US20050188648A1

US20050188648A1 - Unreinforced masonry wall strengthening method and setup

Info

Publication number: US20050188648A1
Application number: US11/068,211
Authority: US
Inventors: Ali Bayraktar; Yaman Kocasinan
Original assignee: SGM SISMIK GUCLENDIRME MERKEZI INSAAT TIC VE SAN
Current assignee: SGM SISMIK GUCLENDIRME MERKEZI INSAAT TIC VE SAN
Priority date: 2004-02-27
Filing date: 2005-02-28
Publication date: 2005-09-01

Abstract

The present invention relates to a method for strengthening unreinforced masonry walls (3) having at least two blocks and at least one joint (mortar) bed with an adhesive layer, between these blocks; with the method consisting of the steps of grooving at least one horizontal channel (6) on the joint bed, positioning at least one strengthening element (1) inside and along the mentioned channel (6), and then filling the channel (6) with mortar (2). The present invention further relates to a special strengthening element (1) embodying this method.

Description

TECHNICAL FIELD

The present invention relates to implementations towards strengthening of existing unreinforced masonry structures constructed from natural or artificial stones, by means of specially-manufactured high tensile strength elements, mortars, and mortar slurries.

BACKGROUND OF INVENTION

Walls are the main supporting systems for traditional unreinforced masonry structures. In unreinforced masonry construction design, walls are designed by taking into account both the vertical and horizontal loads. Accordingly, unreinforced masonry walls are expected both to maintain their strength against compressive stresses resulting from vertical compressive loads and also to exert sufficient strength against shear stresses occurring on wall sections under horizontal seismic load effects. While all those calculations are made, tensile stresses are not desired in any section of an unreinforced masonry structure. The tensile strength of an unreinforced masonry structure is considered to be zero. Therefore, if the tensile stresses can not be prevented in an unreinforced masonry structure or if specific measures are not taken in regions of tensile stresses, permanent deformations and damages occur in the structure. The degree of the damage is proportional to the load's intensity.
Various strengthening methods have been developed to eliminate the tensile stresses that occur in unreinforced masonry structures. The major drawback in earlier techniques is the lack of compatibility between the material used for strengthening and the original materials used in unreinforced masonry structures. As long as this compatibility criterion is not met, strengthening efforts fail to provide long-term benefit and may even cause heavier damage on the reinforced unreinforced masonry structure.
Circular cross-sectional steel rods (rebars) are currently being used individually or in the form of a grid for strengthening of unreinforced masonry structures. However, when exposed to external factors such as moisture and humidity for an extended period of time, these elements corrode and lose their volumetric size. This causes the steel rods to lose their reinforcing function over the years. Concrete mortars are used in this strengthening method to protect the steel rods against corrosion. The ductility and the modulus of elasticity of concrete mortar used for strengthening is quite different compared those of the existing wall mortar. This causes a compatibility problem between the two materials. The concrete mortar used for strengthening purposes increases the mass and rigidity of the unreinforced masonry wall, damaging its homogeneity. The increase in the rigidity of the structure creates bigger forces on the system in the event of an earthquake.
Reinforced concrete elements in the form of concrete walls or columns, designed with steel rods, are generally either placed on the exterior of unreinforced masonry walls or used as supplemental strengthening element. With this supplemental strengthening element; however, significant problems occur at areas of connection to the existing wall. Reinforced concrete elements, more rigid compared to the existing unreinforced structure, attract bigger external forces during earthquakes. Since unreinforced masonry walls, less rigid than concrete, can not resist sizeable forces that occur at connection areas; the system becomes weak at these locations and loses its strength. As a result of all these factors, strengthening methods using reinforced concrete are not appropriate for strengthening unreinforced masonry structures.
Steel profile elements are also used as strengthening materials. The disadvantages inherent in steel rods, however, are also valid for these elements as well. Furthermore, it is practically impossible to incorporate these elements into the unreinforced masonry walls and expect them to behave as one integral system.

BRIEF DESCRIPTION OF INVENTION

The objective of the present invention is to take up the shear stresses and the tensile stresses in excess of the allowable values for unreinforced masonry walls, by means of special strengthening elements, manufactured from carbon-based or other materials of similar properties, and placed at regions where such excess stresses occur.
Another objective of the present invention is to maintain the original appearance of the unreinforced masonry wall and enhance its strength by incorporating the special strengthening element into the wall.
A further objective of the present invention is to obtain a strengthened homogeneous wall structure, thanks to a special mortar application after strengthening elements are integrated in to the wall.
Yet another objective of the present invention is to improve the strength and effective life span characteristics of the existing wall's degraded mortar by a binding mortar slurry injection implementation after the strengthening elements are put in place.
In order to achieve the mentioned objectives, a method has been developed for strengthening unreinforced masonry walls with at least two blocks and at least one bed joint (mortar bed) with an adhesive layer between the two blocks.
The mentioned method involves grooving at least one horizontal channel along the existing bed joint, placing at least one strengthening element into the opened channel, and then filling the channel with mortar. In this manner, no compatibility problem is encountered since the material used for strengthening conforms to the original material used for the unreinforced masonry structure; and furthermore, this mortar material and special strengthening elements incorporated into the wall's structure together form a coherently functioning system. Since no changes occur in the structure's homogeneity or rigidity, potential additional adverse effects and excess stresses are also avoided.
A preferred implementation of the present invention involves positioning at least one hollow tubular element, with one opening facing outside and one facing the inside of the channel, prior to the mortar filling step; and injecting mortar slurry through the mentioned tubular element after the mortar filling step is carried out. Accordingly, the original mortar that has lost its binding capability over the years regains this adhesive characteristic once again.
In another preferred implementation of the present invention, the special strengthening element is made from a carbon-based material. In a further preferred implementation of the present invention, the strengthening element is in the form of a grid manufactured from carbon fibers. Yet, in another preferred implementation of the present invention, the strengthening element is made from polypropylene fiber. The benefits of these specially-manufactured strengthening elements are superior to those of steel rods. Unlike steel, these strengthening elements are not vulnerable against corrosion. They perform their function within the unreinforced masonry walls, without losing their characteristics under the influence of moisture and humidity. They do not rust or corrode under any circumstance. The major advantage of carbon-based strengthening elements, in particular, is their very high tensile strength. Their tensile strength is a minimum 5 (five) times the tensile strength of steel. These special strengthening elements are not glued to or placed on the exterior of the unreinforced masonry structures, but they are incorporated into the masonry wall, becoming an integral part of the original structure.
In one preferred implementation of the present invention, rigid fibers are added into the mortar in order to enhance the mortar's tensile and shear strengths. In another preferred implementation of the present invention, the mentioned rigid fibers are goat hairs, and in another preferred implementation, they are polypropylene-based.
In another preferred implementation of the present invention, expanded clay is added into the mentioned mortar to enhance the mortar's ductility. In this way, the mortar gains the capability to maintain its water content for long periods.

BRIEF DESCRIPTION OF FIGURES

FIG. 1 gives a perspective view of a wall section strengthened by the mentioned special strengthening elements, depicting an implementation of the mentioned strengthening method.
FIG. 2 gives a front view of the unreinforced masonry wall given in FIG. 1.
FIG. 3 gives a cross-section view of the wall given in FIG. 1.

REFERENCE NUMBERS IN FIGURES

1. Strengthening elements;
2. Mortar;
3. Brick wall;
4. Strengthened region;
5. Hollow cylindrical injection tubes; and
6. Channel.

DETAILED DESCRIPTION OF INVENTION

A representative implementation of the subject method is shown in the figures by applying strengthening elements (1) to the front and back faces of a brick wall (3). In this representative illustration, the strengthening elements (1) manufactured from carbon fibers, in the form of a grid, are used to take up the tensile and shear stresses that occur in unreinforced masonry walls (3) due to vertical and horizontal loads. The application region, amount, and interval of the strengthening elements (1) on an existing unreinforced masonry structure, i.e., the height of the region to be strengthened (4) on the wall (3), are determined after the earthquake calculations of the structure are made. As a result of these and other relevant computations, the strengthening elements (1) are placed at the required amount at the regions where tensile stresses and shear stresses exceed allowable values. The building code for unreinforced masonry structures governs all engineering computations.
Three specially-manufactured strengthening elements are used in the representative implementation of the present invention. These are the strengthening elements (1), wall mortar (2), and the wall mortar slurry, all specially manufactured.
The composition of the mentioned specially-manufactured wall mortar (2) used in this representative implementation of the subject method is determined as follows: It is imperative that the mortar (2) used for strengthening must be compatible with the original mortar of the wall. Therefore, the exact composition of the mortar (2) to be used for this strengthening method is determined after the composition of the mortar in the existing wall structure is analyzed via laboratory tests. The mortar type that conforms to the wall's (3) structure is selected in this manner. The following materials are added into the selected mortar (2).
1) Polypropylene or Goat Hair
The amount of material added to the mortar is 2.5 kg/m³. This material increases mortar's (2) tensile strength as well as its shear strength. The material added to the mortar must have a particle length shorter than 10 mm.
2) Expanded Clay
The amount of expanded clay added to the mortar is 2.5 kg/m³. The clay gives the mortar (2) ductility. It further helps the mortar maintain its water content for long periods of time.
The mortar slurry is injected into the wall (3) to improve the characteristics of the existing mortar (2). The purpose of using the mortar slurry is to help the mortar (2) regain its binding capability that it no longer possesses. The mixture proportion of the mortar slurry is determined in accordance with the original mortar of the structure and with the help of chemicals adhesives used for mortar slurries in the relevant technical field.
The implementation principles of the strengthening system according to a representative implementation of the present invention for unreinforced masonry walls are as follows.

- The regions, the number of brick intervals, and the amount of strengthening elements (1) needed for the strengthening on wall (3) sections (4) are determined as a result of engineering calculations.
- At the determined regions, the strengthening elements (1) are positioned within the mortar beds along the wall. To accomplish this task, first 10 cm deep grooves are made in the mortar along the horizontal joint beds between the bricks, and then the existing mortar up to 10 cm depth is removed. Thus, a horizontal channel (6) is established within the mortar bed along the length of the wall where the strengthening elements (1) are to be positioned. This operation is performed symmetrically on both sides of the unreinforced masonry wall.
- The channels (6) are dampened by spraying on the special mortar slurry before the strengthening elements (1) are placed within the joint bed.
- The strengthening elements (1) are placed within the grooved channels (6) tightly in a continuous manner along the wall (3).
- Through the holes opened at the brick joint locations, cylindrical tubes (5) of 5 mm diameter and 15 cm length are secured into the wall (3) along the entire length of the wall, both vertically and horizontally with a spacing of 2 bricks; with the intent of using the cylindrical tubes for the subsequent mortar slurry injection.
- After these injection tubes (5) are secured in place, the channels (6) grooved along the wall (3) are filled with the mortar material (2) containing the materials given above.
- After the mortar within which the strengthening elements (1) are placed becomes hard, the specially prepared mortar slurry is injected into the wall through the previously placed hollow injection tubes (5). The injection pressure and application duration of the mortar slurry is experimentally and specifically determined for the structure to be strengthened.

The way the subject strengthening elements (1) take up the horizontal shear stresses that occur in unreinforced masonry structures is described below.
It is known that shear stresses arise in horizontal cross-sections of unreinforced masonry structures when exposed to horizontal earthquake forces. When the shear force in any wall section is divided by the cross-section area of this region, the resulting expression is in a force/area unit, also known as the “shear stress.” The allowable limit of shear stress in an unreinforced masonry wall cross-section is governed by the building code, specifications, and regulations of a country. If shear stress exceeds the maximum allowable limit given in the regulations, it can be concluded that the wall sections possess insufficient strength. Under standard conditions, the appropriate thing to so is to increase the thickness of the wall or resort to other structural changes that would provide the needed strength.
The manner that the strengthening elements take up the shear stresses can be expressed mathematically with the following formulae.

- V: Theoretical shear force across a wall section
- A_d: Cross-sectional area of the wall
- t_d: Wall thickness
- τ: Shear stress at the wall section due to theoretical shear force
- τ_em: Allowable shear stress safely tolerated by the wall
- V_d: Shear force safely tolerated by the wall cross-section
- V_ç: Shear force taken up by the strengthening elements
- A_sh: Cross-sectional area of the horizontal strengthening elements in vertical cross section of the wall
- d: Wall's useful width
- s_h: Strengthening element interval (i.e. frequency) in vertical direction
- l_d: Horizontal length of the wall
- f_yd: Computed creep strength of the strengthening elements

Dead loads and live loads for an unreinforced masonry structure are analyzed under the effects of earthquake loads. These calculations are governed by the methods and rules given in relevant building codes. First, the largest theoretical shear force value likely to occur in an unreinforced masonry wall cross-section needs to be computed. Then, the theoretical shear force needs to be divided by the wall's cross-sectional area, and the resulting wall shear stress is obtained. If the resulting value is smaller than the value of the wall's allowable shear stress, it can be concluded that the wall is in a safe condition. Otherwise, the shear stress that is not taken up by the wall needs to be accounted for by the stated strengthening elements. These expressions can be formulized mathematically as follows.
If $\begin{matrix} τ = \frac{V}{A_{d}} \leq τ_{em}, & (1.2) \end{matrix}$
where
A_d=l_dt_d, (1.1)
it can be concluded that the wall is safe under the effects of shear stresses. In this case, no strengthening elements are required.
If, however, $\begin{matrix} τ = \frac{V}{A_{d}} > τ_{em}, & (1.3) \end{matrix}$
the shear stress at the wall exceeds the allowable shear stress that can be safely tolerated by the wall.
In this case, strengthening elements (1) must be applied to the wall so as to eliminate the threat posed by the excess shear stress. In order to determine the cross-sectional area and vertical frequency (i.e. interval) of the strengthening elements to eliminate the shear stress along the wall's cross-section, the below-given method is followed.
The theoretical shear force in an unreinforced masonry wall with strengthening elements is taken up by both the wall itself and the mentioned strengthening elements. This concept may be expressed by following formula.
V=V _d +V _ç (1.4)
The maximum shear force that can be tolerated by the wall is calculated as follows.
V _d=τ_em .A _d (1.5)
Hence, the shear force that the strengthening elements need to resist can be found as follows.
V _ç =V−V _d (1.6)
The required strengthening element area and interval (i.e. frequency) in line with the force to be counterbalanced by the strengthening elements (1) can be calculated according to the following formula. $\begin{matrix} V_{ç} = \frac{A_{sh} \cdot f_{y d} \cdot d}{s_{h}} & (1.7) \end{matrix}$
In the above expression, d is computed as follows.
d=0.8l _d (1.8)
According to this result, the physical characteristics of the strengthening elements (1) manufactured from carbon fibers in the form of a grid and given in the representative implementation of the present invention are as follows.

Modulus of elasticity (E) 210,000 N/mm²

Tensile Strength 2,400 N/mm²

Ultimate Strength 2,900 N/mm²

Elongation at Breaking Point 1.2%

Density 1.6 gr/cm³
In the implementation phase, different strengthening elements, possessing the same characteristics as given above, may also be used. The subject strengthening elements may be produced in 5 mm width, 1.5 mm thickness, 7.5 mm²rectangular cross-sectional area, and 80% fiber density; in 100 mm width, 1.5 mm thickness, 15 mm²rectangular cross-sectional area, and 80% fiber density; in 20 mm width, 1.5 mm thickness, 22,5 mm²rectangular cross-sectional area, and 80% fiber density; or in custom sizes according to specific requirements.
The representative implementation of the present invention is fundamentally used for strengthening of existing unreinforced masonry structures. It can further be used; however, in restoring wall cracks that occur in brick-based and stone-based structures, and in newly constructed brick-based and stone-based structures of all kinds.
For example, the strengthening elements needed to strengthen unreinforced masonry minarets can be determined in accordance with the described method. Furthermore, this method may be effectively used to eliminate tensile stresses in all types of unreinforced masonry and reinforced concrete structures; thus this method may be used both as a local or a complete strengthening solution for an existing structure and as an original building material for a new structure.

Claims

1. A method for strengthening unreinforced masonry walls (3) of at least two blocks and at least one joint bed with an adhesive layer, between the blocks; comprising the following steps,

grooving at least one horizontal channel (6) along the mentioned joint (mortar) bed,

positioning at least one strengthening element (1) within this channel (6),

and filling the mentioned channel (6) with mortar (2).

2. The method of claim 1, further comprising the following steps:

positioning at least one tubular element (5) that opens up to the exterior face of the wall on one side, and to the inside of the channel (6), on the other side; before the mortar (2) filling step is carried out on the channel (6);

injecting mortar slurry through the tubular element (5) after the mortar (2) filling step is completed.

3. The method of claim one wherein the subject strengthening element (1) is a carbon-based material.

4. The method of claim two wherein the subject strengthening element (1) is a carbon-based material.

5. The method of claim one, wherein the subject strengthening element (1) is a polypropylene-based material.

6. The method of claim two, wherein the subject strengthening element (1) is a polypropylene-based material.

7. The method of claim one, wherein the subject strengthening element (1) is a mixture of carbon-based material and polypropylene-based material.

8. The method of claim two, wherein the subject strengthening element (1) is a mixture of carbon-based material and polypropylene-based material.

9. The method of claim 3, further comprising a step wherein stiff fibers are added into the mortar (2) in order to enhance its tensile and shear strengths.

10. The method of claim 4, further comprising a step wherein stiff fibers are added into the mortar (2) in order to enhance its tensile and shear strengths.

11. The method of claim 9, wherein the stiff fibers are goat hairs.

12. The method of claim 10, wherein the stiff fibers are goat hairs.

13. The method of claim 9, where the stiff fibers are polypropylene-based.

14. The method of claim 10, where the stiff fibers are polypropylene-based.

15. The method according to claim 1, further comprising a step wherein expanded clay is added into the mortar (2) to enhance its ductility.

16. The method according to claim 2, further comprising a step wherein expanded clay is added into the mortar (2) to enhance its ductility.

17. The method according to claim 3, further comprising a step wherein expanded clay is added into the mortar (2) to enhance its ductility.

18. The method according to claim 4, further comprising a step wherein expanded clay is added into the mortar (2) to enhance its ductility.

19. The method according to claim 5, further comprising a step wherein expanded clay is added into the mortar (2) to enhance its ductility.

20. The method according to claim 6, further comprising a step wherein expanded clay is added into the mortar (2) to enhance its ductility.

21. The method according to claim 7, further comprising a step wherein expanded clay is added into the mortar (2) to enhance its ductility.

22. The method according to claim 8, further comprising a step wherein expanded clay is added into the mortar (2) to enhance its ductility.

23. The method according to claim 9, further comprising a step wherein expanded clay is added into the mortar (2) to enhance its ductility.

24. The method according to claim 10, further comprising a step wherein expanded clay is added into the mortar (2) to enhance its ductility.

25. The method according to claim 11, further comprising a step wherein expanded clay is added into the mortar (2) to enhance its ductility.

26. The method according to claim 12, further comprising a step wherein expanded clay is added into the mortar (2) to enhance its ductility.

27. The method according to claim 13, further comprising a step wherein expanded clay is added into the mortar (2) to enhance its ductility.

28. The method according to claim 14, further comprising a step wherein expanded clay is added into the mortar (2) to enhance its ductility.

29. The method according to claim 15, further comprising a step wherein expanded clay is added into the mortar (2) to enhance its ductility.

30. The method according to claim 16, further comprising a step wherein expanded clay is added into the mortar (2) to enhance its ductility.

31. The method according to claim 17, fuirther comprising a step wherein expanded clay is added into the mortar (2) to enhance its ductility.