LATICRETE® is organized into regions in North America and outside of the North American continent. Our regional managers are trained and ready to assist you in helping you with your concrete floor problems. Contact us to locate the manager nearest to you. We look forward to solving your concrete floor and other concrete related problems.
Hot Weather Concrete
Q: What can we do to prevent the problems we are experiencing with our concrete in hot weather?
A: Concrete is a building material that can be used in all types of weather, throughout the year. In hot weather the craftsmen must handle the concrete with enough care and protection as to imitate "ideal" conditions.
During hot weather concreting, the temperature is one of problem, relative humidity and wind speed are the other components to the problems in hot weather.
History and experience have alerted us to the effects of hot weather on fresh concrete, 1. Increased water demand in the mix. 2. Rapid slump or workability loss. 3. Rapid stiffening or actual setting . 4. Development of surface plastic shrinkage cracks. 5. Fluctuation in a downward trend of air content.
Approaching this from a contractor's point of view, the water demand and slump loss can be addressed in the preconstruction meeting at the start of the project. Discuss these issues with the engineer and the concrete producer. Request proportioning the concrete as specified to meet the changing environment. Use of ice in the mix is possible, but it is an extreme measure and brings its own burden of cost.
Hot weather concreting may have a higher water requirement to maintain the workability. Mainly due to the rapid hydration and evaporation which occurs between batching in hot weather and on site delivery. A mix design performing well in moderate temperatures will need to be proportioned differently for hot temperatures. Remember, if more water is needed, more cement or cementious material will be needed to balance the increase in water.
Speaking to the issue I think you are really asking about, the control of surface plastic shrinkage cracks and surface crusting from rapid surface evaporation, consult ACI 305. It will direct you to a lot of helpful procedures to protect your concrete in hot weather. The one item to focus upon is the use of a monomolecular film on the surface of the fresh concrete. The L&M monomolecular film product is called E-CON™.
Your crew would apply a repeat spray application of this easy to use liquid. The purpose of the monomolecular film is to reduce early evaporation of the surface water from sun and wind. A simple pump sprayer is the normal device to apply the E-CON™. Finishing and tooling are not interfered with and the tools can travel right over the liquid with no consequence.
Summarizing, be proactive and alert the owner and the engineer of your crew are not working in Lab conditions, promote the proportioning changes to the concrete for the hot weather conditions. Finally, use the finisher's friend E-CON™ as an insurance to prevent early crusting of the surface and to resist the formation of surface plastic shrinkage cracks. Finally, consider working at a time of the day when the sun and wind are less a problem, early evening or nighttime.
Concrete Surface Crusting
Q: We have a problem with surface crusting on our fresh concrete slabs. Our sites are almost always exposed to sun and wind. Even though we know that "blessing" the concrete surface with a sprinkle of water by the finisher is not the best answer, we have little else to fall back on. What else can we do?
A: "Surface crusting" is defined as the drying out of the surface of concrete or mortar before setting occurs. This condition gives a false sense of setting. The top of the slab will not have enough moisture at the surface for the finishing operation, which frequently results in a wavy concrete floor surface.
The best independent source I can refer you to is ACI 305, Hot Weather Concreting, (Section 4.2). In this document you will find statements suggesting to the reader a number of practical actions including:
- Avoid placing concrete during the hottest time of the day
- Cool the concrete
- Use a monomolecular film liquid
This last suggestion is, in my opinion, the easiest and most cost-effective step your crew can take to minimize crusting and surface plastic shrinkage cracks that typically go with concrete placements in hot, windy, or dry weather.
Most commercially available monomolecular films are sold in a concentrated form. The dilution ratio I am most familiar with (E-CON™ by LATICRETE®) will provide your crew with up to 50 gallons of monomolecular film liquid from 5 gallons of concentrate. The typical application rate of the diluted material is roughly 200 sq ft/ gallon (5 sq.m/L).
To protect freshly placed concrete, apply the diluted monomolecular film liquid with a durable hand-pump sprayer. It is best when your crew applies it right after the concrete is screeded or bull-floated. Floating and finishing steps will not interfere with the function of monomolecular film liquids. While a finishing tool or finishing machine can pass over the treated surface numerous times without reducing its effectiveness, it is a good practice to reapply if the slab will be left exposed for a long period of time before the next finishing step commences.
Monomolecular films are not curing compounds. They are effective only on freshly placed concrete and last for an hour or two. Subsequent applications can be installed without a problem on difficult concrete pours. In addition to protection of fresh concrete from wind and heat, the installation of mineral and metallic shake-on hardeners is helped greatly with the use of monomolecular film product.
The monomolecular film liquid is named E-CON™, short for Evaporation Control and Economy. A single spray of E-CON™ will reduce the early evaporation of a concrete slab by as much as 80% in wind and up to 40% in hot temperatures. It is economical, too! At an estimated cost of less than a penny a square foot, no job should be without E-Con's protection. I recommend that you include E-CON™ in your regular concrete placement regimen to overcome crusting and other problems associated with the rapid surface drying of fresh concrete. The product concentrate comes packaged in 5 gallon, 1 gallon and a convenient one-quart trial size. For more information contact your local LATICRETE representative.
Concrete Floor Joints
Q: We are an international development company. We specialize in buildings with large areas of concrete floors. Joints in the floors have become a major problem for us. We have a maintenance company on retainer but the repair patch never lasts. Why do we have to have joints at all? Is there a long lasting repair for broken joints?
A: My answer has to be divided into sections and each section will address joint types and a repair option when the joint edges break or become a problem. Do concrete floors need joints? Yes, concrete floors should be jointed. Random cracking will develop in concrete floors without joints. Random cracking occurs because concrete shrinks when it moves from its original plastic or wet volume state to its hardened volume. This shrinkage or volume loss of concrete occurs as it hardens. The friction of the base or soil against the bottom of the shrinking concrete restricts the concrete from moving freely, thus cracks develop. This is also true for pipes penetrating the concrete and other embedment. These embedments restrict the movement of the shrinking concrete and a crack will develop near the embedment or support column.
The first joint we can learn about is the Construction Joint. Your floors have construction joints where one lane of concrete was placed and finished and the next day or later another lane of concrete was placed along side of the older lane. Officially, a construction joint is the division between two successive placements of concrete. Reinforcement may pass through a construction joint. (See diagram A). The construction joints in your floors should be straight, and the edges should be unbroken. The way to maintain and repair construction joints is to clean them out and fill them with a rugged polyurea joint sealant. If the joint faces are not out of vertical alignment but the edges are broken or missing, a rugged polyurea joint sealant can also be the proper repair material.
The second joint we can learn about is the Contraction Joint. Your floors should have contraction joints already in the surface. Contraction joints are also called (crack) control joints or saw-cut joints. Contraction joints are cut into the concrete to prevent random cracking due to volume loss or shrinkage of fresh concrete as it hardens. Contraction joint cutting can be accomplished by a jointer hand tool or, more commonly, by a concrete saw cutting blade. Contraction joints are vulnerable to edge breaking or deterioration after the floor is put into service. This vulnerability can be overcome by filling the contraction joints and their deteriorated edges with a rugged polyurea joint sealant. This joint filling is most effective in non-moving cracks and fully cured slabs. For best performance new floors should be allowed to cure for a minimum of 60 days. Deteriorated contraction joints can be cleaned out and rebuilt with polyurea. (See diagram B)
LATICRETE® can develop a program for your entire organization and your maintenance contractor to protect new joints in your floors and to rebuild or repair the failed joints. The product we offer is a premium, 2-part rugged polyurea called JOINT TITE 750™. This product is a two-component, self-leveling liquid. The most popular color is concrete gray, but other colors are available. JOINT TITE 750™ can be installed in wide range of temperatures and can be opened to traffic in less than an hour. This means that your plant operations will suffer little to no down-time and your material handling machines will be able to roll smoothly across JOINT TITE 750™ filled or repaired joints.
LATICRETE® is organized into regions in North America and outside of the North American continent. Our regional managers are trained and ready to assist you in helping you with your concrete floor problems. Contact us [support-downloads/contact-us]to locate the manager nearest to you. We look forward to solving your concrete floor and other concrete related problems."
Concrete Vapor Barrier
Q: What is the purpose of a "vapor barrier" under a concrete floor, does it make the bottom weak?
A: This is a political football of a question. I will get the easy answer out of the way first. A vapor barrier does not make the bottom of the concrete weak. There are excellent curing conditions in the bottom section of a concrete slab and these tend to make the concrete stronger.
I suspect your question sprang from hearing someone say the bottom of a concrete slab was soft or plastic, when laid over a vapor barrier. This is true for a comparative short time. The comparison is to the top section of the concrete slab. The top section of a concrete slab generally stiffens faster than the bottom section. Both regions will harden, but at differing rates.
Continuing to answer your question in reverse order. The purpose of a vapor barrier is to eliminate rising dampness from the subgrade. Concrete is a stiff and structural material, it is also porous to water or water vapor. Compared to the atmosphere above a slab, the interior of a concrete slab is a high pressure area. Moisture vapor can travel through the interior pore system of the concrete and reaching the surface, condense into liquid water under the floor coverings.
Floor coverings do not perform well when underlain with a minute but persistent supply of liquid water. Carpet dampens, vinyl tile losses its bond, urethanes age due to the high pH of the rising water and epoxies can become cloudy in appearance. Waterproof vapor barriers are intended to prevent any rising dampness from the surface.
Lightweight Concrete Aggregate
Q: What is a general definition for lightweight concrete aggregate?
A: It seems everyone is weight conscious these days? Definition: Aggregate of low specific gravity expanded shale, sintered shale, clay, or slate. These are commonly used to replace some or all of the normal weight coarse aggregate in lightweight concrete.
My experience with lightweight concrete is limited to the actual use and placement of the lightweight concrete. I can tell you the normal concrete cubic foot weight is 150 lbs per cubic foot. The lightweight version is 90 to 120 lbs per cubic foot and this is accomplished by replacing the coarse aggregate with vitrified shale.
Concrete Spalling and Cracking
Q: Occasionally, we are called to repair spalled and scaling concrete on either new or old concrete placements. We've tried mixing our own repair material, mostly “moose milk” (acrylic latex) with Portland cement and fine sand and have had some success, but not always. We are looking for a more reliable way to repair eroded surfaces in concrete work. Call backs are eating up our profits. Any suggestions?
A: This is a very common need and the answer is a product named DURACRETE™. This product was a long time in development, because everyone knows that a “half fast” repair failure causes more dissatisfaction than the original complaint.
DURACRETE is a single component, dry polymer-reinforced, cement based patching and re-surfacing mortar. This product has been formulated to repair, slope or level, and to resurface structurally sound interior or exterior concrete surfaces that are pitted, worn, spalled or scaled. In addition, this product can be “sculpted” to repair vertical and horizontal concrete members, curbs, and deep voids.
The secret to DURACRETE's performance is twofold:
First: Duracrete is “strength matched” to normal concrete. With over 60 years of experience, LATICRETE® has found that patching materials which more closely match their compressive strength to the strength of the concrete substrate adhere better and last longer. In other words, high compressive strength repair materials are generally not better.
Second: DURACRETE contains a technologically advanced, dried polymer, reinforcing and bonding component. At the same time, DURACRETE reduces waste, simplifies installation, and ensures outstanding freeze-thaw durability in colder climates. This particular polymer is key to your search. Once this product is properly bonded to the base concrete, it stays there.
Picture in your mind a scaled driveway surface. The DURACRETE repair approach would be to power-wash the entire driveway with a high-pressure water blaster and selectively apply CITREX™ de-greaser liquid to any oil stains. Allow the standing water to dissipate, yet maintain a damp surface. Fill in all prepared spalls, voids, and scaled areas with a stiff, mortar-like consistency of DURACRETE. Allow to harden for an hour or so.
After setting for a few hours, broom a thick coat of semi-self-leveling DURACRETE, prepared to a slurry consistency, similar to thick buttermilk over the entire area. At this time, your crew can broom, trowel, or even texture the surface with stamping or geometric shapes. Application thickness can range from very thin to many inches thick.
DURACRETE is available in a standard portland cement gray color or in white, which comes job-site ready to accept pigment. Re-build curbs or smooth out dips in gutters with DURACRETE. Your crew will soon become masters at repair with DURACRETE.
Your “call back” will turn into a nice encounter for your company and the property owner at the same time. Please call your LATICRETE representative for more details onDURACRETE and its almost unlimited repair uses.
Concrete Setting and Moisture Loss
Q: We recently installed a large flat concrete floor in a large metal building. The floor project turned out great; however, we had one major difficulty with a long lane of concrete floor along the north side of the metal building's wall. We observed that large areas of the fresh concrete were not setting for hours, while other areas set within a normal time given the conditions of the project. What caused the "spotty setting" of the concrete?
A: Spotty setting, as viewed by your concrete placement crew at the job site, is a variation in the rate of stiffening during the finishing process of the concrete. The two things that most affect the rate of stiffening of concrete are uncontrolled moisture loss from the concrete and the rate of the hardening of the concrete through the hydration process.
Spotty setting of a concrete slab can be caused by a number of things, but typically the most common causes are 1) a variation in the temperature of the concrete at the time of placement and 2) the effect of chemical admixtures on the rate of hydration.
Temperature primarily affects the rate of hydration. It is a generally accepted rule that concrete having a temperature of 90°F will set twice as fast as a concrete having a temperature of 70°F. It is equally true that concrete having a temperature of 50°F will take twice as long to set as a concrete having a temperature of 70°F. The actual temperature of the concrete mix as delivered to the jobsite often varies from load to load and frequently causes a real difference in setting time.
Admixture interaction can play a major role in spotty setting of concrete. Some retarding admixtures can be very sensitive to changes in concrete temperature. A small change in concrete temperature can cause problematic changes in the set from load to load of concrete. Concrete using both a superplasticizer and normal-setting or retarding admixture can have a delayed setting time when the temperature of the concrete falls below 65°F. It is often necessary to vary the dosage rate of set-controlling admixtures in order to get the desired set from load to load as the jobsite conditions change.
Your inquiry was followed up by a telephone call from LATICRETE to your project manager and in the dialog it was revealed that the large metal building had vinyl covered insulation overhead, placed against the ceiling. A number of places along the north wall had water trapped in the insulation that caused a sagging or bulge in the vinyl. The water-filled bulges were punctured and the water drained down upon your granular sub-grade. This produced a number of isolated wet spots in the sub-grade. There was no aggressive cross ventilation in the building and the wet spots did not receive much drying. Your concreting operation began a few days later and the spotty setting became apparent to your concrete crew, especially the finishers. The cold weather conditions outside of the building contributed in a general way, as did the low dew point inside the building.
Your crew suspected that the wet subgrade contributed to the problem. It is our experience that isolated wet spots in the subgrade do not by themselves contribute to a significant differential in setting, but can be a contributory cause, multiplying the effect of other previously mentioned causes. We believe that good practice in this situation would have prompted your crew to uniformly dampen the subgrade, to reduce the variation in dampness.
That being said, I want to commend your onsite crew for the many correct things they did: they frequently checked the temperature of the mix, they inquired about the admixture content of the mix, and they stayed off of the slower setting areas of the floor until they did begin to set, many hours later. The use of E-CON™ [insert link to product page: E-Con], a monomolecular film, on the faster setting concrete was a stroke of genius and permitted the finishers a larger window of time to permit a homogenous finishing of your floor. Good Work!