Strength Testing and Analytical Modeling of Reinforced Autoclaved Aerated Concrete Lintels By: Elizabeth Hungerford Faculty Advisor: Dr. Jennifer Tanner Supporting programs: EPSCoR & Honors Hi, I am Elizabeth Hungerford. Today I will be presenting my work on strength testing and modeling the behavior of reinforced autoclaved aerated concrete lintels. Dr. Tanner with the Civil Engineering department was my advisor.

Autoclaved Aerated Concrete What is AAC? Lightweight, cellular Made of cement, sand, lime, water, and expansive agents Steam cured Autoclaved at 150 psi AAC is a lightweight, cellular, and precast building material. It is made of many of the same ingredients as normal concrete, but it includes an expansive agent (aluminum powder). When steam cured at 320F and kept in an autoclave at 150 psi, the expansive agents creates air voids throughout the material. AAC is 80% air. This gives it the low density and a unit weight of 40 pcf (this compares to the unit weight of normal weight concrete of 150 pcf).

Autoclaved Aerated Concrete High-quality building material Fire resistance Low-density High thermal efficiency Acoustic damping AAC was introduced to the US markets as a building material in the 1990’s. AAC has many advantages. An 8” thick AAC masonry wall has a four-hour fire rating, but its actual performance exceeds requirements for up to 8 hours. As mentioned before – AAC is lightweight. This makes construction with it less taxing. Its lightweight also reduces seismic forces in a building because the overall weight of the structured is lessened. AAC has high thermal efficiency, so it doubles as an insulating material. AAC is manufactured into blocks or panels that are precisely dimensioned, and can be customized for specific projects. During construction, the units are joined with thin-set mortar. The units are used in walls, floors, and roofs. The blocks are easy to cut, making runs for conduit and small-diameter piping easy to create. Because of its porous nature, AAC must be covered in a finish material such as stucco, stone, or siding to prevent moisture from compromising it.

Masonry Lintels Lintels horizontally span wall openings (such as doors and windows) Carry load from the wall above and distribute it across the opening Standard grouted U-block construction with longitudinal steel Lintel Grout Core Reinforcing Steel AAC U-Block Masonry lintels are horizontally span wall openings like doorways and windows. They transfer the load from the wall above to either side of the opening and down the wall. I used standard u-block construction with longitudinal reinforcing.

Previous Research at UW Nathan Stroud Strength testing of AAC lintels Evaluated strength contribution of AAC blocks Nathan Stroud, under Dr. Tanner’s advisement, constructed and tested 12 lintels of various length. He evaluated the strength properties of the beams and the contribution that the AAC has.

Research Goals Additional construction Testing Predict behavior 1, 6ft beam 1, 8ft beam Testing Four-point bending Predict behavior Displacement Design In my research, I was tasked with constructing and testing two additional beams. Once testing was complete, it was the goal of the project to evaluate the behavior of all 14 beams. Using moment curvature analysis, I attempted to predict the displacement behavior of the beams. This research only analyzed the strength of the grout, the contribution from the AAC was neglected.

Construction Construction of lintels Thin-set mortar Grout mixing AAC U-block preparation Curing I constructed two lintels (a 6ft long beam and an 8ft long beam. The units of AAC were joined with thin-set mortar, and allowed at least 24 hours to cure. The longitudinal steel was placed in the lintels with rebar stands. Each beam I constructed used 2 #3 bars. I prepared the grout using fine aggregate, sand, cement, and water – proportioned by the volumetric method. The grout was placed in the lintels and allowed 28 days to cure. Pictures: Nathan Stroud

Testing Four point bending Force applied and displacement measured Hydraulic Actuator Neoprene Pad Spherical Seat Loading Rods and Plate CMU Support Coupled Steel Rod and Plate AAC Lintel After the 28 day curing period, each beam was tested in four point bending. This method is used to eliminate shearing forces in the middle of the beam. The hydraulic actuator puts pressure on the loading plate and rods, and the beam displaces under this force. The displacement in the center of the beam was recorded during the testing using a linear potentiometer. The 6ft beam was displaced 4 inches, while the 8ft beam was displaced 8 inches. Stroud, Nathan, and Jennifer Tanner. Strength Testing of Reinforced Autoclaved Aerated Concrete Lintels.

Testing Masonry class demonstration The testing took place during the Design of Masonry Structures lab as a class demonstration. I oversaw the testing, and the members of the class recorded the displacements as the pressure was applied to the beam.

Testing Basic concrete beam behavior Initial cracking Loss of grout Reinforcing steel During the testing, as the beam is displaced, first, flexural cracks form. These form because the beam is experiencing tension along the bottom. The cracks continue to expand throughout the test. Cracking indicates that the aac and grout are experience tensile forces. Once the grout is in tension, the load is considered to be taken by the reinforcing steel because grout has virtually no tensile strength. As shown on the right hand photo, the grout and AAC is completely missing in the middle section of the beam because of the expansion of the cracks. The grout near the top of the beam experiences compressive forces because the beam is bending up. The compressive forces of the grout are an important part in the strength of the beam.

Companion Material Testing Grout Prisms Replicate water absorption Tested for compressive strength Reinforcing Steel Strain gauges Tested for tensile strength Grade 50 and 60 AAC blocks Grout prism When the grout was mixed and cast into the lintel beams, grout prisms were also cast. The grout was placed in a group of AAC blocks and allowed to cure for the same amount of time as the lintels. The prisms are used to test the strength of the grout and are cast into the groups of AAC block to mimic the water absorption that the grout in the actual lintels experiences. The reinforcing steel in the beams was also tested for tensile strength. Strain gauges are super glued to the rebar, and it is tested in tension by pulling it apart until it completely fails. The graph shows the stress strain relationship of the rebar. Initially in testing, the rebar is elastic. Meaning if the load was removed, the rebar would return to its original length unharmed. After the linearly elastic section of testing, the steel enters its yielding phase (this means that permanent damage has occurred). The yield plateau follows and then the steel strain hardens, which increases its strength just before it fails completely Both of these materials’ strengths are used in calculating the strength of the lintels.

Analysis Force displacement data This graph shows the force displacement relationship during testing of my two beams. The displacement of the beam shown on the bottom axis, is a result of the force applied to the beam (shown on the left axis) The relationship is linear for the early portion in both beams because the steel remains linearly elastic. Ideally, the linear section would lead to a yield plateau, and a strain hardening (just like the stress strain relationship seen in the rebar testing results). Because both beams lose strength as they are displaced further, and there is no yield plateau, this indicates that the steel did not fully yield or enter the strain hardening stress state.

Analysis Reinforcing steel yields Strain hardening Yield plateau Linear elastic This is a force displacement behavior graph of one of Nathan's beam. This beam was reinforced with 1 #3 bar. This allowed the steel to reach its full potential, yielding and strain hardening.

Modeling Moment curvature analysis Predict displacement Assumptions Yield and Ultimate Grade 50 and 60 steel Assumptions Reinforcing steel yields Plastic hinge length Using moment curvature analysis, displacement of the beams was predicted at yield and ultimate strain in the steel. The displacement is assumed to be linear until yielding so it is just a function of the P, the force applied, the Length, the Modulus of Elasticity, and the cracking moment of inertia of the beam. The ultimate displacement is a function of the displacement at yield plus the difference between the yield curvature and the ultimate curvature multiplied by a proportion of the length of the beam. Displacements were calculated for only beams made that contained 1 #3 reinforcing bar. The steel yielded in these cases, and that is a key assumption of the calculations used to model the moment-curvature behavior. Predictions of displacements with beams containing Grade 50 and Grade 60 were done because a mix-up occurred in Nathan’s rebar specimen testing, and once the beams were cast, the Grade of steel being used in the beams was not known. The plastic hinge length was varied during the calculations within reasonable percentages of the length of the beam based on concrete beam analysis. 𝜟= 𝑷 𝑳 𝟑 𝟑𝑬 𝑰 𝒄𝒓 + (𝜱 𝒖𝒍𝒕 − 𝜱 𝒚 ) 𝒍 𝒑 L

Results 𝜟= 𝑷 𝑳 𝟑 𝟑𝑬 𝑰 𝒄𝒓 + (𝜱 𝒖𝒍𝒕 − 𝜱 𝒚 ) 𝒍 𝒑 L This graph shows the actual force displacement behavior of a beam. Also shown are the predicted displacements for a beam reinforced with Grade 50 and/or Grade 60 steel. The yield and ultimate displacements are shown. The squares show the predicted displacement for a beam made with Grade 50 reinforcing steel, and the triangles show the predicted behavior of a beam made with Grade 60 steel. The yield displacement for a beam with Grade 60 is greater than that for a beam with Grade 50 rebar because the yield strength of Grade 60 is higher than the yield strength of Grade 50 steel. The ultimate displacement of a beam made with Grade 60 steel however is less than that for Grade 50 steel because the ultimate strength of the Grade 60 steel is much larger, and the strength of the steel crushes the grout in compression, making the beam less strong ultimately. In practice and design, the amount of steel must be balanced with the size of the beam to avoid over powering the concrete or grout. 𝜟= 𝑷 𝑳 𝟑 𝟑𝑬 𝑰 𝒄𝒓 + (𝜱 𝒖𝒍𝒕 − 𝜱 𝒚 ) 𝒍 𝒑 L

Results 𝜟= 𝑷 𝑳 𝟑 𝟑𝑬 𝑰 𝒄𝒓 + (𝜱 𝒖𝒍𝒕 − 𝜱 𝒚 ) 𝒍 𝒑 L This graph shows the predicted displacements for another beam. It is clear here, that the Grade of reinforcing steel used was 60. The predicted yield displacement is much closer to actual behavior with the analysis done using Grade 60 steel. 𝜟= 𝑷 𝑳 𝟑 𝟑𝑬 𝑰 𝒄𝒓 + (𝜱 𝒖𝒍𝒕 − 𝜱 𝒚 ) 𝒍 𝒑 L

Results 𝜟= 𝑷 𝑳 𝟑 𝟑𝑬 𝑰 𝒄𝒓 + (𝜱 𝒖𝒍𝒕 − 𝜱 𝒚 ) 𝒍 𝒑 L This graph illustrates the predicted behavior for the final beam that was reinforced with 1 #3 bar. In this case, it is not as clear which Grade of steel was used, but Grade 60 seems to fit the actual behavior better than Grade 50. 𝜟= 𝑷 𝑳 𝟑 𝟑𝑬 𝑰 𝒄𝒓 + (𝜱 𝒖𝒍𝒕 − 𝜱 𝒚 ) 𝒍 𝒑 L

Conclusions Plastic Hinge Length Reinforcing Steel Varied between 10% and 20% Reinforcing Steel Grade 60 Actual Capacity vs Predicted Capacity Greater actual capacity Neglected AAC contribution For all the predicted displacements, the plastic hinge length was varied between 10 and 20% to most accurately predict the behavior of the lintels. It is clear in at least one of the beams that the reinforcing steel used was Grade 60. Overall, these results show that the actual capacity of the beams exceeded the predicted capacity. This could be explained through neglecting the contribution of strength from the AAC. This model, though, is an accurate and conservative way to predict the behavior of reinforced masonry lintels.

Thanks NSF EPSCoR Dr. Jennifer Tanner Eisenhauer I would like to thank NSF EPSCoR for funding this project. My advisor, Dr. Jennifer Tanner And the CEAS Shop

Questions?