CRACK CONTROL IN CONCRETE MASONRY WALLS

CRACK CONTROL IN CONCRETE MASONRY WALLS
PROGRAM # This presentation addresses proper application of crack control measures into concrete masonry wall systems as presented in NCMA TEK 10-2B. Most of the principles, statements, tables and illustrations are taken directly from that TEK. TEK 10-2B should be provided as a handout with this presentation. The following TEK are also referred in this presentation and are highly encouraged to be included as handout material in addition to TEK 10-2B: TEK 10-1A TEK 10-3 TEK 1-1C TEK 12-1A TEK 12-2A NCMA/AIA CES Program

Crack Control in Concrete Masonry Walls
Causes of Concrete Masonry Cracks 1. Excessive deflection 2. Structural Overload The basic cause of cracking in masonry buildings is movement. There are many types of movements that occur in buildings. Restraints to this movement such as contact with the earth, intersecting walls, etc. result in tensile forces. As concrete masonry is relatively weak in tension and when the tensile forces exceed the capacity of the masonry, cracking results. Movement due to deflection of supporting elements and structural overload are common occurrences and need to be addressed in proper structural design. TEK 10-1A NCMA/AIA CES Program

Causes of Concrete Masonry Cracks 3. Differential settlement Differential movement can result from uneven settlement of the foundation producing cracking. TEK 10-1A NCMA/AIA CES Program

Causes of Concrete Masonry Cracks 4. Shrinkage Moisture change Temperature Carbonation Length change Concrete materials need a substantial amount of water to effect the hydration of the cement - the hardening process which is a chemical reaction between the cement and water. As the excess water that is not needed for hydration evaporated, the concrete masonry shrinks. Thermal expansion and contraction is the normal dimensional decrease with a decrease in temperature and dimensional increase with an increase in temperature of the material. Carbonation - This is the result of a chemical reaction between the cementitious materials and the carbon dioxide in the atmosphere. It is irreversible and occurs over a long period of time. TEK 10-3 General trend = shrinkage Time (cycles) NCMA/AIA CES Program

Shrinkage Cracking If a wall is restrained at the bottom only, the cracks that form there will tend to be held tightly closed provided that a crack does not form in the foundation. The cracks will open the widest at the top of the wall. Restraint at bottom only NCMA/AIA CES Program

Shrinkage Cracking By the same token, if a wall is restrained top and bottom, the cracks will be held tightly closed at the points of restraint and will open the widest near the center. Restraint at top and bottom NCMA/AIA CES Program

Accommodating Movement
Crack Control in Concrete Masonry Walls Accommodating Movement Control Joints Used in concrete masonry construction Expansion Joints Used with clay brick The most common way of accommodating movements and to control cracking is to provide joints for stress relief. Since the type of volume change associated with concrete masonry is shrinkage, control joints are used in concrete masonry (sometimes referred to as contraction joints). These type of joints allow for opening of the joint rather than closing. Contraction joints are much more aesthetically pleasing than shrinkage cracks and can be much more easily made weather resistant. Clay masonry on the other hand, is a fired product and is at its smallest volume when in comes out of the kiln. From that time on as it picks up moisture from the atmosphere, it increases in size. Therefore expansion joints or a joint that has a gap and allows closure as the material expands is required. NCMA/AIA CES Program

Purpose of Control Joints
Crack Control in Concrete Masonry Walls Purpose of Control Joints Relieve horizontal tensile stresses reduce restraint and permit longitudinal movement Separate dissimilar materials Control joints in concrete masonry provide relief planes for the horizontal stresses and provide horizontal movement. There very seldom is restraint in the vertical direction since gravity works in the that direction. Another purpose of control joints is to separate dissimilar materials and allow relative differential movement such as shrinkage of concrete masonry and expansion of clay masonry. NCMA/AIA CES Program

Goal of Crack Control Provisions Restraint at top and bottom The goal in concrete masonry is to limit uncontrolled cracking to to a maximum of 0.02 inches (0.5 mm) which is the thickness of a 0.5 mm pencil lead. It has been found that most post applied water repellency coatings (including clear water repellents for architectural concrete masonry) are capable of preventing water penetration into cracks up to this this size. Limit crack widths to less than 0.02 in (0.5 mm) able to be bridged by most coatings prevents water penetration TEK 10-3 TEK 10-3 NCMA/AIA CES Program

Specifying Concrete Masonry Units Included in C 90 Compressive Strength Absorption Dimensional Tolerances Density Definitions Linear Drying Shrinkage C 90 The 2000 edition of ASTM C 90 is the specification for loadbearing concrete masonry units. It provides criteria for compressive strength, absorption, dimensional tolerances, density definitions, and linear drying shrinkage. The last is a maximum limit of % for all concrete masonry units. Standard Specification for Loadbearing Concrete Masonry Units TEK 1-1C NCMA/AIA CES Program

Specifying Concrete Masonry Units NOT Included in C 90 Color Texture Density Water Repellency Fire Ratings Thermal Properties Sound Properties C 90 These are items that are not included in the ASTM C 90 specification. They must be addressed in project contract documents. Most architectural units are custom made and different costs associated with each of the different properties. The specifier must work closely with the block producer to convey to him that type of unit that is intended for the specific project. Standard Specification for Loadbearing Concrete Masonry Units Most architectural concrete masonry units are custom made. Work closely with producers to get exactly what you want. TEK 1-1C NCMA/AIA CES Program

Factors Influencing Concrete Masonry Drying Shrinkage Shrinkage increases … as unit cement content increases as aggregate hardness decreases with increasing unit moisture content Virtually the same items that increase the drying shrinkage of concrete have the same effect on shrinkage of concrete masonry units. Increases in cement and water content result in an increase in shrinkage. Decreasing the hardness of the aggregate results in a decrease in the shrinkage as it provides less resistance to the shrinkage effects of the cement paste. ASTM C 90 maximum allowable shrinkage = 0.065% TEK 10-1A NCMA/AIA CES Program

ASTM C 426 Drying Shrinkage Test 10.000 100% Measured shrinkage 70% 30% Typical field shrinkage Distance between gage points CMU moisture content Since units in the field will probably have no higher than 70% of saturation even in the wettest condition and most likely will not dry out to more than 30%, they amount of field shrinkage will not be nearly as much as the 0.065% maximum allowed in laboratory tests. 9.995 10% TEK 10-3 NCMA/AIA CES Program

Type 1 and Type 2 CMU 3.1 Types - Two types of concrete masonry units are covered as follows: Type 1, Moisture-Controlled Units - Units designated as Type 1 shall conform to the requirements of this specification. Type 2, Nonmoisture-Controlled Units - Units designated as Type 2 shall conform to the requirements of this specification with the exception of Table 1. Controlling shrinkage cracking has historically been addressed by focusing on drying shrinkage characteristics of concrete masonry units and by limiting the moisture content of units at the time of placement in the wall. However, this criteria was shown to be difficult to achieve in actual construction since Type I units were not available in many areas of the country. Also special precaution were required at the jobsite to protect against moisture gain. Additionally, this specification only took in account the shrinkage effects of moisture, ignoring the effects of temperature and cement carbonation. Because of the problems associated with the Type I specification, the special provisions for Type I and all reference to unit type were removed from the 2000 ASTM C 90 specification. This section removed. Does not appear in C90-00. TEK 1-1C NCMA/AIA CES Program

Moisture Content Appearance 90 % 70 % 50 % 30 % The above slide shows the relative appearance of concrete masonry units with moisture contents varying from 90% to 30%. According to the MSJC specifications concrete masonry units are not to be prewetted before placing in the wall unless specifically authorized as it will increase the likelihood of shrinkage in the completed wall. On the other hand, it has been found that wet cutting of masonry units adds little to the moisture content of the units and thereby has little effect on the shrinkage potential of the completed product. Also, the percentage of units that are cut on a job is normally a very small percentage. Should concrete block be wetted? Can concrete block be wet-cut? NCMA/AIA CES Program

Control Joint Spacing Recommendations h 25 feet maximum The empirical method utilizes a panel length to height ratio of 1.5:1 up a maximum spacing of 25 ft. To illustrate these criteria: A 20 ft tall warehouse: Length to height ratio = 1 1/2 x 20’ = 30’ > 25’, 25’ controls For an 8’ tall wall: Length to height ratio = 1 1/2 x 8’ = 12’< 25’, 12’ controls As you can see, the control joint spacing for shorter walls is quite close under this provision. Many areas of the country have had success with greater spacings, hence the reason for footnote 3 which allows the spacing to be adjusted up or down based on local experience but no father than 25 ft. 1.5 h maximum So, for h = 8 ft, spacing < 12 feet for h = 20 ft, spacing < 30 feet 25 feet TEK 10-2B NCMA/AIA CES Program

Control Joint Spacing Recommendations 1. Control joint spacings are based on the use of horizontal reinforcement having an equivalent area of no less than in2 / ft of height to keep unplanned cracks closed. The empirical provisions for control joint spacing include a minimum amount of horizontal reinforcement to help keep any cracks that may form between the control joints tightly closed. The amount reinforcement provided must be at least ).025 square inches per foot of vertical wall height. Sum of steel area height TEK 10-2B NCMA/AIA CES Program

Control Joint Spacing Recommendations 1. Control joint spacings are based on the use of horizontal reinforcement having an equivalent area of no less than in2 / ft of height. 9 gage joint reinforcement every other course (16 in.) The in2/ft of wall height amounts to 9 gage joint reinforcement every other course or the equivalent. TEK 10-2B NCMA/AIA CES Program

Control Joint Spacing Recommendations 1. Control joint spacings are based on the use of horizontal reinforcement having an equivalent area of no less than in2 / ft of height. Or, by use of…. #3 bars at 48 in. (4 ft.) #4 bars at 96 in. (8 ft.) #5 bars at 144 in. (12 ft.) The minimum amount of reinforcement can also be provided in bond beams with the minimum amounts of horizontal reinforcement at the maximum spacings indicated. Notice that crack control is achieved with spacings of reinforced bond beams up to 12 ft on center. This is particularly advantageous in reinforce masonry construction where bond beams are required at each floor and roof level. The best results are achieved when spreading the reinforcing steel out within the wall rather than concentrating heavy amounts in more widely spaced bond beams however. TEK 10-2B NCMA/AIA CES Program

The Role of Reinforcement in Controlling Shrinkage Cracking L Length after shrinkage Without joint reinforcement L Length after shrinkage Both of the figures shown indicate a concrete masonry wall after shrinkage has occurred. The length after shrinkage” shows the length that the wall would would be if the ends were not restrained and “DL” would be the amount that the walls did shrink. However, the ends of the walls are restrained and prevented from moving. In the upper figure, no reinforcement was provided resulting in a relatively side shrinkage crack in the wall after the tensile strength of the masonry was exceeded. In the lower figure joint reinforcement (horizontal reinforcement) was provided which held any shrinkage cracks tightly together so that the cracks are much less noticeable and so that the water resistance characteristics of the wall were not compromised. Notice that horizontal reinforcement does not prevent cracking but holds them tightly together so that they are relatively unnoticed and so that the water resistance properties of the wall are not compromised. With joint reinforcement TEK 10-2B NCMA/AIA CES Program

Joint Reinforcement Primary function - control wall cracking associated with shrinkage Secondary Functions metal tie system for bonding structural reinforcement where allowed by code Horizontal joint reinforcement adds tensile strength to the wall and reduces crack width. It does not prevent cracking but produces a series of microscopic cracks throughout the length of the wall rather than permitting one large crack. The reinforcing goes into tension as the crack forms and holds it tightly together. Joint reinforcement can also perform secondary functions such as serving as a metal tie system for bonding multi-wythe wall systems together and as structural reinforcement where allowed by code. At present, this is limited to shear wall reinforcement only by MSJC. TEK 12-2A NCMA/AIA CES Program

Joint Reinforcement Consists of two or more longitudinal wires connected with cross wires forming a truss or ladder configuration Ladder Type Truss Type There are two basic types of joint reinforcement available. Ladder type joint reinforcement has cross-wires spaced at 8 inch centers and are at 90 degrees to the side wires. This is so that the cross-wires will align with webs in standard concrete masonry and will not interfere with the flow of grout. It is the most common type of joint reinforcement used. The other type of joint reinforcement is the truss type. It is not recommended for use when any of the cells in masonry are to be grouted as the diagonal cross-wires restrict the flow of grout. TEK 12-2A NCMA/AIA CES Program

Joint Reinforcement Must be spliced a minimum of 6 inches per MSJC For continuity, joint reinforcement must be lapped at least 6 inches per the MSJC specifications. NCMA/AIA CES Program

Control Joints Sealant - polysulfide, urethane, silicone or epoxy. Avoid oil-based caulks or other materials that dry out rapidly or do not bond effectively to masonry. For all control joints, mortar should be raked out of the vertical joints on both sides of the panels. The mortar should be raked out at least 3/4 in. to allow for back-up rod and sealant but more importantly to assure a plane of weakness at the control joint. Mortar in the control joint may be totally omitted to ensure freedom of movement. Shape factor of the sealant and bond on only two sides is critical to ensure proper performance. If the sealant bonds to 3 surfaces, the back and 2 sides of the joint, potential failure is likely. Placing a backer rod in the joint prior to applying the sealant is suggested to eliminate the three way bonding and minimize the stresses in the sealant. Backer rod NCMA/AIA CES Program

Types of Control Joints with Shear Resistance Preformed gasket Grout shear key One of the most popular control joint systems uses the preformed gasket. This is usually rigid premolded neoprene used in conjunction with a “sash block”. The gasket provides the the key for load transfer between panels and keeps them in relative alignment. There is no bonding to the gasket and raked joints ensure bond is broken or reduced. The grout shear key (commonly called a Michigan joint) is the traditional method of forming a control joint.The connection necessary to key adjacent panels if formed by placing mortar, concrete or grout fill between the ears of the concrete masonry units. Bond across the joint is broken by building paper or other similar material. TEK 10-2B NCMA/AIA CES Program

Types of Control Joints with Shear Resistance Smooth dowel - sleeved or greased on one end Dowels Special unit shape Horizontal reinforcement discontinuous at joint (except for structural bond beams) With the exception of T-joints and bond beams serving as structural diaphragm chord members, no material should be continuous through a control joint. If continuous, the joint reinforcement will restrain the ends of each panel, reducing the effectiveness of the control joint. Control joints may also be formed by using special concrete masonry units. The male end fits within the ears of a conventional eared or special unit to provide the appropriate key between adjacent panels of concrete masonry. TEK 10-2B NCMA/AIA CES Program

Eliminating Control Joints With Reinforcement Control joints can be eliminated completely if enough horizontal reinforcement is provided. ie., where As > An Maximum reinforcement spacing for 8 in. CMU (not solid grouted): #4 bar - 24 inches #5 bar - 40 inches #6 bar - 48 inches In some regions of the country, significant amounts of horizontal reinforcement are required for structural purposes, I.e. high seismic regions. In these areas, cutting of the reinforcement at control joints is undesirable. Studies have shown that horizontal reinforcement in sufficient quantity (0.002 times the net cross-sectional area of the wall) can effectively limit crack width in concrete masonry walls. The net cross-sectional area of a partially grouted wall would be the area of the face shells plus the cross-sectional area of grout in bond beams. TEK 10-3 NCMA/AIA CES Program

4-hour Rated Control Joints Ceramic fire blanket Four hour fire rating in four hour rated concrete masonry walls can be achieved by inserting ceramic fire blanket (insulation) between the mortared head joints. Ceramic fire blanket is not required for fire ratings of three hours or less. TEK 10-2B NCMA/AIA CES Program

Control Joint Locations
Crack Control in Concrete Masonry Walls Control Joint Locations Typically use above grade in concrete masonry walls Shrinkage cracks are an aesthetic rather than a structural concern Generally not used below grade Less temperature and moisture variation Foundation walls traditionally do not include control joints since they are subjected to relatively constant temperature and moisture conditions. TEK 10-2B NCMA/AIA CES Program

Control Joint Locations
Crack Control in Concrete Masonry Walls Control Joint Locations Thus far we have discussed the use of control joints to control potential shrinkage cracking in straight runs of concrete masonry. However, point of stress concentration such as corners, window and door openings, columns, piers, changes in wall thickness and height need control joints as well. Notice that control joints should be placed so that distance from the corner to the control joint does not exceed one half the maximum spacing between control joints determined for straight runs of wall. TEK 10-2B NCMA/AIA CES Program

Control Joints at Openings Cracking will occur in the planes of greatest weakness. In unreinforced masonry openings are particularly vulnerable. Therefore, a control joint should be placed through the opening at one side. Notice that the joint goes around the lintel and allowance for movement between the lintel and the masonry must be provided. Control joints can be placed away from the opening if adequate tensile reinforcement is placed above, below, and beside wall openings. Openings less than 6 feet TEK 10-2B NCMA/AIA CES Program

Control Joints at Openings For openings more that 6 feet in unreinforced masonry, control joints through the opening are recommended on both sides of the opening. Openings more than 6 feet TEK 10-2B NCMA/AIA CES Program

Dissimilar Materials Bond-breaker as a slip plane + 1/4 control joint spacing each side of control joint When using dissimilar materials such as concrete masonry and clay masonry in the same wythe, allowance for the differential movement between the two must be provided for as the concrete masonry will tend to shrink and the clay masonry will tend to expand. The resulting difference in movement will result in cracking of the concrete masonry. One method of allowing the differential movement and minimizing cracking is to provide a slip plane between the two about one-fourth the length of the distance between the control joints. This will provide some amount of structural integrity as well as some freedom of movement. If the wythe containing the clay masonry is a veneer laterally supported by a structural wythe, the slip plane can be the full length of the wall. The joint in the clay masonry must be an expansion joint allowing for a closing of the joint as the clay masonry expands. TEK 10-1A NCMA/AIA CES Program

Control Joints for Multi-wythe Walls If wythes are unbonded... …consider control joint requirements separately for each wythe If wythes are bonded... Joints in multi-wythe walls and veneers should be aligned. Clay brick with concrete masonry backup cavity wall has an expansion joint in the outside brick wythe and a control joint in the interior wythe. Since most compressible fillers have a compressibility of 35 to 50%, the designer is cautioned that a 1/2 in. expansion joint will typically only provide for about 1/4 of expansion. …continue control joints from back-up through face TEK 10-1A NCMA/AIA CES Program

Impact of Mortar Strength Weak mortar The mortar used in the wall should be the weakest possible consistent with the structural requirements. The weaker mortar will result in more frequent, closer spaced but smaller cracks through the mortar joints that are more water resistant as opposed to that of the stronger mortar. The latter tends to result in cracking through the masonry units themselves and in wider, more distantly spaced cracks that are more likely to leak. Strong mortar NCMA/AIA CES Program

Summary Better manage movement through masonry systems by Proper design Using MSJC installation standards Proper location of control joints proper use of bond beams Proper placement of joint reinforcement By utilizing the material presented in this slide presentation and the accompanying NCMA TEK 10-2B movements and crack control can be better managed and accommodated. TEK 10-1A also provides a great deal of information on crack control for causes other than shrinkage. In summary the stages of a project that need attention are: (read list on slide) NCMA/AIA CES Program

CRACK CONTROL IN CONCRETE MASONRY WALLS

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