Like the birth of a child, concrete's first few hours and days can (and will) determine the characteristics of the adult. Unlike the birth of a child, which in most cases takes place in a controlled environment, the delivery room for a concrete slab is the job site, a place that is at the mercy of the weather and the delivery schedule. DNA (chemistry) plays a very important role in the development of both a child and a concrete slab. In the case of the child, at birth gestation has taken place and the child is born with all of his or her parts in place. In the case of concrete, the cement, aggregate, water and admixtures meet for the first time in belly of a concrete mixer and the gestation begins.

Time and temperature

Concrete begins to stiffen in the early stages of the concrete's life. This stiffening is called setting. The time of setting is divided into two time periods, initial set and final set. In a given concrete mix with a temperature of 70F initial set will occur when the concrete is about 4 to 6 hours old, and final set will occur at about 12 to 18 hours. While the American Society for Testing and Materials (ASTM) has developed a test to determine both initial and final set. In the field, initial set is said to occur when it is difficult to stick a knife blade into the concrete, and final set occurs when it is difficult to drive a nail into the concrete.

Depending on temperature, concrete should be placed within an hour to 1 1/2 hours after being mixed. The time of placement is very important, because the concrete chemistry is beginning to become active and is greatly affected by temperature and time. Concrete that is placed at 90°F sets twice as fast as concrete placed at 70°F. On the other hand, concrete that is placed at 50°F with take will take twice as long to set as a concrete placed at 70°F.

Concrete stiffens very slowly at first. Then, as time passes, the rate of stiffening will increase ever faster. We have only a few hours to place and finish concrete. Most specifications should require the concrete to be out of the truck and in the forms before the concrete is 1 to 1 1/2 hours old.

Bull floating, subsidence and bleeding out

After the concrete arrives at the job site and is unloaded, the concrete is leveled or screeded to proper elevation. After screeding, the next step is fill and cut floating, popularly known as bull floating. This operation flattens the surface of the concrete and brings more fine material to the surface for later finishing steps.

The next phase in the life of a concrete slab is subsidence. Subsidence is the forcing of water and entrapped air to the surface, caused by the weight of concrete pressing down upon itself. When water is forced to the surface the concrete is said to be bleeding out. Concrete starts to bleed out approximately 30 minutes to an hour after being placed.

Bleed water might remain on the surface for only a few minutes or a number of hours, depending on the evaporation rate. The evaporation rate is a direct function of relative humidity, air temperature and wind velocity. When bleed water is present on the surface of the concrete slab, all work must stop. If concrete is worked during this time, bleed water will be worked into the surface of the concrete. This greatly increases the water to cement ratio and, as a result, reduces the concrete's strength and durability.

Again, during the time of subsidence and bleeding no finishing activities should take place on the concrete. At this time the concrete is said to be "at rest." Actually, nothing could be further from the truth. As soon as water comes into contact with the portland cement large amounts of calcium ions, hydroxide ions and a form of silica (specifically SiO2) are released.

With the release of hydroxide ions, the pH of the concrete rapidly rises to a highly alkaline pH of 12 to 13. Some people are (or can become) very sensitive to this highly alkaline material, and with extended exposure can suffer severe skin burns. As the pH rises and the concentration of calcium ions and hydroxide ions increases, calcium hydroxide is being formed. Calcium hydroxide reacts with SiO2 to form calcium silicate hydrate (C-S-H). C-S-H and other hydrates bond the aggregates together to form concrete. While are other chemical reactions are taking place that help to bond the aggregates together, C-S-H is the most important one.

Early finishing and evaporation control

When the bleed water has disappeared the next finishing procedure, floating, may begin. Before a floating machine can be placed on the surface of the concrete the concrete must be tested. When the concrete can support the full weight of a workman, and when the heel of that workman's shoe leaves a only a slight impression in the fresh concrete of no more that 1/8 to 1/4 inch, the floating process may begin. Floating further flattens the concrete surface and brings more fine material and cement paste to the surface for later finishing. The floating process leaves the concrete open and allows excess water in the form of vapor and entrapped air to leave the slab.

During these early finishing times the concrete slab is especially vulnerable to plastic shrinkage cracking and surface crusting. Both of these are the result of rapid loss of moisture from the surface of the concrete slab. If the rate of evaporation approaches 0.2 lb/ft2/hour precautions to protect the surface of the concrete are necessary. This is in accordance with ACI 305, Guide to Hot Weather Concreting.

As stated before, the rate of evaporation is a function of wind speed, air temperature and relative humidity. It is commonly known that on hot dry days fresh concrete must be protected during placement. This is true. What is less well known is that fresh concrete also needs to be protected on cool days when the wind velocity is high, and/or when the relative humidity is low.

Aside from erecting the building superstructure and roof prior to pouring the concrete, there are few practical methods for protecting an exposed concrete slab from rapid moisture loss. One of the oldest methods is to place wind screens around the slab to prevent wind from blowing across the slab. A second method infrequently used is the fogging of the slab. Fogging is defined as "the spraying of atomized water over and above the concrete slab." Fogging does not add water to the surface of the concrete slab, as the fog (atomized water) above the slab evaporates before it can reach the surface of the concrete. The fog increases the relative humidity above the slab and increases the vapor pressure at the surface of the slab, thereby slowing the rate of evaporation at the surface of the slab.

A third and immensely more practical method for protection of fresh concrete from plastic shrinkage cracks and surface crusting is the use of L&M's evaporation control product, E-Con. This odorless, water-based liquid is sprayed on to the surface of plastic concrete and forms a thin continuous protective film. By spraying E-Con on the surface of the concrete, the rate of evaporation is reduced by increasing the vapor pressure at the surface of the slab. The use of this product does not interfere with the normal setting of the concrete. E-Con can be applied as often as required to protect the concrete, without leaving a residue. E-Con is used only on plastic concrete. It is not effective as a curing compound, as it lasts only an hour or two.

Hard troweling to "close up" the surface

Meanwhile, the new concrete has continued to become stiffer but has not yet reached initial set. The concrete is still only a couple hours old and the stiffening is due primarily to drying. Very little hydration has taken place at this point. Also, at this time the concrete has reached the point at which subsidence has almost been completed and the final finishing steps to close up the floor surface can begin. This is accomplished through a process known as hard troweling.

While the floating process utilizes large blades or float shoes that are held flat against the surface of the concrete to prevent closing of the surface, the machine troweling process uses narrow blades. At the beginning of the troweling process the trowel blades are held flat against the surface of the concrete. As the troweling process proceeds one edge of the trowel blade is raised off the surface of the concrete at a slight angle, allowing the opposite edge of the blade to contact the surface of the concrete. As the troweling process continues the angle is increased, reducing the contact surface area of the trowel blade with the concrete and thereby increasing the downward pressure on the surface of the concrete. The rotating of the troweling blades and the increasing downward pressure physically compresses and closes up the surface of the concrete slab.

Throughout the mass of the plastic concrete there are millions and millions of microscopic particles of unhydrated cement known as cement grains. During the closing process the action of the troweling blades forces the cement grains closer together and forces out water vapor and entrapped air. With the cement grains now in close proximity to each other as they hydrate, the products of hydration will be tightly bonded. The result is a very hard, dense and abrasion-resistant surface.

When concrete is closed up too quickly, any water vapor and internal air voids trying to leave the surface of the concrete can become trapped in the upper surface of the concrete. This creates surface blisters, paper-thin cement paste bubbles found on the surface of the concrete. They are generally about 1/2 inch in diameter. Any blisters found should be ruptured and the surface refinished. If blisters are not immediately removed during placement, the blistered area will eventually leave a small surface defect in the concrete.

Engineering for shrinkage and curing

A few hours after hard toweling has been completed the joints should be cut into the concrete at the specified places. After final set the rate of hydration will increase and drying shrinkage will occur. As drying shrinkage occurs the concrete is placed in tension and concrete will crack to relieve these tensional forces. Joints are an engineered weakness that allows the concrete to crack where desired, and not in a random pattern.

The concrete slab's first day ends as the curing begins. Curing is a process by which water is added to the surface of hardened concrete or by applying curing compound, such as one of L&M's curing products, over the surface of the concrete, holding in the moisture. In order for concrete to gain strength liquid water must be present in the pore structure of the concrete. Calcium hydroxide and SiO2 can only react in an aqueous (water) environment.

With curing in place, the concrete slab's gestation is now completed. With the concrete finishing midwives having completed their task, the concrete has reached the end of its longest, and most important, day.


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© 2007 L&M Construction Chemicals, Inc. | ConcreteNews January 2007.

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