3.3 MT PERFORMANCE AND COST REDUCTION
3.3.2 WALL CONSTRUCTION STAGES
The wall construction process includes the cost of materials and only four stages are
considered (Table 3.4): Bricklaying (BL), Pointing/jointing (P/J), Rendering/plastering (R/P),
and Wall-strengthening (WS). Painting and decoration is not included, assumed to be the
same as the wall surfaces are well prepared.
The interlocking bricks are assumed perfectly produced and in good condition, likewise the
sand-cement blocks. The bricks are built in the following wall construction stages: -
1. Bricklaying [costs per piece include materials (brick) and bricklaying labour per
piece].
4. Rendering/Plastering (a standardized construction cost per square meter (m2), that
includes mortar and labour). Some saving could be realized here by rendering soil-
stabilized walls with a stabilised-soil plaster that matches the lean mix used for the
bricks themselves; such lean plaster cannot be used on conventional blocks because
it will not adhere properly. This option is not generally considered, but it should be
in practice. Because of the machined MT brick quality, their external surfaces do not
require rendering; only pointing to prevent insects breeding and moisture
penetration. By contrast CB is usually given an external render to improve their
appearance.
5. Strengthening interlocking brick walls by pouring grout through vertical holes.
Hollow/Perforated interlocking brick walls optionally require strengthening by
pouring grout (soil/sand-cement slurry) into the vertical holes through the wall
Kintingu (2003), forming 50mm diameter cores at 300mm centres throughout the wall.
This task (grouting) is normally done after completion of wall erection, while preparing the
wall to receive a ring beam. Before doing so, we insert all conduit pipes in the required
positions and any reinforcement if required. Placement of grout can be accomplished in one
lift for single-story walls less than 8.5 ft (2.60 m) high. Grout lifts must be consolidated with
an internal vibrator with a head size less than 25 mm NCMA TEK 14-22 (2003).
The Hydraform solid interlocking block wall is by contrast strengthened by laying the first
two to three courses and the four last/top courses with mortar like a conventional wall
Hydraform Manual (1988). Thus about a quarter of all courses are mortared and the remaining three-quarter is un-mortared (Table 3.2).
Other costs not included in the calculations are: -
o High-level expert (Engineers or Architects), which may be done on call
(temporary) or on permanent basis.
o The foreman, on a daily routine.
•
Material wastage•
Security of the site.The above three listed items (supervision, material wastage and security) are normally
categorised under ‘sundries’ and assumed to cost not more than 5% of the above four main
wall construction stages.
Table 3.4 Cost comparison of one square metre wall in Tanzanian Shillings (Tsh.)
S/No Stages of wall construction (cost for material and labour)
Wall Type Interlocking Soil- Stabilized Bricks (ISSB) Conventional Sand Cement Blocks (CB) Tanzanian ISSB-T 150mm Hydraform ISSB-SA 230mm Solid CB-1 150mm Solid CB-2 230mm 1 Bricklaying (BL) 7755 11600 14400 22400 2 Jointing/Pointing (J/P) 1 3 1196 1589 3 Rendering/Plastering (R/P) 3675 3675 7350 7350 4 Wall Strengthening (WS)
(filling vertical holes with mortar) 482 0 0 0.0
Total cost for each type of wall including
5% sundries 12509 16042 24093 32906
150 mm wall equivalence 12509 10462 24093 21460
Normalised to ISSB-T costs 1 0.84 1.93 1.72
3.3.3 COST ANALYSIS
Table 3.4, which combines labour-costs with material costs, compares the average 150mm-
equivalent costs (Tsh.11485 and 22776) of MT and CB is summarised in Figure 3.2. It shows
that using mortarless technology we can reduce construction cost by 50% (i.e. MT/CB
=11485/22776 = 0.50) due to the use of cheaper material and elimination of some of the
construction operations.
An alternative approach (Table 3.5) is to look at materials costs and labour separately in the
following order:
(a) The CB material cost to labour cost ratio is assumed as 70:30 UN (1965).
(b) Estimates of MT/CB cost ratios for material (Rm) and labour (RL) are
respectively made
(c) Finally the data is combined to obtain MT/CB overall cost ratio.
For the value of Rm, the material cost ratio, we adopt the approximate value MT/CB = 0.5
from Figure 3.2 the extraction of Table 3.4.
The value of RL, the labour cost ratio, was estimated after a number of considerations were
made. Interlocking bricklaying is three to five times faster than conventional bricklaying
Whelan (1985), Anand and Ramamurthy (2003). This can be best compared in terms of wall area covered per day rather than number of bricks laid per day. Taking an average laying rate
of 1150 pieces per day of interlocking bricks Hines (1993), VanderWarf (1999), and knowing
33 pieces of ISSB-T cover one square meter (Table 3.1), gives that 35m2 of wall can be
completed in a day by one mason and one helper. With conventional blocks and the same
wall thickness (CB-1) the same masons can lay an average of 225 pieces (each weighing over
30 kg), equivalent to only 25m2 of wall per day. Here we can see that the labour productivity
has been increased by 40% if we use the CB-1 as the datum for comparison. (Taking CB-2
this increases to 120 %.). We can support the above arguments by the summarised efforts
towards improving construction productivities reported by Anand and Ramamurthy (2003)
Table 3.5.
Table 3.5 Productivity enhancement as a means of labour cost reduction
S/No. Source Type of interlocking
block
Productivity % increase
Labour cost ratio (RL)
1. Whelan (1985) WHB hollow block 79 0.50
2. Adamus and Spevak (1986) TSZ hollow block 0.35
3. Hines (1993) Haener hollow block 60
4. VanderWerf (1999) Haener hollow block 80 0.35
VanderWerf (1999) Sparlock hollow block 80 0.33
VanderWerf (1999) Azar hollow block 50 0.33
5. Anand and Ramamurthy (2003) SILBLOCK/HILBLOCK 80
Average 71.5 0.37
According to Harris et al. (1992) and Hines (1993) with the combined effect of less skilled
Table 3.6 Costs of materials and labour separated Bricklaying system Material Cost Labour Cost Total Cost CB* 70% 30% 100% Ratio of MT/CB 0.5 0.3 - MT** 35% 9% 44%
NOTE: * Conventional Blocks.
** Mortarless Technology, partial costs expressed as % of CB Total Cost
for given wall area.
MT/BC - assumed ratio of (MT to CB) costs for each input.
The value of MT labour cost being 9% of the conventional total cost, and therefore making
MT total cost equals 44%. However MT realises 56% cost saving compared to CB.