Volume Changes of Concrete

Understanding Volume Changes of Concrete Before Resurfacing
By: Robert Johnson, General Polymers (Published in Concrete Repair Bulletin September/October 1998)

Concrete is known to undergo volume changes as a result of physical and chemical reactions with it’s internal and external environment. All concrete goes through a phase of expansion during the plastic state followed by a phase of drying shrinkage during the hardened state. Unless shrinkage compensating Type K cement is used, the shrinkage will always result in a final volume that is less than the original volume. Even with Type K cements, the final volume will be less than the volume of the expansion phase. Understanding the nature and cause of these volume changes will help us to understand the proper design and installation of industrial coatings and polymer flooring systems on concrete.

PORTLAND CEMENT GEL EXPANSION

The addition of water to Portland cement creates a paste which quickly begins to gel. The subsequent addition of fine aggregate and coarse aggregate to produce concrete, provides a medium within which the cement gel produces the first volume change of concrete. The reaction of calcium sulfate and calcium aluminate in concrete to form calcium sulfoaluminate (ettringite), coupled with the complex exothermic hydration of cement results in an increase in volume of the concrete mass within the first few hours after mixing with water. Because of this expansion, isolation joints are required in locations where the plastic concrete will be placed in contact with a restraint, such as stand pipes, columns, footings, walls, etc. Once the concrete has hardened these isolation joints then act as control joints or expansion joints to accommodate other types of volume changes.

PLASTIC SHRINKAGE

Floors are generally poured into limiting forms, and the concrete is spread and trowelled smooth. Poured concrete floors can present different problems. Overtrowelling by steel or magnesium hand trowels or machine trowelling can result in an extremely smooth, hard, slick surface with little or no tooth for a coating. A “broom” finished surface, where a stiff bristle hand broom is drawn across the partially hardened concrete (Fig. 3), leaves a much better surface for coating, but even this should be checked for laitance. A wood float finish usually has a much better texture for coatings than a steel trowelled finish.

During the pouring of large concrete floors, shrinkage cracking may appear, often in a random pattern over the entire surface. Some cracking is practically inevitable; however, the judicious and planned use of control joints induces a pattern of cracking that can be tolerated by coatings or surfacers.

After they are finished, concrete floors are often treated with clear concrete sealers to control dust, or with concrete hardeners to improve abrasion resistance. Various epoxies and acrylics are used as sealers, and silicate solutions are used as hardeners. Where coatings are applied to the concrete, the specifications should indicate that no sealers or concrete hardeners will be used; however, this does not necessarily assure that they will not be used.

Concrete hardeners are usually sodium silicate solutions or metallic fluorosilicates. Where hardeners have been used, the concrete will usually appear glossy and may be a greyish brown color. The surface generally cannot be scratched with a coin. Hardened concrete cannot be successfully coated unless special methods are used to prepare the surface.

DRYING SHRINKAGE

Once concrete has taken initial set, it is said to enter the hardened state. In this form, drying shrinkage begins. Studies conducted by the U.S. Bureau of Reclamation on the drying shrinkage of 4x4x40-inch beams, that were dried for 38 months, showed that only 38 % of the drying shrinkage occurred in the first month. Over the next 10 months an additional, 52%, shrinkage occurred, and over the next 27 months the final 10% shrinkage occurred. These beams were allowed to dry from all four sides[ii]. What happens in the case of concrete slabs-on-grade?

With concrete slabs-on-grade, drying may only occur from one side. Consider the case of a concrete slab being placed directly on a vapor barrier or vapor retarder. As concrete dries, it will dry faster and therefore loose volume faster at exposed surfaces. For concrete slabs, this means that the exposed top is shrinking faster than the under side, and the edges faster than the center. This causes upward curling or warping of the slab at its perimeters and corners, leaving small voids between the concrete and the sub-grade. Because of concrete’s low flexural and tensile strength, when loads are applied to these unsupported edges by forklifts, hand trucks, pallet jacks etc., they fail by cracking. Proper repair requires that adequate support beneath the concrete be reinstated, and either repair of the crack along with grinding of the surface to a level plain, or complete removal and replacement of the affected concrete.

Another issue with concrete structures is that of long term shrinkage and it’s affect on the performance of joint fillers and/or joint and crack bridging technologies used for industrial coating and polymer flooring systems. For new construction facilities in particular the old rule of thumb to wait at least 28 days prior to application of polymer systems and joint fillers, would be totally inadequate for rigid or semi-rigid systems. Remember that the volume loss or shrinkage of concrete after the first month will at least double over the next 36 months. Therefore any joint or crack bridging technologies employed must be capable of accommodating that movement.

THERMAL MOVEMENT

As with all materials, concrete is subject to the laws of thermodynamics. As such, all concrete has a value associated with it, known as the Coefficient of Thermal Expansion. This value is dependent upon the varied quantity and physical properties of the raw materials used to produce the concrete and will vary accordingly. Typical values for concrete produced with differing aggregates is as follows

Thermal Coefficient of Expansion of 
Concrete Depending on Aggregate Type
                           

Aggregate type
(from one source)

Coefficient, %
per 100ºF

Quartz

 0.066

Sandstone

 0.065

Gravel

 0.060

Granite

 0.053

Basalt

 0.048

Limestone

 0.038

A 100ºF rise or fall in temperature will cause a length change of 0.066% in a given length of floor slab or wall using Quartz aggregate.

Regardless of the mix design, size or shape of the concrete structure, increases and decrease in temperature will cause a corresponding increase or decrease in the volume of the concrete mass. Obviously structures exposed to the elements will have the greatest potential for thermal movement. Materials used for industrial coating and polymer floor systems are also subject to the laws of thermodynamics and have specific coefficients of thermal expansion associated with each type of material. The relative differences between the coefficient of thermal expansion of concrete and that of the polymer system can be cause for concern if it is known in advance that the composite system will be subject to rapid and/or frequent changes in temperature. For protection from the deleterious effects of thermal shock the system must be designed to accommodate the anticipated movement.

SWELLING

Swelling of concrete may be considered reversible shrinkage. Much as drying shrinkage is a function of the loss of moisture, swelling is a function of the gain of moisture. Not all drying shrinkage however is reversible. Initial drying, when water is easily removed from large and medium capillaries and concrete is moving from it’s plastic state to it’s hardened state, is considered irreversible shrinkage and can account for up to 4 % mass loss in the first 12 hours of drying. On subsequent wetting and drying cycles deformations are believed to be essentially reversible.[iv] Volume change due to reversible shrinkage will account for mass loss of 1% to 2 %.

CARBONATION

Carbonation is a process that occurs when carbon dioxide in the air reacts with the moisture in the concrete and converts calcium hydroxides to calcium carbonates. The reaction is dependent on the amount of carbon dioxide in the environment surrounding the concrete. For this reason carbonation occurs slowly in nature over a period of many years. More commonly it is the result of improperly vented space heaters used to help decrease the cure time of concrete while consequentially creating an unnatural supply of carbon dioxide in cold environments. In this situation, the early formation of calcium carbonates will cause a soft chalky surface on the concrete and will result in low resistance to wear.

The presence of calcium carbonate lowers the pH of concrete and robs it of it's protective alkalinity, thus allowing for the corrosive attack of steel. The process of steel corrosion as the result of this loss is the transformation of steel, water and oxygen into ferrous oxide (rust). Rust has a volume 3 to 4 times that of steel. As the rust builds, it creates tremendous pressure on concrete, eventually resulting in cracking and spalling of the concrete.Test kits to easily determine the presence of calcium carbonates are available. A 1% to 2% solution of phenolphthalein in alcohol sprayed onto the concrete surface will change the color of carbonated concrete to a bright pink, while the color of normal concrete will not be affected. Repair to carbonated concrete depends greatly on whether or not the concrete is steel reinforced. If steel reinforced all carbonation must be removed. If non- reinforced concrete is used, removal of the surface carbonation layer by vacuum blasting will be sufficient for industrial coating and polymer floor system application.