Concrete: A Management Perspective
Formwork, reinforcements, composition, production, compaction and curing are subject of concrete management.
Conventionally, concrete is defined as a construction material produced by casting predominantly inert constituents (called aggregates) with binding agents (usually cement) and wherever applicable, (steel or other) reinforcements in a viscous state within a mould of a desired solid geometry. For construction management purposes, installation of forms and reinforcement, constitution, production, compaction and curing of concrete entails administration.
Production and Supply of Concrete
The simplest sequence of production of concrete involves mixing of constituents and placing the same on forms. This may be supplemented by other operations namely, batching, hauling and lifting. The overall performance of production depends on the performance of workforce for each of these tasks. The overall throughput being measured by volume, an evaluation of the throughput of each individual task is presented below:
Mixing: Concrete drum mixers are now almost universally used for all concrete works. Performance of mixers are directly related to the throughput of concrete. Where intermittent concrete operations, that are not supplied by ready-mixed concrete, are required, mixers are generally mobilized for monthly hires. Micro-concrete jobs do not involve mechanical mixing.
Batching: Constituent materials for concrete is batched either by volume or by weight. A stationary or mobile batching plant is used when concrete is batched by weight; volume batching is handled using measuring boxes or loading buckets at the site. Working hours of mobile batching plants are in principle proportional to concrete volume, whereas conventionally both mobile and stationary batchers are mobilized over entire relevant duration of project, as mobilization costs are high.
Loading: Constituent aggregates are loaded into the mixer or batcher (or batching silos in centralized batching plants) by manpower or hoe-loader. Cement is loaded manually in site-mixes or pumped into silos in batching plant. Lead of materials during loading shall be separately accounted if applicable. Both loaders and manpower are not associated with large mobilization costs. Hence, loaders are mobilized in support of drum mixers and small batching plants. Loaders combine scalably with batching plants of large silo capacity and with multiple mixers.
Placing: Simplest placement of concrete is followed in pile concrete, micro-concrete and self-levelling self-compacting concrete where no workforce is actually involved in placing. Otherwise, placement is associated with levelling and compaction of concrete. Levelling and compaction although may not be analytically related to concrete throughput, empirical relation can be evolved for this purpose. Besides, it is necessary to maintain adequate speed of laying to prevent choking of concrete in the pump pipes. Placement speed of 12-16 kL per hour is quite common in the industry.
Compaction: On most concrete jobs, levels are controlled using preset gauges or probes. Freshly placed concrete is levelled using manual tools and compacted using needle vibrator. Needle vibrators (40mm) operate on about 0.25sqm influence area, 400mm (10d) immersion depth for 30s-2min – this makes its capacity to be 8-12 kL/h. On flat concrete surface at grade level, leveling may be achieved with screeding machines and troweling machines.
Lifting: Lifting of concrete on overhead jobs is carried out using concrete pumps or manual labour on scaffolds or hoists.
Transit-Mixer: A transit-mixer is used when the batching plant is located at a distance away from the site, so that mixing takes place while transit by rotation of the drum. The transit mixer performs three stages of operations:
- At the batching plant, transit mixer does not produce any output. Output is solely governed by the capacity of the batching plant. Transit-mixer is always on support to batching plant for this operation, irrespective of its capacity.
- During haulage, transit mixer operates in isolation and depends on the distance and quantity of haulage. Thus for concrete throughput Q and travel time from batching plant to site T, the haulage throughput is QT. If the volume of transit mixer is w units, then the number of transit mixer trips are Q/w engaged for T hours.
- During placement, transit mixer unloading is much faster than the concrete pumping/lifting and actual concrete placement productivity. Unloading is halted when pump/tremie is full, when tremie length is being adjusted or when pump is halted while freshly placed concrete is being leveled. Thus, the transit trucks should stand by during placement of concrete. For continuous placement of concrete, it is necessary that next transit mixer is waiting just-in-time while concrete is placed from one transit mixer. If the placement productivity is q units per hour, transit mixer hours during placement is Q/q.
Therefore, the total transit-mixer-hours required for the project is Q[1/b+T/w+1/q], where b is the batching productivity.
Constitution of Concrete
Composition of concrete designated by proportion of constituents is called Nominal Mix whereas composition designated by the grade of concrete such as say 25MPa and whose mix proportions are designed prior to execution by the supplier is called Design Mix. Design mix is commonly supplied by a vendor from remote batching plant with transit mixers. This arrangement helps to define contractual relationships based on assumption that all concrete will be supplied from a remote batching plant, irrespective of concrete volume. However, for low volume of concrete and during shortage in supply of concrete, transit mixed concrete is replaced by site mix with or without batcher. Volumetric batching without batcher is not entertained for grades above 28MPa.
Nominal proportions, used for structural purposes, (that is equivalent to design mix) designated for example, as 1:1.5:3, are listed along with design mix. If a nominal mix or site-batching equivalent procedure is designated during take-off or procurement plan or inspection, the respective procurement and reconciliation is calculated based on the nominal mix or mix design selected. Concrete used for non-structural purposes are designated nominally. However, these items may also be supplied on transit, site-batch or volume-batch.
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