A Review on the Behavior of Ultra-High-Performance Concrete (UHPC) Under Long-Term Loads

This paper provides a research review regarding the creep of ultra-high-performance concrete with or without the addition of fibers. Unlike other similar studies that mainly considered influential factors and their effects on the creep behavior, this research focuses more attention on the analysis of UHPC creep models. For the creep strain assessments of these concretes, the creep models given in the latest standards cannot be used, but it is necessary to modify them to give reliable results, given the rather complex composition of UHPC. Several proposed creep models for UHPC are presented with comparative analysis. The observation is that by varying key parameters such as compressive strength, relative humidity, cross-sectional dimensions, and temperature, there may be major discrepancies between models, so additional experimental investigations are necessary to perform their calibration. In this paper, the parameters α1, α2, and γ of FIB Model Code 2010 have been modified in order to obtain a match with other proposed models in terms of the final value of the creep coefficient and the creep curve. The creep coefficient of the UHPC decreases when steel fiber content increases, but it is important to consider the excessive fiber addition because very often it causes an increase in creep strain. The application of thermal treatment at a temperature of 90 °C for 48 h significantly improves the time-dependent properties of UHPC. An analysis of the impact of the steel fiber content, fiber type, thermal treatment, and the age of the concrete under load on strains of UHPC specimens and beams under long-term loads is performed.

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