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- ENGINEERED CRACK CONTROL CRITERIA
This TEK supplementsTEK 10-2C Control Joints for Concrete Masonry Walls – Empirical Method (ref. 3). The reader is encouraged to refer to that TEK for other pertinent information such as construction details and location of control joints.
Controlling shrinkage cracking historically has been addressed by limiting the moisture content of units at the time of placement in the wall and indirectly incorporated the effects of variations in temperature and cement carbonation as well as drying shrinkage. In 2000 however, due to problems associated with maintaining moisture controlled units (Type I) in that state until placement in the wall, moisture controlled units (and Type designations) were removed from ASTM C90 (ref. 4).
In view of this, the concrete masonry industry has developed an engineered approach to controlling cracking which examines each of these three parameters separately, and then incorporates them into a single Crack Control Coefficient. In general, this engineered approach is more complicated and requires more detailed knowledge of the masonry characteristics than the empirical approach ofONE 10-2C (ref. 3), which is based on historical solutions that have proven successful over many years of experience for a broad geographic distribution. The empirical method is the most commonly used method and is applicable to most conventional building types. The engineered method is generally used only when unusual conditions are encountered such as dark colored units in climates with large temperature swings.
ENGINEERED CRACK CONTROL CRITERIA
The engineered criteria was developed to produce a more rational approach to crack control in concrete masonry – particularly in areas of high seismicity where relatively large amounts of continuous reinforcing steel are used. Also addressed is additional reinforcement around openings to provide strengthening and allow placement of the control joints at locations other than at the openings. The effectiveness of this method depends onreliable criteria being correctly incorporated into the project design, the materials meeting the requirements of the project specifications, and the masonry being constructed in accordance with the project drawings.
The engineered criteria is based on a Crack Control Coefficient to accommodate internal volume changes. Once the internal movement due to volume change has been estimated, the designer can control crack width to a maximum value by 1) limiting the distance between control joints when used in combination with a minimum amount of horizontal reinforcement or 2) incorporating a predetermined, higher amount of horizontal reinforcement (when needed for structural purposes) to limit crack width without the use of control joints.
Crack Control Coefficient
The Crack Control Coefficient (CCC) is an indicator of anticipated wall movement. Concrete masonry unit shortening per unit length is estimated by including the possible combined effects of movement due to drying shrinkage, carbonation shrinkage and contraction due to temperature reduction. The Crack Control Coefficient value itself is determined by summing the coefficients of these three properties for a specific concrete masonry unit. It is a function of unit mix design and production/curing methods.
The total linear drying shrinkage is determined in accordance with Standard Test Method for Linear Drying Shrinkage of Concrete Masonry Units and Related Units, ASTM C426 (ref. 5). ASTM C 90 (ref. 4) limits total linear drying shrinkage of concrete masonry units to 0.00065 in./in. (mm/mm). Note that this is based a saturated condition (immersed in water for 48 hours). In the field, units will probably be no higher than 70% of saturation. Therefore, the highest realistic drying shrinkage potential realized in the field will be around 0.00045 in./in. (mm/mm) or 0.54 in. in 100 ft (13.7 mm in 30.48 m). It for this reason that the Building Code Requirements for Masonry Structures (ref. 1) stipulates the use of only 50% of the total linear drying shrinkage determined in accordance with ASTM C426 (ref. 5) for design.
Carbonation shrinkage is an irreversible reaction between cementitious materials and carbon dioxide in the atmosphere. It occurs over a long period of time and there currently is no standard test method to determine it. Therefore, it is recommended that 0.00025 in./in. (mm/mm) be used for the carbonation shrinkage coefficient – 0.3 in. in 100 ft (7.6 mm in 30.48 m).
Thermal coefficients for concrete masonry units typically range from 0.0000025 to 0.0000055 in./in./°F (0.0000045 to 0.0000099 mm/mm/°C) (refs. 4 and 5). For design purposes, the value of 0.000004 in./in./°F (0.0000081 mm/mm/°C) be used as outlined in the Building Code Requirements for Masonry Structures (ref. 1). Based on a temperature change of 70 °F (38.9 °C), this would translate to a thermal contraction value of 0.00028 in./in. (mm/mm) or 0.34 in. in 100 ft (8.5 mm in 30.48 m).
The CCC is the sum of the potential length change due to each of these three parameters and for typical concrete masonry units varies from 0.00063 to 0.00108 in./in. (mm/mm). This range corresponds to a 100 ft (30.48 m) long wall shortening 0.76 to 1.30 in. (19.3 to 33.0 mm).
Control Joints with Horizontal Reinforcement
The most common (and usually most cost effective) method of controlling cracks in concrete masonry is to use control joints in conjunction with a minimum amount of horizontal reinforcement between the joints. Reinforcement is often required for wind or seismic resistance and it is prudent to utilize it for assisting incrack control as well. The amount of horizontal reinforcement needed is based on limiting cracks to a width of 0.02 in. (0.51 mm) since water repellent coatings can effectively resist water penetration for cracks of this size. Based on this premise and the CCC criteria discussed earlier, control joint spacing criteria are presented in Table 1 utilizing a minimum horizontal reinforcement ratio of As/An≥ 0.0007. Anis the net area of the vertical cross-section of the wall. For hollow unit masonry and partially grouted masonry it essentially is the total thickness of the face shells times the height of the wall plus the additional area provided by any grouted bond beams. Table 2 presents the maximum spacing of the various sizes of typical horizontal reinforcement to meet the 0.0007 criteria. The wall panel length to height ratio and the maximum length of wall panel criteria in combination with horizontal reinforcement in Table 1 are based on historical field experience and analytical studies.
Table 1—Criteria for Controlling Cracking in Reinforced Concrete Masonry Walls
Table 2—Maximum Spacing of Horizontal Reinforcement to Meet the Criteria As > 0.0007An
Horizontal Reinforcement Only
In some regions of the country, significant amounts of horizontal reinforcement are required for structural purposes, i.e. Seismic Performance Categories D and E. Studies have shown that horizontal reinforcement of sufficient quantity can effectively limit crack width in concrete masonry walls. For standard reinforcing bar sizes, horizontal reinforcement spacings up to 48 in. (1219 mm) o.c. have been shown to effectively control cracking without the use of control joints. It has also been shown that horizontal reinforcement provides internal restraint, which results in transfer of tension from the masonry to the reinforcement, resulting in more frequent but much smaller cracks. As the level of horizontal reinforcement increases, cracking becomes more uniformly distributed and crack width decreases.
When a crack is formed, tension in the masonry is released. This masonry tension is transferred to the reinforcement at the time of crack formation. Therefore, reinforcement should be sized such that the resulting tensile force in the reinforcement does not exceed the yield strength of the steel. This keeps the steel within the elastic range and minimizes the crack width to a point where control joints are not necessary in the design.
To ensure the steel is within the elastic range, the width of a crack at the horizontal reinforcement location would be limited to the yield strain of the steel multiplied by the length of reinforcing bar being strained:
For example, a bar being strained at its yield strain over a length of 10 in. (254 mm) would limit crack width to 0.02 in. (0.51 mm).
In order to meet this criteria of limiting the steel to the elastic range, the tension in the masonry (Tm=FtAn) just prior to crack formation must be less than the yield strength of thesteel (Ts=fyAs):
Ft=average tensile strength of masonry. A verticalcrack would pass through a head joint and then a block in alternate fashion. The tensile strength of typical masonry units is 200 psi (1.38 MPa) and the tensile strength of a typical head joint is 25 psi (0.172 MPa). Average tensile strength is, therefore, 225 psi / 2 or 112 psi (0.772 MPa).
fy= yield strength of steel reinforcement
= 60,000 psi (413 MPa)
Substituting these values, the criteria becomes:
AS≥ 0.0019 An
When this condition is met, there is sufficient horizontal steel to limit masonry cracking to widths of 0.02 in. and control joints may be eliminated as stated in footnote 2 of Table 1. Table 3 indicates the amount of reinforcement that will meet this criteria for various concrete masonry walls.
Table 3—Maximum Spacing of Horizontal Reinforcement to Meet the Criteria As > 0.002An (1)
Control Joints in Vertically Reinforced Walls
In plain masonry walls, control joints are typically placed at an opening as it is a weak point subject to cracking due to the reduced masonry cross section. This requires the control joint above the opening to be aligned with the end of the lintel, cross under the lintel via a slip plane, and then proceed through the opening (ref. 3). In walls containing vertical reinforcement, however, the cell adjacent to the opening usually is grouted and reinforced. Using the same type of detail would require the control joint to cross the vertical reinforcement thereby preventing movement and defeating the purpose of the control joint. However, if the opening is completely surrounded by reinforcement as detailed in Figures 1 and 2, the area through the opening is strengthened and control joints can be placed outside the opening. For best performance the vertical reinforcement should be placed in the cell immediately adjacent to the opening. However, due to congestion in the cell at this location, vertical reinforcement is often placed in the second cell from the opening. On large openings, it is recommended to grout the cell next to the opening as well as the cellcontaining the reinforcement to provide additional resistance for attaching the door or window frame. These details may also be used in unreinforced walls and walls utilizing steel lintels since the area through the opening is strengthened by the additional reinforcement.
When utilizing these details and the wall segments on either side of openings are designed to resist the lateral loads applied directly to them plus those transferred from the opening enclosure, shear transfer devices such as preformed gaskets (seeTEK 10-2C, ref. 3) are not necessary. However, some designers still incorporate them to limit the relative movement between the two panels on either side of a control joint thereby reducing the stress on the joint sealant and providing longer life.
Figure 1—Reinforcement Around Openings Option
- Building Code Requirements for Masonry Structures, ACI 530-02/ASCE 5-02/TMS 402-02, reported by the Masonry Standards Joint Committee, 2002.
- Selden, J. K. Shrinkage Characteristics of Concrete Masonry Walls, Housing and Home Finance Agency Paper 34, pages 1-60, 1954.
- Control Joints for Concrete Masonry Walls – Empirical Method,ONE 10-2C. National Concrete Masonry Association, 20010.
- Standard Specification for Loadbearing Concrete Masonry Units, ASTM C 90-02. American Society for Testing and Materials, 2002.
- Standard Test Method for Linear Drying Shrinkage of Concrete Block, ASTM C 426-99. American Society for Testing and Materials, 1999.
- Toennies, H. T. Concrete Masonry Shrinkage. National Concrete Masonry Association, 1961.
NCMA TEK 10-3, Revised 2003.
NCMA and the companies disseminating this technical information disclaim any and all responsibility and liability for the accuracy and the application of the information contained in this publication.
bond beamsconstruction detailscontrol jointscrack controlengineered methodjoint reinforcementreinforced concrete masonryreinforcing barsshrinkagewall movement
Control joints are typically required in exposed above grade concrete masonry walls, where net aesthetic shrinkage cracking may detract from the appearance of the wall, and to limit moisture or air infiltration.What are the guidelines for masonry control joints? ›
The National Concrete Masonry Association (NCMA TEK 10-2C) recommends that control joints be spaced at 25 feet or 1.5 times the height of the wall, whichever is less.How far apart should control joints be in CMU walls? ›
Below are some standard locations in masonry walls where control joints should be considered. These "rules of thumb" are based on putting 9 gauge horizontal wire every other course. The general rule of thumb is to space the control joints every 1 1/2 the height of the wall not to exceed 25 feet.What are control joints in CMU walls? ›
Control joints are typically used in concrete masonry to reduce the occurrence of shrinkage-related cracking. A control joint is a continuous vertical joint filled with mortar, but with a bond breaker on one side so that tensile stress cannot develop across the joint.How often do you need control joints in masonry? ›
Joints are commonly spaced at distances equal to 24 to 30 times the slab thickness. Joint spacing that is greater than 15 feet require the use of load transfer devices (dowels or diamond plates). Contraction joints may be tooled into the concrete surface at the time of placement.Do you have to put control joints in concrete? ›
Concrete expands and contracts with changes in temperature and moisture. If not properly controlled, cracks can begin to appear. Placement of concrete control joints and expansion joints are crucial when designing and pouring concrete slabs and sidewalks.What are the OSHA requirements for bracing masonry walls? ›
- they must be established prior to the start of construction of the wall;
- they must equal to the height of the wall to be constructed plus four feet, and shall run the entire length of the wall;
- they must be established on the side of the wall which will be unscaffolded;
In vertically reinforced masonry walls with masonry lintels, vertical control joints should be located between openings, or at least 24-inches away from the opening – not to exceed 25-feet on-center (Figure 1).What is the maximum distance between concrete control joints? ›
The maximum joint spacing should be 24 to 36 times the thickness of the slab. For example, the joint spacing for a 4-inch [100 mm] thick slab should be about 10 feet [3 m]. It is further recommended that joint spacing be limited to a maximum of 15 feet [4.5 m].Where are control joints required? ›
A: Control joints shall be installed where a wall or partition runs in an uninterrupted straight plane exceeding 30 linear feet (50 linear feet on center for ceilings with perimeter relief).
Typical CMU mortar joints are 3/8". The nominal dimension works within the 4" grid that other construction materials follow.What is the maximum spacing elsewhere control joints in masonry walls? ›
3.2 c) Because of temperature variations and the shrinkage in a concrete masonry wall construction, it is necessary to provide control joints in blockwork at a maximum spacing of 6m and at points of potential cracking e.g. beside openings and at large steps in wall or footing.What does a control joint of a masonry wall actually control? ›
Control joints are placed in concrete masonry walls to limit cracks due to shrinkage. Control joints are unbonded vertical separations built into a concrete masonry wall to reduce restraint and permit longitudinal movement. They are located where cracking is likely to occur due to excessive tensile stress.What do you fill control joints in concrete? ›
Use QUIKRETE Non-Sag Polyurethane Sealant, and move the nozzle slowly along the length of the joint, pressing the sealant into the joint. Use the tip of the nozzle to smooth the sealant, if needed. Immediately clean up any excess sealant with a commercial solvent or a citrus-based cleaner.What is the difference between a masonry expansion joint and a control joint? ›
Whilst a control joint is used to control small cracks caused by shrinkage, an expansion joint allows concrete to expand and move as a result of temperature changes, without causing the structure to crack.What is the maximum spacing for brick control joints? ›
Typical joint spacing should be 24 – 26 feet maximum for continuous walls. 2. 16 – 20 ft spacing with lots of openings 3. No more than 12 feet from outside corners on each side 4.What is the difference between construction joints and control joints? ›
A: Control and expansion joints are designed to accommodate movement, whereas the aesthetic joint is simply for visual purposes. Construction joints may be used to either facilitate construction practices or the movement of the entire building.Do concrete walls need expansion joints? ›
All concrete will shrink slightly as it dries and, when it's set, will expand or contract depending on the ambient temperature. To prevent cracks from forming, concrete expansion joints should be incorporated to allow for movement, particularly in slabs with a surface area exceeding 6m2.How deep should control joints be? ›
Q.: We're working with a specification that says control joints must be cut to a depth equal to one-third the slab thickness. All the recommendations we can find suggest a depth of one-quarter the slab thickness, with a minimum of 1 inch.Where do you cut concrete control joints? ›
It is recommended to start saw cuts on or at the center of column lines. Joints shall be spaced at 24 to 30 times the slab thickness but this need to be confirmed by a structural engineer. Joint spacing normally ranges between 12 to 18 feet depending on the amount of reinforcement the slab has.
To avoid shrinkage cracks, control joints are placed according to accepted formulas, based on the thickness of concrete and the configuration of the forms. If a slab is 4 inches thick, control joints should be placed on an 8-foot grid.How far apart should wall bracing be? ›
Â. The distance between adjacent edges of braced wall panels along a braced wall line shall be no greater than 20 feet (6096 mm) as shown in Figure R602.Is wall bracing required? ›
The IRC requires certain braced wall lengths to withstand certain lateral loads based on the layout of the building and the calculated lateral forces applied to it.Do walls need diagonal bracing? ›
All framing structural designers know that diagonal bracing is necessary because it keeps framed walls stable, preventing them from collapsing. Let-in bracing is the most widely used bracing system, but this bracing requires the studs be notched so that the brace will be flush with the stud surface.What types of movement joints are required in masonry wall systems? ›
Movement joints are used to allow dimensional changes in masonry and to minimize random wall cracks and other distress. There are various types of movement joints in buildings: expansion joints, control joints, building expansion joints and construction joints.Why are control and expansion joints required in masonry walls? ›
A masonry expansion joint is a complete break in the masonry wall to allow for the entire structure to move under loading, temperature related expansion/contraction, etc. This allowed movement helps prevent distortion and cracking.How far apart should the control joints be for a 6 slab? ›
Historically, the maximum recommended distance or spacing in feet between joints has been two to three times the slab thickness in inches. For a 6 in. thick slab, this recommendation yields a maximum joint spacing between 12 and 18 ft.How wide are concrete control joints? ›
The width of the joint will vary, although a quarter inch (0.64 cm) is the most common. The width of the joint will affect the degree that a joint filler can accommodate the joint expansion as a result of concrete shrinkage.How far apart should horizontal control joint be? ›
Normally, horizontal control joints are placed every floor over the windows. However, they could also be placed at every floor line with loose lintels over the windows. The control joint could be spaced every two floors, however, the size of the shelf angle and the width of the joint would have to be larger.What are the different types of control joints? ›
There are three types of joints: Contraction Joints, Construction Joints and Isolation joints. The most common are contraction joints which control cracks which are caused by restrained shrinkage, loads and other stresses.
The best time to start cutting control joints into concrete is after the poured concrete has dried enough for you to saw through without dislodging or ravelling the particles of the poured mixture. In most cases, control joints should be cut into concrete after it has had 6 to 18 hours of drying time.What is the spacing for movement joints in masonry? ›
As a general rule, the spacing of vertical movement joints in clay brickwork should be between 10m and 12m apart. In calcium silicate brickwork the spacing should be typically between 7.5m and 9m apart. In concrete brickwork the distances are between 6m and 9m.What is the max length of a CMU wall? ›
It is not uncommon to create designs with an h/t from 32 to 50. That could produce wall heights of up to 33 feet for walls built with 8-inch concrete masonry units (CMU), 41 feet for 10-inch CMU, and 50 feet for 12-inch CMU. Regionally, 14- and 16-inch CMU are available, which extend possible wall heights even further.What is the actual thickness of a masonry wall? ›
Size of brick
So, 1/2 brick thick wall ranges from 7 cm to 9 cm. With 1 cm thick mortar, the actual thickness of wall = Thickness of wall + Mortar thickness = 9 cm to 10 cm.
The following recommended tips should be observed: Maximum joint spacing should be 24 to 36 times the thickness of the slab. Joints should be spaced about 10 feet and a maximum of 15 feet. When using joint groove for contraction joints, the joint should be a minimum depth of 1/4 thickness of the slab.What is the AS 3700 masonry code? ›
AS 3700 sets out minimum requirements for the design and construction of masonry elements of the following types: Unreinforced, reinforced and pre-stressed masonry using manufactured units of clay or concrete laid in mortar. Unreinforced masonry using manufactured calcium silicate units laid in mortar.What is the minimum distance for expansion joint? ›
20 to 25mm gap is sufficient to dissipate stresses due to thermal expansion.What are concrete masonry control joints? ›
Control joints in masonry are vertical weak planes intentionally built or cut into masonry to control where cracking occurs. Control joints should be installed in any masonry assembly that expected to experience net shrinkage over time. Usually, we think of control joints in concrete masonry (CMU).What is the best product to fill control joints? ›
Control joints should be filled if floors are subject to heavy, hard-wheeled traffic commonly used in warehouses and distribution centers. Epoxy and polyurea-based joint fillers are two of the most popular filler solutions.Is a control joint an expansion joint? ›
While you might mistake control joints and expansion joints, they are not the same thing. These are two very different and important joints used with building materials.
Expansion joint failure can happen for several reasons, but regardless concrete needs a buffer as it goes through the natural cycles of expansion and contraction. So, this has always been the case, and this is why expansion joints are necessary.Where do you put control joints in concrete? ›
Joints are commonly spaced at distances equal to 24 to 30 times the slab thickness. Joint spacing that is greater than 15 feet requires the use of load transfer devices. Contraction joints may be tooled into the concrete surface at the time of placement.What is the difference between control joint and expansion joint in masonry wall? ›
Control joints can be isolated within the plane being treated (brick facades, concrete sidewalks) while expansion joints must bisect the entire structure thereby creating a gap throughout all of the building elements—foundation, walls, curtainwalls, plaza decks, parking decks, floors, and roof.How many control joints are in concrete? ›
CONTROL JOINT SPACING
Space joints (in feet) no more than 2-3 times the slab thickness (in inches). A 4" slab should have joints 8-12 feet apart.
Movement joints in internal walls are not normally necessary for single dwellings unless the walls are straight and unbroken and over 6m long, in which case the block manufacturer's recommendations should be adopted.What are the 3 types of joints based upon the type of movement allowed? ›
Functionally the three types of joints are synarthrosis (immovable), amphiarthrosis (slightly moveable), and diarthrosis (freely moveable).