Portland cement, pricing and building collapse (2)

on   /   in Columns 12:44 am   /   Comments

Dr. Akintola Omigbodun

The previous week, we  identified that according to European Commission nomenclature, CEM I and CEM II are the types of cement produced in Nigeria.

cement

CEM I is ordinary portland cement whilst the local production of CEM II is portland limestone cement. We identified the three strength classes, 32.5, 42.5 and 52.5. The strength of any cement is determined from tests on concrete made with the cement.

Concrete is made by mixing cement, fine aggregate, coarse aggregate and water in specified proportions. The fine aggregate used locally is usually sharp sand whilst the coarse aggregate is crushed granite. For reinforced concrete, it is usual to have a nominal maximum aggregate size of 20mm.

There is a chemical reaction between the cement and water to form a calcium-silica-hydrate, the cement paste subsequently hardens binding the fine and coarse aggregates together.

In order to obtain the best results, the wet concrete must be fully compacted to make it dense, the strength of the concrete develops with the hardening and the hardened concrete must be cured.

Methods of curing are designed to maintain the concrete in a continuously moist condition for several days, either by preventing evaporation or keeping the surface of the concrete continuously wet. Curing allows the initial reaction between the cement and water to take place over an appropriate period.

Thus, if cement is not applied properly, the strength that would be observed in the concrete would not correspond with the strength class of the cement.

Essentially, the coarse aggregate in concrete should be twice as much as the fine aggregate and by increasing the quantities of cement applied to a specified quantity of fine and coarse aggregates, one would obtain increasing concrete strengths. In building construction, we often specify concrete made with 1 to 2 to 4 mixture of cement to fine aggregate to coarse aggregate.

This would correspond to having 350kg of cement in a cubic metre of concrete. This should be the minimum specification for building construction.

The major elements in a building structure are the foundations, the columns, the beams and suspended floor slabs all built with reinforced concrete. I have often observed that when concrete is being mixed and placed for a suspended floor slab, the workmen feeding the concrete mixer do not put the cement, sharp sand and granite in the correct proportions.

There is always a tendency to put too much sharp sand. On one occasion, I asked one of the workmen why one had to be correcting them repeatedly as to the quantities of the materials fed into the concrete mixer.

The response was that the concrete would be too heavy to carry if the specified proportions were fed into the mixer. I then suggested that the quantities of concrete put into each head pan could be reduced and the response was that it would take longer to complete the work for that day.

The cement manufacturers should join hands with the Building Collapsed Prevention Guild and others in getting the workmen to understand and to adopt the correct practices and standards.

When a blockmaker learns that there is a cement that gives him a higher strength the tendency would be to increase the quantities of sand in the cement/sand mixture such that more blocks are produced from one 50kg cement bag than previously.

Sand/cement blocks from the block makers are not tested for strength and blockmakers should be encouraged to make blocks having a higher density through the adoption of equipment that incorporates a hydraulic press in addition to the vibrating plate which supports the block.

The portland limestone cement that is available from local manufacturers is marked either CEM II A-L or CEM II B-L on the cement bag. A corresponds to 80% to 94% ordinary portland cement clinker and 6% to 20% limestone. With CEM II B-L marking, B corresponds to 65% to 79% ordinary portland cement clinker and 21% to 35% limestone.

The manufacture of clinker requires large amounts of energy and the reduction in the proportion of clinker in a cement type should lead to a reduction in the price of that cement type.

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