CAUSES AND CONTROL OF CRACKS IN CONCRETE STRUCTURES

CAUSES AND CONTROL OF CRACKS IN CONCRETE STRUCTURES

CAUSES AND CONTROL OF CRACKING

Introduction

This post presents a brief summary of the causes of cracks and means for their control. Cracks are categorized as occurring either in plastic concrete or hardened concrete.

Cracking of plastic concrete

• Plastic shrinkage cracking-”Plastic shrinkage cracking (Fig. 1.1) occurs when subjected to a very rapid loss of moisture caused by a combination of factors which include air and concrete temperatures, relative humidity, and wind velocity at the surface of the concrete. These factors can combine to cause high rates of surface evaporation in either hot or cold weather.” When moisture evaporates from the surface of freshly placed concrete faster than it is replaced by bleed water, the surface concrete shrinks. Since plastic shrinkage cracking is due to a differential volume change in the plastic concrete, successful control measures require a reduction in the relative volume change between the surface and other portions of the concrete. These measures include the use of fog nozzles to saturate the air above the surface and the use of plastic sheeting to cover the surface between finishing operations.

• Settlement cracking – After initial placement, vibration, and finishing, concrete has a tendency to continue to consolidate. During this period, the plastic concrete may be locally restrained by reinforcing steel, a prior concrete placement, or formwork. This local restraint may result in voids and/or cracks adjacent to the restraining element (Fig. 1.2). When associated with reinforcing steel, settlement cracking increases with increasing bar size, increasing slump, and decreasing cover (Dakhil et al. 1975). The use of the lowest possible slump, and an increase in concrete cover will reduce settlement cracking.

Cracking of hardened concrete

• Drying shrinkage-A common cause of cracking in concrete is restrained drying shrinkage. Drying shrinking is caused by the loss of moisture from the cement paste constituent, which can shrink by as much as 1 percent. Fortunately, aggregate provides internal restraint that reduces the magnitude of this volume change to about 0.06 percent. On wetting, concrete tends to expand. These moisture-induced volume changes are a characteristic of concrete. If the shrinkage of concrete could take place without restraint, the concrete would not crack. The higher the water content, the greater the amount of drying shrinkage (U.S. Bureau of Reclamation 1975). Drying shrinkage can be reduced by increasing the amount of aggregate and reducing the water content.

• Thermal stresses-Temperature differences within a concrete structure may be caused by portions of the structure losing heat of hydration at different rates or by the weather conditions cooling or heating one portion of the structure to a different degree or at a different rate than another portion of the structure. These temperature differences result in differential volume changes. When the tensile stresses due to the differential volume changes exceed the tensile stress capacity, concrete will crack. Cracking in mass concrete can result from a greater temperature on the interior than on the exterior. Procedures to help reduce thermally-induced cracking include reducing the maximum internal temperature, delaying the onset of cooling, controlling the rate at which the concrete cools, and increasing the tensile strength of the concrete.

• Chemical reaction-Deleterious chemical reactions may cause cracking of concrete. These reactions may be due to materials used to make the concrete or materials that come into contact with the concrete after it has hardened. Some general concepts for reducing adverse chemical reactions are presented here, but only pretesting of the mixture or extended field experience will determine the effectiveness of a specific measure. Concrete may crack with time as the result of slowly developing expansive reactions between aggregate containing active silica and alkalies derived from cement hydration, admixtures, or external sources (e.g., curing water, ground water, alkaline solutions stored or used in the finished structure.) The alkali-silica reaction results in the formation of a swelling gel, which tends to draw water from other portions of the concrete. This causes local expansion and accompanying tensile stresses, and may eventually result in the complete deterioration of the structure.

• Weathering-The weathering processes that can cause cracking include freezing and thawing, wetting, drying, heating and cooling. Cracking of concrete due to natural weathering is usually conspicuous, and it may give the impression that the concrete is on the verge of disintegration, even though the deterioration may not have progressed much below the surface. Damage from freezing and thawing is the most common weather-related physical deterioration. Concrete is best protected against freezing and thawing through the use of the lowest practical water cement ratio and total water content, durable aggregate and adequate air entrainment. Adequate curing prior to exposure to freezing conditions is also important. Allowing the structure to dry after curing will enhance its freezing and thawing durability. Other weathering processes that may cause cracking in concrete are alternate wetting and drying, and heating and cooling. Both processes produce volume changes that may cause cracking. If the volume changes are excessive, cracks may occur.

• Corrosion of reinforcement-Corrosion of a metal is an electrochemical process that requires an oxidizing agent, moisture, and electron flow within the metal; a series of chemical reactions takes place on and adjacent to the surface of the metal (ACI 201.2R). The key to protecting metal from corrosion is to stop or reverse the chemical reactions. This may be done by cutting off the supplies of oxygen or moisture or by supplying excess electrons at the anodes to prevent the formation of the metal ions (cathodic protection). Reinforcing steel usually does not corrode in concrete because a tightly adhering protective oxide coating forms in the highly alkaline environment. This is known as passive protection. Reinforcing steel may corrode, however, if the alkalinity of the concrete is reduced through carbonation or if the passivity of this steel is destroyed by aggressive ions (usually chlorides). Corrosion of the steel produces iron oxides and hydroxides, which have a volume much greater than the volume of the original metallic iron (Verbeck 1975). This increase in volume causes high radial bursting stresses around reinforcing bars and results in local radial cracks. These splitting cracks can propagate along the bar, resulting in the formation of longitudinal cracks (i.e., parallel to the bar) or spalling of the concrete. A broad crack may also form at a plane of bars parallel to a concrete surface, resulting in delamination, a well-known problem in bridge decks. Cracks provide easy access for oxygen, moisture, and chlorides, and thus, minor splitting cracks can create a condition in which corrosion and cracking are accelerated. Cracks transverse to reinforcement usually do not cause continuing corrosion of the reinforcement if the concrete has low permeability. This is due to the fact that the exposed portion of a ba.r at a crack acts as an anode. At early ages, the wider the crack, the greater the corrosion, simply because a greater portion of the bar has lost its passive protection. However, for continued corrosion to occur, oxygen and moisture must be supplied to other portions of the same bar or bars that are electrically connected by direct contract or through hardware such as chair supports. If the combination of density and cover thickness is adequate to restrict the flow of oxygen and moisture, then the corrosion process is self sealing (Verbeck 1975). Corrosion can continue if a longitudinal crack forms parallel to the reinforcement, because passivity is lost at many locations, and oxygen and moisture are readily available along the full length of the crack. Other causes of longitudinal cracking, such as high bond stresses, transverse tension (for example, along stirrups or along slabs with two-way tension), shrinkage, and settlement, can initiate corrosion. For general concrete construction, the best protection against corrosion-induced splitting is the use of concrete with low permeability and adequate cover. Increased concrete cover over the reinforcing is effective in delaying the corrosion process and also in resisting the splitting and spalling caused by corrosion or transverse tension (Gergely 1981; Beeby 1983). In the case of large bars and thick covers, it may be necessary to add small transverse reinforcement (while maintaining the minimum cover requirements) to limit splitting and to reduce the surface crack width (ACI 345R). In very severe exposure conditions, additional protective measures may be required A number of options are available, such as coated reinforcement, sealers or overlays on the concrete, corrosion-inhibiting admixtures, and cathodic protection (NCHRP Synthesis 57). Any procedure that effectively prevents access of oxygen and moisture to the steel surface or reverses the electron flow at the anode will protect the steel. In most cases, concrete must be allowed to breathe, that is any concrete surface treatment must allow water to evaporate from the concrete.

• Poor construction practices-A wide variety of poor construction practices can result in cracking in concrete structures. Foremost among these is the common practice of adding water to concrete to improve workability. Added water has the effect of reducing strength, increasing settlement, and increasing drying shrinkage. When accompanied by a higher cement content to help offset the decrease in strength, an increase in water content will also mean an increase in the temperature differential between the interior and exterior portions of the structure, resulting in increased thermal stresses and possible cracking. By adding cement, even if the water-cement ratio remains constant, more shrinkage will occur since the relative paste volume is increased. Lack of curing will increase the degree of cracking within a concrete structure. The early termination of curing will allow for increased shrinkage at a time when the concrete has low strength. The lack of hydration of the cement, due to drying, will result not only in decreased long-term strength, but also in the reduced durability of the structure. Other construction problems that may cause cracking are inadequate formwork supports, inadequate consolidation and placement of construction joints at points of high stress. Lack of support for forms or inadequate consolidation can result in settlement and cracking of the concrete before it has developed sufficient strength to support its own weight, while the improper location of construction joints can result in the joints opening at these points of high stress. Methods to prevent cracking due to these and other poor construction procedures are well known (see ACI 224R, ACI 302.1R, ACI 304R, ACI 305R, ACI 308, ACI 309R, ACI 345R, and ACI 347R), but require special attention during construction to insure their proper execution.

• Construction overloads-Loads induced during construction can often be far more severe than those experienced in service. Unfortunately, these conditions may occur at early ages when the concrete is most susceptible to damage and they often result in permanent cracks. Precast members, such as beams and panels, are most frequently subject to this abuse, but cast-in-place concrete can also be affected. A common error occurs when precast members are not properly supported during transport and erection. The use of arbitrary or convenient lifting points may cause severe damage. Lifting eyes, pins, and other attachments should be detailed or approved by the designer. When lifting pins are impractical, access to the bottom of a member must be provided so that a strap may be used. The PCI Committee on Quality Control Performance Criteria (1985, 1987) provides additional information on the causes, prevention and repair of cracking related to fabrication and shipment of precast or prestressed beams, columns, hollow core slabs and double tees. Operators of lifting devices must exercise caution and be aware that damage may be caused even when the proper lifting accessories are used. A large beam or panel lowered too fast, and stopped suddenly, results in an impact load that may be several times the dead weight of the member. Another common construction error that should be avoided is prying up one corner of a panel to lift it off its bed or “break it loose.” When considering the support of a member for shipment, the designer must be aware of loads that may be induced during transportation. Some examples that occur during shipment of large precast members via tractor and trailer are jumping curbs or tight highway corners, torsion due to differing roadway superelevations between the trailer and the tractor, and differential acceleration of the trailer and the tractor. Pretensioned beams can present unique cracking problems at the time of stress release-usually when the beams are less than one day old. Multiple strands must be detensioned following a specific pattern, so as not to place unacceptable eccentric loads on the member. If all of the strands on one side of the beam are released while the strands on the other side are still stressed, cracking may occur on the side with the unreleased strands. These cracks are undesirable, but should close with the release of the balance of the strands. In the case of a T-beam with a heavily reinforced flange and a highly prestressed thin web, cracks may develop at the web-flange junction. Another practice that can result in cracks near beam ends is tack welding embedded bearing plates to the casting bed to hold them in place during concrete placement. The tack welds are broken only after enough prestress is induced during stress transfer to break them. Until then, the bottom of the beam is restrained while the rest of the beam is compressed. Cracks will form near the bearing plates if the welds are too strong. Thermal shock can cause cracking of steam-cured concrete if it is treated improperly. The maximum rate of cooling frequently used is 70 F (40 C) per hour (ACI 517.2R; Verbeck 1958; Shideler and Toennies 1963; Kirkbride 1971b). When brittle aggregate is used and the strain capacity is low, the rate of cooling should be decreased. Even following this practice, thermally induced cracking often occurs. Temperature restrictions should apply to the entire beam, not just locations where temperatures are monitored. If the protective tarps used to contain the heat are pulled back for access to the beam ends when cutting the strands, and if the ambient temperatures are low, thermal shock may occur. Temperature recorders are seldom located in these critical areas. Similar conditions and cracking potential exist with precast blocks, curbs, and window panels when a rapidsurface temperature drop occurs. It is believed by many (ACI 517.2R; Mansfield 1948; Nurse 1949; Higginson 1961; Jastnebski 1961; Butt et al. 1969; Kirkbride 1971a; Concrete Institute of Australia 1972; PCI Energy Committee 1981) that rapid cooling may cause cracking only in the surface layers of very thick units and that rapid cooling is not detrimental to the strength or durability of standard precast products (PCI Energy Committee 1981). One exception is transverse cracking observed in pretensioned beams subjected to cooling prior to detensioning. For this reason, pretensioned members should be detensioned immediately after the steam-curing has been discontinued (PCI Energy Committee 1981). Cast-in-place concrete can be unknowingly subjected to construction loads in cold climates when heaters are used to provide an elevated working temperature within a structure. Typically, tarps are used to cover windows and door openings, and high volume heaters are operated inside the enclosed area. If the heaters are located near exterior concrete members, especially thin walls, an unacceptably high thermal gradient can result within the members. The interior of the wall will expand in relation to the exterior. Heaters should be kept away from the exterior walls to minimize this effect. Good practice also requires that this be done to avoid localized drying shrinkage and carbonation cracking. Storage of materials and the operation of equipment can easily result in loading conditions during construction far more severe than any load for which the structure was designed. Tight control must be maintained to avoid overloading conditions. Damage from unintentional construction overloads can be prevented only if designers provide information on load limitations for the structure and if construction personnel heed these limitations.

• Errors in design and detailing-The effects of improper design and/or detailing range from poor appearance to lack of serviceability to catastrophic failure. These problems can be minimized only by a thorough understanding of structural behavior (meant here in the broadest sense). Errors in design and detailing that may result in unacceptable cracking include use of poorly detailed reentrant comers in walls, precast members and slabs, improper selection and/or detailing of reinforcement, restraint of members subjected to volume changes caused by variations in temperature and moisture, lack of adequate contraction joints, and improper design of foundations, resulting in differential movement within the structure. Examples of these problems are presented by Kaminetzky (1981) and Price (1982). Reentrant comers provided a location for the concentration of stress and, therefore, are prime locations for the initiation of cracks. Whether the high stresses result from volume changes, in-plane loads, or bending, the designer must recognize that stresses are always high near reentrant comers. Well-known examples are window and door openings in concrete walls and dapped end beams, as shown in Fig. 1.4 and 1.5. Additional properly anchored diagonal reinforcement is required to keep the inevitable cracks narrow and prevent them from propagating. The use of an inadequate amount of reinforcing may result in excessive cracking. A typical mistake is to lightly reinforce a member because it is a “nonstructural member.” However, the member (such as a wall) may be tied to the rest of the structure in such a manner that it is required to carry a major portion of the load once the structure begins to deform. The “nonstructural element”nthen begins to carry loads in proportion to its stiffness. Since this member is not detailed to act structurally, unsightly cracking may result even though the safety of the structure is not in question. The restraint of members subjected to volume changes results frequently in cracks. Stresses that can occur in concrete due to restrained creep, temperature differential, and drying shrinkage can be many times the stresses that occur due to loading. A slab, wall, or a beam restrained against shortening, even if prestressed, can easily develop tensile stresses sufficient to cause cracking. Properly designed walls should have contraction joints spaced from one to three times the wall height. Beams should be allowed to move. Cast-in-place post-tensioned construction that does not permit shortening of the prestressed member is susceptible to cracking in both the member and the supporting structure (Libby 1977). The problem with restraint of structural members is especially serious in pretensioned and precast members that may be welded to the supports at both ends. When combined with other problem details (such as reentrant comers), results may be catastrophic (Kaminetzky 1981; Mast 1981). Improper foundation design may result in excessive differential movement within a structure. If the differential movement is relatively small, the cracking problems may be only visual in nature. However, if there is a major differential settlement, the structure may not be able to redistribute the loads rapidly enough, and a failure may occur. One of the advantages of reinforced concrete is that, if the movement takes place over a long enough period of time, creep will allow at least some load redistribution to take place. The importance of proper design and detailing will depend on the particular structure and loading involved. Special care must be taken in the design and detailing of structures in which cracking may cause a major serviceability problem. These structures also require continuous inspection during all phases of construction to supplement the careful design and detailing.

• Externally applied loads-It is well known that load-induced tensile stresses result in cracks in concrete members. This point is readily acknowledged and accepted in concrete design. Current design procedures (ACI 318 and AASHTO) Standard Specifications for Highway Bridges) use reinforcing steel, not only to carry the tensile forces, but to obtain both an adequate distribution of cracks and a reasonable limit on crack width. Current knowledge of flexural members provides the basis for the following general conclusions about the variables that control cracking: Crack width increases with increasing steel stress, cover thickness and area of concrete surrounding each reinforcing bar. Of these, steel stress is the most important variable. The bar diameter is not a major consideration. The width of a bottom crack increases with an increasing strain gradient between the steel and the tension face of the beam. The equation considered to best predict the most probable maximum surface crack width in bending was developed by Gergely and Lutz (1968). A simplified version of this equation is: 

in which w = most probable maximum crack width, in.; ? = ratio of distance between neutral axis and tension face to distance between neutral axis and centroid of reinforcing steel (taken as approximately 1.20 for typical beams in buildings); fs = reinforcing steel stress, ksi; dc = thickness of cover from tension fiber to center of bar closest thereto, in.; and A =area of concrete symmetric with reinforcing steel divided by number of bars, in2 A modification of this equation is used in ACI 318, which effectively limits crack widths to 0.016 in. (0.41 mm) for interior exposure and 0.013 in. (0.33 mm) for exterior exposure. However, there is little correlation between surface crack width for cracks transverse to bars and the corrosion of reinforcing, these limits do not appear to be justified on the basis of corrosion control. There have been a number of equations developed for prestressed concrete members (ACI 224R), but no single method has achieved general acceptance. The maximum crack width in tension members is larger than that predicted by the expression for flexural members (Broms 1965; Broms and Lutz 1965). Absence of a strain gradient and compression zone in tension members is the probable reason for the larger crack widths. On the basis of limited data, the following expression has been suggested to estimate the maximum crack width in direct tension (ACI 224R): w = 0.10 fs (dc A)0.33 x 10-3 (1.2) Additional information on cracking of concrete in direct tension is provided in ACI 224.2R. Flexural and tensile crack widths can be expected to increase with time for members subjected to either sustained or repetitive loading. Although a large degree of scatter is evident in the available data, a doubling of crack width with time can be expected (Abeles et al. 1968; Bennett and Dave 1969; Illston and Stevens 1972; Holmberg 1973; Rehm and Eligehausen 1977). Although work remains to be done, the basic principles of crack control for load-induced cracks are well understood. Well-distriiuted reinforcing offers the best protection against undesirable cracking. Reduced steel stress, obtained through the use of a larger amount of steel, will also reduce the amount of cracking. While reduced cover will reduce the surface crack width, designers must keep in mind that cracks (and therefore, crack widths) perpendicular to reinforcing steel do not have a major effect on the corrosion of the steel, while a reduction in cover will be detrimental to the corrosion protection of the reinforcing.

METHODS OF CONCRETE CRACK REPAIR

METHODS OF CONCRETE CRACK REPAIR

Following the evaluation of the cracked structure, a suitable methods of concrete crack repair procedure can be selected. Successful repair procedures take into account the cause(s) of the cracking. For example, if the cracking was primarily due to drying shrinkage, then it is likely that after a period of time the cracks will stabilize. On the other hand, if the cracks are due to a continuing foundation settlement, repair will be of no use until the settlement problem is corrected.

Following is a survey of crack repair methods, including a summary of the characteristics of the cracks that may be repaired with each procedure, the types of structures that have been repaired, and a summary of the procedures that are used.

Epoxy injection

Cracks as narrow as 0.002 in. (0.05 mm) can be bonded by the injection of epoxy. The technique generally consists of establishing entry and venting ports at close intervals along the cracks, sealing the crack on exposed surfaces, and injecting the epoxy under pressure. Epoxy injection has been successfully used in the repair of cracks in buildings, bridges, dams, and other types of concrete structures (ACI 503R). However, unless the cause of the cracking has been corrected, it will probably recur near the original crack. If the cause of the cracks cannot be removed, then two options are available.

One is to rout and seal the crack, thus treating it as a joint, or, establish a joint that will accommodate the movement and then inject the crack with epoxy or other suitable material. With the exception of certain moisture tolerant epoxies, this technique is not applicable if the cracks are actively leaking and cannot be dried out. Wet cracks can be injected using moisture tolerant materials, but contaminants in the cracks (including silt and water) can reduce the effectiveness of the epoxy to structurally repair the cracks.

The use of a low-modulus, flexible adhesive in a crack will not allow significant movement of the concrete structure. The effective modulus of elasticity of a flexible adhesive in a crack is substantially the same as that of a rigid adhesive because of the thin layer of material and high lateral restraint imposed by the surrounding concrete. Epoxy injection requires a high degree of skill for satisfactory execution, and application of the technique may be limited by the ambient temperature.

• Clean the cracks: The first step is to clean the cracks that have been contaminated; to the extent this is possible and practical. Contaminants such as oil, grease, dirt, or fine particles of concrete prevent epoxy penetration and bonding, and reduce the effectiveness of repairs. Preferably, contamination should be removed by vacuuming or flushing with water or other specially effective cleaning solutions.

• Seal the surfaces: Surface cracks should be sealed to keep the epoxy from leaking out before it has gelled. Where the crack face cannot be reached, but where there is backfill, or where a slab-on-grade is being repaired, the backfill material or sub base material is sometimes an adequate seal. A surface can be sealed by applying an epoxy, polyester, or other appropriate sealing material to the surface of the crack and allowing it to harden. If a permanent glossy appearance along the crack is objectionable and if high injection pressure is not required, a strippable plastic surface sealer may be applied along the face of the crack. When the job is completed, the surface sealer can be stripped away to expose the gloss-free surface. Cementitious seals can also be used where appearance of the completed work is important. If extremely high injection pressures are needed, the crack can be cut out to a depth of 1/2 in. (13 mm) and width of about 3/4 in. (20 mm) in a V-shape, filled with an epoxy, and struck off flush with the surface.
• Install the entry and venting ports. Three methods are in general use:

1. Fittings inserted into drilled holes: This method was the first to be used, and is often used in conjunction with V-grooving of the cracks. The method entails drilling a hole into the crack, approximately 3/4 in. (20 mm) in diameter and 1/2 to 1 in. (13 to 25 mm) below the apex of the V grooved section.
2. Bonded flush fitting: When the cracks are not V grooved , a method frequently used to provide an entry port is to bond a fitting flush with the concrete face over the crack. The flush fitting has an opening at the top for the adhesive to enter and a flange at the bottom that is bonded to the concrete.
3. Interruption in seal: Another system of providing entry is to omit the seal from a portion of the crack. This method can be used when special gasket devices are available that cover the unsealed portion of the crack and allow injection of the adhesive directly into the crack without leaking.

• Mix the epoxy: This is done either by batch or continuous methods. In batch mixing, the adhesive components are premixed according to the manufacturers instructions, usually with the use of a mechanical stirrer, like a paint mixing paddle. Care must be taken to mix only the amount of adhesive that can be used prior to commencement of gelling of the material.

• Inject the epoxy: Hydraulic pumps, paint pressure pots, or air-actuated caulking guns may be used. The pressure used for injection must be selected carefully. Increased pressure often does little to accelerate the rate of injection. If the crack is vertical or inclined, the injection process should begin by pumping epoxy into the entry port at the lowest elevation until the epoxy level reaches the entry port above. For horizontal cracks, the injection should proceed from one end of the crack to the other in the same manner. The crack is full if the pressure can be maintained. If the pressure can not be maintained, the epoxy is still flowing into unfilled portions or leaking out of the crack.

• Remove the surface seal: After the injected epoxy has cured, the surface seal should be removed by grinding or other means as appropriate.

• Alternative procedure: For massive structures, an alternate procedure consists of drilling a series of holes [usually 7/8 to 4-in. (20 to 100-mm) diameter] that intercepts the crack at a number of locations. Typically, holes are spaced at 5-ft (1.5-m) intervals. Another method recently being used is a vacuum or vacuum assist method.

There are two techniques: one is to entirely enclose the cracked member with a bag and introduce the liquid adhesive at the bottom and to apply a vacuum at the top. The other technique is to inject the cracks from one side and pull a vacuum from the other. Typically, epoxies are used; however, acrylics and polyesters have proven successful.

• Routing and sealing

Routing and sealing of cracks can be used in conditions requiring remedial repair and where structural repair is not necessary. This method involves enlarging the crack along its exposed face and filling and sealing it with a suitable joint sealant  (Fig. 3.1). This is a common technique for crack treatment and is relatively simple in comparison to the procedures and the training required for epoxy injection. The procedure is most applicable to approximately flat horizontal surfaces such as floors and pavements. However, routing and sealing can be accomplished on vertical surfaces (with a non-sag sealant) as well as on curved surfaces (pipes, piles and pole).

Routing and sealing is used to treat both fine pattern cracks and larger, isolated cracks. A common and effective use is for waterproofing by sealing cracks on the concrete surface where water stands, or where hydrostatic pressure is applied. This treatment reduces the ability of moisture to reach the reinforcing steel or pass through the concrete, causing surface stains or other problems.

The sealants may be any of several materials, including epoxies, urethanes, silicones, polysulfides, asphaltic materials, or polymer mortars. Cement grouts should be avoided due to the likelihood of cracking. For floors, the sealant should be sufficiently rigid to support the anticipated traffic. Satisfactory sealants should be able to withstand cyclic deformations and should not be brittle.

The procedure consists of preparing a groove at the surface ranging in depth, typically, from 1/4 to 1 in. (6 to 25 mm). A concrete saw, hand tools or pneumatic tools may be used. The groove is then cleaned by air blasting, sandblasting, or water blasting, and dried. A sealant is placed into the dry groove and allowed to cure. A bond breaker may be provided at the bottom of the groove to allow the sealant to change shape, without a concentration of stress on the bottom (Fig. 3.2).

The bond breaker may be a polyethylene strip or tape which will not bond to the sealant. Careful attention should be applied when detailing the joint so that its width to depth aspect ratio will accommodate anticipated movement (ACI 504R).

Stitching

Stitching involves drilling holes on both sides of the crack and grouting in    U-shaped metal units with short legs (staples or stitching dogs) that span the crack as shown in Fig 3.3. Stitching may be used when tensile strength must be reestablished across major cracks. The stitching procedure consists of drilling holes on both sides of the crack, cleaning the holes, and anchoring the legs of the staples in the holes, with either a non shrink grout or an epoxy resin-based bonding system.

Additional reinforcement

• Conventional reinforcement

Cracked reinforced concrete bridge girders have been successfully repaired by inserting reinforcing bars and bonding them in place with epoxy. This technique consists of sealing the crack, drilling holes that intersect the crack plane at approximately 90 deg (Fig. 3.4), filling the hole and crack with injected epoxy and placing a reinforcing bar into the drilled hole. Typically, No. 4 or 5 (10 M or 15 M) bars are used, extending at least 18 in. (0.5 m) each side of the crack. The reinforcing bars can be spaced to suit the needs of the repair. They can be placed in any desired pattern, depending on the design criteria and the location of the in-place reinforcement.

• Pre-stressing steel

Post-tensioning is often the desirable solution when a major portion of a member must be strengthened or when the cracks that have formed must be closed (Fig. 3.5). This technique uses pre stressing strands or bars to apply a compressive force. Adequate anchorage must be provided for the pre stressing steel, and care is needed so that the problem will not merely migrate to another part of the structure.

Drilling and plugging

Drilling and plugging a crack consists of drilling down the length of the crack and grouting it to form a key (Fig. 3.6).

This technique is only applicable when cracks run in reasonable straight lines and are accessible at one end. This method is most often used to repair vertical cracks in retaining walls. A hole [typically 2 to 3 in. (50 to 75 mm) in diameter] should be drilled, centered on and following the crack.

The grout key prevents transverse movements of the sections of concrete adjacent to the crack. The key will also reduce heavy leakage through the crack and loss of soil from behind a leaking wall. If water-tightness is essential and structural load transfer is not, the drilled hole should be filled with a resilient material of low modulus in lieu of grout. If the keying effect is essential, the resilient material can be placed in a second hole, the fiat being grouted.

Gravity Filling

Low viscosity monomers and resins can be used to seal cracks with surface widths of 0.001 to 0.08 in. (0.03 to 2 mm) by gravity filling. High-molecular- weight methacrylates, urethanes, and some low viscosity epoxies have been used successfully. The lower the viscosity, the finer the cracks that can be filled. The typical procedure is to clean the surface by air blasting and/or water blasting. Wet surfaces should be permitted to dry several days to obtain the best crack filling.

Water blasting followed by a drying time may be effective in cleaning and preparing these cracks. Cores taken at cracks can be used to evaluate the effectiveness of the crack filling. The depth of penetration of the sealant can be measured. Shear (or tension) tests can be performed with the load applied in a direction parallel to the repaired cracks (as long as reinforcing steel is not present in the core in or near the failure area). For some polymers the failure crack will occur outside the repaired crack.

Grouting

• Portland cement grouting

Wide cracks, particularly in gravity dams and thick concrete walls, may be repaired by filling with Portland cement grout. This method is effective in stopping water leaks, but it will not structurally bond cracked sections. The procedure consists of cleaning the concrete along the crack; installing built-up seats (grout nipples) at intervals astride the crack (to provide a pressure tight connection with the injection apparatus); sealing the crack between the seats with a cement paint, sealant, or grout; flushing the crack to clean it and test the seal; and then grouting the whole area. Grout mixtures may contain cement and water or cement plus sand and water, depending on the width of the crack.

However, the water-cement ratio should be kept as low as practical to maximize the strength and minimize shrinkage. Water reducers or other admixtures may be used to improve the properties of the grout. For small volumes, a manual injection gun may be used; for larger volumes, a pump should be used. After the crack is filled, the pressure should be maintained for several minutes to insure good penetration.

• Dry packing

Dry packing is the hand placement of a low water content mortar followed by tamping or ramming of the mortar into place, producing intimate contact between the mortar and the existing concrete. Because of the low water-cement ratio of the material, there is little shrinkage, and the patch remains tight and can have good quality with respect to durability, strength, and water tightness.

Dry pack can be used for filling narrow slots cut for the repair of dormant cracks. The use of dry pack is not advisable for filling or repairing active cracks.

Before a crack is repaired by dry packing, the portion adjacent to the surface should be widened to a slot about 1 in. (25 mm) wide and 1 in. (25 mm) deep. The slot should be undercut so that the base width is slightly greater than the surface width.

To minimize shrinkage in place, the mortar should stand for 1/2 hour after mixing and then should be remixed prior to use. The mortar should be placed in layers about 3/8 in. (10 mm) thick. Each layer should be thoroughly compacted over the surface using a blunt stick or hammer, and each underlying layer should be scratched to facilitate bonding with the next layer. The repair should be cured by using either water or a curing compound. The simplest method of moist curing is to support a strip of folded wet burlap along the length of the crack.

Overlay and surface treatments

Fine surface cracks in structural slabs and pavements may be repaired using either a bonded overlay or surface treatment if there will not be further significant movement across the cracks. Unbounded overlays may be used to cover, but not necessarily repair a slab. Overlays and surface treatments can be appropriate for cracks caused by one-time occurrences and which do not completely penetrate the slab.

• Surface treatments

Low solids and low-viscosity resin-based systems have been used to seal the concrete surfaces, including treatment of very fine cracks. They are most suited for surfaces not subject to significant wear. Bridge decks and parking structure slabs, as well as other interior slabs may be coated effectively after cracks are treated by injecting with epoxy or by routing and sealing. Materials such as urethanes, epoxies, polyesters, and acrylics have been applied in thickness of 0.04 to 2.0 in. (1 to 50 mm), depending on the material and purpose of the treatment. Skid-resistant aggregates are often mixed into the material or broadcast onto the surface to improve traction.

• Overlays

Slabs containing find dormant cracks can be repaired by applying an overlay, such as polymer modified Portland cement mortar or concrete, or by silica fume concrete. Slabs with working cracks can be overlaid if joints are placed in the overlay directly over the working cracks. In highway bridge applications, an overlay thickness as low as 1-1/4 in. (30 mm) has been used successfully. Suitable polymers include styrene butadiene or acrylic latexes. The resin solids should be at least 15 percent by weight of the Portland cement, with 20 percent usually being optimum.

QUANTITY & RATE ANALYSIS FOR REINFORCED CONCRETE

QUANTITY & RATE ANALYSIS FOR REINFORCED CONCRETE

Today we will see how to prepare rate analysis for Reinforced Concrete (RCC) work. First step to rate analysis is the estimation of labour, materials, equipments and miscellaneous items for particular quantity of reinforced concrete.

The second step is to determine the component of structure for which the RCC rate analysis is required, as the quantity of reinforcement steel varies with slabs, beams, columns, foundation, RCC Roads etc., though the quantity of other materials like sand, coarse aggregate and cement remain the same with the same mix design (mix proportion) of concrete. Labour rates for reinforcement work changes with type of structural component as the quantity of reinforcement steel changes. The Quantity of materials like sand, cement and coarse aggregates vary with mix design such as M15 (1:2:4), M20 (1:1.5:3), M25, M30 etc..

Here we will see the rate analysis for 1m³ of reinforced concrete.

Data required for RCC Rate Analysis:

1. Estimation of materials:

Material estimation include sand, cement, coarse aggregate and steel for a particular mix design. Let us consider a mix design of 1:1.5:3 for our estimation practice. The dry volume of total materials required is considered as 1.54 times the wet volume of concrete, due to voids present in sand and aggregates in dry stage. Therefore, for our calculation, we will consider the total volume of materials required as 1.54 m³ for 1 m³ of wet concrete.

a) Bags of cement required:

Volume of cement required for 1m³ of Concrete =

=0.28 m³

Then number of bags of cement (volume of one bag of cement = 0.0347 m³)

== 8.07 bags of cement.

b) Volume of Sand required:

Volume of sand required =  = 0.42 m³ of sand.

c) Volume of Coarse Aggregate Required

Volume of Coarse Aggregate == 0.84 m³ of coarse aggregates.

d) Estimation of Reinforced Steel:

Quantity of steel required depends on components of structure, i.e. slabs, beams, columns, foundations, roads etc. To estimate the steel required, there are two methods.

First method is, when we have the drawing available, we can calculate the total weight of steel required divided by total volume of concrete for different components. This will give us the weight of reinforcement steel per cubic meter of concrete.

Second method is assuming the percentage of reinforcement for different components. Following are the percentage of reinforcement steel generally required per different components. Its values can vary from structure to structure, and can be assumed from past experiences of similar structure.

• For slabs = 1.0 % of concrete volume.
• For Beam = 2 % concrete volume.
• For column = 2.5 % of concrete volume.
• For RCC Roads, 0.6% concrete volume.

Lets take example of RCC Column, where reinforcement required is 2.5% of concrete volume, weight of steel required will be:

=196.25 kg.

2. Labour Requirement for 1m³ of RCC:

Labours required are presented in terms of days required by particular labour to complete its work for the given quantity of concrete. Following are the various labours required:

a) Mason: As per Standard Schedule of Rates and Analysis of Rates, One mason is required for 0.37 days.

b) Labours: One Unskilled labours required for 3.5 days.

c) Water carrier: One water carrier required for 1.39 days.

d) Bar Bender: Bar bender requirement depends on weight of reinforcement. Lets consider one bar bender required for 100 kg of steel as for 1 day.

e) Mixer Operator: One mixer operator required for 0.0714 days.

f) Vibrator Operator: One vibrator operator required for 0.0714 days.

3. Equipments and sundries:

Equipment and other charges, such as water charges, miscellaneous items, tools and tackles etc can be assumed as some percentage of total cost of materials and labours. Lets say it as 7.5%.

4. Contractor’s Profit:

Contractor’s profit depends on place to place, organization to organization and work to work. It varies from 10 – 20%. For our case lets assume it as 15% of total cost of materials, labours and equipments.

We have calculated the quantity of every item in above 1 – 3 steps. For rate analysis of RCC, we need to multiply each quantity with their rates to get the amount for every item of work. Rates vary from place to place and time to time. It is advisable to assume local rates or standard rates of the place.

The sum total of all the four items above will give the rate or cost for 1m³ of concrete.

DOWNLOAD: RCC Rate Analysis Spreadsheet

http://theconstructor.org/files/RCC%20RATE%20ANALSYSIS.xlsx

FIBRE REINFORCED CONCRETE IN PAVEMENTS

FIBRE REINFORCED CONCRETE IN PAVEMENTS

Fibre reinforced concrete (FRC) is defined as a composite material consisting of concrete reinforced with discrete randomly but uniformly dispersed short length fibres. The fibres can be made of steel, polymer or natural materials. Woven fabrics, long wires, bars, and continuous wire mesh are not considered discrete fibres.

FRC is considered as a material of improved properties and not as reinforced cement concrete whereas reinforcement is provided for local strengthening of concrete in tension region. Since in FRC, fibres are distributed uniformly in concrete, it has better properties to resist internal stresses due to shrinkage. As fibres improve specific material properties of the concrete, impact resistance, flexural strength, toughness, fatigue resistance, ductility also improve.

Fibres generally used in cement concrete pavements are steel fibres and organic polymer fibres such as polypropylene and polyester.

Steel Fibre Reinforced Concrete

Steel fibres have been used for a long time in construction of roads and also in floorings, particularly where heavy wear and tear is expected. Specifications and nomenclature are important for a material to be used as the tenders are invited based on specifications and nomenclature of the items. Such nomenclature is not available in Delhi Schedule of Rates. In a work where steel fibre reinforced concrete was used for overlays just like flooring, the following nomenclature can be adopted for concreting of small thickness.

Providing and laying 40 mm steel fibre reinforced cement concrete in pavement (in panels having area not more than 1.5 sqm) consisting of steel fibre @ 40kg per cubic meter of concrete and cement concrete mix of 1:1.95:1.95 (1 cement: 1.95 coarse sand of fineness modulus 2.42: 1.95 stone aggregate 10 mm and down gauge of fineness modulus 5.99) over existing surface i/c cement slurry, consolidating, tapping, and finishing but excluding the cost of steel fibres which shall be paid separately, complete as per direction of Engineer in Charge (Cement to be used shall be OPC 43 grade and sand and aggregate have to be washed).

Second item of fibres was provided separately as “Providing and mixing steel fibres of dia 0.45 mm in cement concrete duly cut into pieces not more than 25 mm in length.”

Figure-1: Pavement with steel fibre reinforced concrete

Though the item of steel fibre reinforced concrete has been provided with a design mix of concrete, which is almost of 1:2:2 grading, it can now be used of mix like M30 or M35. Since in the executed item, the thickness was to be restricted, the stone aggregates used were of 10 mm size and below however, in case of the concrete of more than 75 mm thickness, stone aggregates of 20 mm grading can be used.

The construction was carried out more than a decade back. It is observed that the performance of the concrete is satisfactory even after many years of construction (Figure 1). Even, no corrosion has been observed in the steel fibres. In fact the concreting has been done just like flooring item in this case over already existing hard surface. In such a case a bonding coat should also be provided like a coat of cement slurry. The fibre reinforced concrete has been provided in small panels considering the workability. Though vacuum dewatered concrete has not been done with steel fibre reinforced concrete but the same is also possible. Vacuum dewatered concrete, though cannot be done in small thickness like 40 or 50 mm but can be used if thickness is 100 mm or more.

Figure-2: A view of PFRC used in parking over lean base concrete

Figure-3: Laying of PFRC in parking over WBM surface

Polymer Fibre Reinforced Concrete

Polymeric fibres are being used now because of their no risk of corrosion and also being cost effective (Sikdar et al, 2005). Polymeric fibres normally used are either of polyester or polypropylene. Polymer fibre reinforced concrete (PFRC) was used on two sites with ready mix concrete and Vacuum dewatering process.

The nomenclature can be used in the works as given here.

"Providing and laying ready mix fibre reinforced cement concrete of M35 grade (The concrete shall also have minimum works test beam flexural strength of 40 kg per sqm at 28 days) in required slope and camber in panels i/c shaping at drainage points as required using cementitious materials not less than 435 kg per cum of finished concrete from ACC/L&T/AHLCON/ UNITECH or equivalent batching plant for all leads and lifts with Fibrecom-CF/Fibremesh/Recron or equivalent (100 % virgin synthetic fibre size 12 mm long) to be mixed @ 900 grams per cum of concrete i/c finishing with screed vibration, vacuum dewatering process, floating, trowelling, brooming and normal curing etc. complete as per standard manufacturer’s specifications and as per direction of Engineer’s in charge (All related equipment shall be arranged by the contractor. Cost of centering, shuttering, grooving etc. shall be paid separately. Design Mix shall be got approved from the Engineer in Charge).

In both the sites, vacuum dewatered concrete was used. Both the sites are to be used for parking. In a site, fibre reinforced concrete was used over a base cement concrete of lean mix of 1:4:8 (Figure 2) while in other site it was laid over water bound macadam (WBM) (Figure 3).

When dewatered concrete it has no problem of water being coming out on surface during compaction process but when it is done over WBM, a lot of concrete water is soaked by WBM and thus the concrete loses the water to WBM and the water which comes out during dewatering/compaction process is not in same quantity asin case of lean concrete. It appears that it is better to provide base concrete than WBM as the base. The groove was made in one case before setting of concrete and also panels were cast with expansion joints in one direction. No cracks were observed in the direction in which expansion joints were provided assuming this is longitudinal direction. In lateral direction, no joints were provided and the width of such panel was about 12 m. It was later observed that cracks have developed in this direction (Figure 4).

Figure-4: A closer view of crack due to no expansion joint provided in PFRC on lean concrete base.

Figure-5: A closer view of crack due to no expansion joint

As it is known that the width of 12 m is too long for expansion/ contraction. It has been observed that almost at about one–third of the panel width, such cracks developed i.e. size of panel from one side is about 4 m and from other side it is about 8m. From the site observation, it is therefore inferred that the panel should have the size of about 4m x 4m in the temperature conditions of Delhi however small variation can also be made as per site conditions. In other case, the contractor delayed the cutting of grooves and thereafter the area was occupied due to some urgent requirements, the cracks in both the directions developed. The cracks were almost in line. Later on the grooves were made through cutters. It has been observed that the distance of cracks in one side was almost near to 4 m and on other side at about 7 to 9 m (Figure 5). Thus from this case study also, inference can be made that grooves if made in panels of 4m x 4m, it would be appropriate.

In both the cases, no lateral grooves were made, as working was not a problem due to use of vacuum dewatering process. In both the cases, horizontal line cracks have been observed indicating that the grooves in other direction are also essential. From this, it is imperative that polymer fibre reinforced concrete should be laid in panels or grooves should be provided so that concrete acts like in panels. Cutting grooves is easy as it can be made after casting of the concrete. But it should not be delayed for long and should be made before concrete achieves its desired strength. The size of panels may be kept around 4m x 4m.

Thus, fibre reinforced concrete has advantage over normal concrete particularly in case of cement concrete pavements. Polymeric fibres such as polyester or polypropylene are being used due to their cost effective as well as corrosion resistance though steel fibres also work quite satisfactorily for a long time. It appears that fibre reinforced concrete should be laid on base concrete of lean mix such as 1:4:8 cement concrete rather than over WBM and provided with grooves in panels of about 4m x 4m to avoid expansion/ contraction cracks. Grooves can be made after casting of concrete through cutters.