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Why Concrete Cracks?
Cracks in concrete are extremely common but often misunderstood. When an owner sees a crack in his slab or wall, especially if the concrete is relatively new, he automatically assumes there’s something wrong. This is not always the case. Some types of cracks are inevitable. The best that a contractor can do is to try to control the cracking. This is done by properly preparing the subbase, assuring that the concrete is not too wet, utilizing reinforcement where needed, and by properly placing and spacing crack control joints and expansion joints. However, sometimes cracks happen in spite of any precautions taken.
The American Concrete Institute addresses this issue in ACI 302.1-04. “Even with the best floor designs and proper construction, it is unrealistic to expect crack-free and curl-free floors. Consequently, every owner should be advised by both the designer and contractor that it is normal to expect some amount of cracking and curling on every project, and that such occurrence does not necessarily reflect adversely on either the adequacy of the floor’s design or the quality of its construction (Ytterberg1987; Campbell et al. 1976)”.
Diagnosing 6 Types of Concrete Cracks
Plastic Shrinkage Cracks
Probably the single most common reason for early cracks in concrete is plastic shrinkage. When the concrete is still in its plastic state (before hardening), it is full of water. This water takes up space and makes the slab a certain size. As the slab loses moisture while curing it gets a bit smaller. Because concrete is a very rigid material, this shrinking creates stress on the concrete slab. As the concrete shrinks, it drags across its granular subbase. This impediment to its free movement creates stress that can literally pull the slab apart. When the stress becomes too great for the now hardened concrete, the slab will crack in order to relieve tension. Especially in hot weather, shrinkage cracks can occur as early as a few hours after the slab has been poured and finished.
Often, plastic shrinkage cracks are only a hairline in width and are barely visible. However, even though a crack is hairline, it extends through the entire thickness of the slab. It’s not just on the surface as one might think.
One factor that contributes significantly to shrinkage is mixing the concrete too wet. If excessive water is introduced into the mix, the slab will shrink more than if the correct amount of mix water were used. This is because the additional water takes up more space, pushing the solid ingredients in the mix further apart from each other. It’s similar to over-diluting a pitcher of Mi-Wadi. By doing so, a weaker solution is created. When the excess water leaves the slab, the solid particles have larger voids between them. These empty spaces make the concrete weaker and more prone to cracking. Unfortunately, wetter concrete is easier to place and finish, especially in hot weather. This is one reason that many concrete finishers add water to concrete mixer trucks: it makes their work easier. A few litres per cubic metre will not significantly affect the mix. However, if an excessive amount of water is added, one can unwittingly reduce the concrete’s strength.
Plastic shrinkage cracks can happen anywhere in a slab or wall, but one place where they almost always happen is at re-entrant corners. Re-entrant corners are corners that point into a slab. For example, if one were to pour concrete around a square column, you would create four re-entrant corners. Because the concrete cannot shrink around a corner, the stress will cause the concrete to crack from the point of that corner (See Figure 1).
Figure 1: Shrinkage cracks originating at re-entrant corners
A rounded object in the middle of a slab creates the same problem as a re-entrant corner. This is commonly evidenced around slab penetrations such as pipes, plumbing fixtures, drains, and manhole castings. The concrete cannot shrink smaller than the object it is poured around, and this causes enough stress to crack the concrete (See Figure 2).
Figure 2: Shrinkage crack at slab penetration
To combat random shrinkage cracks, control joints (often mistakenly referred to as expansion joints) are incorporated into the slab. Control joints are actually contraction joints because they open up as the concrete contracts or gets smaller. They are simply grooves that are tooled into fresh concrete, or sawed into the slab soon after the concrete reaches its initial set. Control joints create a weak place in the slab so that when the concrete shrinks, it will crack in the joint instead of randomly across the slab (See Figure 3).
Figure 3: A successful crack control joint
For a crack control joint to be effective, it should be ¼ as deep as the slab is thick. That is, on a typical 100mm thick slab, the joints should be no less than 25mm deep; a 150mm thick slab would require 38mm deep joints, etc. To minimize the chances of early random cracking, these joints should be placed as soon as possible after the concrete is poured. If the control joint is not deep enough, the concrete can crack near it instead of in it (See Figure 4).
Figure 4: A crack next to a too-shallow joint
Crack control joints should be placed at all re-entrant corners and slab penetrations, and evenly spaced throughout the rest of the slab. A good rule of thumb for 100mm thick residential concrete is to place joints so that they separate the slab into roughly equal square sections, with no joint being further than about 3 metres from the nearest parallel joint. Following these guidelines, a 1.2 metre wide footpath would be cross- jointed at 1.2m intervals. A 4.8 m x 19.2m driveway would have one joint running up the centre length ways, and joints cut across it every 2.4 metres . This pattern would create sixteen 2.4m x 2.4m sections. If a driveway is metre wide or less, the centre joint up its length can usually be safely omitted, and the cross joints spaced the same distance as the driveway is wide (for example, an 3 metre wide driveway would have no centre joint and cross joints every 3 metres). If joints are not placed where they need to be, the concrete will create its own joints by cracking. It’s interesting to note that it often cracks in the same pattern as it should have been jointed (See Figure 5).
Figure 5: Driveway cracks where joints should have been placed
Another reason that concrete cracks is expansion. In very hot weather a concrete slab, like anything else, will expand as it gets hotter. This can cause great stress on a slab. As the concrete expands, it pushes against any object in its path, such as a brick wall or an adjacent slab of concrete. If neither has the ability to flex, the resulting force will cause something to crack.
An expansion joint is a point of separation, or isolation joint, between two static surfaces. Its entire depth is filled with some type of compressible material such as tar-impregnated cellulose fibre, closed-cell poly foam, or even timber (See Figure 6). Whatever the compressible material, it acts as a shock absorbed which can “give” as it is compressed. This relieves stress on the concrete and can prevent cracking.
Figure 6: Foam expansion joint separating driveway and curb.
Expansion joint material can also prevent the slab from grinding against the abutting rigid object during periods of vertical movement. During these times of heaving or settling, expansion joint material prevents the top surface of the slab from binding up against the adjacent surface and flaking off (See Figure 7).
Figure 7: Expansion joint between these slabs would have prevented chipping
Cracks Caused by Heaving
Another factor which contributes to cracking is ground movement brought on by freeze/thaw cycles. During such cycles, the frozen ground can lift as much as several inches, and then settle again when the ground thaws. If the slab is not free to move with the soil, the slab will crack. The presence of large tree roots can also cause concrete to heave. If a tree is located too close to a concrete slab, the growing roots can lift and crack the concrete (See Figure 8).
Figure 8: Tree roots lifted and cracked this sidewalk
Cracks Caused by Settling
Conversely, if a large tree is removed from near a concrete slab the buried roots will decompose. The resulting void can cause the ground to settle and the concrete to crack. Settling is also called subsidence.
Subsidence is very common over trenches where utility lines and plumbing pipes are buried. Often times, the utility trench is not compacted when it is refilled. If concrete is placed atop a poorly compacted trench, the void created by subsidence can cause a crack across the unsupported concrete slab (See Figure 9).
Figure 9: Crack across the unsupported concrete slab.
Another place where concrete commonly subsides is near a house. Whether the home is built on a basement or crawlspace, the over-dig is subsequently back filled. Unless the back fill material is compacted in lifts as the over-dig is filled, it will settle over time. This settling will cause any concrete poured atop it to settle along with it. Many times this settling will cause the concrete to crack and tilt back toward the house, creating negative slope (See Figure 10).
Cracks Caused by Overloading the Slab
Another factor which contributes to cracking is placing excessive weight on top of the slab. Although it is a very strong material, concrete still has load limits. When you hear someone speak of 4,000 psi concrete, they are referring to the fact that it would take 4,000 pounds per square inch of pressure to crush it. Residential concrete, however, is rarely overloaded as far as compressive strength is concerned. That is to say, the weight doesn’t usually pulverise or crush the concrete. What is more common is that the excessive weight is too much for the ground underneath the concrete. This is especially true after periods of heavy rain or snow melt when the ground is saturated and soft.
When groundwater migrates under the concrete it causes the underlying soil to become soft or spongy. Excessive weight on the slab at this point can press the concrete down. Since the flexural strength of concrete is less than its compressive strength, the concrete bends to its breaking point. Homeowners who place large recreational vehicles or dumpsters on their driveways are more likely to see this type of cracking. Driving heavy vehicles off the edge of a slab creates a similar type of crack.
Figure 12: A heavy truck drove over this sidewalk, cracking the edge
Cracks Caused by Premature Drying
Crazing cracks are very fine surface cracks that resemble spider webs or shattered glass. They can happen on any concrete slab when the top loses moisture too quickly. Crazing cracks can be unsightly, but are not a structural problem. They are so fine that there is no way to repair them (See Figure 13).
Crusting cracks often happen during the concrete stamping process. They usually occur on sunny or windy days when the top of the slab dries out sooner than the bottom. The top becomes crusty so when the stamp is embedded, it pulls the surface apart near the stamped joints causing small cracks around the outside edges of the “stones”. Although they are cosmetically unappealing, crusting cracks present no structural problem but may be patched if desired
Figure 14: Crusting cracks caused by premature surface drying
The Importance of Reinforcement
The use of synthetic fibers, reinforcing wire mesh, or rebar can add some extra support to concrete, but none of them will prevent cracking. In fact, too much steel can actually cause a slab to crack by restraining normal concrete shrinkage. However, if cracks happen, reinforcement can hold the different sections together.
The presence of reinforcement can be the difference between a crack remaining hairline in nature or separating and becoming wider and unsightly. Steel reinforcement can also keep the concrete on both sides of a crack on the same horizontal plane. This means that one side doesn’t heave or settle more than the other, which could cause a tripping hazard. It is sometimes impossible to determine exactly what caused a particular crack. However, proper site preparation and good concrete finishing practices can go a long way towards minimizing the appearance of cracks and producing a more aesthetically pleasing project.
Source : Ardex Building Products
Is Water Good or Bad in Concrete?
Trick question! The answer is both. Water is needed, but after the need is met, it begins to hurt the concrete.
There are two types of water in concrete: water of necessity (also known as water of hydration) and water of convenience.
Water of Necessity
The main ingredients of concrete are fine aggregate (sand), coarse aggregate (gravel or stone), cement and water. When water comes in contact with cement, it begins a chemical process called hydration. This process forms crystals that bind the coarse aggregates together. Hydration is how concrete gains its strength. A certain amount of water is required to full hydrate the cement so that all the cement is used to bond the concrete mix together. This is commonly referred to as a water to cement ratio. The ratio is calculated by taking the weight of the water and dividing it by the weight of the cement. The minimum water to cement ratio required for hydration is a 0.28. Any additional water is water of convenience.
Water of Convenience
While a low water to cement ratio is optimized for performance, the mix is very stiff and difficult to work with. It has what we call a low slump. That is a measure of how far a cone of wet (or plastic) concrete will fall when the cone is removed. A low slump of an inch is very stiff, whereas a high slump of 6 inches would flow quite easily. Think of it like a pancake batter. If the pancakes are too stiff, adding more milk to make it flow more so it is more workable. Unfortunately, that has a significant down side.
How Does Water Hurt Concrete?
So now we know why we need water for hydration, what is the down side to adding as much water of convenience as we can? All of the excess water that is in the mix will need to leave the concrete in the form of what is called bleed water. This has negative effects. Adding just one gallon of water to a cubic yard of concrete will:
- Increase the slump about 1 inch
- Decrease compressive strength about 200 to 300 psi
- Increase shrinkage potential by as much as 10% (increased cracking) – Think about how the concrete is changing size while all that water bleeding out at the same time it is trying to get hard.
- Decrease resistance to de-icing salts
- Decrease wear resistance to traffic
- Increase dusting and other surface defects
- Increase time needed to cure the concrete
How Can I get the Good Parts of Water without the Bad?
The good news is that you can have your cake and eat it too! There are chemical admixtures called normal water reducers and high range water reducers that we can put in the concrete. Concrete designed for slumps greater than 5 inches typically will require the use of a high range to avoid having a high water cement ratio. Depending on the required slump, these water reducers will increase the slump and fluidity of the concrete without extra water and all its negative effects.
It’s important to know how much slump you want with your concrete when you order it, so we can help you pick a mix optimized for your needs so you don’t feel that you have to add water at the job site and decrease the quality of your concrete.
Source : Chaney Enterprises
Inspection of concreting works is an important step to achieve greater strength and durability of the structure. Although it is easy to remember number of checks during inspection of concrete member, a checklist is always required for record of the placement of concrete and quality control measures taken at site.
Stages of Inspection of Concrete Works
Inspection of concrete is done in 3 stages,
- During placement
Type of inspection of concrete depends on type of concrete, i.e. PCC or RCC, type of elements to be casted, such as RCC slab, columns, footing, beams, walls etc. It is both beneficial to contractor as well as engineer to maintain the record of checks, so that they can produce it in case of any discrepancy.
It also allows becomes a proof of quantity of concrete work done by the contractor, so that no discrepancy arises during billing.
The checklist also notes the number of cubes taken for the given work and its id is noted in the checklist, so that when cube test results arrive, it becomes easy to identify the structural elements for the given cube test results.
1. Concrete Pre-Placement Checklist
Table 1: Standard Inspection Checklist for pre-placement of concrete.
|1||Centre line||As per drawing|
|2||Formwork & Staging||As per drawing & in exact plum|
|3||Construction joint location*||As per drawing|
|4||Steel reinforcement diameter / spacing & coating*||As per drawing|
|5||Cover to the reinforcement and overlap*||As per drawing|
|6||Shuttering||Aligned as per drawing and in plumb|
|7||RLs and reference levels||As per drawing|
|8||Embedment part check, i.e. insert plates, nipple etc||As per drawing|
|9||Placement of water stoppers, if any*|
|10||Location of construction joint|
|11||Water tightness of shuttering, if required||No water seepage allowed|
|12||Quality of water||Potable and clean|
|13||Measuring jar for water pouring||As per water content requirement|
|14||Quality of materials||As per specification|
2. Inspection during Placement of Concrete
Table 2: Standard Inspection Checklist during placement of concrete
|1||Water cement ratio||As per specification|
|2||Surface preparation by mortar bedding||Fresh mortar|
|3||Slump testing*||As per requirement|
|4||Adequacy of vibration||40mm and 60mm needle required|
|5||Segregation of aggregates||Not allowed|
|6||Removal of temporary spacers and ties*||To be removed|
|7||Check for shuttering prop displacement / settlement||Not Allowed|
|8||Number of cubes taken for testing with identification No.: ________ Id: __________||3 No’s for RCC > 6m33 No’s for every 5m3|
|9||Continuity of operations||No break in between concreting|
3. Post placement Concrete Inspection Checklist
Table 3: Standard Inspection Checklist for Post placement of concrete.
|1||Observation for honeycombing||Not Allowed|
|2||Line and Level||As per drawing|
|3||Surface finish||As per drawing|
|4||Cracks and air bubbles||Not Allowed|
|5||Method of curing||As specified|
|6||Checked for stripping/Removing of formwork support etc. after specified duration of stripping time.||Yes||No||NA|
|7||Checked for position of embedment||Yes||No||NA|
|8||Repair and finish all surface defects by specification / approved method||Yes||No||NA|
|9||Surface of part of structure is alright and allowed for subsequent activity / backfilling||Yes||No||NA|
|10||Others, if any|
* The above checklist are valid for Reinforced Concrete works only.
- All the empty space has to be filled and remarks if any shall be written in remarks column.
- Date of inspection of pre-concrete, during placement and post-concrete placement shall be noted as these steps may not be performed on the same day.
- Signature by contractor supervisor and engineer is done after each inspection.
Source : The Constructor