RIVERSIDE BUILDERS SUPPLY INC.

CONCRETE TROUBLESHOOTING


DUSTING CONCRETE SURFACE
SCALING CONCRETE SURFACE
CRACKING CONCRETE SURFACE
DISCREPANCIES IN YIELD
LOW CONCRETE CYLINDER STRENGTH
STRENGTH OF IN PLACE CONCRETE
CURING IN PLACE CONCRETE
HOT WEATHER CONCRETING
CONCRETE BLISTERING ON SLABS
FINISHING CONCRETE FLATWORK
CHEMICAL ADMIXTURES IN CONCRETE
CURLING OF CONCRETE SLABS
DISCOLORATION



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 :

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 :

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 :

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 :



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



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 :
Under American Concrete Institute standards (ACI), 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

Curing methods


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

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 :

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

Rules to Finish Concrete



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



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



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




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