Strain-Based Approach for Measuring Structural Build-Up of Cement-Pastes in the Context of Digital Construction (2018-11)¶

As a first step toward characterising structural build-up of high-strength, printable concrete mixes, the structural build-up of cement pastes of varying compositions and rheological properties is investigated. It is demonstrated that applying low shear rates over short measurement periods does not always result in the achievement of flow-onset in stiffer cementitious materials, commonly used in digital construction. For such materials, a characteristic delay exists before the effective shear rate reaches the applied shear rate. This leads to effective strains in the materials tested and consequently to the erroneous characterisation of structural build-up. A strain-based approach is suggested here as a more appropriate method for characterising structural build-up in the case of stiff materials. Maintaining a low, constant shear rate among various measurements is not necessary if the total effective strain is kept constant. Investigations on pastes with different compositions show that pastes in which a portion of the cement was replaced with micro-silica and fly ash exhibited high structuration rate. The use of secondary cementitious materials (SCM) appears to be similarly appropriate measure to make cementitious materials ‘printable’ when compared to using set-accelerators, in the absence of inline mixing of accelerators in the printhead.