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Non-Invasive
Imaging of Distribution of Coarse Aggregate in Hardened States Concrete Using
Advanced Gamma Ray Computed Tomography

As
the key composition of the concrete, the homogeneous distribution of aggregate
particles and its resistance to segregation during the casting process of
concrete are critical for the development of adequate mechanical properties and
durability of self-consolidating concrete. Ensuring homogenous distribution is
dependent on the rheology (i.e., viscosity and yield stress) of the suspending
material. Achieving proper rheological properties and uniformity of these
highly fluid mixtures can foster more efficient and safe structural
applications and designs. To establish relationship between rheological
properties and aggregate distribution in hardened concrete is therefore
important.

This
work has been done using gamma-ray computed tomography (CT) technique, as shown
in Figure 1, to measure the cross-sectional distribution at selected levels of
aggregate particles segregation (by adjusting rheological properties of
suspending mortar), and mortar mixtures. This technique has a strong
penetration power and can be implemented in the field using the industrial
methods for concrete pouring and casting. In this technique, the Cesium-137
(initially of 250 mci) is used as collimated sources with a set of 15 NaI(TI)
scintillation detectors in front of this source. The source Cs-137 and detectors
are rotated by stepping motors in 360ᵒ in small steps around the object
and can be moved at various levels.

There
are three different types of concrete sample, no segregate, highly segregate,
and totally segregate, by using a different dosage of high range water reducer
to adjust rheological properties of suspending mortar. In other hand, there are
three levels of scans for each sample to track the percentage of the volume
fraction for the aggregate to quantify the distribution of aggregate in hardened
concrete. 

By
using the Alternating Minimization algorithm (AM), has used extensively to
process gamma ray CT data to image aggregate distribution in highly segregate
sample as given in the Figure 2 and show that the linear attenuation for the
mortar and the aggregate close comparing with air in the blue edges of the
figure. Also, The distribution of the aggregate was calculated as shown in the
Figure 3, the red color represents the aggregate in the concrete sample and the
blue color no aggregate in this spot, therefore, we can find the distribution
of the aggregate with different heights of the same sample.

Figure 1 Gamma Ray Computed Tomography

Keywords: Aggregate, Concrete,
gamma-ray
computed tomography (CT).