The decline in the number of workers in the construction industry is a severe problem in Japan. Formwork is indispensable for concrete structures, but due to the shortage of carpenters, formwork assembly tends to be slow and can cause project delays. To deal with this problem, new workers are being hired and formwork carpenters are being trained, but the payoff is not immediate because the acquisition of the required skills takes several years of education and experience. The “formless construction method” that eschews
formwork, might be a possible solution (Fig. 1), substituting an outer shell formed with sprayed mortar. The
reinforced concrete structure is then created by placing
reinforcing bar and casting self-compacting concrete
(SCC) inside the outer shell.
In this approach, the lateral pressure of the fresh concrete stresses the outer shell during concrete casting, so
the shell must have high tensile strength. Since Japan is
an earthquake-prone country, structures are required to
have strong deformation performance and must resist
the large bending and compressive stresses generated during earthquakes. Furthermore, to reduce the life
cycle cost of the structure and increase its sustainability,
both high durability and maintenance-free design of the
structure are required. Thus, the outer shell must have
a high resistance to chloride ion penetration and other
aggressive exposures.
To satisfy the performance requirements, we decided
to use Ultra High Strength Fiber Reinforced Concrete
New Application Method of
Sprayed UHPFRC
By Satoru Kobayashi
Fig. 1: Concept of formless construction method
(UHPFRC) as the sprayed material. UHPFRC is a highstrength and high-ductility material with compressive
strength of 22,000 to 36,000 lb/in2
(150 to 250 MPa)
and tensile strength not lower than 1200 lb/in2
(8 MPa).
It is also characterized by a highly dense concrete matrix
with very low water and air permeability and thus high
chloride resistance. In Japan, UHPFRC has been used
mainly for factory produced precast products. Recently,
with the advent of mass manufacturing of precast members with large sections, the application of UHPFRC
to civil engineering structures has been increasing. The
largest such project to date is the application of UHPFRC for the floor slabs of Runway D at Tokyo International Airport with a UHPFRC volume of approx. 26,000
(20,000 m3
Thus far, UHPFR has rarely been used with sprayed
placement, and forming the outer shell of a structure
with sprayed UHPFRC is a novel challenge. This article
outlines the experimental method developed and used
to form the outer shell of a structure with sprayed
We focused on columns as the target structure. This
formless construction method requires a core material
that is easy to install and remove. Air tubes were adopted as the core material. Figure 2 shows the installation
of the air tubes. The thickness of the member was 16 x
16 in. (400 × 400 mm), and the thickness of UHPFRC
was 1.6 in. (40 mm). The height of the columns was 59
in. (1500 mm).
In this experiment, a mortar pump (squeeze type),
maximum discharge rate 8 yd3
/hr (100 L/min) and a
delivery hose with a diameter of 2 in. (55 mm) were used.
Fig. 2: Installation of air tubes Spring 2020 | Shotcrete 33
The diameter of the tip nozzle was
0.6 in. (15 mm). The sprayed material was required to stick on vertical
surfaces without sagging. To this
end, a non-alkali hardening accelerator was added at the nozzle.
Figure 3 shows the spraying of
UHPFRC. The material adhered to
the vertical surfaces of the air tubes
without sagging, and coverage of
the sprayed material to the required
thickness and height of 5 ft (1.5 m)
was achieved without problem.
Figure 4 shows the removal of
the air tubes that was easily accomplished.
As the next steps, reinforcing
bars will be set and SCC will be
cast inside the outer shell. The
structural performance will be evaluated by flexural strength testing.
Fig. 3: Spraying of UHPFRC on air tubes Fig. 4: Removal of the air tubes
Satoru Kobayashi
is a senior
for Kajima
Technical Research
Institute based
in Japan. He
graduated from
Hiroshima University where he studied
the durability of concrete. He is highly
skilled in concrete, for example, selfcompacting concrete, anti-washout
underwater concrete, dam concrete,
and shotcrete. Recently his research
project focuses on the new application
method of UHPFRC and various
ways to use it at the construction site
effectively in order to improve the
durability of the structure and the
productivity of the construction process.