Masonry Edge the storypole Vol7 No1 : Page 12

Photo by Elizabeth Young Embodied Energy of Concrete Masonry S Say Goodbye to f' m = 1500 psi by David T Biggs, PE, SE, Dist M ASCE, HTMS Table 2 -Compressive strength of masonry based on the compressive strength of concrete masonry units and type of mortar used in construction Net area compressive strength of masonry, psi 1 (MPa) 1,350 (9.31) ustainability has risen to the top of the concerns for every industry. Masonry experts have done an excellent job promoting the sustainable characteristics of masonry. As structural engineers, we try to incorporate sustainability into our designs. However, that usually means specifying alternate materials for aggregates, supple mentary cementitious materials (SCM) and additives. Personally, I have felt that options to increase the sustainability of my projects have been limited. However, structural engineers have the ability to make the greatest impact on sustainability by harnessing the embodied energy of concrete masonry. We need to use masonry as eﬃciently as we can with our designs. Net area compressive strength of concrete masonry units, psi (MPa) Type M or S mortar — 1,900 2,800 3,750 4,800 1 Type N mortar 1,900 (13.10) (14.82) (21.03) (27.92) (36.20) become the method most engineers seem to use, due to its simplicity and time savings. (13.10) (19.31) (25.86) (33.10) 2,150 3,050 4,050 5,250 1,500 (10.34) 2,000 (13.79) 2,500 (17.24) 3,000 (20.69) For units of less than 4” (102mm) height, 85% of the values listed. Table 2 from TMS 602 illustrates the minimum strength for CMU allowed by ASTM C90, 1900 psi, combined with Type S mortar results in an f' m of 1500 psi. Unit Strength Method The Unit The defacto design strength used by many engineers is f' m = 1500 psi. However, in many regions of the country, 1500 psi is an extremely conservative value based upon the concrete masonry units (CMU) available. Generally, f' m equal to or greater than 2000 psi is easily achievable because higher strength units are the norm. Using masonry more eﬃciently will enhance the economic advantage of reinforced masonry and improve sustainability. Strength method is recognized by the masonry standard, Speciﬁcation for Masonry Structures (TMS 602) 1 as one of two valid methods to verify the compressive strength of masonry. The Prism Test method involves a mason contractor constructing a prism, two CMU high with one mortar joint, then having the prism crushed by a testing ﬁrm. Both are acceptable for new construction. Unit Strength has How did we arrive at f' m 1500 psi as the defacto standard? That comes from the Unit Strength method. ASTM C90, Standard Speciﬁcation for Loadbearing Concrete Masonry Units 2 , requires that CMU have a minimum strength of 1900 psi. Also, Type S mortar is commonly speciﬁed for structural masonry. The Unit Strength table was derived from the results of over 329 prism tests (Figure SC-2) and was developed using outdated ASTM test methods to be overly conservative. It is not uncommon to perform prism tests in the ﬁeld only to the ﬁnd the actual f' m is 25% to 30% higher than was obtained from the Unit Strength method. The National Concrete Masonry Association recognizes the conser -vatism in the Unit Strength method and has embarked on a research project to perform current testing procedures on the population of prisms and reassess Table 2. Initial results indicate that the f' m can be increased while remaining conservative. However, until those results are published and accepted, we are left with the current Table 2 results. Table 2 (TMS 602) illustrates the Unit Strength method for concrete masonry. This method requires the engineer to specify the minimum net area compres sive strength of the masonry units and specify the type of mortar. From these two pieces of information, we arrive at a net compressive strength of the masonry ( f' m ). Somehow, f' m = 1500 psi became the standard for design. However, it’s a conservative value. Using Table 2 with the Unit Strength method, the f' m is 1500 psi. Greater Strength CMU Using Table 2, the f' m for 3000 psi units and Type S mortar would be 2105 psi (from linear inter po la tion). Compared to f' m = 1500 psi, that's more than a 40% strength increase that goes unrealized. Compared to the structural steel industry, that's like designing for A36 (F y 36 ksi) steel, but actually getting V50 (F y 50 ksi) material. The unrealized capacity in that case is about 38%. Most engineers would not let that capacity slip away with steel, but are doing just that with masonry. Concrete masonry manufacturers throughout the US often produce to a higher standard than the minimum compressive strength listed in ASTM C90 (please verify speciﬁcs with your CMU manufacturer). CMU with a minimum strength of 3000 psi are common in many regions, especially in cold climate areas. Thus, the base price for many units already includes higher strength masonry. The higher strength is often used by manufacturers to provide a better mix for their molding process and to provide greater durability. So, while engineers may be specifying and designing with a minimum of 1900 psi units, they are likely getting units with a far higher strength on their projects. Are you one of them? 12 MASONRY EDG E / thestorypole Vol 7 No 1 Masonry Technology | Innovation masonryedge.com

Embodied Energy of Concrete Masonry

David T Biggs

Sustainability has risen to the top of the concerns for every industry. Masonry experts have done an excellent job promoting the sustainable characteristics of masonry. As structural engineers, we try to incorporate sustainability into our designs. However, that usually means specifying alternate materials for aggregates, supple mentary cementitious materials (SCM) and additives. Personally, I have felt that options to increase the sustainability of my projects have been limited. However, structural engineers have the ability to make the greatest impact on sustainability by harnessing the embodied energy of concrete masonry. We need to use masonry as efficiently as we can with our designs. The defacto design strength used by many engineers is f'm = 1500 psi. However, in many regions of the country, 1500 psi is an extremely conservative value based upon the concrete masonry units (CMU) available. Generally, f'm equal to or greater than 2000 psi is easily achievable because higher strength units are the norm. Using masonry more efficiently will enhance the economic advantage of reinforced masonry and improve sustainability. Unit Strength Method The Unit Strength method is recognized by the masonry standard, Specification for Masonry Structures (TMS 602)1 as one of two valid methods to verify the compressive strength of masonry. The Prism Test method involves a mason contractor constructing a prism, two CMU high with one mortar joint, then having the prism crushed by a testing firm. Both are acceptable for new construction. Unit Strength has become the method most engineers seem to use, due to its simplicity and time savings. Table 2 (TMS 602) illustrates the Unit Strength method for concrete masonry. This method requires the engineer to specify the minimum net area compressive strength of the masonry units and specify the type of mortar. From these two pieces of information, we arrive at a net compressive strength of the masonry (f'm). The Unit Strength table was derived from the results of over 329 prism tests (Figure SC-2) and was developed using outdated ASTM test methods to be overly conservative. It is not uncommon to perform prism tests in the field only to the find the actual f'm is 25% to 30% higher than was obtained from the Unit Strength method. The National Concrete Masonry Association recognizes the conservatism in the Unit Strength method and has embarked on a research project to perform current testing procedures on the population of prisms and reassess Table 2. Initial results indicate that the f'm can be increased while remaining conservative. However, until those results are published and accepted, we are left with the current Table 2 results. How did we arrive at f'm 1500 psi as the defacto standard? That comes from the Unit Strength method. ASTM C90, Standard Specification for Loadbearing Concrete Masonry Units2, requires that CMU have a minimum strength of 1900 psi. Also, Type S mortar is commonly specified for structural masonry. Using Table 2 with the Unit Strength method, the f'm is 1500 psi. Somehow, f'm = 1500 psi became the standard for design. However, it’s a conservative value. Greater Strength CMU Concrete masonry manufacturers throughout the US often produce to a higher standard than the minimum compressive strength listed in ASTM C90 (please verify specifics with your CMU manufacturer). CMU with a minimum strength of 3000 psi are common in many regions, especially in cold climate areas. Thus, the base price for many units already includes higher strength masonry. The higher strength is often used by manufacturers to provide a better mix for their molding process and to provide greater durability. So, while engineers may be specifying and designing with a minimum of 1900 psi units, they are likely getting units with a far higher strength on their projects. Are you one of them? Using Table 2, the f'm for 3000 psi units and Type S mortar would be 2105 psi (from linear inter po la tion). Compared to f'm = 1500 psi, that's more than a 40% strength increase that goes unrealized. Compared to the structural steel industry, that's like designing for A36 (Fy 36 ksi) steel, but actually getting V50 (Fy 50 ksi) material. The unrealized capacity in that case is about 38%. Most engineers would not let that capacity slip away with steel, but are doing just that with masonry. In several states, such as New York and Iowa, 2800 psi CMU are common and the industry in those states promotes f'm = 2000 psi. This has created a greater awareness among engineers of higher strength units that can be produced. No longer do masonry knowledgeable engineers rely on f'm = 1500 psi. They have learned to first ask the manufacturers what they produce. Benefits from Increasing Strength The benefits from increasing the f'm should be obvious: sustainability and hidden economy in every design. Let’s look at a few of the ways. • Less grout required for partially grouted walls. • Less reinforcement required for all walls. • Reinforcement lap lengths reduced. • Embedded anchors with greater capacities. Several example designs illustrate potential for construction economy. The following examples are based upon Allowable Stress Design (ASD) methods and were prepared using the NCMA wall design software3 and TMS 4024. The CMU has a density of 115 pcf. These design examples illustrate that increasing f'm will provide greater economy because it will: • Increase axial and flexural wall capacity. • Increase capacity of shear walls. • Decrease lap lengths for reinforcement splices. • Increase flexural and shear capacity of masonry beams, columns and pilasters. • Increase the stiffness of masonry elements by increasing the modulus of elasticity. • Increase tension and shear capacities of embedments. • Reduce reinforcement required. In many low seismic zones, typical spacing for vertical bars is 48" oc. That comes from 6t and 8" CMU (6 x 8=48"). Engineers need to take advantage of 6t with 12" CMU (6x12=72"). The examples also indicate that simply increasing designs to a minimum of f'm = 2000 psi can improve the economy of masonry projects and thereby use the embodied energy of the masonry more fully. Sustainability. Good Economics Whether you justify it as sustainability or good economics, the embodied energy of CMU needs to be captured along with the economy that comes with it. Clearly, increasing your f'm to 2000 psi, as a design minimum, will improve capacities for the same amount of material, decrease required amount of wall grouting, and reduce the amount of reinforcement in partially grouted walls. Isn’t that sustainability? CMU manufacturers should publish their mini - mum unit strength, the higher strengths they offer and the results of prism tests using their units. Engineers should demand this information. Engineers may even find through prism test results that a minimum f'm greater than 2000 psi is available regionally; all without an increase in unit cost. It’s been there all the time and not used. Let’s say goodbye to f'm = 1500 psi and switch to f'm = 2000 psi as a minimum to make those designs more economical. It’s one of the most truly sustainable decisions an engineer can make.

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