Concrete pros share ways to optimize project productivity
Construction industry professionals and observers have seen the coronavirus pandemic as a catalyst for change. However, if the insights from the American Concrete Institute’s Virtual Concrete Convention are any indication, the industry still has a long way to go to achieve the transformation in productivity seen in industries such as manufacturing. Many of the lessons learned during the convention seemed evergreen in a good way and a bad way. It is good in the sense that the construction industry has a collective wealth of experience dealing with persistent issues related to productivity. It is bad in the sense that a 2017 McKinsey analysis of construction’s productivity problem is still relevant in 2021.
The value of constructability
Cary Kopczynski, who will serve as president of ACI in 2021-2022, says the industry’s productivity narrative is “not all doom and gloom” thanks to the increased use of modular construction and prefabrication, but says concrete professionals should heed one of the seven recommendations in the 2017 report -- rethinking design.
Specifically, Kopczynski said, there is a need for a greater focus on constructability. Oscar Antommattei, chief concrete engineer and materials engineering manager at Kiewit, said early constructability considerations are important in fast-tracked projects. One example he noted was the high-profile SoFi Stadium project in Los Angeles. Kiewit was responsible for handling the excavation as well as footings and other structural components, including structural columns and a mechanically stabilized earth wall. According to Antommattei, there was no time to follow the typical design and shop-drawing review process, and the shop drawings were underway before the engineer had the final reinforcement design. Early collaboration between the construction manager, design engineer, the main contractor and subcontractors was critical.
Kopczynski noted that constructability does not refer to the well-executed completion and coordination of construction documents, but rather the effective integration of construction knowledge into the planning, layout, design and construction of a building. He offered one example of a project in Las Vegas that was demolished prematurely due to a failure to address constructability early in the design process. In a more positive example -- a building in Seattle -- after his team chose the formwork system, they determined that columns needed to be aligned with the transverse direction across the building. That decision needed to happen before determining the location of the columns.
Mike Schneider from Baker Concrete Construction also touched on the need to emphasize formwork early in the design process. He said that even though formwork is not a “tangible” part of a finished building, it can account for more than 50% of a site-cast concrete frame. He added that applying three basic principles of formwork economy - design repetition, dimensional standards and dimensional consistency - can help designers integrate constructability into a project. However, incorporating those principles does not mean asking a designer to assume the role of formwork planner, and it does not make the structural design a “slave to formwork considerations,” he said.
Matt D’Ambrosia, principal and co-founder of MJ2 Consulting, touched on formwork failure that occurs because of lateral fluid pressure. Improper formwork can lead to loss of life and costly litigation, according to D’Ambrosia.
He said some of the common pitfalls with formwork include vibration, admixture overdosing (particularly with fly ash) and disturbance of forms. Some of the ways to mitigate formwork pressure risk factors include selecting concrete with a fast buildup of static yield stress, placing concrete in lifts and using rheology to determine limits on placement heights and rates.
Another problem that can have deadly results is pumpability failure. Pumpability, according to D’Ambrosia, is based on viscosity and stability. High viscosity increases pressure and reduces flow. Unstable mixtures can cause blocking. Some of the common issues leading to pump difficulty include trapped air, segregation, aggregate volume, line design and admixture compatibility. One common solution is to add water and increase the slump, but D’Ambrosia says that is “absolutely the wrong direction to go” because if you push the slump too high, the material will start to segregate. He recommends staying away from high coarse aggregate content and unstable concrete mixes and moving toward things such as viscosity modifiers and optimized aggregate gradations to minimize demand for paste. He also recommends examining paste content carefully through ACI’s 211 and 304.2 standards, as well as ensuring mixture stability through ASTM C1712.
Contractors, material suppliers and specifiers must also consider a plethora of factors when planning, constructing and designing successful concrete repairs in hot weather. In particular, pre-packaged repair mortars require careful attention, said John Fauth, vice president of marketing and business development at ChemMasters. According to Fauth, fast-setting repair mortars may be more susceptible to plastic shrinkage cracks and reduced working time and finish quality. However, it depends on the depth of repair and the physical properties of the material. Thinner repairs may be more vulnerable amid high temperatures, while fast-setting materials generate increased exothermic temperatures.
Fauth added the industry needs to make greater use of initial curing methods such as fog-spraying. He cited the ACI 308R-16 Guide to External Curing of Concrete, which notes that initial curing methods can help when concrete “exhibits a tendency to bleed, when evaporative conditions are severe, or both.”
This is part 1 of a two-part recap from the ACI Virtual Concrete Convention. Part 2 will be online April 19.