Accelerated sprayed concrete
Sprayed concrete, also known as shotcrete, is a special type
of concrete conveyed under pressure through a pneumatic hose or pipe and
projected into place at high velocity, thereby combining the placement and
compacting of the material in only one process. Compared with concrete without
accelerator (normal concrete), accelerated sprayed concrete has a shorter final
setting time and higher early-age compressive strength. Thus, sprayed concrete
is widely used in many applications, such as tunnel primary supports, roadways,
foundation pit slop supports and bridge reinforcement structures.
The microstructure of concrete is affected by raw material
composition, hydration processing and age, and the mechanical and durability
properties of concrete depend on the microstructure of concrete. Researchers
have recently studied the hydration of cement paste with different
compositions. Lv et al. employed environmental scanning electron microscopy
(SEM) to continuously observe Portland cement hydration and elucidate the
mechanism of this process. Andrew et al.
and Rikard et al. analysed the mechanical properties and early hydration
physical chemistry of Portland cement paste using X-ray diffraction (XRD),
infrared spectroscopy, SEM and Vicat techniques or ultrasound reflection. Sun
et al. and Jiang et al. applied XRD, thermal analysis, SEM and atomic force
microscopy to study the relationship between fly ash (FA) amount and bond
water, between microstructure and mechanical properties. Ou et al. analysed cement paste hydration and change
regulation using thermal analysis. Zeng et al.
applied mercury intrusion porosimetry and nitrogen adsorption to analyse
the relationship between the pore structure of cement pastes and water–binder
(w/b) ratio, FA amount, and curing age. Hamlin
built a microstructural model of a calcium silicon hydration (CSH) gel
through theoretical analysis.
Despite the availability of information on hydration and
cement paste, however, few studies have specifically focused on the hydration
of accelerated cement pastes and sprayed concrete. Zhang and Fang investigated a cement–accelerator–water
system and found that cement is accelerated by Al2O3 and Ca(OH)2 while NaAlO2
catalyses the reaction to form ettringite (AFt) and secondary gypsum. Liu et
al. reported that C3S hydration is
promoted by alkali from an accelerator to form CSH gel. High hydration rates
require large amounts of free water combination and reductions in the fluidity
of cement slurry. To date, however, the hydration process of accelerated cement
and the mechanical properties of sprayed concrete remain unclear.
Civil Scholar is a Blog by Aravind Samala on Construction, Environment, Technology, Trends and Innovation.