239 / 2022-06-15 13:37:05

Rheological research and applications of ultra-high-performance concrete

UHPC

Ultra high performance concrete (UHPC) is a pioneer concrete, which occupies remarkably mechanical properties, especially a tensile strength above 7 MPa with significant remaining post-cracking bearing capacity. However, the introduction of an extremely low W/B and high content of varying ultrafine powders, superplasticizer and steel fiber makes UHPC have higher viscosity and longer mixing time, compared to ordinary concrete. Both exert difficulties in on-site pouring of UHPC. In this paper, the effects of superplasticizer and ultra-fine powders on rheological properties of UHPC are investigated and analyzed. The packing density and viscosity of interstitial solution between particles are found to be the two important parameters.



At a low W/B ratio of paste of bellowing 0.16, it is found that packing density and the water film thickness cannot be improved by further increasing superplasticizer dosage above a certain value of 0.4% (Fig.1 and Fig. 2). This will result in a very small interparticle spacing of particles in paste. Besides, a large dosage of residual superplasticizer is found to be existed in paste solutions and entangled within the small interparticle spacing (about 35~40nm) (Fig.3). Both make UHPC increase viscosity rapidly. Moreover, ultrafine powder with a rounder shape and a suitable diameter can reduce the pulping time of paste more significantly (Fig.4). Meanwhile, to improve particle dense packing and releases entrapped water in agglomeration by selecting the satisfied shape and size of ultrafine powders, a low viscosity and high flow rate of paste can be obtained (Fig. 5 and Fig. 6).



Basically, two solutions are proposed for tailoring the rheological of UHPC. One is the mix optimization design to optimize particle packing and release entrapped water in agglomeration, by introducing satisfied ultrafine powder combination. The other is the specialized SP design, with features of longer side chain, enhanced adsorption capacity and excellent compatibility with various surfaces, to disperse particles better with a lower dosage, which should be beneficial for enhancing the particle separation and lowering the viscosity of interstitial solution.



With the help of the proposed solutions, two excellent applications are employed. One is the Fifth Nanjing Yangtze River Bridge, a cable-stayed bridge of three towers with a streamlined steel-UHPC composite beam, where the UHPC is prepared with a slump flow of 420 mm and the ingredients are regulated to be distributed uniformly by adjusting a satisfied viscosity and yield stress (Fig. 7). The other is the UHPC pavement of two steel bridges on the Ning-Hang Highway, where the UHPC is successfully tailored with a good self-consolidating property during pumping and a highly thixotropic property after pumping (Fig. 8).