How to avoid common, expensive construction errors

By Fidelis Sakwa

fidelis sakwaredc


Over the years, collapse of buildings has been witnessed across the country, with most typically at construction stage but some have occurred in buildings under habitation.  The failures have resulted in fatalities, insurmountable financial losses, severe injuries, and consequential damage to surrounding buildings, infrastructure and environment.

The quality of constructed buildings can be represented as an entire spectrum of varying degrees from a collapsed building to a perfectly well constructed structure and every time we go to an office, hotel, visit friends and relatives we are at some point on that continuum – either very safe or about to be exposed to an imminent potential disaster.

A national Construction Authority 2020 survey study showed that poor workmanship accounted for 35% of cause of failures, substandard material for 28% and poor structural design for 25%. A Critical evaluation of the root cause will show that these problems emanate from non-involvement of qualified key construction professionals – engineers and architects. It’s unfathomable that one would put up a structure costing tens or hundreds of millions of shillings and avoid these professionals simply because they wish to avoid professional fees.  Other gaps exist in design, supervision to enforce designs and controls at sites touching on various facets of the entire construction process.

The second aspect of quality of construction materials will form the core of this discussion. One of the key materials used in construction is concrete with constituents comprising cement sand, aggregates and water. The use of these materials and the builders knowhow play a significant role on how the structure will be able to resist applied loads.

To prevent errors, qualified construction professionals should be engaged

Cement invariably constitutes the highest cost of a concrete mix. Some sites will deliberately to use less of it to save costs. Cement is the primary binding agent and if the use is compromised, the structure will lack the requisite strength to stand on its own and support loads.

A key attribute of cement is consistency of quality determined by a manufacturer’s quality controls process. When cement strengths are high, builders will tend to use less and when the strengths dip the concrete will be doubly affected by the low strengths and reduced quantity. The cement in the market must meet specifications of existing standards and not be substandard due to its major role in strength of concrete.

River sand has been traditionally used in concrete over time. River sand is known to be contaminated with clays and silts especially in the rainy seasons where surface runoff will contaminate. Clays and silts absorb water which forms air spaces in concrete on drying and in combination form weak zones that compromise concrete strength and hinder bonding of cement to aggregates. Through their absorption they increase water demand and any increases in water in concrete will result in significants reduction in strength.

A number of sites have resorted to the use of quarry sand which was a waste material in aggregate for a long time. Quarry sand is an acceptable construction material but due care must be exercised in the mix design due to a higher content of fine material that can potentially increase absorption water and lower concrete strengths.

Aggregates quality will affect concrete strengths

Aggregates mined from weathered rock or overburden material will have soil and clay in them causing similar problem. The ideal aggregate particle is rounded or cubicle shaped with a good particle size distribution to increase the packing density through intimate contact of the particles.

However extremes exist in the market where flaky or crisps-like aggregates or aggregates with some size fractions missing that are sold often cheaply to unsuspecting end users. The net effect is a highly compromised packing density with lots of air spaces that will reduce concrete strength.

readymix concrete pouring 4

The biggest challenge in concrete is on the mix design. Sections of the industry are using prescribed mixes of ratios such as 1:2:4 of cement to sand to aggregates to make a Class 20 concrete. The water content is undefined and the more water you add to concrete the weaker it becomes. Aggregates, sand and cements vary as described countrywide and these mixes should be discouraged as they will not achieve reasonable strengths. In extreme cases, some sites use the ratio to depict 1 bag of cement to 2 wheel barrows of sand and 4 wheel barrows of aggregates rather than equal volume ratios resulting in very low concrete strengths.

Projects should conduct mix designs in approved testing laboratories using the actual materials they will use at site and this must be supplemented by physical testing of the concrete to ascertain strengths well before the project commences.

Concrete quality

Quality enforcement at sites portends another major concern. Labourers especially when tired add more water to concrete to make application easy, lose count of buckets they use to mix individual components with the net effect of dangerously lowered concrete strengths. They may scoop aggregate with soil and mix all of it to the detriment of structure under construction. Few sites are able to check the moisture in aggregates and compensate with free water that will achieve desired strengths.

A trial and error eyeball approach is used to determine water requirements with no tests thus impairing the concrete strengths. Some concreting proceeds during rains resulting in excess water in the concrete and the strengths will dip. Some sites do not vibrate concrete or the poker breaks down and they continue casting. Vibration is a process in which air introduced in concrete during mixing is driven off. 1% of air in concrete can reduce strengths by a massive 5 MPa.

To prevent all this from happening qualified construction professionals when engaged are able to supervise and check on the quality of all materials. A strict regulatory framework is needed for all materials in terms of regular quality testing to ascertain absolute adherence to standards and consistency. Concrete mix designs must be made, pre-construction tested and rigidly enforced in all construction sites.

Any change in materials must be accompanied by changes in mix designs. The best cement will make useless concrete when sites use unregulated and dirty river sand, flaky aggregates and sewage water to cast the concrete in unknown proportions.

Fidelis Sakwa is the Innovation & Technical Services Manager at Bamburi Cement.