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Civil Engineering Magazine THE MAGAZINE OF THE AMERICAN SOCIETY OF CIVIL ENGINEERS

Concrete Test Blocks Top ‘Living Building’

By Catherine A. Cardno, Ph.D

Over the next few decades, researchers will monitor the performance of 200 innovative concrete blocks used to construct wall sections atop the University of Cambridge's new high-tech James Dyson Building.

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The University of Cambridge, England, will soon open a high-tech “living building” with sensors that will monitor the building’s behavior from the rooftop to the foundation piles. Monitors will be included in blocks that were handmade from innovative concrete compositions by the university’s department of engineering and used to create internal walls in the building’s rooftop mechanical room. Antonis Kanellopoulos

December 15, 2015—The University of Cambridge's Department of Engineering has a new high-tech addition to its facilities: the James Dyson Building. Made possible by a donation from the James Dyson Foundation, which in headquartered in Malmesbury, England, the structure has been dubbed a "Living Building" because in addition to providing additional space for the department, it will also be used as a full-scale field test for the department's latest innovations. From its rooftop to its foundation piles, the building has been equipped with an array of sensors to monitor all aspects of the building's structural behavior for decades to come.

Included in this testing are 200 full-size, lightweight concrete blocks that were created to test the performance of a wide range of industrial by-products and innovative concrete compositions. The handmade concrete blocks have been used to build nonstructural wall sections in a covered portion of the building's rooftop plant room. 

Of the 200 blocks being tested, half are composed of self-healing concrete mixes. The remaining blocks use a variety of concrete types, including those made without Portland cement (also called clinkerless), those that include industrial waste, and those that are designed to remain free of cracks, according to Abir Al-Tabbaa, Ph.D., CEng, FICE, a professor of civil and environmental engineering at the University of Cambridge in England, who wrote in response to written questions posed by Civil Engineering online. The rooftop project is being overseen by Al-Tabbaa, and the materials being tested were generated by her many current researchers.

Each particular mix has been used to create approximately four or six blocks, according to Al-Tabbaa. "The self-healing blocks include a number of self-healing systems and mechanisms including microcapsules, glass tubes containing a healing agent, powder expansive minerals, and coated, self-healing mineral pressed pellets," she explained. "The crack-free concrete uses reactive magnesia as an expansive additive, the clinkerless cement concrete is based on alkali-activated materials, and green activators and the waste-based concrete uses industrial by-products."

The most innovative concrete being tested is the self-healing one that incorporates the microcapsules, Al-Tabbaa explained. "We are the first group to develop the type of microcapsules used," she said. "These are [roughly] 200 microns and contain sodium silicate as a healing agent. The main novelty is that the microcapsule shells are initially 'rubbery' to survive the aggressive concrete mixing stage, but once the concrete sets they polymerize, forming a thin, rigid, and fragile shell that easily fractures upon crack initiation." 

In addition to the variations in the mixes themselves, some of the self-healing concrete blocks have incorporated corroded steel bars so that the response of the self-healing microcapsules to the corrosive environment can be assessed. "While the developed microcapsules are designed to be triggered on the initiation of a crack, we are currently working on developing microcapsules that will deal with corrosion so the microcapsule shell will dissolve or disintegrate in the presence of chloride ions or CO 2 ," Al-Tabbaa said.

The focus of the concrete research is on enhancing the sustainability, performance, durability, and longevity of concrete as a material. Thus the in-situ monitoring of the blocks will include measuring their strength, durability, volume stability, and air permeability. The 400 by 215 by 100 mm blocks have been placed with both control blocks and commercially available lightweight blocks for comparison purposes, according to Al-Tabbaa. "This provides field validation on-site so generations of students will be able to observe these blocks perform [in] real time and on-site," she said.  

In addition to monitoring the aging process that is typical for concrete, the project will also assess severe exposure conditions that will be locally imposed on selected blocks to compare their behavior with the control blocks, according to Al-Tabbaa. Damage scenarios will be introduced to the self-healing blocks so that their self-repair processes and crack closures can be monitored over time with ultrasonic measurements and microscopic analyses.

The wall sections have been designed such that individual blocks can be removed and replaced, if that proves necessary. In addition, "there is no concern about the structural integrity of the wall…the cracks expected or damage imposed will be too small to compromise the integrity of the wall," Al-Tabbaa noted. 

As a whole, the building will also boast what the university refers to as a "central nervous system." Fiber-optic strain sensors and fiber Bragg grating (FBG) sensors located throughout the building's concrete frame, columns, beams, slabs, and piles will enable the health and behavior of the building's structural elements to be monitored. The sensors will monitor the temperature of the foundation's piles and the setting of concrete during construction, as well as load changes in the structural elements over the course of construction and over the building's life span. The sensors can be interrogated at any time to create a "strain profile," according to the university. This profile will indicate how loads are being transferred through different structural elements and down to the piles.

The James Dyson Building is expected to be completed in January. The monitoring and testing of the concrete block walls within the building's plant room will begin almost immediately.

Al-Tabbaa and her team are already looking toward the future of their self-healing concrete: efforts to scale-up the production of the microcapsules to commercial levels are already under way. A large-scale field trial of retaining walls made with this concrete mix is also under way, in Wales, facilitated by the Maidenhead, United Kingdom-based contractor Costain and developed with academic partners from Cardiff University and the University of Bath.

"From both [the James Dyson Building and the Wales] trials, collectively we hope to validate the self-healing performance of the concrete and provide details on the process and time scale of the healing under field conditions," Al-Tabbaa explained. The expectation is that the results of the two full-scale field trials will show that the impermeability of such concrete can be successfully restored, and this, it is hoped, will speed up the commercialization of the self-healing concrete.


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