1 De Haro
1 De Haro is a 170,000 SF four-story office building in San Francisco’s Design District and is comprised of three levels of mass timber construction (cross laminated timber [CLT] and glulam framing]) over one level of concrete. While its upper three levels are for office usage, the ground-level concrete podium is for light industrial work. The milestone project holds multiple titles, including the first CLT building in the City of San Francisco and the first multi-story mass timber building in California. The structure was topped out in February 2020 and the building was ready for occupancy in Summer 2021.
Mass Timber and Concrete Focus
Due to poor existing soil conditions the project team needed to reduce the weight of the structure, so they explored the option of mass timber. Mass timber also provided a sustainable component to the project as it equates to roughly 2,000-metric tons of stored carbon (in CO2e).
By utilizing mass timber framing above the concrete podium in lieu of an all-concrete structure, the team was able to significantly reduce the weight of the structure and eliminate a third of the pile foundations. The reduction of foundations not only saved cost and labor, but also decreased the embodied carbon associated with the building due to the concrete and rebar materials no longer required for the removed piles.
For the structural elements that required concrete to satisfy strength and durability requirements, the design team took strides to tailor their specifications to facilitate the procurement of low carbon concrete. DCI extended test ages from 28 days to 56 and 90 days for elements that would not experience full design loads until much later in construction or affect schedule. In addition, the exclusion of overall supplementary cementitious material limits allowed the concrete supplier freedom to minimize cement content and associated embodied carbon while still meeting the necessary performance criteria.
During procurement, DCI received two different sets of concrete mix submittals for the cast-in-place concrete portion of the building:
Vendor A: mid-grade set with 15-30% cement replacement.
Vendor B: high-grade set with 70% cement replacement in the foundations, plus 20-50% for all other applications. All high-grade mixes also contained the carbon injection admixture, CarbonCure.
Both vendors utilized high strength Orca Aggregates and provided Environmental Product Declarations (EPDs) for their mixes.
To learn more about the impacts of design decisions and determine the most effective strategies to reduce the embodied carbon concrete structures on future projects, DCI performed a retroactive life cycle assessment (LCA) to compare the differences between the two concrete mix submittals. The initial LCA was completed using Tally, which yielded approximately 10% overall cradle-to-gate GWP reduction of the entire structure for both Vendor A and B when compared to the NRMCA Pacific Southwest Baseline.
Although Tally was correctly incorporating the compressive strength and cement replacement content, the preset mix options were not considering the additional reductions associated with extended test ages, high-strength aggregates, and specialty admixtures. When directly associating the concrete material quantities with their respective EPDs, we found the GWP associated with all mixes were overestimated by Tally. The mixtures from Vendor A were relatively close with a 6% difference, but the mixtures from Vendor B was actually 25% lower than what Tally suggested. See Figure 1.
Once the concrete material impacts were updated, the associated embodied carbon for the entire structure was reduced by 15% with the use of Vendor B’s concrete mixes when compared to the NRMCA Pacific Southwest Baseline. See Figure 2. Because Tally’s estimation of Vendor A’s mixes closely aligned with the EPD values, the overall embodied carbon reduction for the structure was still approximately 10% when utilizing this set for all concrete applications and compared to the same baseline.
However, perhaps the greatest impact on the overall embodied carbon associated with the building is the use of wood from sustainably managed forests. When assuming the requirements associated with including biogenic impacts are fully realized, the project was able to save approximately 2,400 metric tons of carbon in comparison to the 350 metric tons of carbon savings associated with the use of low-carbon concrete mix designs. See Figure 3.
Table 1. Overall Tally results of Concrete vs Timber structure including all life cycle stages (A-D) and different considerations of biogenic carbon*.
*Refer to Tally and GaBi documentation for more information on differences between calculations including and excluding biogenic carbon.
Figure 1. Tally v EPD Concrete Embodied
Carbon Analysis (Stages A1-A3).
Figure 2. Overall Embodied Carbon
Comparison (Stages A1-A3).
Figure 3. Overall Embodied Carbon Comparison
(Stages A1-A3) including stored carbon.