Common cracks in masonry Analysis and Prevention




Abstract: In engineering practice, cracks found in a large number of masonry structure, the impact of light were beautiful, re-use of those affected; and therefore has been how to cracking of masonry structure reinforcement and processing it is very urgent. This paper made a number of cracking of masonry reinforcement of prevention measures, but also for the construction and design personnel in the future to prevent the accumulation of masonry cracking some useful experience.




Keywords: Analysis of cracks to prevent cracks in masonry structure reinforcement measures






Introduction of masonry, cracks are very common, one of the quality of the accident.
Affect the appearance of minor cracks in masonry and the use of small features, serious cracks may affect the carrying capacity of masonry, and even cause collapse. In many cases, the occurrence of cracks and development of a major accident is often a precursor, which need careful analysis, properly addressed. Masonry cracks occurred in the main are: uneven settlement of foundation, foundation, uneven frost, temperature changes caused by stretching, building materials and construction of structures dealing with the use of improper and unfair.


1. Foundation cracks caused by uneven settlement of

1.1 Analysis of uneven ground settlement occurred after the settlement of a large part of the masonry and the masonry part of the settlement have a relatively small displacement, so that the additional tension generated masonry or shear, when such additional internal forces of more than the strength of masonry When it cracks in masonry. Such cracks are often left with the ground into a 45 ° angle, the width under the narrow, inclined towards the depression seam (sinking large parts).


1.2 preventive measures to prevent uneven settlement of foundation cracks caused by the main measures are:

1.2.1 Setting a reasonable settlement joint. Size Housing complex, in particular, a high degree of difference between the large, should be set up settlement joint. Settlement joint separated from the ground up, and there is sufficient width, construction joints should be maintained within a clean, to prevent broken bricks, mortar and other debris body falling into the slit in the East.


1.2.2 enhance the overall stiffness of the upper structure to improve the shear capacity of the wall, so that even the adjustment of masonry can be adapted to the uneven settlement of foundation. The doors and windows to reduce the building end hole, increasing the end-to-wall side of the hole the width of the wall to strengthen the ring beam arrangement, can strengthen the integrity of the structure.


1.2.3 strengthen the foundation work inspection slots were found to be timely and properly handle the bad foundation, and foundation construction may be undertaken.


1.2.4 is not appropriate to set the building on the foundations of a different stiffness, as a section of the building, part of the natural foundation, part of the pile and so on with. Must use a different foundation, we should properly handle and make the necessary calculation and analysis.


2. Foundation cracks caused by frost heave

2.1 Analysis of the upper foundation soil temperature dropped below 0 ℃, the upper began to freeze, the lower part of the water rising due to capillary action in the formation of ice crystals in freezing level, volume expansion, so that the soil upward bulge. Uplift of the extent and thickness and the freezing of the water table level, the general uplift of up to a few millimeters to a few millimeters, which converted frost heave force of up to 2 - 10Mpa, and often uneven. Building self-respect is often difficult to resist the uplift forces of frost heave, which was the building of a local top-up, and the ground caused by uneven settlement similar to the way housing cracking.


Such frost cracks in the frost region, one, a small two-story buildings are common.
Designers lack of knowledge of the danger of frost heave that is a small building, infrastructure can be buried in a shallow 1:00; or poor quality of construction workers have encountered frozen ground is very hard, difficult excavation, unauthorized raising foundation depth, resulting in frost heave cracks. In addition, some buildings of the subsidiary structures, such as the door fighting, steps, flower beds and other design or construction are often not well, not enough depth, often caused by frost heave cracks.


2.2 precautions

2.2.1 Be sure to base buried deep into the frozen line, not because it is a small building or a subsidiary structure of the foundation above the frozen place. Sometimes, the designer of the interior wall foundation, but have not placed due to heating frozen line, while in the construction or delivery prior to use when there is the possibility of frost heave, it shall promptly take appropriate preventive measures.


2.2.2 When the foundation can not do the frozen line should be taken when changing soil (replaced by the non-frost-heave soil) and other measures to eliminate the soil frost heaving.


2.2.3 in a separate foundation, foundation beam bear the weight of the wall, the foundation beam below a certain degree of porosity should be left in order to prevent frost heaving of soil top of the foundation and brick walls cracked.


3. Cracks caused by temperature changes

3.1 Analysis of thermal expansion and contraction is the most basic physical properties of objects, masonry is no exception. Uneven due to temperature changes so that the non-uniform shrinkage of masonry, or masonry stretching bound by the uneven temperature stress exceeds the strength of masonry, which leads to cracking of masonry.


Commonly masonry length is too long in the upper stretch of masonry at the base at large, small and cause cracking.
Therefore, specifications should be set up joints.


In addition, concrete roof and concrete ring beam and the masonry of different coefficient of thermal expansion changes in temperature cause the wall cracks.


3.2 preventive measures

3.2.1 In accordance with relevant regulations promulgated by the state, according to the actual situation of the building (such as whether the heating and locations of temperature change, etc.) set the joints.


3.2.2 in the construction joints to ensure that a reasonable approach, so that it can work.


3.2.3 of the entire roof, such as pouring concrete, or, having fabricated the roof, but there are a whole pouring concrete surface layer, will have to stay in good with the construction until the middle of a period of time and then poured with concrete, it can avoid the shrinkage of concrete and two kinds of materials, coefficient of linear expansion due to temperature differences caused by co-deformation, thus avoiding cracks.


3.2.4 Construction of the roof insulation layer, such as when the temperature during the season, it is susceptible to rapid temperature changes caused by roof cracking. It is best to avoid the hot roof construction season.


3.2.5 the event of a long-situ concrete roof EAVES can be sub-construction, set aside the expansion joints, concrete in order to avoid the adverse effects of stretching on the wall.


4.
Improper use of building materials

4.
1 Analysis of cracking of masonry in the southern regions of the statistical analysis showed that the use of cinder brick masonry cracks easily, mainly because cinder bricks of raw materials and production processes and common brick is different from its linear expansion coefficient and clay are also different.


4.
2 preventive measures on the cinder brick masonry cracks only in the design of ways to select other materials, structures constructed on the prevention and control measures, remains to be further studied.


5.
Tectonic measures to deal with different

5.
1 Analysis of number of walls of masonry structures due to improper layout, structural columns to set unreasonable, irrational, etc., causing beam pad designed masonry cracking.


5.
2 preventive measures to strengthen the design review drawings

6.
To deal with cracks in masonry reinforcement causes cracks in the masonry analysis it is clear, they were harmful levels should be cracks in masonry reinforced with different reinforcement measures.


6.
1 grouting is a way that cracks smaller, fewer joints, cracks, and has been basically stable, grouting methods can be used. The strength of the grouting, if necessary, do test to test. Test method is: using the same material to do two or four pilot masonry column. Is divided into two groups, one group with the press before fracturing, re-grouting, and then the two groups do destructive testing of masonry columns to compare, such as grouting of masonry with the same intensity of the original masonry, reinforcing the view that qualified . Based on previous experiments have shown that post-reinforced masonry grout to achieve and even exceed the original masonry strength.


Grouting materials used are pure mud, cement mortar, sodium silicate mortar or cement mortar.
In the masonry repair, the multi-purpose water slurry; due to availability of irrigation water slurry of good can be successfully run through the pores exposed, for a width of about 3.0mm crack pouring real. The actual crack width is greater than 5.0mm, the cement mortar can be used. Small cracks when the pressure grouting can be used. Here are some of pouring material with the ratio (see Table 1), for reference. Slurry a column in the table apply to 0.3-1.0mm wide cracks; thick paste applied to cracks in 1.0-5.0mm; mortar for cracks wider than 5.0mm.


 

                Grouting slurry mixture ratio in Table 1










Mortar

Type


Cement




Water




Sand




107 glue




Binary Emulsion




Silicate




Polyvinyl acetate






The first formula




Slurry


Thick paste


Mortar




1

1

1



0.
9

0.
6

0.
6



 


 


1



0.
2

0.
2

0.
2



 




 




 






The second formula




Slurry


Thick paste


Mortar




1

1

1



0.
9

0.
6

0.
6 ~ 0.7



 


 


1



 




0.
2

0.
15

0.
15



 




 






The third formula




Slurry


Thick paste


Mortar




1

1

1



0.
9

0.
7

0.
6



 


 


1



 




 




0.
01 ~ 0.02

0.
01 ~ .0.2

0.
01



 






The fourth formula




Slurry


Thick paste


Mortar




1

1

1



1.2

0.
74

0.
4 ~ 0.7



 


 


1



 




 




 




0.
06

0.
055

0.
06


With pure water mud reinforced, for example, its construction order:


Steps to clean up a crack, so that cracks run through the channel without congestion;


Step 2 of adding a coagulant of 1:2 cement mortar caulking to prevent the grouting, the slurry spill;


Step three electric drill or hand hammers made of filling in cracks, holes or grouting the top of the mouth;


Step 4 with 1:10 dilute slurry washing cracks again, and check for cracks in the flow channel, while the surrounding masonry cracks soaked;


Step 5 3:7 or 2:8 of pure water poured into the mud;


Step 6 will be cracks in reinforced local conservation office.


Use of pressure grouting of the construction sequence is similar to the above, process as shown.
Grouting pressure of 0.2 - 0.25Mpa.


The level of pass for a long crack along the crack drilling, bond sales made in order to strengthen both sides of the masonry together.
Pin button diameter 25mm, pitch 250 - 300mm, the depth should be smaller than the thickness of the wall 20 - 25mm. Done selling keys after grouting, grouting method ibid.


6.
2 embedded reinforcing method, but a small number of wide cracks, the cracks intersect in crevices in the ash, with high elevation of mortar and fine steel fillers, can also be embedded block reinforcing method, that is, both ends of the cracks and central wedge of reinforced concrete or Pa, according to reinforcement. Wedge or Grilled saw with wall thickness, or wall thickness of 1 / 2 or 2 / 3.


6.
3 External reinforcement method when the cracks are more available when local steel net to be reinforced using concrete and mortar. Reinforced Mesh available Φ6 @ 100 - 300 (two-way) or Φ4 @ 100 ~ 200. With concrete wedge or expansion bolts fixed on the wall, the wedge or bolt spacing of 500mm or so, showing plum-shaped layout. Construction of the wall paint layer should be removed before the clean, wiping masonry cement mortar should be pre-wet, using concrete and mortar should be conservation of at least 7 days later.


6.
4 Law of the wall due to an additional level of steel rod thrust, uneven settlement, temperature changes caused by stretching things happen for reasons such as flash, cracks in the wall or to produce a larger outer longitudinal walls and transverse walls inside the Rachel bad, the available reinforcement or a steel rod to be reinforced. Such as the use of steel wire rod, preferably a long-pass Rachel, and set up along the wall on both sides. The middle of a long rod should be added to flower blue screw to tighten the rod, welding rod lengthening should be used. Exposed in the wall of the rod or plate nut may be appropriate for construction deal. Rod and the plate should be coated anti-rust paint. Rachel horizontal layer in the Office, the outer ring beams can be increased to enhance the reinforcing effect. The diameter of steel bars can be used are as follows: When a bay plus a bar when 2Φ16 (housing into the deep 5-7m), 2Φ18 (housing into the deep 8 ~ 10 m) 2Φ28 (housing into the deep 11-14m). The corresponding plate dimensions can be used in Table 2.


Bar diameter and steel plate or steel size relationship table 2












Rod Diameter




ф16



Φ18



Φ20





Angle plate


Channel plate


Steel plate





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