Optimising construction with self-compacting concrete

Abstract

Self-compacting concrete or self-consolidating concrete (as it is known in North America) (SCC) is used on the basis of its unique properties of flowability, passability and resistance to segregation. It requires no external energy to achieve full compaction, so is advantageous on site, but there is evidence that its higher cost is a significant barrier to greater adoption. The research entailed work measurement of 14 UK single-family home residential projects (eliciting data on construction time and labour productivity) and cost modelling of three slab scenarios (exploring the relationship between material and labour costs). The study found SCC was placed up to 73% faster than conventional concrete and, when labour and material costs are included, the supplier is able to price SCC to closely match conventional concrete, hence making SCC more viable for the contractor. This relationship between as-built costs for SCC and conventional concrete is clarified by developing Pmax, providing a new mechanism for understanding project profitability and viability of SCC.

Full text

Optimising construction with
self-compacting concrete
David Rich MEng
Research Engineer, Centre for Innovative and Collaborative Construction
Engineering, School of Civil and Building Engineering, Loughborough
University, UK
Jacqueline Glass BA, PhD, DipArch, CertHE, DipBRS, FHEA
Professor of Architecture and Sustainable Construction, School of Civil and
Building Engineering, Loughborough University, UK
Alistair G. F. Gibb BSc, PhD, CEng, MICE, MCIOB
Professor of Construction Engineering Management, School of Civil and
Building Engineering, Loughborough University, UK
Christopher I. Goodier BEng, PhD, PGCert, MCIOB, MICT, FHEA
Lecturer in Structures and Materials, School of Civil and Building
Engineering, Loughborough University, UK
Graham Sander BSc, PhD
Professor of Hydrology, School of Civil and Building Engineering,
Loughborough University, UK
Self-compacting concrete or self-consolidating concrete (as it is known in North America) (SCC) is used on the
basis of its unique properties of flowability, passability and resistance to segregation. It requires no external
energy to achieve full compaction, so is advantageous on site, but there is evidence that its higher cost is a
significant barrier to greater adoption. The research entailed work measurement of 14 UK single-family home
residential projects (eliciting data on construction time and labour productivity) and cost modelling of three
slab scenarios (exploring the relationship between material and labour costs). The study found SCC was placed
up to 73% faster than conventional concrete and, when labour and material costs are included, the supplier is
able to price SCC to closely match conventional concrete, hence making SCC more viable for the contractor. This
relationship between as-built costs for SCC and conventional concrete is clarified by developing P
max,
providing a
new mechanism for understanding project profitability and viability of SCC.
Notation
aslab area
C superscript indicating conventional
concrete construction
dslab depth (m)
eadministrative costs to employ labour
Llabour cost (£)
Mmaterial cost (£)
O
V
overhead cost (£)
O
V
C
overhead cost for conventional concrete
construction (£)
O
V
S
overhead cost for self-compacting concrete
construction (£)
P
C
conventional concrete price (£/m
3
)
P
max
maximum permissible premium (£/m
3
)
P
S
self-compacting concrete price (£/m
3
)
pconcrete price (£/m
3
)
rrate of worker pay (£/min)
Ssuperscript indicating self-compacting
concrete construction
T
PT2
total pour time per m
2
T
PL2
total labour time per m
2
ttime to place 1 m
2
of slab
1. Introduction
Self-consolidating concrete, also called self-compacting
concrete (SCC), has been available in Japan, North America
and Europe for over 20 years. It is used in many mainstream
construction projects because of its flowing nature and early
maturity; it is generally considered to be an innovative material
that is more expensive than conventional concrete mixes.
Higher initial material cost seems to be the most significant
barrier to greater adoption (Concrete Society and BRE, 2005;
Rich et al., 2010, 2012), but its placement is claimed to be
faster and more reliable than conventional concrete (Khayat
et al., 2001; Okamura and Ouchi, 2003). Therefore, directly
comparing SCC with conventional mixes, simply on the basis
of material cost, is inappropriate and inaccurate.
Most extant SCC research focuses primarily on understanding
and optimising physical and structural properties; with little
examination of its effect on commercial out-turn measures,
such as the construction cost (including materials, labour and
plant). This paper reports on research into SCC and conven-
tional concreting methods in 14 UK single-family home pro-
jects. Work measurement and cost modelling through scenario
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Construction Materials
Volume 170 Issue CM2
Optimising construction with self-
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Rich, Glass, Gibb, Goodier and Sander
Proceedings of the Institution of Civil Engineers
Construction Materials 170 April 2017 Issue CM2
Pages 104–114 http://dx.doi.org/10.1680/coma.14.00025
Paper 1400025
Received 30/05/2014 Accepted 19/03/2015
Published online 12/05/2015
Keywords: buildings, structures & design/slabs & plates
ICE Publishing: All rights reserved
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analysis captured data on material costs, placement rates,
workers’activities and plant/truck movements, to identify any
significant time and cost differences between SCC and conven-
tional concrete.
2. Commercial status of SCC in construction
2.1 Physical properties and claimed benefits of SCC
SCC requires no external energy to achieve full compaction
and is characterised by four distinct properties: resistance to
segregation, flowing ability, passing ability and early strength
gain (Goodier, 2003; Skarendahl and Billberg, 2006). An
extensive, global research base on SCC’s structural and physical
performance aspects has built on earlier Japanese research
(e.g. De Schutter et al., 2008; Khayat, 1999; Okamura and
Ouchi, 2003; Shobha et al., 2006) and accepted test methods
and standards/codes of practice have emerged (e.g. ASTM
and Rilem). This is essential to encourage use and ensure the
consistency and durability of an emerging technology. SCC
also facilitates more complex structures owing to its ability
to flow, fill formwork and compact without external input
(Goodier, 2003; Grimes, 2005). Reports suggest improved
quality and homogeneity, resulting in improved durability, and
resistance to degradation (Goodier, 2003; Henderson, 2000;
Skarendahl and Billberg, 2006). Furthermore, virtual elimin-
ation of surface defects, such as blow-holes and honeycombing
(Gaimster and Foord, 2000; Goodier, 2003) is claimed through
the removal of the compaction operation, which is dependent
on operative ability (Concrete Society and BRE, 2005; Holton,
2003), and should reduce remedial work (Gaimster and Foord,
2000; Grimes, 2005). SCC enables the contractor to use less
site equipment (Damtoft et al., 2008) such as vibrating tools
that are synonymous with hand–arm vibration syndrome
and the generation of noise, which can present a hazard to oper-
atives and a disturbance to local communities (Bartos and
Cechura, 2001; Skarendahl and Rilem, 2003; Walraven, 2003).
Further to this, the simplification of the construction operation
can enable operative numbers to be reduced (Damtoft et al.,
2008; Goodier, 2003). Yet, despite these claims, there is rela-
tively little evidence and few data on SCC’s practical application
within construction (e.g. Damtoft et al., 2008; Gaimster and
Foord, 2000; Goodier, 2003; Henderson, 2000), as summarised
in Table 1. Although significant work has been undertaken
in Europe, notably the Brite EuRam project (Grauers,
2000), which explored many aspects of SCC application includ-
ing productivity and economy, which quantify SCCs’effect
in conventional construction processes, there remains a lack of
UK research. However, extant publications are often subjective
and imprecise, lacking detail about data derivation and often
citing values from elsewhere without justification. Hence, it is
inadvisable to make a case for SCC based on such incomplete
data.
Benefit Level of improvement cited Detail and reference source
Reduction of costs 15% reduction Early decision to use material in Swedish construction (Concrete
Society and BRE, 2005)
21·4% reduction French comparison undertaken by Lafarge, SCC compared with
conventional concrete (Concrete Society and BRE, 2005)
5–15% reduction SCCs at design stage derived from ‘experience’within Europe
(Holton, 2003)
5–15% reduction Comparison between SCC and conventional bridge construction
(Goodier, 2003)
Reduction in
construction time
2·5 months French comparison undertaken by Lafarge, SCC compared with
conventional concrete (Concrete Society and BRE, 2005)
20% reduction (2·5 years build
reduced to 2 years)
Live construction of Akashi-Kaikyo Bridge in Japan (Okamura and
Ouchi, 2003)
22 months to 18 months SCC on construction of LNG tank for Osaka Gas Company
(Okamura and Ouchi, 2003)
Labour reduction 150 operatives reduced to 50 Live construction of Akashi-Kaikyo Bridge in Japan (Concrete Society
and BRE, 2005)
Energy saving 20–30% reduction. Greenhouse
gases also reduced
SNRA report, result of reduced resources in construction (Goodier,
2003)
Productivity
improvements
60% improvement over
conventional concrete
Observation of Swedish works on 19 bridges and house slabs
(Persson, 2001)
Table 1. Summary of the benefits of SCC, from the literature
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2.2 Factors affecting the decision to use SCC
SCC is considered a specialist material, only suitable for certain
applications (Clear, 2006; Holton, 2003), but this view is slowly
being dispelled through continued use in general construction
(Williams, 2008) and high-profile projects, such as The
Collection, Lincoln, UK (Grimes, 2005) and The Hepworth,
Wakefield, UK. Here, SCC was selected in response to specific
challenges, for instance to create very high-quality, complex and
detailed facades. SCC is rarely selected as a ‘first choice’con-
struction option in its own right, yet the literature confirms that
the material has a broader range of benefits than this ‘single
attribute’decision making might suggest (Williams, 2008). SCC
should not be considered on first-cost alone (Concrete Society
and BRE, 2005; Holton, 2003), however, misconceptions relate
to poor knowledge and an apparent increase to project costs
as a result of material price (Concrete Society and BRE, 2005;
Rich et al., 2011, 2012), as SCC tends to be more expensive
than conventional concrete in cost per sales by volume. Rich
et al. (2012) developed this idea further through case studies
and industry surveys by identifying three distinct circumstances
for use
&strategic change from conventional methods as part of a
balanced assessment of SCC and its effect on construction
&reactionary response to a specific issue or problem
(problem solving)
&specification of SCC as a pre-conceived construction option.
Rich et al. (2012) conclude that strategic change is a direct
result of considering SCC as a construction method rather
than a material, invoking engagement with SCC earlier in the
construction process during design and planning with greater
potential to adapt construction to realise other benefits.
2.3 Lack of practical evidence on construction
process
Concrete slab construction has six steps, whether using SCC
or conventional concrete: material discharge, manipulation of
material, compaction, levelling, finishing and curing. However,
SCC ‘performs’the compaction process itself and the manipu-
lation, levelling and finishing are simplified. This simplification
can remove several steps compared to conventional
construction (Figure 1). SCC’s free-flowing nature reduces
the requirement for manual manipulation. SCC exhibits self-
levelling traits, whereas conventional concrete requires exten-
sive manipulation with rakes and shovels prior to compaction
and striking off. Conventional concrete slab finishing requires
two stages: initial finishing by manual floating; then, once ade-
quate curing has occurred, power floating. SCC finishing uses
a‘dappling’process; small surface waves are created to achieve
a smooth and level finish, equivalent to power-floated flatness.
Both methods then require curing.
A good research base exists on SCC’s physical and structural
properties, but there is still a need to explore the effects of
SCC’s different approach on the construction process systema-
tically on live projects. No one has yet explored SCC’s effect
on project time and cost, including casting rates, labour effi-
ciency and quality of construction. The next section explains
the research approach through work measurement and cost
modelling of UK residential projects.
3. Research approach
The aim of the research was to compare SCC with con-
ventional concrete, identifying differences in time and cost,
using a series of residential projects to provide real-life data.
There were two phases: work measurement and cost modelling.
3.1 Work measurement
Work measurement establishes the time for a ‘qualified’
individual to complete a specified task at a defined level of
performance (Currie, 1977; Drewin, 1982). The process com-
prises the following stages
(a) Selection of work to be measured (A.1): clarify exactly
what is to be measured. Here, the intention is to compare
SCC with conventional concrete in the construction of
single family homes.
(b) Defining the method (A.2): select the optimised con-
struction process. In this case, use is made of traditional
slab construction with conventional concrete and SCC
construction methods.
(c) Establish work elements (A.3): break down the methods
into elements which are easily measured, identifiable
and transferable between construction options. Final
quantification can only be confirmed when observations
are complete.
(d) Measurement of elements (A.4): direct timing of an
element of work. Success depends on clearly defined
work elements with distinct start and finish points,
incorporating a defined and detailed method of data
capture. Standard rate of working and rest time are
factored in.
(e) Obtaining a standard time (A.6): determine an
overall time for the construction processes combining
all measured elements and adjustments. Rationalisation
of time measurements was on a unit area and volume
basis.
3.2 Selection of project cases and method used
The study element must be built regularly with very similar or
standardised methods; ground floor residential slabs satisfied
these criteria, providing directly comparable, robust results that
could compare the claimed benefits of SCC with conventional
concrete. There are two distinct slab construction methods for
the ground floors of houses in the UK: ground bearing in situ
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(GB) or suspended (SS). The SS method typically uses precast
beam and blocks with an in situ topping and SCC is used
more often than GB slabs. The management structure for such
projects is simpler than structural frame projects, leading
to easier permissions for site observations. Although a number
of parties are involved, it is only the contractor responsible
for the placement of the concrete who experiences any changes
to methods of working through using SCC. The contractor’s
primary concern is the strength and finish specifications
(British and Eurocode standards along with National House-
Building Council (NHBC, 2011) standards), it is therefore only
this contractor’s work that needs to be observed and
considered. In total, 14 separate slabs were investigated, six
using conventional concrete mixes (ranging from C16/20 to
C28/35) and eight using SCC over a 3-month summer period
in 2010. The sites were within reasonable travelling distance
from base, were comparable in size and design, and all
involved experienced contractors. The processes were filmed to
record the movement of materials, labour and plant. For each
pour, records were made of total pour time, time from initial
material discharge to curing, activity times, time for each
individual construction activity to be completed, along with
labour times, time on the pour and time actively working. To
identify significant differences in construction protocols so that
Arrival of material
Minimal manipulation
to fill form
Minimal manipulation
to fill form
Surface finishing
(dappling process)
Minimal adjustment
to final level
Minimal manipulation
to fill form
Easy float to
basic finish
Screeding to
final level
Compaction of slab
according to specification
Initial adjustment to
approximate levels
Minimal manipulation
to fill form
Minimal manipulation
to fill form
Material discharge
Conformance checking
Self-compacting
concrete
Conventional
concrete
Figure 1. Breakdown of conventional concrete and SCC placing
methods
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any anomalies through the analysis of times may be interro-
gated to determine the drivers, several additional observations
were made, including slab dimensions and construction details,
material design, labour, plant and ambient conditions.
3.3 Developing cost model scenarios
The cost differences between the two methods were determined
using both the timings and labour/plant movement data cap-
tured on film along with additional cost data, facilitating a
more holistic understanding. Cost modelling was an appropriate
method, enabling rationalised comparisons based on realistic
scenarios. Two conventional concrete scenarios illustrated
variability in the power-floating operation, where completion is
dependent on the rate of curing (dictated partly by ambient con-
ditions). Only one scenario was needed for SCC as it does not
require any break point once pouring has commenced. Each
scenario was based on slabs of a similar type to those used in
obtaining the time data. The slab design was an idealised
suspended ground-floor residential slab, common to UK house-
building, constructed using an insulated block and beam system
with a concrete topping layer (to a size of 9 m × 5 m× 0·075 m=
45 m
3
), finished smooth to suit final finishes (Figure 2).
As-built cost data were developed from construction time data
to describe the construction process. The study was extended
by further and more detailed observations of the construction
process, with an understanding of each step in the construction
process and its requirements with respect to auxiliary items
such as plant, labour, materials, and so on. This was where
additional site observations provided deeper understanding of
each construction activity. For each process, the cost to con-
struct was developed considering material price (concrete and
supplementary (curing agent, form release agents, and so on)),
operative pay rates, volume of labour, plant and equipment
costs (hire or purchase), site overheads, out-of-hours working
and regional cost variations. Data were collected from material
suppliers, contractors and pricing books, and were current at
the first quarter of 2011. Currency of the data, the units of
measurement and outlying factors that might influence the
costs on a live project were verified. The eventual costs of each
element were collated into a single cost to determine the differ-
ence between conventional and SCC methods.
4. Results
4.1 Time study
These results are based on direct observations of the construc-
tion of 14 residential slabs similar to that shown in Figure 3.
In the majority of cases, the SS structures incorporated
inverted T precast beams, with either concrete or insulated
polystyrene infill blocks. Brick or block work was initially con-
structed from foundation level to two brick courses below the
damp-proof course level. Precast beams were laid typically
spanning between perimeter foundation walls to intermediate
supporting columns or walls. Brickwork was laid to final
Concrete topping
Insulating block Precast beam Damp-proof membrane
75·0 mm
Figure 2. Representation of SS design
Figure 3. One of the residential sites used in the time study
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finished slab level, and the infill blocks were installed. A
damp-proof membrane was placed onto the block and beam
structure overlapping onto the brickwork at damp-proof
course level. In both approaches, concrete was discharged to
the slab from the bucket of a 360° excavator with no significant
variation in discharge rate. Compaction of conventional con-
crete typically used mechanical poker vibrators or vibrating
tamping rails.
The total construction time for each slab was determined
from the film analysis from initial discharge to the final ap-
plication of curing protocol, along with intermediate break-
downs of each distinct phase: placement, tamping, screeding,
floating, power floating, dappling and curing. This provides a
descriptive account, but does not enable direct comparisons to
be made owing to the varying slab areas and depths, so the
data have been rationalised. First, from a total time to an
average time per unit area (m
2
) and per unit volume (m
3
)for
each slab (the lower two rows in Tables 2 and 3) and second,
the average construction time, taken across all instances of
SCC slabs and conventional slabs, can be seen at the far right-
hand columns in Tables 2 and 3.
Direct comparison of the average construction times show that
SCC produces a significant overall time saving over that of
conventional concrete (36 s/unit area (SCC) compared to 2 min
26 s (conventional); and 7 min 35 s/unit volume (SCC), com-
pared to 30 min 16 s (conventional)). Figure 4 illustrates pour
time (T
PT2
= total pour time/m
2
) and labour time (T
PL2
= total
labour time/m
2
) differences. SCC is 73% quicker and uses 70%
less labour.
4.2 Understanding the influence of material, plant
and labour costs
It was stated earlier that the high material cost of SCC is
a barrier to adoption, so the time study data have been
extended to include cost information. Three scenarios were
developed from observations, as described in the research
approach section (Figure 5, which includes both time and
labour requirements)
&‘best case’conventional concrete slab
&‘worst case’conventional concrete slab
&SCC slab.
Slab code
number
Pour time:
min:s
Area:
m
2
Volume:
m
3
Rationalised construction
time: (min:s)/m
2
Rationalised construction
time: (min:s)/m
2
CON1 82:54 41·83 3·14 01:59 26:24
CON2 86:05 37·80 2·84 02:17 30:19
CON3 80:15 37·80 2·84 02:07 28:16
CON4 47:57 16·50 1·24 02:54 38:45
CON5 47:07 16·50 1·24 02:51 38:04
CON6 84:15 56·77 4·26 01:29 19:47
Average construction time for all six slabs 02:16 30:16
Table 2. Rationalised construction times for slabs using
conventional concrete
Slab code
number
Pour time:
min:s
Area:
m
2
Volume:
m
3
Rationalised construction
time: (min:s)/m
2
Rationalised construction
time: (min:s)/m
3
SCC1 28:40 36·73 3·67 00:47 07:49
SCC2 23:15 49·49 4·95 00:28 04:42
SCC3 45:40 69:31 5·20 00:40 08:47
SCC4 23:15 42·80 3·21 00:33 07:15
SCC5 30:50 42·85 3·21 00:43 09:36
SCC6 22:15 46·33 3·47 00:29 06:25
SCC7 23:30 41·46 3·11 00:34 07:33
Average construction time for all seven slabs 00:36 07:26
Table 3. Rationalised construction times for slabs using SCC
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Construction with conventional concrete is susceptible to sig-
nificant variation in the cost to construct, mainly due to the
power-floating operation. In the worst-case scenario costs are
raised by 72%, which, if encountered on a number of slabs,
would significantly increase total project costs. SCC provides a
substantial saving, £291·45 for completed construction, over
the worst-case scenario of conventional concrete. However,
in the best-case scenario, SCC is more expensive by £28·63
(Table 4). All costs were current at first quarter of 2011.
This comparison enables prevalent factors in method choice
to be determined. SCC reduces construction overheads and
placement labour through the simplified construction process;
however, they are not significant in the balance between
options. Material price is the most significant factor for SCC;
this reflects existing literature, which considers price as a major
barrier to uptake (Concrete Society and BRE, 2005; Holton,
2003; Rich et al., 2012). The determining factor for
conventional concrete is labour, where out-of-hours working is
most influential. However, the relationship between out-of-
hours working on conventional projects and the material price
for SCC determines the tipping point between each method.
4.3 The importance of P
max
It has been possible to establish the maximum permissible
premium (P
max
) per unit volume (m
3
) that can be applied to
SCC by the supplier, such that the contractor can achieve parity
between the as-built costs for both conventional and SCC
methods (and therefore be at no overall disadvantage from
using SCC). Here, P
max
is determined with respect to slab size
and labour rate, based on the construction of a 75 mm deep
topping on block and beam construction (Figure 6).
In the ‘best-case’scenario for conventional concrete, parity can
be achieved by providing SCC at a P
max
of £29·46/m
3
above
the price of conventional concrete. Figure 6 shows the
00:00:00 00:00:43 00:01:26 00:02:10 00:02:53 00:03:36
Time
Rationalised average construction time: min/m2
Ave. SCC
Ave. conventional
concrete
TPL2
TPT2 00:02:16
00:01:03
00:00:36
00:03:10
Figure 4. Comparison of average construction time for
conventional and SCC methods, including power floating
operation
08:00 09:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00 22:00 23:00 24:00
Working day Double time
Time and a half
Conventional:
best case
SCC
Conventional:
worst case
T
T
L
L
L
T
2
1
4
411 11 11
2
Placement
Power floating
Curing agent
application
T = Pour time
L = Labour on slab
Figure 5. Gantt chart of idealised construction scenarios, with
time and labour requirements
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relationship between slab size and P
max
, as slab size increases
P
max
decreases, effectively reducing the cost benefit of the SCC
method. However, at low labour rates P
max
is small because of
the relationship that exists between material and labour cost,
material price is the significant variable with SCC and labour
with conventional concrete. When labour rate is low the differ-
ence between material and labour is at its greatest, which
reduces P
max
as this is a function of the difference in as-built
construction costs determined by material and labour costs. As
labour costs increase for the same sized slab, material costs
remain constant. This in turn reduces the as-built cost differ-
ence between methods and enables P
max
to be maintained at a
higher value.
As slab size increases, P
max
decreases. However, for the same
slab, there can be two different timings and costs from ex-
tended working hours due to varying ambient conditions.
Therefore, the relationship between P
max
and slab size in these
circumstances may vary. If out-of-hours working is incurred
when constructing with conventional concrete (Table 4) the
relationship between material cost and labour rate varies,
where initially the difference in constructed costs will reduce to
a point where conventional construction costs will exceed SCC,
the worst-case scenario. The amount of out-of-hours working
is determined by two factors: ambient conditions and slab
size. If ambient conditions remain constant for slabs of increas-
ing size, the increase in out-of-hours working would be propor-
tional to slab size. P
max
is directly related to these factors and
would see a change in the relationship to slab size, increasing
rather than decreasing (Figure 7), improving the as-built cost
benefit of SCC once out-of-hours working is required.
The maximum permissible premium, P
max
, can now be calcu-
lated for residential suspended slab construction to find the
tipping point between extra material cost of SCC and the extra
labour costs by using conventional concrete. P
max
can deter-
mine when SCC would become a viable selection.
For single-family homes, where the slab areas do not exceed
65–75 m
2
with a depth of approximately 300 mm, the P
max
relationship can be described mathematically based upon the
total cost to place concrete into the slab, T(£), shown in
Equation 1.
1:T¼OVþLþM
where O
V
denotes the overhead costs (£), Lis the labour cost
(£) and Mis the material cost (£).
Considering each term in detail, O
V
is the construction over-
head costs (£) derived from on-site observations representing
cost of plant and equipment. Lis the labour cost (£), a product
of the size of the slab and the time taken to place 1 m
2
, labour
operatives’rate of pay and the cost to employ, therefore
2:L¼taer
Costs: £ SCC
Conventional
Worst Best
Overheads 2·00 43·33 43·33
Placement labour 15·22 57·02 57·02
Power float labour 0·00 2·37 14·20
Out of hours labour premium 0·00 334·00
Material 380·63 252·59 252·59
Curing labour n/a Included in O/H 2·09
Curing agent 6·75 6·75 6·75
Total 404·60 696·05 375·97
Saving from using SCC compared with conventional concrete n/a +291·45 +72% −28·63 −7%
Table 4. Costs for the three slab scenarios
5
10
15
20
25
30
10 20 30 40 50 60 70 80 90 100 110
Labour rate: £/h
Pmax: £/m3
Slab size: m2
100
75
10
15
20
30
50
40
Figure 6. Graph of P
max
for varying slab size and labour rate (slab
depth = 0·075 m)
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where tis the time to place 1 m
2
of slab (rationalised to a min/
m
2
rate), ais the slab area (m
2
), ris the rate of worker pay
(£/min) and eis the administrative costs to employ labour.
Material cost (M) is a factor of slab area, slab depth and the
retail price of concrete, as follows
3:M¼adp
where ais the slab area (m
2
), dis the slab depth (m) and pis
the concrete price (£/m
3
). The overarching formula for calculat-
ing total placement cost remains constant but the contributing
times and costs between conventional and SCC methods vary,
therefore, for conventional construction
4:T¼OC
VþLCþMC
and for SCC construction
5:T¼OS
VþLSþMS
Parity between construction methods, when it is possible to
derive P
max
, is achieved when the total placement costs for
both conventional and SCC are equal, therefore when
Equation 4 is equal to Equation 5, as shown in Equation 6
6:TC¼TS
Rearranging Equation 6 produces Equation 7, which is used to
attain the value of P
max
7:pSpC¼OC
VOS
V
ad þer tCtS

d
where p
S
−p
C
=P
max
, the maximum permissible premium
to maintain parity between both construction methods.
Respectively t
C
and t
S
can be replaced by the values of 340/60
and 72/60, respectively, which have been determined through
detailed analysis of observations of suspended block and beam
slabs construction.
By not exceeding P
max
, the supplier helps the contractor
exploit the benefits of SCC to deliver a faster, higher quality
slab at the same overall cost as conventional concrete. The
existence and importance of this calculation has not been
acknowledged in previous SCC research.
5. Discussion
Using time study of 14 residential slabs and cost modelling of
three scenarios, SCC has been shown to improve construction
times significantly as first proposed by Henderson (2000),
Goodier (2003) and Holton (2003). In this research, SCC
reduces construction by up to 70% compared to conventional
concrete owing to its ease of placement and simplified con-
struction process. Conventional methods combine compaction
and levelling processes through tamping and screeding, a
highly intensive and physically demanding activity; this inten-
sity was apparent in the manipulation of the concrete –it
requires more physical effort to manually level and power float
the slab, whereas SCC’s fluid nature requires minimum work
(dappling) to achieve a similar result. The literature also claims
that material cost was a prohibitive factor in SCC’s further
adoption (Concrete Society and BRE, 2005; Rich et al., 2012).
The three cost modelling scenarios have demonstrated that
financial savings can be realised through SCC, subject to
several conditions. The ‘best-case’scenario for conventional
construction was marginally cheaper than SCC considering all
factors (i.e. material, plant and labour costs), but when out-of-
hours working was required to complete the conventional con-
crete placement, SCC would result in significant savings. This
offers new and insightful findings to a research field previously
dominated by physical and structural testing. This research has
also identified an important new variable, P
max
, describing the
relationship between labour and material costs, such that the
potential cost saving per m
3
from using SCC can now be ident-
ified and understood.
There is also a wider implication: the combination of SCC’s
time and potential cost savings alongside its reliability
and predictability mean that construction variations and
associated risks can also be reduced. The inherent variability
of conventional, in situ concrete construction justifies the use
of the more predictable and reliable SCC method. SCC could
increase the confidence of the project team in pre-project plan-
ning and cost estimation, thereby avoiding expensive out-
of-hours working. Considering the wider consequences of
delays, there is potential for delays to follow-on trades, which
may have an impact on the project’s critical path. Such delays
Pmax: £/m3
Slab area: m2
£ 8/h £ 10/h £ 12/h £ 14/h
+ve
+ve
Figure 7. Variation of P
max
resulting from out-of-hours working,
with respect to increasing slab area, for a range of operative pay
scales
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potentially incur costs which would far outweigh the additional
material cost of SCC (e.g. total site running costs and potential
penalties for late completion). SCC can shorten the critical
path: it is faster with fewer operatives.
6. Limitations of the study
The main limitation is that the cases were all based in the UK,
so caution should be exercised before extrapolating the results
to other countries. Furthermore, work measurement typically
has limitations relating to process, subject of the study and
generalisation as explained further below.
&The study observed operatives with the potential for work
rates to increase from constant observation. In mitigation,
instances of obvious enhanced speed were noted and
negated during analysis.
&Site observations, data capture and analysis and
interpretation of results were all by the same person to
provide continuity. Although checking procedures were in
place, additional observers could have been used to verify
the data.
&Activity timings were based on observations of when each
activity started and finished, with times being recorded to
the nearest 5 s, relying on the observer to identify the end
points.
&A sample size of 14 slabs, although modest, has provided
sufficient data to address concerns of unrepresentative
sampling. The selection of a generic building type
responded to gaps in the literature. This has produced
robust findings, but any further generalisation of results
would require additional analysis.
&The time study required the breakdown of the
operation into individual elements or activities; these
were individually timed and combined to establish total
construction time. However, there was always more than
one operative and often a number of concurrent activities,
so construction was filmed and timings were noted when
reviewing the films.
&Construction is inherently complex, involving numerous
elements and the input of many parties, even for simple
projects. While the optimum situation would have been the
evaluation of a whole project from inception to completion,
this was not practicable.
7. Conclusion
This research aimed to identify whether or not SCC had a
quantifiable effect on construction, ostensibly because there
was a lack of robust evidence in the literature to establish if
this was the case. Using data from 14 residential concrete slabs,
SCC has been shown to present significant time savings and,
even if these may not always result in actual cost savings, then
SCC can be priced to closely match conventional concrete
project costs overall. The relationship between costs for SCC
and conventional concrete for slabs has been clarified and a
new mechanism for understanding profitability and viability of
SCC (P
max
) has been presented.
The results also reinforce the case for considering SCC as a
method rather than a material (Rich et al., 2012). Where oper-
ations are adapted to make the most of SCC’s benefits, it can
have a positive effect on construction time and cost. This was
evident from the residential projects selected; the market for
SCC for such slabs in the UK has been growing steadily, albeit
the reasons for this were previously not well understood.
Further research to examine the effects of SCC on flat slab con-
struction is underway, where risk reduction may have a more
significant effect and the larger slab sizes may prove more
competitive for SCC. While extending the work to different
slab types and other construction elements such as walls and
footings would also be worthwhile, the development of an
ideal or best practice model for design and planning of SCC
and how to adapt construction practices would also be a
useful contribution to knowledge and practice.
Acknowledgements
The authors would like to acknowledge the contribution from
Lafarge Aggregates Ltd (now Lafarge-Tarmac) and all the
project staff. This work is part of an engineering doctorate
funded by the UK Research Council (EPSRC) by way
of Loughborough University’s Centre for Innovative and
Collaborative Construction Engineering.
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