Self Compacting Concrete

1. Self-Compacting Concrete
Er. Binay Charan Shrestha
SDE, DUDBC
The term Self-Compacting Concrete (SCC) refers to a special type of concrete mixture, characterized
by high resistance to segregation that can be cast without compaction or vibration. This type of high-
performance concrete has been in use only since the late 1980s. The first prototype was developed in
Japan in 1988 as a response to the growing problems associated with concrete durability and the high
demand for skilled workers.
The self-compacting behavior is achieved by adjusting the aggregate content and using a combination
of chemical and mineral admixtures. These admixtures, typically consists of polycarboxylate-based
high-range water-reducing (HRWR, or super-plasticizing) and viscosity-modifying admixtures. Filler
materials are often used for replacing some of the aggregates and modifying the viscosity. High doses
of super-plasticizer produce mix with high fluidity and allow for a reduced water-powder ratio. For the
purposes of SCC, powder can be described as particles smaller than 0.25mm, including some fine
aggregate, cement, fly ash, limestone powder, blast furnace slag, and silica fume. Without a viscosity-
modifying admixture, the mixture would tend to segregate. However, the viscosity-modifying
admixture enhances the viscosity of the mixture, which reduces aggregate segregation, settlement, and
bleeding.
The basic compositions of the self-compacting concrete are essentially similar to the ordinary concrete
except that the proportion of ingredients is varied to improve the workability of SCC. Main
constituents of the SCC include Binder(Cement & fine powder), Water, fine and course aggregates
and Chemical Admixtures. As observed from Figure 1, Self-Compacting Concrete differs from
ordinary concrete mainly with respect to the increase in amount of finer materials and reduction of
course aggregates.
Figure 1 Comparison between SCC and Conventional Concrete.
Before any SCC is produced at a concrete plant and used at a construction site, the mix has to be
designed and tested. A self-compacting concrete mixes must meet three key properties:
1. Ability to flow into and completely fill intricate and complex forms under its own weight.
2. Ability to pass through and bond to congested reinforcement under its own weight.
3. High resistance to aggregate segregation.

2. To achieve these performances, the following three different aspects need to be accomplished:
1. Reduction of the coarse aggregate content in order to reduce the friction, or the frequency of
collisions between them, increasing the overall concrete fluidity.
2. Increasing the paste content to further increase fluidity.
3. Managing the paste viscosity to reduce the risk of aggregate blocking when the concrete flows
through obstacles.
The main characteristics of SCC are the properties in the fresh state. SCC mix design is focused on the
ability to flow under its own weight without vibration, the ability to flow through heavily congested
reinforcement under its own weight, and the ability to obtain homogeneity without segregation of
aggregates. The SCC is characterized by being highly flowable and stable with the following
properties:
o Slump flow > 600 mm
o Remain flowable ≥ 90 minutes
o Withstand a slope of 3%
o Pumpable ≥ 90 minutes through pipes ≥ 100 m long
Laboratory and field tests have demonstrated that the SCC hardened properties are similar to those of
HPC.
 SCC compressive strengths are comparable to those of conventional vibrated concrete made
with similar mix proportions and water/cement ratio.
 Tensile strengths and the ratios of tensile and compressive strengths are in the same order of
magnitude as the conventional vibrated concrete.
 Bond strengths are higher than those of conventional concrete.
 Permeability is lesser than similar conventional concrete.
Several test methods have already been developed worldwide to characterize the performance of a
fresh SCC. Among the various test methods only the common tests used for evaluating the compacting
characteristics of fresh SCC are described below.

3. i) Slump Flow Test using the traditional slump cone, is the most common testing method for
flowability. The basic equipment is the same as for the conventional slump test. When the
slump cone has been lifted and the sample has collapsed, the diameter of the spread is
measured (Figure 2). The spread can range from 455 mm to 810 mm.
(a) (b)
Figure 2 Slump Flow Test
ii) Funnel Test: is done to evaluate the material segregation resistance of SCC and viscocity,
using a V-shaped funnel. SCC is allowed to flow through an orifice under its own weight,
and the time taken to drain the content is measured. Usual value of flow time is 8-15
seconds.
(a) (b)
Figure 3 Funnel Test
iii) T50 Test is another test method for evaluating the material segregation resistance of SCC,
whereby the time for the flow to reach 500 mm is measured in the slump flow test.

4. iv) U-Type and L-Type Tests are performed to determine the self-compactability or filling
ability and passing ability of the SCC. These two tests simulate the casting process by
forcing an SCC sample to flow through obstacles under a static pressure (Figure ). Once
the tube is filled with fresh SCC, the sliding door is opened to let flow the concrete to other
chamber. The final height H and H2/H1 for the U-box and the L-box respectively are
recorded. They provide indication on the static and dynamic segregation resistance of an
SCC as well as its ability to flow through reinforcements. They are frequently used in the
field as acceptance test methods. Concrete with the filling height of over 300 mm can be
judged as self-compacting. Acceptable value of the passing ability ratio, H2/H1, is
normally 0.80 - 0.85 but values as low as 0.60 have sometimes been shown to give
acceptable results in the actual structure.
Figure 4 U & L Type Test
The structural design requirements of SCC are no different from those for standard concrete. Existing
codes and standards can be used when designing products using SCC. However, it has been argued
that the fluidity of the mix may increase the hydrostatic pressure on the formwork.
The benefits of using SCC are grouped into two catergories: financial and engineering benefit.
Financial Benefits
1. Labor savings.
2. Placed at a faster rate with no mechanical vibration, resulting in savings in placement costs.
3. Shorter construction periods.
4. Reduced energy consumption from vibration.
5. Reduced wear and tear on forms from vibration.
6. Reduced wear on mixers due to reduced shearing action.
7. Elimination of vibrator noise.
Engineering Benefits
1. Improved architectural surface finish with little to no remedial surface work.
2. Ease of filling restricted sections and hard-to-reach areas. Opportunities to create structural and
architectural shapes and surface finishes not achievable with conventional concrete.

5. 3. Improved consolidation around reinforcement and bond with reinforcement.
4. Improved pumpability.
5. Improved uniformity of in-place concrete by eliminating variable operator-related effort of
consolidation.
6. Increased jobsite safety by eliminating the need for consolidation.
7. Reduced permeability.
8. Superior strength and durability.
SCC does have limitations to consider. One of the most significant limitations concerns the apparent
lack of established reliable test standards that can quantify the physical properties of SCC. In order for
SCC to be accurately specified and to assure quality, uniform standards must exist that can be
accepted and used by those in the industry.