If there are cracks in load-bearing structures buildings and structures, it is necessary to organize systematic monitoring of their condition and possible development in order to find out the nature of deformations of structures and the degree of their danger for further operation.
Monitoring the development of cracks is carried out according to a schedule, which in each individual case is drawn up depending on the specific conditions.
Cracks are identified by inspecting the surfaces of structures, as well as selective removal of protective or finishing coatings from structures.
It is necessary to determine the position, shape, direction, distribution along the length, width of the opening, depth, and also determine whether their development continues or has stopped.
A beacon is installed on each crack, which breaks when the crack develops. The beacon is installed in the place of greatest development of the crack.
When observing the development of cracks along the length, the ends of the cracks during each inspection are fixed with transverse strokes applied with paint or a sharp instrument on the surface of the structure. The date of inspection is indicated next to each stroke.
The location of the cracks is schematically marked on the drawings of the general view of the walls of the building, noting the numbers and date of installation of the beacons. For each crack, a schedule of its development and opening is drawn up.
Cracks and beacons are periodically inspected in accordance with the observation schedule, and based on the inspection results, a report is drawn up, which indicates: the date of inspection, a drawing with the location of cracks and beacons, information about the condition of cracks and beacons, information about the absence or appearance of new cracks and installation on them lighthouses.
The width of crack opening is usually determined using an MPB-2 microscope with a scale division of 0.02 mm, a measurement limit of 6.5 mm and a MIR-2 microscope with a measurement scale of 0.015 to 0.6 mm, as well as a magnifying glass with scale division (Brinell loupes ) (Fig. 1) or other devices and tools that provide measurement accuracy of at least 0.1 mm.
Rice. 1. Instruments for measuring crack opening a - reading microscope MPB-2, b - measuring the crack opening width with a magnifying glass: 1 - crack; 2 - division of the magnifying glass scale; c – dipstick
The depth of cracks is determined using needles and wire probes, as well as using ultrasonic devices such as UKB-1M, Beton-3M, UK-10P, etc. The scheme for determining the depth of cracks using ultrasonic methods is shown in Fig. 2.
Rice. 2. Determination of the depth of cracks in the structure 1 - emitter; 2 – receiver
When using the ultrasonic method, the depth of the crack is determined by changing the transit time of pulses both during through sounding and by the longitudinal profiling method, provided that the plane of crack formation is perpendicular to the line of sounding. The depth of the crack is determined from the relations:
where h is the depth of the crack (see Fig. 2); V is the speed of propagation of ultrasound in the area without cracks, μ/s; ta, te - time of passage of ultrasound in the area without a crack and with a crack, s; a - measurement base for both sections, see
An important tool in assessing deformation and crack development are beacons: they make it possible to establish a qualitative picture of deformation and their magnitude.
The lighthouse is a plate 200-250 mm long, 40-50 mm wide, 6-10 m high, made of gypsum or cement-sand mortar, placed across a crack, or two glass or metal plates, each with one end fixed on opposite sides of the crack , or lever system. The rupture of the beacon or the displacement of the plates relative to each other indicate the development of deformations.
The lighthouse is installed on the main material of the wall, having first removed the plaster from its surface. It is also recommended to place beacons in pre-cut grooves (especially when installing them on a horizontal or inclined surface). In this case, the grooves are filled with gypsum or cement-sand mortar.
Beacons are inspected a week after their installation, and then once a month. In case of intense cracking, daily monitoring is required.
The opening width of cracks during observation is measured using crack gauges or crack gauges. The design of the gap gauge or crack gauge can be different depending on the width of the crack or seam between the elements, the type and operating conditions of the structures.
The simplest solution is a plate beacon (see Fig. 3). It consists of two metal, glass or plexiglass plates that have marks and are fixed to the solution so that when a crack opens, the plates slide over one another. The edges of the plates should be parallel to each other. After attaching the plates to the structure, mark on them the number and date of installation of the beacon. By measuring the distance between the risks, the size of the crack opening is determined.
Rice. 3. Plate beacon made of two painted plates 1 - plate painted white; 2 - plate, painted red; 3 - gypsum tiles; 4 – crack
We have already written earlier on the website in the article “” how cracks in load-bearing structures can be dangerous and what are the main reasons for their formation. Some idea of monitoring can also be obtained from the article "", published earlier. And today’s publication is devoted to specific monitoring methods and devices used for these purposes - the so-called “beacons”. At the end of the article you can watch a presentation with photographs and diagrams of the lighthouse structures described.
In what cases is it usually necessary to monitor cracks in a building?
- As part of comprehensive monitoring of building deformations
- In the presence of load-bearing structures that have limited operability and emergency conditions
- If the building falls into the zone of influence of new construction or reconstruction
The main task when monitoring cracks is to record ongoing changes in their parameters for objective monitoring of the technical condition of structures.
The goals of monitoring may be different, but their essence is the same - timely receipt of information about ongoing changes for decision-making. Based on the results of monitoring, decisions can be made on the possibility of further operation, the need and type of repair measures, prompt elimination of factors affecting the development of cracks (for example, dynamic influence from a nearby construction site), prevention of emergency situations, etc.
Monitoring objectives, technical condition and design features influence how crack development is monitored. When choosing the method and methods of observation, the following main factors must be taken into account:
- The need to take into account temperature and humidity influence
- The need to quickly obtain information
- Required measurement accuracy
- Cost, reliability and durability of the monitoring system and its components
- The complexity of taking readings and servicing the system
What beacon designs are used for observing (monitoring) cracks and what are the features of their use?
Electronic sensors and monitoring systems
To take into account temperature and humidity influences on structures, it is necessary to make appropriate measurements. And for objective assessment Such influences may require indicators of temperature/humidity of air and structures both outside and inside the premises. A sufficient amount of such data can only be provided by electronic system continuous monitoring with task-appropriate sensors. It is also possible to obtain the necessary data in fragments using manual measurements with instruments at the time of taking readings from beacons installed on cracks. But this approach should still be considered uninformative, since it does not provide enough data to assess the influence of temperature and humidity on changes in the parameters of cracks in structures.
Electronic measuring systems with the ability to transmit information remotely also have the greatest efficiency in obtaining measurement results. They also generally have the greatest measurement accuracy - they record the width of the crack opening to hundredths of a millimeter. The disadvantages include the impossibility of measuring with one sensor the movement of parts of a structure relative to each other in the vertical and horizontal directions simultaneously.
Accurate electronic measuring monitoring systems allow short-term (2-15 days) observation cycles to be carried out, providing information on current trends in the development of deformations and allowing prompt decisions to be made. Such systems are becoming increasingly widespread, but the main obstacle to their widespread use remains their high cost and low resistance to vandalism. However, this is definitely promising direction development of deformation monitoring tools, with the help of which it is already possible to solve a wide range of monitoring tasks.
Gypsum beacons
Of all the methods, the traditional design of a gypsum beacon for observing cracks has the least cost. However, it has a number of disadvantages:
- Ineffective for use in outdoor structures and places where significant temperature fluctuations are possible. In such conditions, the gypsum beacon is “triggered” by temperature deformations, which does not make it possible to unambiguously determine the presence of other factors influencing the crack.
- Low durability and intensive destruction under unfavorable external conditions, high damageability.
- Labor-intensive installation, impossibility of installation at subzero temperatures.
- Dependence of the beacon's performance on the quality of the installation. Failure to comply with the recommended requirements for surface preparation, dimensions and design of the beacon leads to its inoperability.
- Due to the low reliability of the received data, the installation of a large number of beacons is required. Typically at least two per crack and at least one per 3 meter crack.
- The accuracy of crack width measurements is very low due to unevenness at the measurement site. For the same reason, there is no possibility of using high-precision measuring instruments.
- The main thing is that the gypsum beacon is disposable. In most cases, when it is triggered (a crack appears in the body of the beacon), it is necessary to install a new beacon nearby.
Plate beacons
Plate beacons do not have many of the disadvantages of their gypsum counterparts. One of their main advantages is the ease of installation - this is done with quick-curing epoxy glue, or on dowels, or by combining these two methods. Depending on the design, these beacons can be implemented additional features, not available in beacons of other designs:
- The ability to measure the movement of structures along two axes (when using a special design, three) relative to each other - in the vertical and horizontal directions.
- Possibility of using high-precision measuring instruments to measure hundredths of a millimeter change in crack opening width.
- Ease of use, including the ability to apply additional information to the lighthouse.
Signal measuring scale allowing without additional tools visually evaluate the changes in the crack opening width.
Currently, this is perhaps the most effective design in terms of the ratio of installation cost, labor intensity of observations and the quality of the results obtained.
Point beacons
Another type of beacons for monitoring cracks are point devices that allow observations to be made at two, three or four points fixed on the structure. Design Such devices can be extremely varied from simple dowel-nails to special installation devices. Such devices can be made inconspicuously in the color of the wall finish or transparent (made of plexiglass). The advantage of some of them is that there is no need to prepare the surface and clear the finishing layers. The use of special calculation methods makes it possible to track movements in both vertical and horizontal directions. The accuracy of measurements is limited only by the accuracy of the instruments used. The undoubted advantage of most representatives of this type of lighthouse structures is the extremely high resistance to vandalism, achieved by rigid fastening to the structure, with small dimensions of the device.
Sentinel beacons
In addition to those mentioned above, sentry-type beacons (messuras) are common, having a measuring scale and relatively high accuracy measurements without the use of additional tools. These are the most intuitive devices to use, allowing you to easily navigate the changes taking place and take readings. For some reason, it is this type of beacon that most attracts vandals; sometimes even special protective structures do not help. In addition, their cost is significantly higher than plate, point, and even more so gypsum, which significantly reduces the scope of their application. Greater efficiency can be achieved by fixing two points on the structure and using the measurements only as a measuring tool to perform control measurements of the distance between the fixed points.
There are other types of beacon designs, but in conclusion I would like to once again warn against the use of paper and glass beacons, since their designs do not meet the requirements and can be misleading when making observations.
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The critical stage is to study cracks, identify the causes of their occurrence and the dynamics of their development.
According to the degree of danger for load-bearing and enclosing structures, cracks are divided into three groups:
- cracks are not dangerous, worsening only the quality of the front surface;
- dangerous cracks that cause significant weakening of sections, the development of which continues with unabated intensity;
- cracks of the intermediate group, which worsen the operational properties, reduce the reliability and durability of structures, but do not contribute to their complete destruction.
If there are cracks in the load-bearing structures of buildings and structures, it is necessary to organize systematic monitoring of their condition and possible development in order to determine the nature of the deformations in the structure and the degree of their danger for further operation.
Cracks are identified by examining surfaces, as well as selectively removing protective or finishing coatings. It is necessary to determine the position, shape, direction, distribution along the length, width of the opening, depth, and also determine whether their development continues or has stopped.
A beacon is installed on the crack, which breaks when the crack develops. The beacon is installed in the place of greatest development of the crack. When observing the development of a crack along its length, the ends of the crack are fixed with transverse strokes during each inspection. The date of inspection is indicated next to each stroke. The location of the cracks is schematically plotted on the drawing of the walls of the building or structure, noting the numbers and date of installation of the beacons. For each crack, a schedule of its development and opening is drawn up.
Based on the results of systematic inspections, a report is drawn up, which indicates the date of inspection, a drawing with the location of cracks and beacons, information about the absence or appearance of new cracks.
The lighthouse is a plate 200-250 mm long, 40-50 mm wide, 6-10 mm high, placed across the crack. The lighthouse is made from gypsum or cement-sand mortar. Two glass or metal plates are also used as a beacon, each attached at one end to different sides cracks, or lever system. The rupture of the beacon or the displacement of the plates relative to each other indicates the development of deformations.
The lighthouse is installed on the main material of the wall, having first removed the plaster from its surface. It is recommended to place beacons also in pre-cut grooves. In this case, the fines are filled with plaster or cement-sand mortar.
Beacons are inspected a week after their installation, then at least once a month. In case of intense cracking, daily monitoring is required.
The opening width of cracks during observations is measured using crack gauges or crack gauges. The observation log records the number and date of installation of the beacon, the location and layout, the initial width of the crack, and changes in the length and depth of the crack over time. If the beacon is deformed, a new one is installed next to it, which is assigned the same number, but with an index. Lighthouses on which cracks have appeared are not removed until the end of observations.
If no change in the size of the cracks is detected within 30 days, their development can be considered complete, the beacons can be removed and the cracks can be repaired.
And about. Head of the Department of Engineering Surveys and Inspection of Building Structures Belskaya Yu.S.
Methods for observing cracks in stone and concrete structures
Cracks in buildings and structures can form for various reasons. They might just spoil appearance, but may indicate a serious safety threat to people.
Defects that seem insignificant at first glance, if not corrected in a timely manner, can progress and, ultimately, cause complete destruction of structures. Such defects include cracks in stone and concrete structures.
According to the type of development, cracks can be stabilized or unstabilized over time. In order to determine whether the development of a crack continues or has stopped, a beacon is installed on it at the place of greatest development of the crack. When observing the development of a crack along its length, the ends of the crack are fixed with transverse strokes during each inspection. The date of inspection is indicated next to each stroke. The location of the cracks is schematically plotted on the drawing of the walls of the building or structure, noting the numbers and date of installation of the beacons. For each crack, a schedule of its development and opening is drawn up. Based on the results of systematic inspections, a report is drawn up, which indicates the date of inspection, a drawing with the location of cracks and beacons, information about the absence or appearance of new cracks. The rupture of the beacon or the displacement of the plates relative to each other indicates the development of deformations. Beacons are inspected a week after their installation, then at least once a month. In case of intense cracking, daily monitoring is required. The opening width of cracks during observations is measured using crack gauges. The observation log records the number and date of installation of the beacon, the location and layout, the initial width of the crack, and changes in the length and depth of the crack over time. If the beacon is deformed, a new one is installed next to it, which is assigned the same number, but with an index. Lighthouses on which cracks have appeared are not removed until the end of observations. If no change in the size of the cracks is detected within 30 days, their development can be considered complete, the beacons can be removed and the cracks can be repaired.
Gypsum (cement) beacons
Of all the methods, the traditional design of a gypsum or cement beacon for observing cracks has the least cost. Dimensions of beacons: length 250-300 mm, width 70-100 mm, thickness 20-30 mm. Beacons are installed across cracks in places of their greatest development and are securely fixed to the load-bearing part of the walls on both sides of the crack (see Fig. 1).
Beacons are placed in places cleared of plaster, allowing for daily observations. Each beacon is assigned a number and the date of its installation is indicated. It is not allowed to install gypsum beacons in damp places - in this case it is necessary to install beacons made of cement mortar.
Plate beacons
The design of the beacons allows their use in a wide range of weather, temperature and humidity conditions. Readings can be taken both visually and using measuring instruments.
The deformation scale represents 2 plastic plates, one of which is marked with a millimeter grid and a reading scale, and the second with a control crosshair.
The method of using the deformation scale is the most simple solution to monitor cracks that may form as a result of the following phenomena:
Uneven foundation settlement;
- temperature deformations of long walls;
- overloading of individual sections of walls as a result of dismantling the structure without complying with technical requirements.
The deformation scale consists of two plastic plates. They are attached to both sides of the crack so that when the crack opens, the plates slide over one another, and the red crosshair of one plate moves relative to the millimeter scale of the other plate, allowing you to take a report on the scale and enter it in the observation log. The plates must be secured parallel to each other. After attaching the deformation scale to the building, it is assigned a number and the number and date of installation are noted on the scale. By measuring the distance between the scale marks, the size of the crack opening is determined.
Visual monitoring is possible both along the vertical and horizontal axes.
Observation of cracks at 3 – 4 points
In some cases, plate and electronic beacons cannot be used when monitoring cracks. For example, in cases where the risk of damage to beacons is high, or the installation of beacons is undesirable for aesthetic reasons. In these cases, observation of cracks in building structures can be performed using fixed observation points. On each side of the crack, two points are secured using dowels or other devices. The installed devices are usually unnoticeable and at the same time securely fixed. With this method of monitoring cracks, measurements are made using high-precision measuring instruments - digital calipers. The distances between fixed points are measured, and the measurement results are entered into spreadsheets. After processing the data, we obtain the amount of movement of parts of the structure separated by a crack relative to each other along two axes - vertical and horizontal. This method of monitoring the deformations of buildings and structures does not have the capabilities of visual observation, and calculations are required to obtain results.
However, three- or four-point observation is the only reliable and at the same time highly accurate method of observation in places where there is a high probability of losing other types of beacons due to vandals.
Rice. 1. The procedure for installing the beacon.
Install gypsum or cement beacons on cracks and organize observation withregistration of results at certain intervals in a specialmagazine. Dimensions of beacons: length 250¸300 mm, width 70¸100 mm, thickness 20¸30 mm.
Beacons are installed across cracks in places of their greatest development and reliablyare fixed to the load-bearing part of the walls on both sides of the cracks (see drawing). Lighthousesplaced in places cleared of plaster, allowing for daily observations.
Each beacon is assigned a number and the date of its installation is indicated. If within the periodobservation, a crack will not appear on the lighthouse, which means uneven settlement of the walls andthe formation of cracks in them has stopped and the crack can be repaired after cleaningsolution. If the beacons are destroyed, then the deformation of the walls continues. In thatIn this case, the journal with the results of observations should be sent for study for acceptance. solutions. It is not allowed to place plaster beacons in damp places - in this caseinstall beacons made of cement mortar.
Observations
behind the cracks
Observations on the development of cracks in walls duringtime are carried out using plaster, glass or plate beacons.
1 - crack; 2 - plaster or glass lighthouse; 3 - metal plate; 4 - risks;
5 - nail
The crack opening width is measured using:- graduated magnifiers and microscopes (MIR-2, MPB-2) with 2.5-24x magnification;- celluloid or paper stencils, with lines of different thickness from 0.05 to 2 mm , by aligning the lines with the edges of the crack;- scale bars when cracks open more than 2 mm (measurement accuracy ± 0.3 mm).
During long-term observations, the crack opening width for the period under consideration is determined using portable indicators with a division value of 0.01 mm and calipers with a division value of 0.1 mm. The opening value is taken to be equal to the difference between two measurements of the distance between the pins (benchmarks) with a centering device embedded in the structure on both sides of the crack.
The depth of development of non-through (blind) cracks htr is determined:- By trace of a crack on the surface of a core drilled from the body of the structure;- using steel calibrated probes of variousthickness according to the formula:
+ 5 mm, (2)
Where:
dн - opening of the crack from the outside in mm (average of three measurements);
dш, hш - the thickness of the probe and the depth of immersion of the probe into the crack in mm without force (the average of three measurements when the probe is displaced along the crack by 1-2 cm);
Using ultrasonic devices (UKB-1M; UK-10P; UZP-62, etc.) in accordance withinstructions of the RTU of the Ukrainian SSR 92-62.
The depth of the crack is determined by the time differencepassage of ultrasonic pulses into the ISS along the base length a - with and without a crackcracks according to the formula:
, (3)
where: tl, ta - travel timeultrasound, respectively, in the area
with and without crack.
Beacons are numbered and the date of installation is written on them. Cracks and beacons according to scheduleobservations are periodically examined (at least once every 2 days), and according toresults of the inspection, a report (journal) is drawn up, which indicates: dateinspection, a drawing with the location of cracks and beacons, information about the condition of cracks andbeacons, information about the absence or appearance of new cracks and installation on themlighthouses (in the observation log (report) it must be recorded -location of the beacon, its number, installation date, original width cracks).
In case of deformation (rupture) of the beacon next to ita new one is installed, which is assigned the same number, but with an index.Beacons on which cracks have appeared are not removed until the observations are completed.
If within 30 days there is no change in the size of the cracks
fixed, their development can be considered complete, beacons can be removed and cracks close up.
LIGHTHOUSE OBSERVATION JOURNAL
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