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Loadbearing Masonry - Construction |
by
Richard C. Schumacher, P.E. |
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| Loadbearing concrete
masonry is a proven system that provides structural support and attractive
walls at an economical cost. Construction methods used are based on
simple, one-story structures, with some modifications depending on the
building height and location. An added benefit is the architectural system
may be part of the structural system.
Project Planning Coordination and planning are the keys to fast, quality construction at an economical cost. Prior to foundation construction, meetings held between the general contractor, mason contractor, architect, structural engineer and masonry supplier created a valuable understanding of the construction requirements. These meetings insure that the project starts on time and the transition between building trades is smooth. Before placing the foundation, the mason contractor and foundation contractor should coordinate vertical wall reinforcement. This will insure that dowels are placed at the location of the block cores. If this alignment is not accomplished, misplaced dowels may be bent at a 1:6 ratio from the intended position to match wall reinforcement. If bending is not possible, the block webs must be cut out. Masonry Units Since concrete masonry units are manufactured using automated equipment under controlled conditions in a factory, variations between units are minimized. Factory settings provide an opportunity to test units prior to construction. Concrete masonry unit strength is the major factor determining the strength of loadbearing masonry walls. Current manufacturing methods result in the production of concrete masonry units which are 2 - 3 times higher than the compressive strength specified by ASTM C90. This allows the use of different strength units for different levels of the building. The net compressive strength of masonry, f1m, can be determined by two methods: unit strength method and prism test method. Using the unit strength method, block are sampled from the construction site and tested in a laboratory in accordance with ASTM C140. The net unit strength is correlated on a table (Table 1) with the appropriate mortar type to determine the f1m of the masonry. Table 1 Compressive strength of masonry based on the compressive strength of concrete masonry units and type of mortar used in construction.
Following the prism strength method, a two-block high prism is constructed on site using full mortar bedding and the same mortar as used to lay block. The prism is then tested in accordance with ASTM C1314 in a laboratory to determine the f1m. (Note: It is advisable to build a prism on a wooden plank and strap another plank on top to prevent cracking the mortar joint during handling and transit.) Normally, 28-day strengths are required, but seven-day strengths are commonly permitted if a correlation can be shown by historical performance. If multiple strengths of concrete masonry units are used on the same project, it is advisable to identify the units or cubes of units. Proper labeling will insure that the correct strength units are used. Marking can entail the use of molded marks, color pigmentation or spray, stamps or as a minimum, marking individual cubes of block. Inspection Contract documents specify the level of inspection necessary which depends upon the facility function and its design method. The details of the minimum submissions and inspection are defined in Section 1.14 of the Masonry Standards Joint Committee (MSJC) code. Level I: applies to nonessential buildings designed under the empirical design method. Certificates that show the materials comply with the specifications are the only product submissions required. Level II: applies to essential buildings designed under the empirical design provisions and for nonessential buildings designed under other design provisions. In addition to certification of materials, verification of the net compressive strength is required prior to construction. Level III: applies to essential buildings designed under the design provisions. In addition to prior verification of f1m, the strengths should be verified every 5000 sq. ft. (464.6 m2) during construction. Tolerances The masonry elements must be built to the appropriate tolerance to insure the structural performance of the masonry structure. Section 3.3G and section 3.4 of the MSJC specifications contain the acceptable construction tolerances. Mortar The mortar type is specified by the designer. The field properties of the mortar should be compatible with the weather and masonry product properties. As an example, low water content and high initial absorption or stiff mortars make vertical alignment difficult. Likewise, fluid mortars make alignment difficult and prevent rapid unit placement and wall construction. Avoid mortars with higher cement contents than specified. High cement content mortars tend to have more water which can cause the mortar to become brittle and shrink resulting in cracks.
Since mortar is normally prepared on site, it has the largest potential
for variation. Consistency of mortar, through accurate control of
proportions, will insure a quality structure, control material costs and
possibly If available, the use of prepackaged mortar and silo mortar systems should be prepackaged mortar and silo mortar systems should be seriously considered to improve mortar consistency. The ability to produce mortar quicker using newer equipment and systems creates the increased possibility that mortar can begin its initial set. This mortar can be used if retempered on the mortarboard. This permits using the mortar 2 - 3 hours after mixing, depending on the temperature. Most basic structural units are available with either a smooth or architectural face (as shown in photo). Grout Grout is a cementitious mix of sand, cement and possibly fine aggregate. It is used to fill bond beams and cores containing reinforcing steel. Normally, only spaces containing steel are filled. The grout should be the same strength as the concrete masonry units. Concrete should not be used in place of grout. There are two types of grout defined in ASTM C476. The selection of the type of grout depends on the grout pour height and the minimum grout space requirements. Grout should be placed within 1-½ hours of mixing. When placed, grout should have a slump of 8" - 11" (200 mm - 280 mm) to insure complete filling of the desired voids. In contrast to concrete, excess water is beneficial to proper grouting. The excess water assists in complete filling and providing a good bond to the reinforcement. The masonry units absorb the excess water causing initial grout settlement which creates an ideal environment for curing grout. Dry grout is subject to bridging, especially if the walls of the entire grout cell are not smooth and continuous. Grout may either be pumped or placed with buckets. Grout pours must be consolidated by mechanical means or puddling. Pours more than 12" (300 mm) must also be reconsolidated after settlement has occurred; excessive vibration should be avoided. The top surface of a grout lift should be kept about 1-½" (38 mm) below beams flush with the top of the unit. If using an open bottom bond beam, it is desirable that the webs align with the webs of the unit below and mesh, screen or any other barrier is used to prevent grout from filling the cores. Reinforcement Reinforcement should be placed before grout is placed and not worked into the grout later. This insures proper location and embedment. Provision should be made to provide the proper length for lapping future reinforcement in the walls above. Prior to grouting the last lift of a wall and filling the bond beam, reinforcement should be placed to provide a continuous connection to the floor system. Depending on the grout type, 1/4" or 1/2" (6 mm or 13 mm) spacing should be maintained between the reinforcement and the masonry unit. When the masonry wall is exposed to the weather, 1-½" (38 mm) of cover is required between the reinforcement and the exposed surface. Section 1.14 of the MSJC code provides detailed information on placement tolerances. Execution Concrete masonry units with the correct geometry should be selected. If the project requires full mortar bedding, use units with webs that align when laid in the specified bond. In running bond, units laid must lap the underlying units by 1/4" - 1/2" (6 mm - 13 mm) of the unit length. If vertical steel and grout are used, the cell containing grout must be sealed from adjacent cells by a mortar dam or mortar on the cross webs. Care should be taken to construct the grout cell as vertical as possible with minimum projections into the grout core or changes in vertical grout face. Mortar joints must be maintained as close to 3/8" (10 mm) for both strength and appearance. Joint reinforcement in the mortar must be lapped as a crack control measure to insure structural integrity and performance. If high lift grouting will be used, clean-out holes on the face of the wall should be provided for the cores to be inspected. Cutouts can be used for plain block, but face shells should be removed to permit inspection and cleaning. The openings should be filled prior to grouting. Weather Cold: In cold weather construction, there are two main concerns: preventing the mortar from freezing when it is saturated and protecting the wall during the time immediately after construction. These provisions usually require special care for raw materials and protect the wall during and after construction. Heating may be required. The National Concrete Masonry Association’s TEK 3-1A provides detailed information on the degree of protection for different conditions. Hot: In hot weather construction, the temperature of the mortar should be kept below 120o F (38o C). Smaller batches of mortar may be used to decrease the length of time between mixing and laying units. In hot weather, it is advisable to use cool water to prolong mortar board life. To prevent mortar from drying out after units are laid, the wall should be protected or covered if winds are present. Precipitation: Completed walls should be protected from rain until the mortar joints and grout have been satisfactorily cured. This will insure consistent color and appearance. Summary Constructing economical, high quality mid and high rise loadbearing masonry structures is similar to constructing ordinary structures. However, multi-story projects require additional planning and other provisions may be made. Proven performance of concrete masonry units for mid and high rise buildings has led to a steady increase in use. References:
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lower labor costs. It will also allow masons to lay block faster
and more accurately.