5.1.1 Concrete Mixture
Concrete is one of civilization’s oldest building materials and most often is a material
already widely used in most rural communities. Teaching the local laborers the
importance of proper mixing techniques and mixture types will improve the quality of
all concrete construction and thus may be one of a project’s primary successes
68 Concrete is produced by mixing water, Portland cement and sand and gravel. To
produce a good concrete block, care needs to be taken in the quality of the sands and
gravels used in the mixture. Construction quality control of the sand and gravel
materials often requires preparatory work as natural conditions rarely leave well-
graded deposits. The fine aggregate with a diameter less than 5 mm, more commonly
referred to as sand, is often available on rural construction sites. No silt or clay
passing a #200 sieve or about 0.074 mm may be used. Similar to the process for soil
classification, if sand is sourced locally out of a riverbed, a mesh screen must be used
to ensure proper grain-size. Sands need to be washed and sifted through a screen with
5 mm openings. Coarse aggregate or gravel is a mixture of rock with a range of 6-20
mm diameter which may be found in-situ or created by crushing larger locally
available rock. The gravels and sands should have regular grain-size grading without
one specific size dominating the size distribution: with sand, particularly too many
fines (Ruskulis, 1996).
Water content controls the workability of the mixture and chemically reacts with the
cement to bond the resulting concrete. One of the critical components of quality
control is to ensure that the proper ratio is maintained during construction for
increased portions of water will improve workability but decrease material strength
(Engineer, 2002). For hand-mixing, a water to cement ratio of about 0.55 produces a
workable and durable concrete (Davis, 2002) but for more specific cement ratios,
69 Table 7 Concrete Ratios by Volume (Adapted Engineers, 2006)
Mixing technique is another aspect of quality concern. Many rural laborers are
familiar with mixing concrete but local methods of mixing are often inferior as there
is a lack of quality control standards. Common is the volcano approach, in which
aggregates and cement are mixed by hand, forming a pile. A hole dug out of the top
provides a bowl-form for the water to be poured and mixed. Although common, this
approach is not appropriate as it is difficult to attain an even mixture. Alternatively,
to hand-mix concrete, one must specify that the water is to be splashed into the
mixture in lifts while being manually mixed using a shovel.
Once set, the fresh concrete must be kept wet during the curing period. Concrete will
set in three days but reaches workable strength after seven days (Hazeltine,2003).
For greater detail on appropriate methods for concrete mixtures, reference Engineers
Without Borders, Concrete Mixes Guidelines (Engineers, 2006).
5.1.2 Steel Cable
Steel cable has two types of elongation: elastic stretch that fluctuates with the applied
load and the permanent stretch that corresponds with the cable strands rearranging
Mix Ratio by Volume (Cement:Sand:Gravel:Water) Typical Use on Bridges Approximate Yield (m³) 1 : 3 : 6 : 1.6 Tower Foundations, Block Anchors 0.24 1 : 2.5 : 5 : 1.6 Tower Foundations on poor soil 0.21 1 : 2 : 4 : 1.6 Non-structural Approach walls 0.17 1 : 2.5 : 3.5 : 1.6 Structural Column in Tower 0.17
70 and tightening in cross-section. The type of cable purchased dictates the amount of
hoisting sag. Cable may be purchased as either non-prestretched or prestretched, the
latter which will be considered herein. It is pertinent to not that if non- prestretched
cable is used, the design engineer must increase the anticipated sag onset from
loading which would have a greater impact on the hoisting sag set.
Cable handling is of paramount importance. It is critical not to unwind the cable
incorrectly, as this may cause kinks in the cable which result in weak points in the set
cable and thus potential failure points. Figure 26 shows the proper way to unwind the
cable.
Figure 25 Cable Uncoiling Procedure (Helvetas, 2001)
Cable transport from the drop point to the bridge site is also critical. Figure 27 shows
the proper way to transport cable.
71
5.1.3 Cable Clamps
U-bolt clamps, often referred to as bull-dog clamps, are used to tie the cable around
the anchors. The singularity of the clamping method is one of the few design aspects
that does not include redundancy. As such, the material properties of the steel used to
create the clamps and the process used to attach the clamps is critical for the quality
assurance a bridge project.
The structural integrity of the clamps used to connect the steel cable is an area of
concern, as clamp failure is the source of the only known bridge failure to date
(Nepal, 2008) as shown in Figure 28.
Figure 27 Failed Nepali bridge: Clamp Slippage
Malleable steel clamps are most common but are inadequate for continuous load-
72 forged are of superior quality for bridge-type loadings but are often difficult to locate
in developing countries.
The difference between the two is the process used to create the clamp. As with all
steel, the principal mechanical properties of interest to designers are strength,
ductility and hardness all of which are dependent on the process used to create the
clamp. In the casting process to create malleable clamps, the mold has the shape of
the desired component and the liquid metal flows into the desired shape. Malleable
clamps are able to attain the same efficiency ratings based on breaking strength of
wire rope, but are apt to continuously loosen with continued load and thus reduce
their ‘grip’ on the cable. With forged steel, the original shape is an ingot that is
forged into shapes by presses. The resulting product has a greater material strength
and lower ductility. As such, the torque specified to reach maximum efficiency rating
is greater than a malleable clamp of comparable diameter but once torque, the clamp
is far less likely to slip. It is the engineering field supervisor’s responsibility to
ensure that the clamps used on-site are per specification.