CONCRETE TROUBLESHOOTING
DUSTING CONCRETE SURFACES
Dusting concrete surfaces, or surfaces that powder or chalk easily, are caused by a weak wearing surface. The weakness of the surface can be caused by a number of things, including :
- Finishing the surface while bleedwater is still present. When you trowel or work this water back into the top surface, you are weakening the top ¼" by greatly increasing the water / cement ratio, resulting in a surface much weaker than the remainder of the slab.
- Placing concrete over a non-absorbing subgrade or polyfilm, which reduces normal absorption by the subgrade, thereby increasing bleedwater moving to the surface.
- Improper or no curing of the concrete.
- Inadequate ventilation in closed in areas. Carbon dioxide from heaters, gasoline engines, or mixer engines can cause a chemical reaction known as carbonation, which greatly reduces the strength and hardness of the concrete surface.
- Inadequate protection from rain, snow or winds that can dry out the surface.
Preventing Dusting
To help minimize dusting, use concrete with a moderate slump of 5 inches or less. If you need a higher slump, use a superplasticizer. Avoid sprinkling dry cement on the surface to dry up the bleedwater. If you need to remove bleedwater, drag a hose or something similar across the slab. NEVER perform finishing operations with water present on the surface.
Provide proper curing by using a liquid membrane curing compound or by covering the slab with WET CLEAN burlap. When placing concrete in cold weather, use heated concrete and an accelerator to speed up the setting of the concrete.
REPAIRING DUSTING
To minimize dusting, apply a floor hardener, or treat the surface with boiled linseed oil. Be aware that these methods can change the appearance of the concrete. If the damage is severe, the surface can be wet-grinded off, and a concrete topping course can be applied.
If this proves impractical, a floor covering or carpet may be a less expensive option.
SCALING CONCRETE SURFACES
Scaling of the concrete surface is usually caused by freezing and thawing (freeze/thaw cycles).
The amount of scaling and the degree of the scaling damage can vary. Light scaling does not expose the coarse aggregate (gravel or stone). Moderate scaling exposes the coarse aggregate, and may involve the loss of up to 3/8" of the concrete surface. In severe cases of scaling, more surface has been lost and the aggregate is clearly exposed. Some causes of scaling are as follows :
- Use of non-air entrained concrete for applications exposed to freeze / thaw cycles.
- Applying calcium, de-icing salt, ammonium nitrate or other harsh de-icers can cause scaling as well as causing severe chemical attack on the surface of the concrete.
- Finishing concrete while bleedwater is on the surface is a common cause of scaling. When you finish or work that bleedwater back into the top ¼" of the concrete, you greatly increase the water / cement ratio, which causes a very weak surface.
- Inadequate or no curing.
PREVENTING SCALING
To help prevent scaling, always order air-entrained concrete, and place that concrete at a moderate slump of 5" or less. If a wetter mix is desired, use superplasticizer. DO NOT use
De-icers such as salt or calcium chloride. Properly cure the concrete with a liquid membrane curing compound. This maintains moisture in the concrete, which is needed to hydrate the cement, which enables the concrete to reach its full strength potential. If the concrete is allowed to dry out in a few days, the strength gain from hydration is stopped, resulting in low strength concrete. DO NOT perform any finishing operations with water on the surface.
Protect the concrete from the harsh winter environment by applying a sealer specifically made for use on concrete. Boiled Linseed Oil can be used as well, but tends to darken the surface of the concrete. These sealing treatments should be applied in late summer.
REPAIRING SCALED SURFACES
Scaled surfaces can be repaired by resurfacing with either a Portland cement mixture, or a latex modified concrete. Before applying these products, you must remove all loose and scaling surfaces to assure the new topping will adhere properly. Also remove any oil or paint on the surface.
CRACKING CONCRETE SURFACES
Concrete surface cracks are usually caused by improper design and construction practices, such as :
- Omission of isolation joints and control joints, and / or improper jointing practices.
- The use of high slump concrete or addition of water on the jobsite.
- Improper subgrade preparation.
- Improper finishing.
- Inadequate or no curing.
Minimizing Surface Cracking
All concrete has a tendency to crack and it's not possible to consistently produce completely crack-free concrete. It is possible to reduce the occurrences of surface cracking by following these safeguards :
- Properly prepare the subgrade, removing all topsoil and soft spots. The material under the concrete should be well compacted by rolling, tamping or vibrating. Smooth, level subgrades help prevent concrete cracking.
- Polyethylene vapor barriers increase bleeding of the concrete, greatly increasing the chance of cracking. The vapor barrier should be covered with one to two inches of damp sand to minimize the bleedwater, which will help reduce the chance of cracking.
- Use concrete with a moderate slump of 2" to 4". Try to place concrete quickly enough to reduce the chance of adding water to the mix (retempering). Be sure to specify air entrained concrete for all concrete placed outdoors, subject to freeze-thaw cycles.
- DO NOT perform finishing operations with water present on the surface. If the surface is drying too quickly due to evaporation, try to erect wind breaks to keep the wind from drying out the surface too fast. In extreme conditions, consider the use of an Evaporation Retarder.
- Start curing as soon as possible. Spray the surface with a curing compound when finishing operations are complete. A second application of curing compound the next day is a good idea.
- Install control joints by sawing, forming or tooling a groove about ¼ the thickness of the slab, no further apart than about 25 to 30 times the thickness. (ie: 4" thick slab = control joints about 9 to 10 feet apart.) Isolation joints should be provided when slab is against other structures. These joints should be full depth of the slab. Expansion Joint is usually used for isolation joints.
DISCREPANCIES IN YIELD OF CONCRETE
( I RAN SHORT !!)
Concrete Yield
Concrete yield is the volume of ready mixed concrete produced from a known quantity of ingredients. One cubic yard equals 27 cubic feet. Many factors can cause the
Actual "yield" of the concrete to change. The moisture content of the raw materials, for instance, can cause a considerable change in the actual yield of the concrete. It takes more "pounds" of wet sand and gravel to create the same volume as the drier sand and gravel. That's why the concrete producer checks the moisture content on a regular basis,
usually more than once a day.
Another factor can be the "air content" of the concrete. A typical air entrained mix will contain about 6 percent air, meaning 6 percent of the volume
in that cubic yard of concrete is actually just that, AIR. As the concrete sits in the truck during delivery and unloading, the air content tends to drop. To adjust for this, the concrete producer will batch the concrete with more than 6 percent air at the batch plant, trying to predict the actual air loss for that load. If the truck sits on the job for an extended time, the actual air content may be only 4 percent, meaning 2 percent of the concrete has been lost.
How to help prevent yield discrepancies
- Measure the formwork accurately. A small difference in measurement can make a big difference in actual yield.
- Check for overexcavation. The subgrade must be very level to accurately measure its depth, and if the depth is not uniform, a yield problem can occur. These yield problems can go either way. There are many yield problems that involve the customer having too much concrete, wasting money.
- Striking off the surface, or "straight-edging", can greatly influence the amount of concrete needed for a slab. A bowed straight edge, depending on which way it is bowed, can either leave a hump of concrete in the middle of the slab, or a valley. It may not be visable to the eye, but just ¼" variation can add up on a large slab.
- Always allow a little for waste. Even though the truck seems empty, a small amount will usually stick to the mixer drum. When added to the amount spilled, and stuck to boots, tools, etc., It can add up fast. Most authorities on concrete will recommend 4% to 7% extra. Concrete producers realize that they ask for a great deal of trust, when dealing in cubic yards of concrete. Just keep in mind, that the vast majority of concrete suppliers would not jeopardize their reputation, and their relationships with their customers, to "save a buck" and short the customer.
LOW CONCRETE CYLINDER STRENGTH
The two major reasons for low compressive strength tests are improper handling and testing, and reduced concrete quality to due an error in production, or the addition of too much water at the jobsite. High air content can lead to low concrete strengths, stressing the need for accurate test results from the jobsite, when tests are being performed.
Collect all test reports and carefully analyze the results before taking action. Look at the slump, air content, air and concrete temperature. Check how many days the test cylinders were left at the jobsite, and any noted cylinder defects. Some of the most common causes of mis-handling are as follows :
- Cylinders left in the field too long without curing. The cylinders must be taken to be cured in 24 hours.
- Frozen cylinders. Test cylinders on the jobsite must be maintained between 60 degrees and 80 degrees. Falling out of that temperature range will severely affect the cylinder strength.
- Impact during transport. Always take great care to handle the cylinders properly. Any bouncing around could damage the cylinder, even if it looks okay. Be Careful!
- Improper cylinder capping. The test cylinder must be capped on both ends prior to being placed in the compression machine. If these caps are not properly installed, the strength data from the specimen could be completely useless.
- Improper testing of the compressive strength. When breaking test cylinders, the proper rate of compression must be used. Using too much pressure, too quickly, can lead to false readings. Once again, useless test data.
Under American Concrete Institute standards, concrete is acceptable if no one test is lower than the specified strength by more than 500 psi and the average of three consecutive tests equals at least the specified strength. If a test falls below by more than 500 psi, an investigation should be made to determine the problem. Always distribute copies of the test results to the concrete producer, as he may see a problem before it becomes serious.
STRENGTH OF IN-PLACE CONCRETE
Why measure in-place strength?
Testing of in-place concrete strength may be needed when standard cylinder strengths are low, and are not attributable to faulty test practices. In-place testing can be done by rebound hammer, also known as "Swiss hammer", probe penetration resistance testing, and core testing.
Rebound hammer testing
Test the strength of the in-place concrete using a rebound hammer, checking both the areas in question, and areas where the strength is not in question. Sometimes these results will show similar readings from both areas, avoiding the need for further testing. This rebound hammer method should be performed by someone experienced in the procedure.
Probe penetration resistance testing
This method is not often used, as it can be expensive, and difficult to perform. It consists of driving special probes into the concrete, checking its resistance. A strength curve can be developed for the concrete under investigation.
Core strength testing
A common type of "final" testing, cores of the in-place concrete are drilled out of the slab, and used as strength specimens for compressive strength. These cores can also be examined for approximate cement content, air content, water/cement ratio, foreign substances or impurities in the concrete, as well as deficiencies in the placement and finishing of the concrete. A minimum of 3 cores should be taken. Be aware that drilled cores test LOWER
than properly made test cylinders. ACI Building code states that core strength is considered adequate if the cores average at least 85 percent of the specified strength, with none below 75 percent. Cora testing can also prove expensive, and should be used only as a last resort.
CURING IN-PLACE CONCRETE
Curing is the maintaining of a satisfactory moisture content and temperature in concrete. Curing begins after finishing so that the concrete may develop the desired strength and hardness, leading to greater durability. Without an adequate supply of moisture, the cement in the concrete will not "hydrate", to form a quality finished product. Temperature is an important factor in curing concrete, since the rate of hydration is temperature dependent. For concrete exposed to weather, humidity and wind conditions also play an important part, contributing to the moisture loss from the concrete. A windy day with low humidity is a concrete nightmare.
Reasons to cure
- Predictable strength gain. Concrete not properly cured can lose as much as 50 percent of its potential strength. Placing concrete in hot weather may result in high strengths at early ages, but the ultimate strength will be lower, perhaps much lower.
- Improved durability. Well cured concrete has a harder surface, and is more watertight.
- Better appearance and serviceability. If the concrete is allowed to dry out prematurely, it will have a soft surface, with poor resistance to wear and abrasion. Proper curing reduces craze cracking, dusting and scaling.
Curing methods
- Liquid membrane-forming compounds. These should be applied per manufacturers instructions, after the bleed water has disappeared from the surface. Usually about one hour after finishing operations have been completed.
- Plastic sheeting or waterproof paper. This method can lead to a marred surface, and should be done carefully. The edges of the sheets should be sealed so the moisture will be trapped.
- Burlap or cotton matting can be used, but must be clean, and kept CONSTANTLY WET. If allowed to dry out, the burlap will actually absorb water FROM the concrete, actually being worse than not using it at all.
- Sprinkling on a CONTINUOUS BASIS is suitable, as long as air temperatures are well above freezing. The soaking must be continuous, as repetitive drying and wetting can damage the concrete.
The importance of proper curing cannot be overstated. You may get lucky. You may leave your concrete uncured and never have a problem. At the price of your installed concrete, don't bet on it! Spend a little money, and a little time. It can save you big money later.
HOT WEATHER CONCRETING
Placing concrete during periods of hot weather requires special attention to detail. Hot weather conditions can produce a rapid rate of evaporation of the moisture from the concretes surface. It can also greatly accelerate the concretes setting time. High humidity can reduce the effects of high temperature on concrete. The higher the humidity, the slower the evaporation of water from the surface.
High temperature causes increased water demand in the concrete, resulting in lower strengths in higher temperatures. The same mix that gave you 4700 psi in April, may only come up to 4200 psi in August. Depending on the conditions, the difference in strength can be even greater than that. As the hot temperatures cause faster setting times, you must be prepared to place the concrete faster than in cooler weather. Slow placement in hot weather means loss of slump, which leads to added water, which results in lower strength. Shrinkage cracking can result in hot weather, especially if there are windy conditions. Low humidity will add to the problem.
If concrete placed on a hot day is subjected to a cool night, thermal cracking can occur.
General rules for hot weather concreting
- Adequate manpower. The concrete must be placed quickly.
- When ordering the concrete, ask for a set-retarder to be added to the mix. This is a low cost additive that extends the set time of the concrete. In hot conditions, even the 30 year professional uses retarder.
- Limit the addition of water at the jobsite. When the truck arrives, have him adjust the slump of the concrete to your requirements, and then try to avoid adding water again. The utilization of more manpower, and ordering smaller, more manageable loads can be a big help.
- Don't place slabs directly on polyethylene sheeting. Always cover the sheeting with one or two inches of damp sand.
- Finish the concrete as soon as the sheen of the bleed water has left. Start curing after the finishing is completed.
- Before placing concrete, moisten the subgrade. Avoid standing water, but get it good and damp. This will prevent the subgrade from soaking up moisture from the concrete, allowing the concrete to use that moisture to hydrate and gain strength.
- Keep all test cylinders in shaded areas, and cap or cover with plastic to prevent them from drying out. Keep them between 60 and 80 degrees.
The bottom line is "Don't bite off more than you can chew". Many people who thought they could handle the big pours in high temperatures, were very sorry. Don't be afraid to cut down on the amount you pour. Get smaller loads. And don't hesitate to specify set-retarders in your concrete.
It could be the difference between a good job and doing it over again.
CONCRETE BLISTERS
Concrete blisters are hollow, low-profile bumps on the concrete surface, typically from the size of a dime, to a couple inches in diameter. A dense troweled skin of mortar about 1/8" thick covers a void, which moves under the surface while troweling. There are basically two theories as to the cause of these "blisters". Some think incidental air voids rise and are trapped under the dense surface skin produced by troweling. Others believe that bleed water rises and collects to form a void under the surface. Eventually this water is absorbed into the concrete, leaving a void.
Blisters are more likely to form if :
- The subgrade is cool, causing the concrete at the bottom to set more slowly.
- Entrained air is used, or is higher than normal.
- A dry shake is used, particularly over air entrained concrete.
- The slab is thick.
- The concrete was placed directly on polyethylene.
- Excessive use of a jitterbug or vibrating screed.
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To help avoid blisters, do not seal the surface before the bleed water has escaped and evaporated. Avoid dry shakes on air entrained concrete. In cold weather, use heated and accelerated concrete to promote even setting throughout the slab. DO NOT place concrete directly onto polyethylene sheeting. If blisters are forming, try to either flatten the trowel blades, or tear the surface with a wooden float, and delay the finishing process as long as possible.
FINISHING CONCRETE SLABS
Finishing makes concrete attractive and serviceable. The final texture, hardness, and joint pattern on slabs, floors, sidewalks, patios, and driveways depends on the concrete's end use. Industrial floors usually need to be level and smooth, while an office buildings floors may be covered with carpet, and don't need to be as exact. Exterior slabs must be sloped to carry away water, and must provide a texture which will not be slippery when wet. Having the proper manpower and equipment on hand, as well as properly timing the operations is critical.
Guidelines to placing concrete
- Select the proper concrete mix for your application. Consult with your concrete supplier if you're not sure.
- Whenever possible, place concrete directly from the chute, or use wheelbarrows or buggies. If pumping the concrete, alert your concrete supplier, as this may require a different mix. If unsure, have your pump operator contact the concrete supplier to work out the details.
- Spread the concrete with a square end shovel or concrete rake. Using a typical garden rake will cause the stone in the mix to segregate.
- Use a wooden or metal straightedge, or screed, to strike off and level the concrete. Be sure the straightedge is indeed "straight", or an uneven surface will result.
Rules to Finish Concrete
- Float the concrete as soon as it has been straightedged. A wood float, or a metal bull float can be used to further level the surface, and embed any stone or gravel near the surface. Floating the surface must be completed before bleed water appears on the surface. Never float bleed water back into the surface. This water would severely weaken the surface, and seriously effect the durability of the slab.
- WAIT. You must wait for the bleed water to evaporate from the surface before starting any other finishing operations.
- Edge the concrete all the way around the perimeter. Proper edging will result in a more pleasing appearance.
- Apply joints to the slab. Control joints can be installed by using a jointer, or groover tool. The depth of the joint should be about ¼ the depth of your slab.
- Trowel the surface if a smooth surface is desired. Troweling would not be a good idea for driveways or sidewalks, as the surface would be slippery when wet. Excessive troweling of the surface can result in a "burned" appearance.
- Texture the surface. If you do not want a smooth troweled surface, add texture to the slab. You can "broom" the surface, leaving fine broom marks in the surface. You can expose aggregate on the surface, add dry-shake color to the surface, or even stamp patterns into your concrete. The options are almost endless.
- NEVER sprinkle water or cement on the concrete while finishing. This is a leading cause of scaling and dusting.
CHEMICAL ADMIXTURES FOR CONCRETE
Admixtures are natural or manufactured chemicals which are added to concrete before or during mixing. The most commonly used admixtures are air-entraining agents, water reducers, retarders and accelerators. The function of admixtures in concrete is to enhance the durability, workability or strength characteristics of the mix.
Types of admixtures
- AIR ENTRAINING AGENTS are liquid chemicals added during the batching process to produce microscopic air bubbles in concrete. These bubbles greatly improve the concrete's durability and increase its resistance to damage from freezing and thawing, as well as de-icing salts. Air entraining agent also improves workability, and reduces bleed water. Concrete that will be exposed to freeze / thaw cycles should contain from 4.5% - 7.5% air content.
- WATER REDUCERS are used for two purposes. The first, and most obvious is to lower the amount of water needed in the mix, thereby increasing the strength of the mix. The second purpose is to increase the slump (wetness) of the concrete, while maintaining the original strength. The average amount of water reduction when using a standard range water reducer is between 8 and 11 percent. This reduction increases the potential strength of the concrete considerably.
- RETARDERS are chemicals, which delay the initial set of the concrete by an hour or more. Retarders are particularly useful in hot weather to counter the faster set times in hot temperatures. Most retarders are also water reducers, which enhance the strength of the concrete.
- ACCELERATORS reduce the initial set time of concrete. Accelerators are mostly used in cold weather, to offset the slower set times in cold temperatures. It must be pointed out that accelerators used in their normal dosages ARE NOT ANTI FREEZE. They simply cause the concrete to set faster than normal, aiding the early strength development of the concrete. There are a few accelerators on the market, which can act as an anti-freeze, but the high dosages needed usually make the cost prohibitive. The common types of accelerators are calcium chloride and accelerators that contain no chlorides, "Non-Chloride Accelerators".
- HIGH RANGE WATER REDUCER, also known as "superplasticizer", acts in a similar way to normal water reducer except the amount of water reduction is much greater. Reduction between 10% and 25% can be accomplished using superplasticizer, which can greatly increase the strength of the concrete, or can increase the slump (wetness) of the concrete by a considerable amount. When added to a mix at a 3 inch slump, the superplasticizer can increase its slump to 8 to 9 inches, making the concrete much more pumpable and workable.
CURLING OF CONCRETE SLABS
Curling is the distortion of a concrete slab into a curved shape by upward or downward bending of the edges. This distortion can lift the edges of the slab from the base, leaving an unsupported edge or corner which can crack under heavy loads.
Causes of Curling
Typically, curling is caused by shrinkage or contraction of the top surface, relative to the bottom. When one surface changes size more than the other, the slab tends to warp at the edges. This curling is most noticeable at the sides and corners. Most curling is the result of moisture and temperature gradients in the slab.
How to minimize curling
- Use the lowest practical slump and avoid adding extra water at the jobsite.
- Use the largest practical maximum size aggregate and/or the highest practical coarse aggregate content to minimize drying shrinkage.
- Take precautions to avoid excessive bleed water.
- Do not place the concrete directly on polyethylene sheeting. Place one or two inches of damp sand over the polyethylene.
- Avoid using mixes with high cement content. Dense concrete will produce larger top to bottom moisture differentials, increasing curling.
- Cure the concrete thoroughly, including joints and edges. If you use a membrane curing compound, apply it twice in two applications at right angles to each other.
- Use a thicker slab. Thin slabs tend to curl much more.
CONCRETE DISCOLORATION
Surface discoloration is the non-uniformity of color on the surface of a single concrete placement. It may take the form of dark blotches or mottled discoloration on the slabs surface. Some of the main factors influencing discoloration are the use of calcium chloride in the mix, variation in cement alkali content, admixtures, hard troweled surfaces, inadequate curing, incorrect finishing procedures, and changes in the concrete mix.
How to prevent discoloration
- Don't use high alkali cement.
- Limit the use of Calcium Chloride when color is important.
- Eliminate trowel burning of the concrete surface.
- Curing with polyethylene can cause discoloration.
- Improper or uneven curing can cause discoloration.
- Moisten subgrade, properly cure the surface, and protect surface from excessive drying due to wind and sun.
- Color changes from one day to the next, is common. The amount of sunshine, humidity, temperature, subbase, and many other factors can affect the color.
- Use the same finishing techniques and timing on all placements.