Anyone who has been around concrete knows that things can and do go wrong, be it light surface irregularities or large-scale cracking. This can be due to one culprit but is more commonly a combination of many factors, and it is rarely black and white. For the purposes of this article, a distinction has been made between elements that occur during production and those which are handled on-site.
Production
Material Quality – The most complex and nuanced of everything on this list, material types have a huge impact on the quality of concrete. Aggregates need to not only be robust and inert to provide strength and resist chemical breakdown, but also of the right shape and size. Cement, at its most basic level, needs to be capable of creating a strong bond between the materials in concrete, while producing no detrimental secondary effects (e.g., excessive heat or bleeding). Antoun performs routine quality control in the form of visual inspections and lab testing to ensure on-going material quality.
W/C Ratio – All else equal, this is arguably the most important mix design factor for durability. Water to cement ratio usually sits around 0.5 for mid-strength (20 – 32 MPa) concretes, and near 0.4 for higher strength grades. Lower ratios not only lead to increased strength, but to drastically improved durability. Despite the pronounced effects, it is not uncommon to see extra water added to a batch of concrete on-site. In isolated cases, this may be necessary, however is usually ill-advised and detrimental. Antoun volumetric mixer operators are trained to control water content through a combination of metering equipment, visual inspections, and slump tests.
Fit-for-Purpose – Concrete mixes are designed and optimised around their intended application and delivery method. As an example, lets presume a 40 MPa concrete mix is required to construct a stretch of pavement via slip-form paving, hand-placement from an agitator chute, and pumping in areas with limited access. The slip-form mix would have a low slump for machine compatibility and mix efficiency, the hand-placed mix would have more workability for ease of placement and labour cost, while the pump mix would require extra cohesiveness to prevent segregation. It is important to know what your project needs are and select an appropriate mix type. Antoun has a technical team dedicated to providing advice to clients for their project needs.
On-site
Compaction – With the exception of self-compacting mixes, mechanical vibration is necessary to remove air pockets from fresh concrete. These voids cause weak points where the concrete will tend to crack and propagate damage to surrounding areas. Even a single air pocket, the size of an almond, can propagate cracks upwards of 100mm to the surface of a slab. To avoid this, it is recommended to follow a steady-paced, consistent, and methodical pattern for compacting the concrete, ensuring no spots are missed.
Curing – Curing is the name given to procedures that control the moisture and temperature of hardened concrete. It is necessary to control these during the early stages of hardening to produce good concrete. Ideally, concrete is cured by keeping the surface wet with water, however curing compounds have also proven to be a highly practical approach. Temperature in an outdoor setting need not be controlled entirely, best practice is simply to avoid extreme conditions such as freezing temperatures and hot, dry winds. The Antoun team maintains in-situ ambient condition monitoring devices and acts appropriately to prevent adverse weather effects.
Design adherence – It should go without saying that following the drawings is important, however issues can and often do arise. Even if everything mentioned thus far has been met in spectacular fashion, concrete is still inherently meant to contract, expand, and crack. It is only through the careful set-out of joints, reinforcement, and support from sub-base, formwork etc. that everything may come together and form a structure that stands the test of time. It is important to keep an open line of communication between contractor and client to straighten out any kinks before they are (quite literally) set in concrete.