It is universally accepted that concrete is a versatile and durable construction material. Though concrete is very strong in compression, and compressive strengths greater than 20,000 psi (138 MPa) have been achieved in various high-rise building applications, its tensile strength is only about one-tenth of the compressive strength. Consequently, concrete is prone to cracking unless precautions are taken to minimize factors that can cause cracking. In addition to affecting aesthetic appeal, cracking can have a profound impact on the durability of a concrete structure. The issue of cracking has taken on new prominence with the focus on sustainability in concrete construction with regards to service life and the potential for repairs or replacement.
Why does Concrete Crack?
Concrete cracks due to one reason and one reason only; when its inherent tensile strength is exceeded by the net tensile stresses induced within its matrix. There are several reasons why tensile stresses are induced in concrete and therefore to minimize the incidence of cracking in concrete it is important that the fundamental cause(s) of cracking are clearly understood. This requires knowledge of several factors including: 1) the environmental exposure conditions during and after construction, 2) the characteristics of the concrete mixture, and 3) structural loads. Cracking can occur while concrete is still in the plastic state or, as is typical, in the hardened state.
Cracking can occur in plastic concrete when rapid evaporation of bleedwater from the concrete surface causes the surface to dry out. Factors that influence the rate of evaporation of bleed water either separately or collectively include: 1) high ambient and concrete temperatures, 2) low relative humidity, and 3) high wind speed. The cracks that form are classified as either craze cracks or plastic shrinkage cracks, depending on their appearance. A type of cracking, termed settlement cracking, can occur directly over reinforcing steel where the steel restrains downward movement or settling of solid materials in plastic concrete. Plastic cracks in concrete can be minimized by simply adding synthetic microfibers, either monofilaments or fibrillated, to the concrete mixture, such as BASF’s MasterFiber® microfibers. These microfibers reinforce the plastic concrete when it has no inherent tensile strength. They also help to minimize settling of the solid materials, thus mitigating settlement cracking. In addition to these synthetic microfibers, evaporation reducers, such as BASF’s MasterKure® ER 50 product can be applied over the concrete surface under adverse environmental conditions to minimize the loss of bleedwater; thus minimizing the potential for plastic shrinkage cracks.
There are several reasons why tensile stresses occur in hardened concrete. The primary reason is due to structural loads, which is expected. Secondarily, tensile stresses may be induced in concrete because of restraint to volume changes, either contraction or expansion. Potentially destructive volume changes may be due to drying shrinkage, thermal expansion and contraction, carbonation shrinkage, corrosion of embedded reinforcement, freezing and thawing in cold weather regions, alkali-aggregate reactivity, delayed ettringite formation (DEF), sulfate attack, and the hydration of free expansive components such as calcium oxide lime (CaO) or magnesium oxide (MgO). Concrete-making materials that can be used to mitigate cracking in hardened concrete are discussed in the section titled “Impact of Materials on Cracking.”
Types of Cracking
Cracks in concrete can be described in multiple ways. These descriptions can be used individually or in combination to describe the exact nature of the crack; that is, the surface appearance, orientation, depth, width, and movement characteristic of the crack. They include:
|Potential Underlying Cause
||Depth & Width
||Checking / Crazing
||Map or Pattern
||Hairline / Fine
The surface appearance of the crack(s) can yield valuable information with respect to the cause of cracking. Map or pattern cracks represent a series of interconnected and uniformly distributed short cracks that sub-divide the affected concrete surface area into smaller, irregularly-shaped pieces. Map cracks may be relatively shallow in depth as in the case of crazing or checking cracks, or they may be deeper and throughout the matrix as in the case of cracking due to alkali-aggregate reactivity. Conversely, the cracks may be isolated or individual in nature, usually running in the same general direction as in the case of plastic-shrinkage cracks. As such, the terms diagonal, longitudinal, transverse, vertical and horizontal are often used to describe the orientation of isolated or individual cracks.
Knowing the depth and width of a crack is useful in helping to assess the potential impact of the crack on the durability properties of the concrete and to determine the proper repair technique. Typically, surface and shallow cracks will have no significant impact on durability compared to partial-depth and through cracks that will provide deep access for aggressive agents into the concrete matrix. Regarding width, cracks can be described as hairline or fine in nature when less than 1 mm wide, medium between 1 and 2 mm, or wide when greater than 2 mm in width. Structural cracks will typically be isolated, wide and deep in nature.
Active and dormant are used to describe the movement characteristic of a crack. The movement characteristic is an important consideration in determining what crack repair to use. As implied by the name, active cracks are those for which the underlying factor or mechanism that led to the cracking is still present, resulting in continued movement of the crack. Dormant cracks on the other hand, are those that are no longer moving or have insignificant movement because the underlying mechanism is no longer a factor. Finally, cracks can be described with respect to the potential cause leading to cracking. These terms include plastic shrinkage, drying shrinkage, thermal, D-cracking, structural and settlement.
Impact of Design Decisions on Cracking
Improper structural design will obviously lead to distress and structural cracking in overstressed elements. This includes improper selection and detailing of reinforcement and restraint of members subjected to temperature- and moisture-induced volume changes, and improper foundation design. In addition, improper detailing such as failure to provide adequate contraction joints in one form or another, or failure to provide appropriate reinforcement at reentrant corners may lead to cracking.
Impact of Materials on Cracking
Concrete-making materials can impact cracking depending on the degree to which they impact bleeding in the plastic state, and volume change in the hardened concrete. As such, ingredients that reduce bleeding in concrete, such as silica fume can lead to plastic-shrinkage cracking, unless precautions are taken to protect the surface of the concrete immediately after placing and during early finishing operations. Fogging or the use of evaporation retarders to minimize the rate of evaporation of bleedwater from the fresh concrete surface are appropriate measures with silica fume concrete.
In the hardened concrete, it is essential that the concrete does not inherently have high drying shrinkage, and that measures are taken to minimize the potential for cracking due to other causes, such as corrosion of embedded reinforcement, aggregate reactivity or sulfate attack. Low drying shrinkage can be achieved by first limiting the mix water content of the concrete. Significant reductions in drying shrinkage can be achieved by adding shrinkage- or crack-reducing admixtures to the concrete mixture, such as BASF’s MasterLife® CRA 007 or MasterLife® SRA 035 admixtures. These admixture technologies are increasingly being used in liquid-containment structures, bridge decks, parking structures and slabs-on-ground, as well as other applications where crack mitigation is required.
With respect to cracking due to causes other than drying shrinkage, low permeability and the addition of appropriate durability-enhancing materials is a must. Low permeability concretes can be effective in improving the durability of concrete exposed to chlorides and sulfates or other aggressive environments. Low permeability can be achieved by using high-range water-reducing admixtures to lower mix water contents, pozzolans (such as: fly ash, silica fume, metakaolin) and slag cement. The durability of concrete in chloride environments can be enhanced further by using corrosion-inhibiting admixtures that effectively delay corrosion initiation of embedded reinforcement or prevent corrosion outright over the design service life of the structure. In addition to these measures, a concrete mixture should not have high residual amounts of free calcium oxide or magnesium oxide or other materials that can lead to expansion at later ages. As much as possible, the use of aggregates that are potentially reactive should be avoided or, if their use is inevitable, measures should be taken to inhibit alkali-aggregate reactivity. These measures include the use of appropriate pozzolans, slag cement or lithium-based admixtures. In cold weather regions, concrete that will be exposed to the elements should be air-entrained with an adequate air-void system. The use of materials with dissimilar thermal coefficients of expansion and contraction should also be avoided.
Impact of Construction Practices on Cracking
Retempering concrete, that is adding water at the job site, in excess of the design water content is undesirable mainly because of the accompanying decrease in compressive strength of the concrete. Retempering; however, increases the magnitude of drying shrinkage because of the additional water added to the concrete mixture. The combination of lower strength and increased drying shrinkage increases the potential for cracking of the concrete. Retempering also increases the potential for settlement cracking. Retempering can be eliminated by using workability-retaining admixtures or hydration-controlling admixtures, such as BASF’s MasterSure® Z 60 admixture.
Other construction practices that impact the potential for cracking include improper or lack of curing, inadequate compaction of subgrade, inadequate formwork support, inadequate consolidation, and improper jointing.
The information shared in this article provides an overview that minimizing the potential for cracking of concrete is a shared responsibility among the design engineer, concrete producer, and the contractor.
The design engineer's responsibility lies in the proper design and detailing of the structure, and the development of clear, concise specifications regarding the use of proper construction practices and the use of the right concreting materials.
It is important for the concrete producer to understand the impact of concrete-making materials on the potential for cracking in concrete. It is also the producer’s responsibility to use materials that are in accordance with the specifications for the project. Additionally, the concrete producer should propose the use of alternative materials where necessary as solutions to potential cracking problems. Some of these solutions include the use of 1) shrinkage-reducing admixtures to minimize drying shrinkage, especially in slabs and liquid containment structures, 2) corrosion-inhibiting admixtures to delay the onset of reinforcement corrosion in aggressive environments, and 3) pozzolans, slag cement or lithium-based admixtures to inhibit alkali-silica reactivity if the use is reactive aggregates cannot be avoided.
Following good construction practices is a must for the contractor to ensure that the design and detailing of the structure and the concrete mixture provided are not compromised. To this end, subgrade preparation should be adequate, and formwork should be properly designed and supported. Most importantly, concrete should not be retempered as the additional water will increase shrinkage and the potential for cracking, in addition to reducing strength. Proper concrete consolidation, curing and the provision of appropriate joints, where necessary, are also a must.
Concrete indeed does have a low tensile strength and can be prone to cracking. However, with proper design and detailing, the use of good concreting materials, including specialty mitigating materials where needed, and adherence to good construction practices, the incidence of cracks in concrete can be minimized.
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