Materials

Cracking In The Reinforced Concrete Elements – Codal Provisions On Design Requirements, Repairs And Remedial Measures

__
<p>In some of the construction projects, the massive reinforced concrete elements like bridge abutment caps show unforeseen cracks even after compliance with all the design requirements as per the relevant standard codes of practice.&nbsp;</p><p>Despite the dense reinforcement steel provided in the structural elements, these cracks appear during the construction phases even before the element is subjected to its intended loads and may sometimes be wider than the crack widths indicated in the design codes. These cracks can be repaired, and dismantling can be avoided.</p><p>The crack widths indicated in the codes are design phase crack widths for checking the concrete section adequacy, and the same shall not be compared with the post-concreting phase crack widths.&nbsp;</p><p>In most situations, the crack width measured at the site is compared with the design phase crack widths, and dismantling of cast RCC elements is being demanded, and further progress of works is being stopped, which can be avoided. &nbsp;</p><p>The following is the listing of the guidelines from standard codes of practice which will help provide clarifications to apprehensions raised by the customers and avoid time delays in the completion of the projects:</p><p>&nbsp;</p><h2><span style="font-size: 14pt;">Technical Specification Requirements indicated in Bid Document</span></h2><p>Generally, in the contract document technical specifications provided by the customer, there will be no mention of the dismantling of hardened concrete on account of cracks, but on the other hand, it provides for rectification of the defects appearing in the permanent works.</p><p>&nbsp;</p><p><strong>Codal Stipulations On The Minimum Design Requirements&nbsp;</strong></p><p>&nbsp;</p><p><strong>As per Indian Railway Standard Concrete Bridge Code 2014</strong></p><ul><li>As per Table 4(b) for durability requirements, the minimum grade of concrete required for moderate exposure is only M20.</li><li>As per Cl 14.9 and Cl 15.9.9, the shrinkage and temperature reinforcement shall be provided in the direction of restraints to such movements. The reinforcement shall be spaced at not more than 150mm c/c.</li><li>As per CL 15.9.4.2, in the predominantly tensile area of a solid slab or wall, the minimum area of secondary reinforcement shall not be less than 0.12% of btd.&nbsp;&nbsp;</li></ul><p>&nbsp;</p><p><strong>As per IS 456-2000&nbsp;</strong></p><ul><li>As per Cl 26.5.1.1, the minimum percentage of tensile reinforcement is given as As/bd =0.85/ fy</li><li>As per Cl 26.5.2.1, for slabs, the minimum reinforcement in either direction in slabs shall be 0.12% in the case of HYSD bars.</li><li>As per Cl 34.5.2, the nominal reinforcement for concrete sections of thickness greater than 1m shall be 360mm2 per meter length in each direction on each face.</li></ul><p>&nbsp;<br /><strong>As per ACI -224R-01</strong></p><ul><li>Table 4.1 indicates that the crack widths indicated are only a guide for design under service loads. Further, it states, "It should be expected that a portion of the cracks in the structure will exceed these values. With time a significant portion can exceed these values. These are general guidelines for design to be used in conjunctions with sound engineering judgement".</li><li>CL 4.4 states that the crack width values indicated in Table 4.1 are intended to serve as a guide for proportioning reinforcement during the design phase. A larger cover, even if it leads to a larger surface crack width, may be preferable in certain environments. It clearly states that an "Acceptable level of cracking is primarily an aesthetic issue".&nbsp;&nbsp;</li></ul><p>&nbsp;</p><p><strong>Codal Provisions On Cracking in Concrete&nbsp;</strong><br />&nbsp;<br />As per IRS Concrete Bridge Code 2014</p><ul><li>Cl 10.2.1 "Cracking of concrete shall not adversely affect the appearance or durability of the structure". Further, in this clause, design crack widths are provided in a tabular form depending upon the environmental exposure conditions.</li><li>Cl 6.4.3 "As soon as forms are removed, a list of major/minor defects noticed in concrete should be prepared." &nbsp;</li><li>Cl 5.5.1 "When concrete gets hardened, it shall have the required strength, durability, and surface finish.&nbsp;<br />&nbsp;<br /><strong>As per IS-456 2000 </strong></li><li>As per Cl 35.3.2, the practical objective of calculating the crack width is merely to guide the designer in making appropriate structural arrangements. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;&nbsp;<br />&nbsp;<br /><strong>As per IR Bridge Manual 1998</strong>&nbsp;<br />As per clause 514, encasing/jacketing can be provided where the bed block has cracked.&nbsp;</li></ul><p>&nbsp;</p><h2><span style="font-size: 14pt;">Preventive Measures&nbsp;</span></h2><p>&nbsp;</p><ul><li>Care should also be taken to minimize any cracks that may collect or transmit water</li><li>Cracking can be minimized by reducing the restraints on the free movement of the structure</li><li>Ultrasonic Pulse Velocity Method&ndash; can be used to determine the homogeneity of the concrete and the presence of cracks</li><li>Some hairline cracks which may form during casting or curing, if superficial, have no detrimental effect on the structural capacity of the member. However, to prevent any possible corrosion through hair cracks and deterioration of concrete suitable surface treatment shall be given to the exposed surface</li><li>Crack-resistant topping layers can be provided using the following:<ul><li>Fiber-reinforced concrete (FRC), Latex- and epoxy-modified concrete</li><li>Polymer-impregnated concrete (PIC), Epoxy, and other polymer concrete overlays</li></ul></li><li>The techniques commonly used for repairs of masonry are : Cement pressure grouting, epoxy grouting, and Shot creting/Guniting</li><li>Nano technology-based water-soluble waterproofing agent, which penetrates the substrate and seals even the minor cracks, shall be used to repair the cracks</li><li>Fine and medium-sized cracks which are not yet deep may be grouted and sealed using epoxy resin through the pressure injection method. If the cracks are deep, cement grouting may be resorted to.</li></ul><p>&nbsp;</p><h2><span style="font-size: 14pt;">Discussions&nbsp;</span></h2><p>The guidelines indicated in the various Indian and International standard codes of practice are provided above. It is seen from these guidelines that the crack width values indicated in the codes are to be adopted as a guideline in the detailed design for detailing reinforcement in RCC elements. The cracks observed, if any, in the concrete elements after their casting should be addressed primarily as an aesthetic issue and repaired.&nbsp;</p><p>&nbsp;</p><h2><span style="font-size: 14pt;">Conclusion</span>&nbsp;</h2><p>Dismantling of the structure due to the presence of cracks is not warranted in all cases and needs to be examined on a case-to-case basis. If the required strength, durability, and surface finish for the intended use are not impaired, repairs and remedial measures must be attempted, and the structural element shall be made good and restored. Project completion time delays can be avoided by adopting suitable repair and remedial works.</p><p>&nbsp;</p><p><span style="font-size: 10pt;"><em>This article was contributed by our expert <a href="https://www.linkedin.com/in/rajashekar-ananthula-a6b635188/?external_page=LPC.Immersive&amp;external_control=ViewProfileLink&amp;external_app_instance=53b509de-d9c1-40b9-b116-0b6531af63ad&amp;external_page_instance=&amp;experiment=displayLinkedInDataPrebind" target="_blank" rel="noopener">Rajashekar Ananthula</a></em></span></p><p>&nbsp;</p><h3><span style="font-size: 18pt;">Frequently Asked Questions Answered by Rajashekar Ananthula</span></h3><p>&nbsp;</p><h2><span style="font-size: 12pt;">1. What are the major implications of crack formation in reinforced concrete elements in the construction industry?</span></h2><p><span style="font-size: 12pt;">Crack formation in reinforced concrete affects the appearance and durability of concrete.</span></p><p><span style="font-size: 12pt;">For proper appearance, protection against corrosion of reinforcement steel, and performance of the structure, cracking should be kept within reasonable limits by paying attention to reinforcement detailing.</span></p><p><span style="font-size: 12pt;">Cracking leaves the reinforcement steel exposed to air and moisture, which may cause it to rust and weaken the concrete.</span></p><p><span style="font-size: 12pt;">Cracking reduces the stiffness of the slab members, especially slabs of unbraced frames.</span></p><p><span style="font-size: 12pt;">The extent of cracking permitted depends on factors such as environment, aesthetics, and occupancy.</span></p><p><span style="font-size: 12pt;">In the cracked state, the concrete is assumed to be unable to resist either tension or shear. Thus, equilibrium is attained by means of tensile resisting forces in reinforcement and compressive resisting forces in concrete.</span></p><p><span style="font-size: 12pt;">Crack widths are good indicators of the state of the structure and should be observed to help determine whether the structure is satisfactory.<br />&nbsp;<br />If the required strength, durability, and surface finish of concrete for its intended use are not impaired, the acceptable level of cracking is primarily an aesthetic issue.<br /></span></p><p>&nbsp;</p><h2><span style="font-size: 12pt;">2. What are some of the current innovations to address the issue?</span></h2><ul><li>Use of Ultra-High-Performance Concrete</li><li>Cementitious concrete composite-Its dense structure reduces the permeability and potential risk of corrosion of reinforcement steel. Has ductile behavior</li><li>Use of High-Volume Fly ash Concrete for mass concrete works</li><li>Use of Fiber Reinforced Concrete- reduces the thermal and shrinkage cracks</li><li>Use of Temperature controlled and Triple Blend Concrete (comprising of Ordinary Portland Cement+ Flyash+Ground Granulated Blast Furnace Slag)</li><li>Use of insulated formworks</li><li>Use of polymers in improving cracking characteristics</li></ul><p>&nbsp;</p><h2><span style="font-size: 12pt;">3.&nbsp;What are the reasons for concrete cracking, and how does it affect durability?</span></h2><p>Cracks may occur due to loss of strength of masonry/concrete because of age, excessive dynamic impact, settlement, or any other damage to foundations</p><ul><li>Loading- Direct or flexural tension in concrete due to external service loads and temperature gradients due to solar radiation</li><li>Shrinkage and temperature effects, including temperature rises caused by the heat of hydration released by the cement</li><li>Cracks develop due to improper seating or installation of load transfer elements (like bearings in the bed block of a bridge)</li><li>The formwork should be removed without shock, as the sudden removal of wedges is equivalent to an impact load on the partially hardened concrete.</li></ul><p>&nbsp;</p><h2><span style="font-size: 12pt;">4. What measures can construction industries take in terms of Reinforced Concrete Elements?</span></h2><p>Workmanship to obtain full compaction and efficient curing of concrete shall be ensured. The low permeability of concrete is a prerequisite for the durability of concrete.</p><p>Special attention shall be given to the curing of concrete to ensure maximum durability and minimize cracking.</p><p>Maintain accuracy of positions of reinforcement steel as per design drawings and as per the standard codes of practice. Steel reinforcement shall be well distributed over the zone of maximum concrete tension.</p><p>During the initial stages of construction, concrete surfaces shall be covered by polyethylene or tarpaulin sheets to avoid surface drying.</p><p>Provide the required cement content (per environment exposure condition), minimum concrete grade, and maintain the water-cement ratio.</p><p>It is necessary to control cracking arising from shrinkage and temperature effects, including temperature rises caused by the heat of hydration released by the cement. Reinforcement steel shall be provided in the direction of any restraint to such movements.</p><p>Stresses in concrete and reinforcement steel shall be within limits specified in the standard design codes of practice for serviceability limit states.</p><p>&nbsp;</p><p>&nbsp;</p>
KR Expert - Rajashekar Ananthula