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Canadian Building Digest

CBD-22. Concrete Floor Finishes

Originally published October 1961

H. B. Dickens

The most common industrial floor finishing material used today is concrete. When well laid it provides a hard and durable surface capable of meeting most floor service requirements at relatively low cost.

Yet many concrete finishes fall far below expectations. A survey by the American Institute of Plant Engineers in 1960 indicated that floors are the third most troublesome item in plant maintenance. The importance of this problem in Canada is perhaps reflected in the large number of inquiries that are directed each year to the Division of Building Research for advice on the causes of concrete floor deterioration and on methods of repair.

The premature failure of an industrial floor is a matter of real concern to the architect, the contractor and the owner of a building as well as to the user; a "weekend" patching job will rarely suffice and often the entire floor topping has to be removed and a new surface laid, resulting in costly interruptions in plant use.

Experience suggests that most failures can be attributed to improper design or poor workmanship and could have been avoided with careful attention to the factors that make for a quality installation. It is the purpose of this Digest to discuss these factors in relation to new concrete floor finishes. Some information is also provided on the more commonly used surface treatments for concrete floors.

The Base Slab

For simplicity, it will be assumed that the base slab on which a finish is to be installed has adequate structural strength for the applied. loads. The concrete should be made with hard well-graded aggregates and should normally contain not more than 4¾ gallons of water for each sack of cement. It should be a plastic mix that can be placed without voids and without accumulation of excess water on the surface. The proportions of fine and coarse aggregate should be adjusted to obtain a mix of desired workability but should normally be in the range of 1 part cement to 2½ parts of fine aggregate to 3½ parts of coarse aggregate. The aggregate should be well graded, the fine material ranging in size from ¼ inch down and the coarse material from ¼ inch up to ½ inches. The concrete should be thoroughly compacted by vibrating or by tamping and spading. Subsequent steps will vary with the type of floor finish selected.

Selecting the Finish

The method of finishing a new floor should depend on the service requirements to which it will be exposed. For light duty, where abrasion is not a serious problem, it may be sufficient to finish the base slab simply by floating and trowelling. If a power float is used, such a floor should be satisfactory for trucks with rubber tired wheels and even for light trucks with steel tires. The concrete should be floated to the desired grade and allowed to stand until all water sheen has disappeared. Final trowelling should be done only when the concrete is so hard that no mortar accumulates on the trowel and a ringing sound is produced as the trowel is drawn over the surface.

There are many industrial uses, however, for which the plain finish floor is not adequate and it becomes necessary to provide an improved surface by applying a special wear resistant topping. This topping may be placed either monolithically during the laying of the base slab and before it has set, or as a separate layer applied after the base slab has hardened. Both methods are used and each has its specific advantages.

One advantage cited for a monolithically applied topping is that it more readily ensures a good bond between the topping and the base. Separation and curling of the wearing layer that result from inadequate bonding combined with the effects of shrinkage arc, common faults in separately applied toppings; they are aggravated as the toppings are made thicker.

In contrast to this a monolithic finish must usually be laid much earlier in the construction schedule and is therefore vulnerable to damage during subsequent construction operations unless carefully protected. It may also have to be applied under relatively poor placing conditions since it must be fitted into the regular schedule of construction. A separate topping can be placed under good conditions when the building is closed in and the heating system available for use if required. In addition it is claimed that separate toppings permit more accurate control of the water cement ratio, a factor that has a most important effect on the quality of the finish.

In laying concrete toppings there are certain specific requirements that should be observed for each type of finish. In monolithic construction the base slab should be made with concrete no leaner than 1 part of cement to 2½ parts of fine aggregate to 5 parts of coarse aggregate so that it will have sufficient strength to withstand the shrinkage stresses that are set up by the richer topping. It should be of a stiff consistency to prevent free water from working up through the topping and increasing its water cement ratio; alternatively the topping should not be placed until the base has partially set and no free water is available.

When a separate topping is to be applied, the base slab should be swept with a wire broom before it hardens to roughen the surface by exposing the aggregate and to remove all laitance, dirt or loose aggregate. It should be thoroughly wetted just before the finish is placed, but there should be no pools of water left on the surface. A thin coat of neat cement grout should be brushed well into the surface just before the topping is spread. This will assist in obtaining a strong bond between topping and base. Waterproofing admixtures in the base slab should be used with care as these may tend to restrict penetration of the cement slurry and reduce bond.

Control of Quality

The main principles of good concrete floor construction are not new. They are simply the application of current concrete technology to the particular requirements of floors. Much of the discussion contained in CBD 15, "Concrete," is of direct interest to this problem which involves;

(i) the quality of the aggregate,
(ii) the proportions of the mix and amount of mixing water used,
(iii) the operations of placing and finishing,
(iv) the provisions for proper curing of the newly placed floor and its protection until it has attained sufficient hardness for service.

It is to remember that good concrete is made of cement, water and aggregate and that poor concrete is made of the same materials. The difference lies in the grading of the aggregate, the proportioning of the mix and the care with which the vital operations of placing, finishing and curing are carried out.

Since aggregates represent nearly 75 per cent of the mass of the concrete and provide the main wear resistance of the floor finish, they must be of proper quality. They should be clean and free from dust, clay, silt or frozen material, have sufficient hardness and satisfy the standard tests for durability. Grading is as important as hardness since poorly graded aggregates require an excessive amount of cement paste to fill the voids and result in crazing and dusting at the surface as well as reduced wear resistance. Too much fine material will require a large amount of mixing water and a high water cement ratio that in turn means a low-strength non-durable concrete subject to excessive shrinkage. Additional cement may be added to maintain a low water cement ratio but this too contributes to increased shrinkage.

Grading also affects the case with which topping can be placed. It is recommended that fine aggregate should consist chiefly of 1/16 to ¼-inch particles with not more than 5 per cent of the grains passing a 100-mesh sieve and not more than 15 pet- cent passing a 50-mesh sieve; coarse aggregate should range in size from 1/8 to 3/8 inch with all particles passing a 2-inch sieve. Special aggregates offering increased abrasion resistance and non-slip properties may be added to the surface finish by dusting on a mixture of the aggregate and cement and then floating the material into the freshly placed concrete topping.

In proportioning a floor topping there are two main requirements to be kept in mind. The mix should be as lean as possible, containing a minimum of cement and water to give low shrinkage and lessen the risk of cracking, curling and dusting. The mix must be sufficiently workable for it to be placed without the prolonged tamping or vibration that cause segregation of the aggregate and bring fine material to the top, creating a weak surface layer.

A high quality wearing surface will have uniform distribution of coarse aggregate through the entire depth of finish and up to the wearing surface with no film of laitance or weak mortar at the surface layer. The proportions of the topping mix largely govern the degree to which this standard of finish can be attained. Although the best finishes are obtained from a relatively harsh stiff mix, a topping mix can be too dry for proper placing. The desired consistency will depend on the method of placing, whether hand labour or mechanical equipment is to be used.

A relatively lean dry mix that is difficult to work with hand tools can be readily compacted and smoothed with power driven equipment. Such a mix is obtained by using one part of cement, one part of fine aggregate, 2 parts of coarse aggregate by volume, and a water cement ratio of only 3½ gallons per sack of cement. This provides a mix with very low slump, It is not easy to spread this dry mix uniformly, but with power tools such as the vibrator screed and the mechanical float it can be done satisfactorily, and the improved wearing qualities that result fully justify the use of such equipment. If it is necessary to work it by hand, the water cement ratio must be increased to 4 or 4½ gallons per sack of cement and, in addition, a slightly richer mix may be required.

Power machines are particularly effective in overcoming one common cause of defective floors: incorrect timing of finishing operations. Richer mixes and the increased water content required to make a mixture sufficiently workable for placing with hand labour necessitate a waiting period while the topping becomes stiff enough to trowel. This waiting period is a source of much difficulty in floor finishing. Floating, which is done to compact the surface, fill the holes and level out the humps left after screeding, if done too soon will increase the bleeding action that produces laitance and a soft layer of material at the surface. Trowelling, which provides the final smooth finish to the floor, will also tend to bring moisture and fine materials to the surface if carried out before the mix has attained sufficient stiffness. With the stiff mix made possible by power equipment these dangers are largely eliminated; there is little or no waiting period and the finishers can begin work almost as soon as the topping is placed. Close supervision of all operations is, however, a necessary prerequisite of a high quality floor. Such supervision must ensure that no short cuts are taken; to hasten drying of the surface of the mix by dusting on dry sand or cement, or to add water to make trowelling easier once the mix has attained a stiff consistency can have particularly serious results and must be avoided.

Finally, there is the vital factor of adequate curing. Its neglect is a prime cause of early deterioration. The chemical reactions between cement and water that cause them to harden continue only if moisture is present and the temperature is favourable. The internal structure of the concrete is built up to provide strength, resistance to wear and water tightness. Floor finishes, however, present such large surface areas that loss of moisture through evaporation takes place rapidly unless measures are taken to prevent it. Rapid drying not only stops the chemical reactions but may cause dusting and cracking of the surface because shrinkage takes place before the concrete has much strength. To prevent: drying, water for curing should be applied to the concrete as soon as is possible without marring the surface.

Frequent sprinkling or covering of the surface with wet sand are common methods of curing. The application of polyethylene sheet will greatly assist curing if the joints are tightly sealed to prevent evaporation. Ponding is another method sometimes used; the floor slab is surrounded by small dykes of sand and the enclosure kept filled with water to a depth of at least 1 inch. The curing period should be at least a week in mild weather and longer in cold weather, although this can be reduced where high early strength cement is used. For best results temperature should be above 50°F since concrete hardens very slowly at lower temperatures. At freezing temperatures all concrete should be protected until it bas gained sufficient strength. When necessary, heat should be furnished. These precautions are particularly important for floors because of their large exposed area and limited thickness.

Artificial heating devices must be used with great care, since the high temperatures near them cause too rapid drying unless the concrete is well protected. Heaters should be raised and the floor underneath for a distance of several feet on all sides should be covered with 3 or 4 inches of sand saturated with water during the curing period. In addition, fuel-burning heaters should be vented to prevent: accumulations of carbon dioxide within the enclosure. It can be harmful to fresh concrete during the first 24 hours by causing excessive carbonation of the surface.

Surface Treatments

Even the best floor will tend to produce some dusting of the surface in use. This can be reduced by applying a material that will assist in hardening and binding, but these treatments should not be regarded as cure-alls for poor materials or careless workmanship. Although treatment will improve many surfaces it cannot be expected to make a good wearing surface of a poorly built floor. A number of proprietary hardening materials are available.

Solutions of fluosilicate of zinc and magnesium in water have been used with success either separately or as a mixture of 20 per cent zinc and 80 per cent magnesium. Linseed, china wood or soybean oil may also be used as surface treatment. By sealing the floor finish they help to protect the floor from some materials used in industry that will attack concrete. In the more severe industrial exposures, such as in chemical laboratories and acid plants, these treatments will be of little use and special acid proof coverings may be required.

Where appearance and decorative effect are important, concrete floor paints or inorganic chemical stains may be applied. Paints require protection in areas of heavy traffic and stains may need several applications before the desired colour effect is obtained. Colour may also be achieved by mixing pure chemical pigments with the concrete topping at the time of placing. Occasionally, a dusted-on colour mixture has been used but it is only suitable for floors subject to light foot traffic. Its application involves a difficult operation and it is not easy to obtain satisfactory results. All of these surface treatments may be further protected with an ordinary paste wax.

Instead of a surface treatment, a concrete slab may be covered with one of the many common flooring materials including asphalt, linoleum or vinyl tile, slate, concrete or ceramic tile, terrazzo or hardwood flooring. The selection and application of appropriate material is beyond the scope of this Digest but there are certain basic requirements that should be mentioned. These relate mainly to problems of moisture. Concrete floor slabs in contact with the ground (either below or at grade level) require a flooring material resistant to moisture and alkali. In addition, it is wise to include a moisture resistant membrane beneath the base slab and to make adequate provision for drainage in the sub grade.

With a suspended concrete floor, ground moisture is not a problem but the concrete itself must be allowed to dry thoroughly before coverings such as asphalt, linoleum, vinyl or wood are installed. A simple test to determine whether a concrete floor is dry is to place several pieces of a material such as polyethylene, rubber or linoleum over the floor and weight them down to maintain close contact. If there are no damp spots after 24 hours the flooring can be safely applied.


This Digest has considered the factors that affect the quality of a concrete floor-finish. Much useful information as well as suggested specifications may be found in the publication of the Portland Cement Association entitled "Concrete Floor Finishes" that is distributed in Canada by the Canada Cement Company. It is wise to, remember, however, that even with a sound specification the quality of a concrete floor is still very much dependent on the workman on the job. The aggregate may be carefully selected, the materials properly proportioned, the base slab well prepared, yet a poor quality floor may still result from the use of too wet a mix or incorrect timing of the finishing operations.

It is unfortunate that the easiest way to lay a concrete floor is with a wet mix since such a mix readily allows the aggregate to segregate, produces laitance on the surface and has a high drying shrinkage and low strength. The end result is usually limited abrasion resistance, surface dusting, shrinkage cracking and bond failure. Avoidance of such failures depends as much on quality control at the site as on the correct design of the floor.

Date Published: 1961-10-01
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