Essential knowledge for concrete floor construction
In a webinar hosted by The Big 5, part of the American Concrete Institute (ACI) Webinar series, we examine the classes and types of concrete flooring and deep-dive into the traits of good concrete flooring.
Salah Abuobaid, Concrete Floor Specialist, Yibna Construction & Contracting Est.
- Concrete floor construction is guided by ACI 302.1R-15 Guide to Concrete Floor and Slab Construction, ACI 360R-10 Guide to Slabs-on-Ground, and ACI 117.1R-14 Guide for Tolerance Compatibility in Concrete Construction.
- Concrete flooring can make up 15-20% of the cost of the entire project and is the most used item in the structure.
- The traits of a good concrete floor include durability, flatness & levelness, minimised cracks and dust proofing through curing & surface treatment.
What are concrete floors?
Concrete floors are rigid pavements bound by cement, as opposed to flexible pavements, which are aggregates bound by Bitomine. Concrete floors are used for warehouses, industrial institutions, car parking lots, aprons, ports and harbours, cold stores and freezer rooms, as well as roads and residential buildings.
Table 4.1 in ACI 3013 outlines the classes of floors, anticipated traffic type, the intended use of the floor, specific considerations as well as the final finish. Concrete floors can either be Slab on Grade/Ground (SOG) where the concrete floor rests on compacted fill, or Topping, where the concrete floor rests on concrete.
There are two types of Topping: Bonded, which behaves as one unit with the concrete under (thickness 2cm – 5cm) and Un-bonded, which behaves independently (thickness can be over 7.5cm for foot traffic or 10cm for vehicular traffic). It is important to remember that making concrete Topping of only 4cm-5cm needs additional action such as roughing the substrate and using a trustable bonding agent.
Essential knowledge of concrete floors
Designers and contractors must focus on concrete floors because concrete flooring can make up 15%-20% of the cost of the entire project, as it is the most used item in the structure. Floors are extremely important as they must sustain new industry development in racking systems, and in MHE (mechanical handling equipment).
The traits of a good concrete floor include:
Durability comes from a proper design by the designer as well as the appropriate concrete mix design, suitable reinforcement and joint selection. ACI 360 has many chapters talking about joints, substrate and methods of design.
The designer must know the point load from racking or distributed load, as well as the wheel load from the MHE and the K-value (coefficient of subgrade reaction). The designer will be able to give clarification on the thickness of the floor, the concrete strength and type of reinforcement (conventional or fibres).
The concrete mix is responsible for 50% of the equation, and concrete floors have a special mix design that is different from concrete for footings, columns and slabs. The properties of plastic concrete include workability (easy to place, compact and strike off), finishability (easy to straight edge, float and trowel), 1-3% bleeding, and a setting time of 3-6 hours. Meanwhile, the properties of hardened concrete include surface abrasion, wear-resistant, high impact resistant, flexural strength and shrinkage characteristics that minimise potential cracking.
Concrete components consist of aggregates, cement, potable water and admixtures (Type F and G are commonly used in floor construction). Aggregates consist of coarse, intermediate and fine, and uniformly graded materials provide high plastic and hardened concrete performance at the lowest paste content. Meanwhile, cement types include Type 1 (ASTM C150), which is commonly used on floors, and PPC, which is suitable for hot weather.
For good concrete proportion, the cement content is between 280-415 depending on the aggregate size. A high amount of cement makes concrete cohesive. For the water-cement ratio, ACI 302 recommends between 0.47-0.55. Meanwhile, the air content should not be between 3%, and air-entrained admixtures are used when freezing is a possibility. For aggregate, the workability factor should be between 32%-40%, the paste fraction by volume should be between 25%-28%, coarseness factor 52%-72%, and the mortar fraction by volume should be 53%-57%.
When looking at types of reinforcement for concrete, we have conventional rebar or wire mesh, and synthetic or steel fibres. Micro-fibres are used for plastic shrinkage and settlement cracks. At the same time, steel fibres provide reinforcement in the way of increased strain strength, impact resistance, flexural toughness, fatigue endurance, increase in contraction joint spacing and crack with control.
There are three types of joints in concrete floors: construction joints, isolation joints and contraction joints. Using steel fibre in floors reduces spacing of contraction joints even though we can do jointless floors (without contraction joints) by increasing the dosage of steel fibres.
- Flatness & Levelness
The flatness refers to the degree of undulation on a floor while levelness refers to the degree of inclination. Flatness and levelness offer better movement for machines and efficient use of space when storing at heights.
According to ACI 117, there are two methods of testing flatness and levelness, including the straight edge method and the new F-Numbers system. The advantages of the straight edge methods are that it is easy to use, availability, does not need training and offers immediate results. However, the disadvantages are that it may not be accurate, different testing gives different results and the floor may be level but not flat. Table 126.96.36.199 talks about the manual straight edge method.
The F-Number system is the new American Concrete Institute (#117) and Canadian Standard Association (#23.1) standards for the specification and measurement of concrete floor flatness and levelness (Ff for flatness and FL for levelness). Flatness relates to the bumpiness of the floor, and levelness describes the tilt or pitch of the slab. The higher the F-Number, the better the characteristic of the floor. Table 188.8.131.52 in ACI 117 talks about F-Number tolerances. Many instruments in the market can be used to test the F-Number system accuracy in flatness and levelness. Table R4.8.4 outlines methods to evaluate flatness.
- Minimising the appearance of cracks
ACI 302 states the following: “Even with the best floor designs and proper construction, it is unrealistic to expect completely crack and curl-free floor. Consequently, every owner should be advised by both the designer and contractor that it is completely normal to expect some amount of cracking and curling on every project and that such an occurrence does not necessarily reflect adversely on either the accuracy of the floor’s design or the quality of its construction.” (Ytterberg 1987).
Concrete cracks are caused by the tensile stress inside the concrete. In general, tensile strength is around 10% of the compressive strength of concrete. So when the tensile stress inside the concrete exceeds the tensile strength of the concrete, the concrete will crack.
Cracks are caused by volumetric change, thermal stress, restraint of the slab, excessive loading, premature loading, and vibration and movement. Cracks can occur in plastic concrete (plastic shrinkage cracks and plastic settlement cracks), as well as in hardened concrete (drying shrinkage and thermal contraction cracks, differential settlement structural cracks and curling).
For shrinkage cracks in plastic concrete, this is caused by direct sun and strong wind which will cause hairline cracks on the surface. This can be prevented by dampening the base when no vapour retarder is used, making windbreakers and sunshades, fog spraying the surface, covering the concrete with polythene sheet as troweling is completed, adding microfibers in the concrete and preventing rapid drying.
Having no cover above reinforcement causes settlement cracks in plastic concrete. Increasing the cover above reinforcement and increasing water-cement ratio can prevent this. Dry shrinkage cracks start to appear after 4.12 months and are caused by excess water in the concrete mix, and the volume change cause micro-cracking to develop into full cracking. This can be prevented by reducing water in the concrete mix and using large aggregates.
The most efficient way to reduce cracks in concrete is to use a good mix design, a polythene sheet not less than 250 micron thickens under the concrete floor, use steel or synthetic fibre in the concrete, avoid pouring concrete under direct sunlight or windy weather, avoiding using water during finishing, use all the right joints, and immediate curing.
- Dust proof: Curing & Surface Treatment
Many products in the market now combine dust proofing and curing compounds to work together. Methods of concrete curing/surface treatment include curing covers/Wet Cure. This works by covering the floor with reusable or single covers such as saturated covers of burlap usually for 3-7 days. Under this method, water is applied to the surface, but although it is a good curing method, it is also costly. With Membrane Curing, the curing compound should meet the provisions of ASTM C309. It should retain the existing water inside the concrete, resin-based curing compound is used when the floor is expected to have coatings. Wax-based curing compound must be mechanically removed before the application of any coatings.
In warehouses, production facilities and car parking, some floors get surface treatments such a liquid surface treatment which is applied on new floors and includes certain chemicals such as sodium silicate. When such chemicals penetrate the slab, they react chemically with hydroxide, a product of cement hydration. The effect is a densified, glossy surface and reduced dusting. However, such products do not replace curing compounds or surface hardeners.
The purpose of coating surface treatment is to give the floor the following characteristics: dust proof, shiny surface, increase in abrasion resistance, easy maintenance and the availability of colour options. Some examples of coating treatments include Epoxy, Polymer-resin coatings, and Polyurethane coatings.
Concrete floors have many uses and offer numerous benefits in terms of strength, rigidity, span, fire resistance, acoustics, maintenance and longevity. As concrete flooring can make up 15%-20% of the cost of the entire project, it is extremely important to understand the traits of a good concrete floor, such as durability, flatness & levelness, minimising the appearance of cracks, and curing & surface treatments, as well as the latest industry developments in concrete flooring.