection between masonry and concrete structure is not strengthened. After five years of use, it was found that the long transverse window protruded in many places, and cracks appeared at the contact between the masonry and the floor, with the maximum width of 7 mm. It was decided by relevant parties to rework the enclosure structure, but the cost exceeded 2 million yuan. Common quality problems caused by design have caused shocking economic losses and social impacts [6]. Therefore, there are many provisions in the prevention and control measures.

3. Some provisions in the design code with poor construction operability are supplemented or adjusted. As stipulated in article10-2002 (GB 500 10.2. 1), the horizontal spacing of the steel bars in the longitudinal lower part of reinforced concrete beams should not be less than 25mm and d. If the reinforcement is designed according to its lower limit, the steel bars at the bearing are often too dense, which will affect the quality of concrete construction. It is common that there are holes in the concrete under the steel bar at the bottom of the bearing. Therefore, the following relevant provisions are made for prevention and control measures:

(1) Multi-span continuous beams (including frame beams) with the same section should be provided with some long bottom bars to reduce the number of stressed nodes, as shown in figure (15.3):

(2) For frame structures with special-shaped columns whose beams and columns are 200mm wide, the number of longitudinal reinforcing bars in each row should not exceed two (Article 15.5).

4. The quality of some working procedures in construction is easily disturbed by the construction of subsequent working procedures, which reduces the quality and forms common quality defects. For example, the two rows of longitudinal bars on the beam surface are not installed firmly, which leads to the sliding of the bars when pouring concrete and reduces the bearing capacity; The thickness of the protective layer does not meet the requirements, which reduces the durability and bearing capacity of the structure; The requirements of concrete construction were not considered during the installation of steel bars, and the steel bars were randomly pried during concrete pouring to reduce the quality. The "Prevention Measures" has made specific regulations on these issues to ensure the construction quality. For example:

The arrangement of longitudinal reinforcement on beam surface shall meet the following requirements:

Figure 4.3 Schematic diagram of longitudinal reinforcement arrangement on beam surface

Note: a: the spacing between steel bars is ≥d and not less than 25mm.

B: beam width

C: construction clearance ≥60mm.

D: steel bar diameter

(1) When the beam width b is less than or equal to b≤400mm, there is a construction operation gap;

2 when 400 mm < b ≤ 800 mm, leave two construction operation gaps;

(3) when 800 mm < b ≤1200 mm, leave three construction operation gaps.

5. Carry out the notice of the Ministry of Construction on resolutely preventing balcony collapse [7]. The possibility of cantilever structures such as balcony in Zhuhai still exists, and the provisions in preventive measures 14. 1.2 and 14.2.4 reflect the spirit of this notice.

6. Carry out the provisions of Guangdong Province on eliminating common quality defects in construction and installation projects [8], which is one of the main bases for the appearance quality inspection of model projects in Guangdong Province. The serial numbers of prevention and control measures are 1, 5 and 6, which reflect the specific requirements of the Regulations.

7. Eliminate backward materials, technologies and processes, and recommend new materials, technologies and processes. For example, in the past, water-soluble materials such as cement expanded perlite and cement expanded vermiculite were used as the insulation layer of roof engineering in our city, but the results were almost tried and tested, and now it is prohibited; In the past, cement mortar precast slabs with protruding feet and clear height of 40 ~ 60 mm were widely used as roof insulation boards, which had poor insulation effect and caused a large number of roof structural layers and top walls to crack and leak, which was extremely harmful and was also prohibited. It is recommended to use rigid foam polyurethane, polystyrene and other materials with thermal conductivity λ≤0.05kw/m.h.c as thermal insulation materials for roofing projects. The clear height, exposed size and application scope of overhead insulation layer are clearly defined.

8. According to the engineering practice in our city, some mature practices are provided. For example, the super-long exterior wall should be painted with high elastic paint (Article 3.4.4); When the span of the outdoor cantilever slab is greater than 400mm and the length is greater than 3m, anti-crack reinforcement shall be set at the bottom of the slab (article 14.2.7); The external wall concrete decorative board suspended under the beam shall be provided with sufficient longitudinal reinforcement to limit the crack width, regardless of the whole pouring or post-pouring (Article 15.6).

9. Some clauses related to use safety in current design codes and construction quality acceptance codes are extracted, which are often ignored by design, construction, supervision and engineering quality management personnel, resulting in common quality problems, such as clauses 10. 1 1, serial number 1 1.

Third, the equipment installation part

1, which is not mentioned in the design specification, is clearly required in the current construction acceptance specification. At present, there are provisions such as serial numbers 20 and 24, and the main considerations are:

(1) The content of serial number 20 has been put forward in article 3.3. 13 of the acceptance specification GB 50242-2002. In the construction, because the design is not put forward and not included in the budget, this regulation is often not implemented, and these parts are prone to leakage. Therefore, the serial number 20 of this measure clearly requires the specification, form, installation position and caulking of the casing in the picture and text. What needs to be added is that the grounding leakage of the drainage pipeline and the branch pipe of sanitary ware is short, and the other end of the straight pipe is not fixed, so there is enough space to compensate the deformation caused by the linear expansion of the pipeline, so there is no need to set a sleeve.

⑵ The content of No.24 has been put forward in Article 5.2.4 of Code for Construction Acceptance (GB 50242-2002), but there is no such requirement in the current Code for Design of Building Water Supply and Drainage, nor in most design drawings. Considering that this clause involves the fire safety of buildings, Document No.24 still requires that UPVC drainage pipes with pipe diameters greater than φ 1 10 in high-rise buildings must be provided with fire rings, and

2. There are relevant requirements in the design specification, but the serial number 2 1 is not mentioned in the construction acceptance specification. Article 3. 10.6 of Code for Design of Water Supply and Drainage in Buildings (GBJ15-88 (version1997) stipulates that gutter rainwater should be set at the entrance of rainwater drainage pipeline and rectifier grid should be set at rainwater hopper. In the construction of roof rainwater pipeline, the construction unit used floor drain instead of rainwater bucket to directly discharge rainwater pipeline, and the side rainwater pipeline didn't even have rainwater bucket, only the bent pipe was directly connected to the rainwater pipeline, which led to the rainwater discharge not meeting the design requirements and caused many disputes. The serial number 2 1 has made special requirements for this, and clearly stipulated the installation of side drainage rainwater pipes in the form of charts. It is also explicitly required to add an overflow port at the end of parapet, gable or gutter to prevent the rainstorm exceeding the design displacement from gathering in the air and ensure the safety of the roof structure.

3, more stringent than the current specification requirements are:

(1) No.25 document comprehensively considers the use effect, service life, appearance quality, unified installation standard and other factors, and stipulates that all exposed pipes on the roof must be supported by fixed steel brackets.

⑵ Most exposed lightning protection zones in this area are made of galvanized steel bars. Considering that the bracket is directly welded with the lightning protection belt, the galvanized layer of round steel is destroyed and the lightning protection belt is rapidly corroded, which affects the service life; It is also possible that the welder's skill is not very good, and the solder joint bites the meat, which reduces the circulation cross section of the lightning protection belt and affects the use safety and appearance quality. Therefore, the serial number 32 specifies the connection mode between the lightning protection belt and the bracket in graphic form.

4. Quote some clauses in the current design and construction quality acceptance specification that are often ignored by the responsible subject of engineering quality, such as serial number 27 to serial number 3 1.

Four. conclusion

In the preparation of preventive measures, we insist that the prevention and control of common quality defects in construction projects must start from the source of design and must be comprehensively prevented from the whole process of design to construction. This is the main difference between the prevention and control measures of common quality defects in construction projects in our city and other cities, and it is also a highlight of project quality management in our city. If this "preventive measure" can be fully implemented, the quality of construction projects in our city will be improved by leaps and bounds.

Limited to the level, the prevention and control measures are inevitably wrong. Please give timely feedback from readers and users of prevention and control measures for further revision in the future.

refer to

(1), Zhuhai Municipal Construction Committee. Zhu Fu Jian [1999] No.477.

(2) Zhuhai Municipal Construction Committee. Zhu fu Jian [2000] No.296

(3) Zhuhai Construction Bureau. Zhu Fu Jian [2003] No.312

④ Chen Zhaoyuan. Query on reliability design method of structural design code. Architectural structure. Issue 4, 2002.

5], Wang Tiemeng. Crack control of engineering structure. China Building Industry Press. 1998

[6] Lin,. Harm and prevention of common design quality problems. Engineering quality. Issue 5, 2003

Once, the Ministry of Construction. Notice on resolutely preventing balcony collapse accidents. [87] Urban Construction Quality ZiNo. 13

Being. Guangdong Provincial Construction Committee. Notice on printing and distributing the Provisions of Guangdong Province on Eliminating Common Quality Problems in Construction and Installation Projects. Pre-knot knot word [1995] No.

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