Concrete Problems and Cracks

Concrete Problems

When concrete is applied, many things may happen and must be managed:

•  Air entrainment

Most air problems arise when there isn't enough entrainment to prevent freeze-thaw damage. Air entrainment prevents concrete from scaling and spalling. In southern climates it helps to control excessive bleed water from coming off the surface and causing finishing problems. Too much air can cause scaling during machine troweling.

Low air entrainment that results in spalling and scaling of the surface destroys finishes. When dry-shake color is used, scales typically have color on one side and plain concrete bonded to the colored layer on the back side of the scale. This is because dry shake color hardeners provide a densified layer that effectively protects the colored layer from freeze-thaw damage. When air entrainment is excessively high, strength goes down, and there is virtually no bleed. It can also be difficult to properly "wet out" dry shake color hardeners.

• High water/cement ratios

A water-cement ratio that is too high is characterized by lower than specified design strength, porous and weak surfaces, carbonation, excessive efflorescence, and increased shrinkage.

Because of the more porous surfaces that result from too much water, colored finishes, including chemical stains, diffract more light, giving the impression of weaker coloration. Due to the weaker surface, traffic wear patterns can develop. In the case of chemical staining, wear call remove the colored layer.

• Lack of curing

Too little curing can result in low strength, excessive amounts of unhydrated portland at the surface, carbonation, dusting, and excessive surface wear of high traffic areas.

These symptoms can be similar to high w/c ratio conditions. Unhydrated cement does not develop calcium hydroxide, so there is less of it for some decorative products to react with. Colored surfaces will appear less intense than well-cured concrete with the same amount of color. Stained surfaces also appear less intense. Dusting and traffic wearing patterns problems can also result. However, most decorative finishes can't tolerate the same curing methods used for plain concrete.

• Low strength

Problems can stem from poor mix design, excessive water in the mix, or lack of curing. Low-strength mixes can increase the possibility of cracking, excessive surface wear, and porous concrete. When concrete is below 27 MPa strength in freeze-thaw climates, scaling can result. Surface strength is reduced when finished with bleed water present.

When there isn't enough cement paste in a mix, integral color isn't properly restrained in the paste, and color can be lost from the surface. Chemical stains may not have enough calcium hydroxide to react with, resulting in less coloration. Low strength in decorative finishes can cause traffic to wear into the finishes.

• High moisture levels

Waterproof coatings applied before moisture levels are low enough can result in blemishes, blistering, and spalling.

Chemical stains react differently in areas of a slab that have higher relative humidity. Decorative treatments, which do not have good moisture vapor transmission properties, can peel off the surface, blemish, turn cloudy white, or cause blisters to develop.

• Cold weather conditions

During cooler weather, the time lag between placement of the concrete and initial set is longer, causing extended bleeding times. Using unvented construction heaters can cause rapid carbonation of freshly placed concrete.

Long initial set times and excessive bleed water mean that more calcium hydroxide comes to the surface where it reacts with carbon dioxide from the air to form efflorescence. More laitance also comes to the surface from silica in the aggregates, causing hard white silicates to form. Also, slab finishes are often wet in appearance. In cold weather, concrete is usually covered with curing blankets or plastic, causing unsightly efflorescence markings.

• Hot weather conditions

When conditions are really hot, there is less time to perform all the added steps needed for some decorative finishes. In the case of stamped concrete it is more likely that impressions will be "mushy" at the beginning of the stamping process and too light, with little texture, at the end.

• Cracks

There are several tips to help towards preventing cracks. From this section you should get ideas to discuss with the designer and concrete contractor to achieve for example a slab that meets the requirements you need:

• Compaction of subbase and subgrade

Excavations beneath the slab that are not to be filled with concrete (usually plumbing or other mechanical trenches) should be brought back to grade in compacted lifts. This means a 6'000 mm trench would be backfilled 150 mm at a time, each "lift" being mechanically vibrated so it is left compact.

If these excavations are not compacted when backfilled, this loose soil will settle over time leaving the concrete over that area with no earth under it. This becomes a prime place for concrete to settle. Since the soil next to these excavations is native soil, the uncompacted trench can literally become a thoroughfare for water.

Most rental yards have compacting equipment and it is worth the investment to use it.

Excavations from the house to the street for utilities should also be backfilled and compacted in the same manner so areas under the driveway concrete don't sink. Uncompacted areas under lawns can be identified by areas of sunken grass-so it is good idea to compact trenches even under areas not receiving concrete.

• Low Water/Cement Ratio

A low water to cement ratio is the number one issue effecting concrete quality.

Low water cement ratio impacts all of the desired properties of concrete listed in the desired properties of concrete section.

Use a maximum 0.50 water to cement ratio when concrete is exposed to freezing and thawing in a moist condition or to deicing chemicals.

Use a maximum 0.45 water to cement ratio for concrete with severe or very severe sulfate conditions.

Water permeability increases exponentially when concrete has a water cement ratio greater than 0.50.

Durability increases the less permeable the concrete mix is.

Strength improves with lower water cement ratios. A 0.45 water cement ratio most likely will hit 30 MPa or greater. A 0.50 water cement ratio will likely reach 27 MPa or greater.
 

• Properly Curing Concrete

Why cure concrete. Curing serves two main purposes.

• It retains moisture in the slab so that the concrete continues to gain strength
• It delays drying shrinkage until the concrete is strong enough to resist shrinkage cracking.
• Properly curing concrete improves strength, durability, water tightness, and wear resistance.

How to cure concrete.

• Water cure:
  The concrete is flooded, ponded, or mist sprayed. This is the most effective curing method for preventing mix water evaporation.
• Water retaining methods:
  Use coverings such as sand, canvas, burlap, or straw that are kept continuously wet. The material used must be kept damp during the curing period.
• Waterproof paper or plastic film seal:
  Are applied as soon as the concrete is hard enough to resist surface damage. Plastic films may cause discoloration of the concrete-do not apply to concrete where appearance is important.
• Chemical Membranes:
  The chemical application should be made as soon as the concrete is finished. Note that curing compounds can effect adherence of resilient flooring, your flooring contractor and/or chemical membrane manufacturer should be consulted.
• All the desirable properties of concrete are improved by proper curing!

Allow proper time to water cure

• After concrete is placed, the concrete increases in strength very quickly for a period of 3-7 days. Concrete which is moist cured for 7 days is about 50% stronger than uncured concrete.
• Water curing can be done after the slab pour by building dams with soil around the house and flooding the slab. The enclosed area is continually flooded with water. Ideally, the slab could be water cured for 7 days.Some builders on a tight schedule water cure for 3 days as this achieves approximately 80% of the benefit of water curing for 7 days.
• Consider planning your job to pour at the end of the week, build berms, then flood over the weekend. You get the benefit of water curing without losing too much time in the schedule.

• Placing of control joints

Control joints are planned for cracks which allow for movements caused by temperature changes and drying shrinkage. In other words, if the concrete does crack-you want to have an active role in deciding where it will crack and that it will crack in a straight line instead of randomly.

Space joints properly. Space joints no more than 25-35 times the slab thickness. A 100 mm slab should have joints 2’500 to 3’500 mm apart.

Cut joints deep enough. Cut joints 25% of the depth of the slab. A 100 mm thick slab should have joints 25 mm deep.

How to cut joints. Grooving tools cut joints in fresh concrete. Saw cutting cuts joints as soon as the concrete is hard enough that the edges abutting the cut don't chip from the saw blade.

Cutting joints soon enough. In hot weather, concrete might crack if joints are not cut within 6-12 hours after finishing concrete. In this condition, if you don't want to use a grooving tool to cut joints, there are early-entry dry-cut lightweight saws that can be used almost immediately after finishing. These saws cut 25 to 75 mm deep, depending on the model.

Place joints under walls or under carpet areas. Under walls they won't be seen. Under carpet areas the joints won't have a chance to telegraph through vinyl areas.

Avoid re-entrant corners; planning the joint pattern can sometimes eliminate re-entrant corners.

Most plans don't have joint spacing marked on them. So don't leave this important part of concrete construction to chance.

Jointing is often not taken seriously enough and the "sawcutter" comes to your job and puts the cuts where he feels they belong or where it is convenient for him.

Be active in deciding where control joints will be placed!

• Preventing Concrete Cracks

When installed properly, concrete is one of the most durable and long lasting products. But it is important that concrete contractors follow well-established guidelines with respect to concrete placement. Durable, high strength, and crack resistant concrete does not happen by accident.

Why Concrete Cracks

1- Excess water in the mix

Concrete does not require much water to achieve maximum strength. But a wide majority of concrete used in residential work has too much water added to the concrete on the job site. This water is added to make the concrete easier to install. This excess water also greatly reduces the strength of the concrete.

Shrinkage is a main cause of cracking. As concrete hardens and dries it shrinks. This is due to the evaporation of excess mixing water. The wetter or soupier the concrete mix, the greater the shrinkage will be. Concrete slabs can shrink as much as 12 mm per 30 m. This shrinkage causes forces in the concrete which literally pull the slab apart. Cracks are the end result of these forces.

The bottom line is a low water to cement ratio is the number one issue effecting concrete quality- and excess water reduces this ratio.

What you can do about it:

Know the allowable water for the mix the contractor is pouring - or be very sure you have chosen a reputable contractor who will make sure the proper mix is poured. It is more expensive to do it right - it simply takes more manpower to pour stiffer mixes.

2- Rapid Drying

Also, rapid drying of the slab will significantly increase the possibility of cracking. The chemical reaction, which causes concrete to go from the liquid or plastic state to a solid state, requires water. This chemical reaction, or hydration, continues to occur for days and weeks after you pour the concrete.

You can make sure that the necessary water is available for this reaction by adequately curing the slab.

What you can do about it:

Read about the methods to cure concrete and understand how your contractor will cure the concrete.

3 - Improper concrete poured on the job

Concrete is available in many different strengths. Verify what strength the concrete you are pouring should be poured at.

Talk to the ready mix supplier

4- Lack of control joints

Control joints help concrete crack where you want it to. The joints should be of the depth of the slab and no more than 25 - 35 times of the thickness of the concrete. So 100 mm concrete should have joints 2'500 - 3'500 mm apart.

Other reasons:

Never pour concrete on frozen ground.

The ground upon which the concrete will be placed must be compacted.

The sub grade must be prepared according to your soil conditions. Some flat work can be poured right on native grade. In other areas 150 mm of base fill is required along with steel rebar installed in the slab.

• Shrinkage Reducing Admixtures

Hydrated cement paste shrinks as it loses moisture from its extremely small pores. As the moisture is lost in these small pores, the surface tension of the remaining water tends to pull the pores together which results in a loss of volume over time.

Shrinkage reducing admixtures are designed to decrease the effects of drying shrinkage by reducing the surface tension in these pores.

It should be noted that the type of aggregate and the cement properties themselves can influence the amount of cracking that can occur. Thus, it is important to test local project-specific materials if doing shrinkage testing.

Curing also affects cracking. In slabs, the top tends to dry out first and shrinks while the lower sections still have a higher moisture content. This difference in moisture can be altered by use of Shrinkage Reducing Admixtures, which alter the way water migrates through the concrete and results in a more uniform moisture profile.

• Thermal Cracking

Hydration of cement is an exothermic process meaning it generates heat. As the concrete cools it contracts and in extreme conditions may contract in three days as much due to cooling as it could in a year due to drying conditions

A temperature differential of 35o F within 1 ft is usually considered enough to cause cracking. However, within 24-hours of placement, concrete temperatures can reach anywhere from 20o to 50o F hotter than ambient temperatures

Admixtures can work to alter the rate of heat generation but at the same time can result in lower concrete strengths. In mixes where low water/cement ratios are required for durability (such as bridges, parking structures or marine facilities), concrete is often substantially over-designed for strength. In cases like this, heat reducing admixtures can reduce the likelihood of cracking.

• Plastic Shrinkage Cracking

Plastic shrinkage occurs as fresh concrete loses its moisture after placement but before any strength development has occurred.

This type of shrinkage is affected by environmental effects of temperature (concrete and ambient), wind and relative humidity. It is a particular problem in hot weather concreting.

At present, there are no specific admixtures developed to handle this type of cracking.

• Admixtures Reduce Autogenous Shrinkage

This type of shrinkage is difficult to measure or separate from drying shrinkage which is a much greater concern.

Autogenous shrinkage takes place as the portland cement hydrates and without loss of water from the mix as opposed to the drying shrinkage process. Admixtures may reduce this type of shrinkage but are not specifically designed to do so.

• Early Entry Sawing

The timing of the early-entry process is timed to be prior to development of significant tensile stress in concrete.

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