Autoclaving of ultra‐high performance concrete (UHPC)

Abstract

By the combination of an optimized granulometry, a reduced water-cement ratio and the use of superplasticizers ultra-high performance concrete achieves a compressive strength of over 150 N/mm 2 and a high resistance regarding acids and water-soluble salts. In different research projects the effect of an autoclavation on the mechanical strength and the phase composition was analyzed. In systematic studies, the concrete mix and the process parameters were varied.The results show that autoclavation leads to an improved pozzolanic and hydraulic reaction and significantly improves the mechanical strength compared to a conventional thermal treatment. If a minimum time-span for hydration is ensured, the achievable strength level is not dependent on the prestorage time. However, the duration of the autoclaving is significant. After the maximum strength is reached there is only a very slight decrease, even if unrealistically long autoclaving times are applied. K E Y W O R D Sheat treatment, hydroxylellestadite, tobermorite, ultra-high performance concrete, zonation UHPC [4].Due to the better controlled conditions, precast plants are well suited for producing structural members, facade panels, and other building components from UHPC. Like for prefabricated members from ordinary concrete, thermal treatment accelerates hardening and reduces shrinkage [5]. Aim of several research projects at the Federal Institute for Materials Research and Testing (BAM) was to study how the mix design and the process parameters of thermal and hydrothermal treatment affect the performance in terms of strength and durability but also in terms of economic and ecological efficiency. In the following sections, some results of this work are presented.

Selbstvorspannende ultrahochfeste Betonelemente

“…In werden Ergebnisse vorgestellt, die eine nochmalige Erhöhung der Druckfestigkeit bei einer weiteren Temperatursteigerung auf 200 °C belegen. In wurde verglichen, wie sich eine Wärmebehandlung in einem Klimaschrank zu einer Wärmebehandlung in einem Autoklaven verhält. In beiden Fällen konnte auch die Druckfestigkeit durch die Temperaturbehandlung gesteigert werden, was auf eine Verdichtung der Mikrostruktur im thermisch nachbehandelten UHPC zurückzuführen ist.…”

“…In wurde verglichen, wie sich eine Wärmebehandlung in einem Klimaschrank zu einer Wärmebehandlung in einem Autoklaven verhält. In beiden Fällen konnte auch die Druckfestigkeit durch die Temperaturbehandlung gesteigert werden, was auf eine Verdichtung der Mikrostruktur im thermisch nachbehandelten UHPC zurückzuführen ist. Grundsätzlich ist eine Wärmenachbehandlung bis etwa 400 °C empfohlen, wobei sich ab rund 200 °C festigkeitssteigernde kristalline CSH‐Phasen, vorwiegend Xonotlith (C 6 S 6 H), ausbilden .…”

UHPC–AAC facade elements: Structural design, production technology, and mechanical behavior

“…The first one involved casting the AAC slurry onto a prefabricated UHPC layer and combined autoclaving of UHPC and AAC after the aluminum‐driven expansion process of the latter. The advantages of autoclaving UHPC are reported elsewhere . The second approach was based on gluing prefabricated AAC blocks (60 cm × 40 cm × 18 cm) onto the UHPC surface by using cement‐based mortar as an adhesive.…”

“…Procedure for two-step production of box-shaped UHPC elements: (a) cast of exterior layer; (b) placement of a rigid frame as internal formwork on hardened exterior layer; and (c) cast of upturning edges4 PRODUCTION TECHNOLOGYThe first trials based on a one-step production (simultaneous manufacturing of UHPC exterior layer and upturning edges with cast-in insulation) were not successful due to the high values of buoyancy reached by a rigid AAC block acting as a floating body. The advantages of autoclaving UHPC are reported elsewhere[13][14][15]. One day after the cast of the exterior layer, the upturning edges were cast.…”

Elephant skin formation on UHPC surface: Effects of climatic condition and blast furnace slag content

“…Some reasons behind this scant usage of field-cast UHPC can be listed as follows: uncontrolled climatic conditions (sun, wind, hot weather, etc. ), extreme shrinkage values [3], lack of knowledge about the rheological properties which are very important during transporting and casting stages [4], its curing sensitivity comprising high hydration heat [5], and lack of standardization [6]. On the other hand, UHPC has not been widely used in engineering practice due to its high cost and environmental impact [7].…”