A DIAGNOSTIC ASSESSMENT OF CRACKING OF PLINTH BEAMS IN A STORAGE BUILDING

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1 A DIAGNOSTIC ASSESSMENT OF CRACKING OF PLINTH BEAMS IN A STORAGE BUILDING A. K. Azad*, King Fahd University of Petroleum & Minerals, Saudi Arabia H. I. AI-Abdul Wahhab, King Fahd University of Petroleum & Minerals, Saudi Arabia o. S. B. AI-Amoudi, King Fahd University of Petroleum & Minerals, Saudi Arabia 28th Conference on OUR WORLD IN CONCRETE & STRUCTURES: August 2003, Singapore Article Online Id: The online version of this article can be found at: This article is brought to you with the support of Singapore Concrete Institute All Rights reserved for CI Premier PTE LTD You are not Allowed to re distribute or re sale the article in any format without written approval of CI Premier PTE LTD Visit Our Website for more information

2 28 th Conference on OUR WORLD IN CONCRETE & STRUCTURES: August 2003, Singapore A DIAGNOSTIC ASSESSMENT OF CRACKING OF PLINTH BEAMS IN A STORAGE BUILDING A. K. Azad*, King Fahd University of Petroleum & Minerals, Saudi Arabia H. I. AI-Abdul Wahhab, King Fahd University of Petroleum & Minerals, Saudi Arabia o. S. B. AI-Amoudi, King Fahd University of Petroleum & Minerals, Saudi Arabia Abstract In this paper, the assessment of the causal factors for cracking of the concrete plinth beams at the foundation of a storage building is presented to highlight the case of shrinkage and thermal cracking in restrained members. The investigation is carried out by a detailed investigation of pertinent factors including monitoring of cracks and thermal fluctuations, chemical analysis, strength, crack depth measurements and structural analysis. The study shows that development of high tensile stress in the beams due to drying shrinkage and thermal contraction has been the principal cause of cracking. A repair scheme is also proposed. Keywords: Investigation, monitoring, shrinkage, strains, repair 1. Introduction The plinth concrete beams used at the foundation level between the exterior columns of a storage building developed numerous vertical cracks soon after its construction. The beams were constructed monolithically with the concrete pedestals along the perimeter of the building without a construction joint. The beams support the exterior block walls. The cracks, mostly vertical are well-formed, randomly spaced and are distributed along the length of the beams, with crack width extending 0.5 mm in most cases. Past repair by epoxy injection proved to be a failure, as cracks reemerged at many of the old locations and more new cracks developed. Concerned with this problem of cracking for which no satisfactory answer was made available to the client by the consultant, the owner called for an independent detailed investigation of the problem. In this paper, the investigative work that was carried out has been presented to illustrate an interesting case study of what was eventually diagnosed as shrinkage and thermal induced cracking. The study shows that the primary causal factors pertaining to cracking are related to shrinkage and thermal. The construction of the beam as a continuous member around the perimeter without any expansion/contraction joint has resulted in a highly restrained member. Excessive tensile stress caused by the drying shrinkage and temperature variation led to the development of cracks in the beams. 2. The Structure and The Problem Figure 1 shows a layout of the ground beams of the storage building whose typical column footing is shown in Figure 2. The column footings are spaced at 7.5 m centres. The ground beam of size 300x1500 mm, whose cracking is the focus of this study, is cast as a continuous beam between the column pedestals, encompassing the entire perimeter of 120x90 m building without any construction joint. The ground beam directly supports the exterior block wall and a substantial height of the beam is above the ground level. Steel columns, erected directly over the column pedestal supports the roof of the building and a travelling crane. 217

3 E 0 en /I E It) CI c: '0 ta C. en N... A B c 0 E F G H J K L M I Lower Level Wall 17 A B c 0 E F G H J K L M N N m = 120m I 17 ~I Figure 1. Layout of the plinth beams G'"rundBeam (300 x 1500) ~ 300 I+- Roorslab GL 2400 Lean Calcrete t Figure 2. Typical column footing Cracks in the ground beams were observed shortly after the completion of curing. With time, some cracks became wider and well-formed and more new cracks emerged in the beam. Past monitoring of some cracks by the owner showed that the cracks are active, as the crack width varied with temperature variations. All visible major cracks were repaired with epoxy injection at cooler weather, when the crack opening was wider. However, the repair proved to be ineffective, as cracks reemerged soon after repair and more new cracks developed. Concerned with the development of these cracks in a newly constructed, unoccupied structure, for which no satisfactory explanation was in the offing, the client decided to undertake a thorough, independent study of the causes of the cracks. 3. Detailed Investigation A detailed investigation was planned for this work and this included all aspects of investigation related to the causes of cracking [1-3]. It basically consisted of condition survey, in-situ measurements and monitoring of the crack movement and temperature variation on the exposed beam surface, collection of core samples and determination of in-situ concrete strength, chemical analysis of concrete samples, soil sample analysis and geotechnical investigation, and finally structural analysis of the beams. The condition survey of the cracked beams and the exposed areas of the foundation revealed that the cracks are mostly vertical, well-formed and randomly distributed along the entire length of the beam (Figure 3). Except cracking of beams, no other form of damage to building was noted. 218

4 ~14 ~14 ~14 ~14 ~ \.,1 I O Figure 3. Typical crack pattern in beams Figure 4. Crack depth measurement No evidence of corrosion of reinforcement was seen and the exclusion of this as the contributing factor to cracking is further supported by the results of the chemical analysis of concrete samples which show that chloride is far below the threshold value of 0.3% by weight of cement [4]. The geotechnical investigation has shown that the building was built on a backfill material whose bearing capacity is estimated as 150 kn/m 2. Under the dead load of the structure, the column footings are expected to undergo a very small uniform settlement without any appreciable amount of differential settlement. The leveling measurements taken at the top of the beam also indicate the absence of a differential settlement that may be a cause for cracking. The composition of concrete mix used and the chemical analysis of concrete samples taken from the beams point out that there is no notable material-related problem other than the richness of the mix due to the use of 400 kg/m 3 of ordinary Portland cement (Type I). Such a rich mix produces higher heat of hydration and is a source of early thermal cracking. According to quality control reports, prompt curing was implemented by the contractor. Record shows the use of water-cement ratio of 0.37 for mixing. Core tests reveal an average compressive strength of about 35 MPa, exceeding the specified value of 30 MPa. Cores were taken through wider cracks to observe the through-thickness depth of crack. It was found that in most cases, the cracks extended across the full thickness of the beam, as shown in Figure 4. The initial prognosis suggested that the cracks are probably caused by the combined effect of shrinkage and thermal effects [5, 6], as no other factors could be cited as the major contributing factors. In order to verify this postulation, structural analysis of the beam was undertaken by taking into account the effect of dead load, thermal and shrinkage. 4. Thermal Range and Shrinkage For the thermal effects, the following surface ambient temperature fluctuations on the exterior and interior faces are considered from the initial construction temperature of 22 C. (i) The ambient temperature of the exterior face varies from 55 C to 4 C. (ii) The ambient temperature of the interior face varies from 14 C to 35 C. The temperature ranges are established from the results of in-situ monitoring of the surface temperature of the beam through embedded thermocouples. As the exterior wall faces the direct sun in hot summer months due to its orientation, the ambient temperature reaches almost 55 C occasionally. The following minimum temperature fluctuations can be established for design based on the above temperature range: (i) a temperature increase of 33 C at the exterior face and 13 C at the interior face, with a thermal gradient of DoC to 20 C across the beam's thickness; and (ii) a temperature drop of 1BoC at the outside face and BOC at the inside face, with a thermal gradient of DoC to -10 C across the beam's thickness. As the exact shrinkage characteristics of the concrete mix used are not known, a value of ultimate drying shrinkage strain of 300 microstrains under the prevailing condition can be assumed for the purpose of this calculation [7). This value, after allowing for initial plastic shrinkage, is on the lower side, and the actual value may well exceed this. As a decrease in temperature has a cumulative effect with shrinkage, the combination of temperature drop plus drying shrinkage which is critical for the beams is investigated. The critical environmental loading therefore consists of a drop in temperature by 18 C at the outside face and a drop in temperature of BOC at the inside face, together with a minimum drying shrinkage strain of 300 microstrains. 219

5 5. Cracking Force Due to the support restraints provided by the monolithic construction of the beams and the supporting pedestal, an axial tensile force, F, would develop in the beam due to temperature drop and shrinkage. The tensile force would be divided into steel reinforcement and concrete in proportion to their axial rigidity prior to cracking of concrete. Taking the weakest reinforcement in the beam, as shown in Figure 5, the area of longitudinal steel, As = 3267 mm 2 and the concrete area Ae equals 4.47*10 5 mm @300ctc Figure 5. Reinforcement in beam With modulus of elasticity for steel Es = 200 GPa and that for concrete Ee = jf[ in psi = 28.0 GPa (with fe' = 35 MPa = 5076 psi), the proportion of tensile force F carried by concrete and steel is calculated as 0.95F and 0.05F, respectively. The tensile strength of concrete in direct tension can be taken as f t = 6..jf[ ( fc' in psi) = 427 psi = 2.95 MPa, with fe' = 5076 psi (35 MPa) [4]. The cracking force, F, is given as 0.95F = 2.95 Ae = 1318 kn. Thus, an axial force in the vicinity of 1318 kn would be required to cause the beam to crack. The corresponding stress based on the gross concrete section is 1318*10 3 /(300 * 1500) = 2.93 MPa. Thus, a maximum tensile stress of 2.93 MPa can be applied to the gross uncracked concrete section. 6. Resistance of the Pedestal Footings Against Slip Each column is founded on an individual single footing, laid on compacted soil at a depth of 2.8 m below the top of the floor slab. A typical footing is shown in Figure 2. As the construction dictates, a beam can be assumed to be fixed at each pedestal with regard to its longitudinal movement. However, the maximum longitudinal force that can develop within a beam is equal to the resistance of the footing and the pedestal against movement provided by the compacted soil. If the longitudinal force exceeds the resistance provided by the foundation, the pedestals will slip. The foundation resistance against the slip comes from the frictional resistance of the footing against sliding and the passive resistance of the backfill material provided against the pedestal and the footing itself. In the absence of any expansion joint, it would be appropriate to consider that the footings are virtually immovable and therefore the longitudinal movement of the beams is not possible, as the entire foundation with the perimeter plinth beam formed an enclosed box. This immobility allows the full development of thermal and shrinkage forces in the beams, without any possible relief from the movement of the pedestals. 7. Estimation of Thermal and Shrinkage Stresses in the Beams From a cracking point of view, temperature drop is critical. This is considered in estimating the possible 'free movement' (if beams were free to move) due to temperature drop and shrinkage. As indicated earlier, a temperature drop of 8 C in conjunction with a linear thermal gradient of O C to -10 C across the beam's thickness is considered as critical. A simplified hand-calculation (a more exact finite element analysis was also carried out to confirm the findings), is presented here for an estimate of the level of shrinkage and thermal stress. The small difference between the values of linear coefficient of thermal expansion for concrete and steel is neglected. Taking the coefficient of thermal expansion for concrete as 10*10-6/oC, the uniform thermal strain, Ct, due to a temperature drop of 8 C is Ct = -10*10-6 * 8 = -80*10-6. As stated earlier, the drying shrinkage strain can be taken as, Csh = -300*10-6. Thus, total concrete strain, Ce is Ce = Ct + Csh = -380*10-6. As the beams ends are considered fixed, the thermal plus shrinkage tensile stress due to Ce = -380*10-6 is C>tel = 380*10-6 * 28.0*10 3 = MPa. 220

6 Additional thermal stress due to a thermal gradient of O C to -10 C across the beam's thickness is given as CJtc2 = ±S * 10*10-8 * 28.0*10 3 MPa = ±1.40 MPa. Thus, the maximum tensile stress on the cooler face (outside) is CJtc = MPa = MPa. This stress is calculated without considering the creep of concrete under tensile stress. As the stress, CJtc, develops slowly over time, the creep of concrete allows some axial deformation to take place and this results in a stress relief [7]. Thus, the expected tensile stress will be less than the computed value of MPa. The exact magnitude of creep relief is difficult to predict. Since the calculated stress is much higher than the cracking stress of 2.93 MPa, it can be concluded that the actual tensile stress within the beam had certainly exceeded the tensile strength of the concrete, thereby producing cracks in the beams. It should be noted that at the construction stage when the building was not sheltered, the ground beams were subjected to a more critical case of having the same temperature at both faces. Additionally, the ground beams are also subjected to bending due to the dead load of the block wall and self weight, which introduces additional tensile stresses in the beam due to bending. 8. Post-cracking Behavior of Beams Crack depth measurements taken from cores extracted at locations of significant cracks have starkly revealed that the cracks are full-depth, covering the entire thickness of the beam, which is 300 mm. A beam spanning between two pedestal supports is therefore effectively transformed into several segments of reinforced concrete elements tied together by the longitudinal bars. At a crack, the concrete is unable to sustain any tensile stress, but it is capable of transferring compressive and shear stresses. At the cracked section, the entire tensile force is now absorbed by the longitudinal steel. The maximum thermal and shrinkage stress in steel reinforcement reaches to CJts = (380+S0)*10-8 * 200*10 3 = 86 MPa. Each uncracked concrete segment between the cracks can be assumed to behave essentially like an unrestrained element under a thermal gradient. For a uniform temperature drop, the longitudinal reinforcement absorbs the full tensile force at a crack. The in-situ measurements of the crack-mouth opening show that the cracks are not dormant but the mouth opening fluctuates with the fluctuation in the outside temperature at the site. The maximum movement recorded in the crack width was about 0.2 mm due to a temperature change of 2SoC. The fluctuation in the width of the crack at the surface occurs due to increased steel strains and predominantly due to the curvature of the concrete element under a thermal gradient. For a thermal gradient of say +10 C across the thickness of the beam, the radius of the circular arc for a beam segment between two vertical cracks (assuming the ends of the segment are free due to cracking, which allows the beam to curve out of the plane) is As this radius is at the centerline of the segment, the additional increase in the length of the outside surface for a segment length of I mm is given as For a segment length of say I = 1000 mm, It 0= (R+tl2)/IR -I =- 2R This shows that the width of the crack is likely to increase by more than O.OS mm for a segment length of 1 m or so as the additional increase in width due to increased steel strain from uniform temperature drop of 8 C has to be added. The in-situ measurements have shown that the width of the crack may increase by over 0.1 mm. Although the beams are cracked, they are capable of resisting bending moments and shear forces due to in-plane vertical loads as reinforced concrete elements. However, the axial tension 221

7 capacity of the beams is provided only by the longitudinal steel. The analysis of the beams shows that the beams are capable of sustaining the imposed dead load from the block walls. 9. Repair In seeking a durable repair that would perform well, the cause and the movement of the cracks must be taken into consideration. Monitoring of the widths of the crack at two selected locations has reaffirmed the initial prognosis that the cracks are indeed 'active', as their widths fluctuate under the changing temperature. In proposing a repair scheme, the following considerations are taken into account: (i) It should be functionally effective, long-lasting and have sufficient flexibility to allow the breathing of the beams, thereby absorbing the thermal movement. (ii) It must provide a highly effective protection against ingress of chloride ions, moisture, and acidic gases from outside. (iii) As the beams are structurally adequate, the repair may not have to provide any additional strength to fully restore the structural integrity of the ground beams. Based on the above strategy, the proposed repair scheme is as follows: (i) All wide cracks (width> 0.3 mm) should be routed or grooved first and then sealed at the crack mouth with a good quality sealant material. (ii) The exposed faces of the grade beams (both exterior and interior) will receive a flexible (elastic), high-performance surface coating with excellent properties to protect the concrete from the attacks of chloride ions, acidic gases, and moisture. The coating must be flexible enough to absorb a cyclic movement of at least 1.0 mm and must be able to bridge the cracks having a width of 1.0 mm or more. 10. Conclusions The following conclusions are drawn from this study: 1. The combined effect of drying shrinkage and temperature drop has caused excessive tensile stress in the restrained ground beams, resulting in the cracking of the ground beams. The absence of any expansion joint in the construction of ground beams has created a highly restrained structure, aiding the development of high tensile force in the beams. 2. The strength of the cracked ground beams calculated under the combined action of dead load, thermal, and drying shrinkage indicates that the beams are structurally safe and adequate to sustain the imposed load. 3. As the cracks are 'active' under thermal response and the ground beams are structurally safe under the action of dead load, shrinkage, and thermal forces, the cracks need not be sealed by an injecting material due to the fact that, in such a case, the cracks may reappear for an effective temperature drop of over 10 C. From a durability viewpoint, it is preferred to provide a flexible, protective coating or cover on both side faces of the beams, which would allow breathing of the beams and inhibit any ingress of moisture and contaminants. Acknowledgement This work is part of a project undertaken by the department of Civil Engineering at King Fahd University of Petroleum & Minerals. The support of the University for this work is acknowledged. References [1) "Guide for evaluation of concrete structures prior to rehabilitation", ACI R-94, American Concrete Institute, Detroit, [2] "Guide for making a condition survey of concrete in service", ACI R-92, American Concrete Institute, Detroit, [3) "Causes, evaluation and repair of cracks in concrete structures", ACI Committee 224 Report, ACI Journal, May-June 1984, [4) "Building code requirements for structural concrete", ACI , American Concrete Institute, Detroit, [5) "Causes, mechanism and control of cracking in concrete", ACI SP-20, American Concrete Institute, Detroit, [6) "Non-structural cracks in concrete", Technical Report No. 22, Concrete Society, [7) Mehta, P. K., Concrete Structures, Properties and Materials, Prentice-Hall, New Jersey,