Assuring the Long-Term Performance of Additively Constructed Structures

Summary

The use of 3D Concrete Printing (3DCP) is growing, along with the complexity of the structures that are being built with the technology. The availability of reliable test methods for assuring performance, however, is lagging, and becoming a barrier to adoption into building codes, which can slow industry growth. The problem is that many of the pre-existing test methods for conventional concrete construction were developed with an implicit assumption of the materials and construction methods used, and might not be directly applicable to 3DCP materials and methods.

Given the lack of experience with specifying requirements for 3DCP and the lack of evidence in the applicability of existing testing methods, the building code community is developing the practice of requiring contractors to print companion walls that are to be cut up into smaller specimens for testing, which can consume additional time and resources, resulting in more effort than is required to adequately assure the structural performance of the constructed object.

This project will identify and address current measurement needs by developing mechanical and durability test methods that adequately quantify performance for 3DCP, and will develop sampling strategies that achieve a balance of time, materials, and assurances of performance. To obviate the need for printing companion walls, real-time material property measurements will be developed for identifying whether material properties have deviated from prescribed minimum values. Moreover, the testing will address the material properties that are used to assure both structural and durability performance. Ultimately, the project will develop the fundamental measurement science and new standardized test methods that will form the foundation for reliable building codes that enable AC industry growth.

Description

Objective
Develop the metrology, best practices, and standardized test methods that are needed to develop building codes that can ensure adequate structural performance throughout the intended service lifetime of AC structures.

Technical Idea
[Focus on the Layer Interface]
These problems can be solved by developing the needed test methods and sampling strategies for structures built using AC. Moreover, even when addressing durability, test methods are needed for both the mechanical and durability properties because only those degradation processes that affect the structural properties are a concern. Moreover, because the layer interface is most affected by exposure to the weather, it is the “weakest link” from the perspective of both mechanical properties and durability properties. Therefore, the primary focus should be the characterization and testing of the layer interface properties.

[Sampling for Oriented Defects]
Sampling strategies of companion walls are needed for identifying how many specimens, and from where, to cut from the wall. This requires knowledge about the interplay between variations in layer interface properties and the overall performance of the printed structure. Even if a layer interface property is far below the target minimum value, if the length of the deficiency is very short, it is likely the overall structural performance is sufficient. Another contributing factor is the density of these deficiencies. The structural performance might become insufficient only when the lineal density of layer interface properties reaches a critical value. Therefore, a sampling plan must take these factors into account when developing a strategy plan.

[Exploit ongoing Flexural Testing]
There is ongoing work to develop a standardized flexural test method for printed walls. Completing this work would benefit durability research because the flexural test concentrates the tensile stresses near a single layer interface. Moreover, the testing procedure involves cutting specimens from a printed wall, so it will inform improved strategies for sampling plans that can estimate the likelihood of critical defects in the constructed wall.

[Combine FEM, Petrography, and Stereology]
These improved sampling plans could be constructed from a combination of structural analysis, mechanical testing, and petrographic analysis. Structural analysis of layered walls could be combined with stereology to estimate the size and density of critical defects required to fail. Mechanical and petrographic testing could be combined to measure the width of the layer interface and to measure the layer properties. Information learned from the mechanical and petrographic tests could inform the development of specific durability tests for AC structures. As for conventional concrete construction, the test methods would be specific to a degradation mechanism (e.g., freeze-thaw, corrosion of steel reinforcement).

[Need for Lab Scale Printing]
Eventually, a means of attacking these problems effectively and efficiently is to study the effects of the materials selection and construction process on properties the laboratory. This allows for control of processes and environmental exposure. This is the accepted practice used in conventional concrete construction whereby laboratory cast cubes and cylinders are measured for the purpose of estimating the expected properties of the concrete when cast in the field. Presently, there is no commonly accepted equivalence for studying AC materials. Moreover, validation by comparison of laboratory specimens to field AC materials is complicated by the inherent variability in properties in field AC.

[Using the ACE Consortium]
The Additive Construction by Extrusion (ACE) consortium was developed to help ensure rapid development of standards and promote industry collaboration through research, education, and engagement. The consortium meets approximately 7 times a year, and during a Summer Workshop at NIST. There are currently 6 active Task Groups (TG) with one additional that is in review. These TGs explore independently, ways to foster innovation, develop testing methods and carry out data evaluation, contribute to a properties database, participate and contribute towards current and future standards and design guidance, and provide education. During FY26, consortium members will be engaged in the flexural testing method development, ASTM test method promulgation, and the subsequent interlaboratory study (ILS) to develop a Precision and Bias statement for the method.

[Why NIST?]
Providing the underpinning to reliable codes and standards should overcome barriers to specifying and accepting novel AC structures, which should help grow the industry. The advances would leverage NIST unique expertise in metrology and concrete materials, from both a structural and durability perspective. It also leverages our engagement with industry through the ACE consortium. Furthermore, the long-term solution will require a measure of persistent commitment to this work.

Research Plan
The research plan involves four steps:

[Mechanical Test]
The first step is to complete the ongoing development of a standardized flexural test method for AC walls, and to also ensure there is a precision and bias statement. In existing code requirements of AC practice, acceptance is based on mechanical tests of specimens cut from a printed wall. For broad industry adoption, this needs to be an approved standardized test method. In addition, to characterize the variability among values taken across a wall, the uncertainty in each specimen measurement is needed.

Therefore, the standardized test method needs a precision and bias statement. This work has been underway, in collaboration with members of the ACE consortium, and will be completed in FY26. The industry has identified flexural testing as an immediate critical need for evaluating AC structures. In collaboration with ACE consortium members, a test has been proposed, based on 4-point bending tests on small sections cut from a printed wall, that is a candidate for an ASTM standard test method. The method attempts to measure the strength of the layer interface. The work is underway, and in FY26 the last pair of walls will be printed and tested, and the results published in a peer-reviewed journal article, which will form the bases for an ASTM standard test method. The ASTM test method will be balloted, and upon approval, an inter-laboratory study (ILS) will be coordinated with ACE consortium members, and conducted in the remainder of FY26. The test method will provide an industry-accepted practice for characterizing relevant mechanical properties of the layer interface in hardened AC materials.

[Wall Sampling Strategy]
The Second step is to determine the requirements of a companion wall sampling plan. This requires considering the length scale of defects, the orientation of the defects, and a spatial sampling plan that reduces the likelihood of missing a critical deviations. The aforementioned ASTM test will provide an industry accepted method of characterizing the layer interface in the hardened material. The experimental design will be developed in close collaboration with SED because there are simultaneous questions of “how many” and “where” to sample, given that the material properties can be changing along the printed filament. This, in turn, will be informed by FEM analysis to determine critical defect lengths and critical numbers of defects.

It is anticipated that multiple mechanical properties of the layer interface will be needed to assess long-term durability. Additional test methods will need to be developed that can probe the properties at the interface, and these will be informed by petrographic analysis of cross sections. Microstructure and hardness will serve to help characterize the width of the interface, which will inform mechanical test method development.

[Field Measurement Development]
The third step is to develop in-situ field measurements for characterizing mechanical and durability properties. It is possible that reducing the probability of missing a critical defect would result in an onerous testing plan. One viable alternative is to have in-situ measurements, providing immediate feedback on variations in specified properties. The in-situ measurements do not need to be direct measures of specific properties, but the variability in these measurements must be relatable to the variability in the results from tests on hardened specimens, for the purpose of detecting “defects”.

[Industry Challenge Prioritization]
The fourth step is identifying near-term and long-term AC industry needs for materials characterization to support construction, structural performance, and long-term durability. From these, identify ongoing work, possible gaps, unique NIST roles, and possible impacts if successful. The outcome will inform FY27 project development.