Foundation Formwork<\/a><\/p>\n\n
The question is: how can an apparently simple task, such as installing a few wooden boards or metal panels around foundation concrete, threaten the 50-year lifespan of a building? Why do engineers with years of structural design experience sometimes make the most disastrous decisions when it comes to foundation formwork execution?<\/p>\n
In this article, the Zemanco<\/strong> technical team examines the hidden causes of foundation cracks and demonstrates how the foundation formwork system affects the durability, safety, and service life of the entire building.<\/p>\nThe Role of the Foundation in Preventing Structural Cracks<\/h2>\n
The foundation is the most important load-transfer element in a building. All structural loads are transferred to the ground through this component. If there is weakness or an execution defect in this section, additional stresses will develop and ultimately appear as cracks in the foundation, followed by cracks in other parts of the building.<\/p>\n
The Load Transfer Path in a Building<\/h3>\n
Every building has a specific load transfer path:<\/p>\n
Roof load \u2192 beams \u2192 columns \u2192 foundation \u2192 soil beneath the foundation<\/p>\n
The foundation is the last structural member and the most sensitive load-transfer element. Any defect in this part leads to uneven force distribution and the formation of cracks in the foundation and upper parts of the structure.<\/p>\n
Mechanisms of Crack Formation in Foundations<\/h2>\n
Foundation cracks are usually the result of stress concentration, excessive deformation, or reduced concrete strength. The main mechanisms leading to cracks are discussed below.<\/p>\n
Differential Settlement of the Foundation<\/h3>\n
One of the most important causes of foundation cracking is uneven settlement. This problem occurs when the foundation dimensions are not executed accurately or when there are errors in the formwork.<\/p>\n
In such cases, the contact surface between the foundation and the soil is not uniform, and pressure is transferred unevenly. As a result, part of the foundation settles more, creating bending stresses. These stresses cause cracks in the foundation and subsequently in the walls and columns.<\/p>\n
Signs of differential settlement include:<\/p>\n
\n- \n
Diagonal cracks in walls<\/p>\n<\/li>\n
- \n
Cracks at the beam-column connection<\/p>\n<\/li>\n
- \n
Cracks in the floor slab<\/p>\n<\/li>\n
- \n
Level differences in various parts of the building<\/p>\n<\/li>\n<\/ul>\n
Reduced Concrete Cover and Rebar Corrosion<\/h3>\n
Adequate concrete cover protects reinforcing steel from moisture and corrosive agents. If the foundation formwork is not properly executed, rebars may be positioned too close to the concrete surface.<\/p>\n
Reduced concrete cover results in:<\/p>\n
\n- \n
Faster moisture penetration to the rebar<\/p>\n<\/li>\n
- \n
Onset of rebar corrosion<\/p>\n<\/li>\n
- \n
Increase in rebar volume<\/p>\n<\/li>\n
- \n
Internal pressure within the concrete<\/p>\n<\/li>\n<\/ul>\n
This process ultimately leads to internal cracking of the foundation concrete. This type of crack is highly dangerous as it gradually reduces the structural strength.<\/p>\n
Honeycombing in Foundation Concrete<\/h3>\n
Another significant cause of foundation cracks is the formation of voids or honeycombing in the concrete. This problem usually occurs due to leakage of cement paste from the formwork joints or inadequate concrete compaction.<\/p>\n
Honeycombing results in:<\/p>\n
\n- \n
Reduced compressive strength of the concrete<\/p>\n<\/li>\n
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Open paths for water and corrosive agents<\/p>\n<\/li>\n
- \n
Reduced foundation durability<\/p>\n<\/li>\n
- \n
Increased likelihood of cracking<\/p>\n<\/li>\n<\/ul>\n
These voids cause stress concentration and initiate cracks in the foundation.<\/p>\n
The Direct Impact of Formwork on Foundation Cracking<\/h2>\n
The quality of formwork has a direct impact on the shape, dimensions, and structural capacity of the foundation. If the formwork suffers from the following issues, the likelihood of cracking in the foundation increases:<\/p>\n
\n- \n
Formwork displacement during concrete pouring<\/p>\n<\/li>\n
- \n
Lack of proper leveling of the formwork<\/p>\n<\/li>\n
- \n
Leakage of cement paste<\/p>\n<\/li>\n
- \n
Formwork deformation due to concrete pressure<\/p>\n<\/li>\n
- \n
Reduction of rebar concrete cover<\/p>\n<\/li>\n<\/ul>\n
According to a report by the Construction Engineering Organization<\/span><\/span>, an investigation of 280 buildings showed that more than 43% of foundation problems were caused by improper formwork execution. This statistic clearly demonstrates that correct formwork implementation plays a vital role in preventing foundation cracks.<\/p>\nWhy Foundation Cracks Are Dangerous<\/h2>\n
Cracks in the foundation are not merely cosmetic issues; they are serious warnings regarding the structural health of a building. The most important consequences of foundation cracks include:<\/p>\n
\n- \n
Reduction in the load-bearing capacity of the structure<\/p>\n<\/li>\n
- \n
Increased risk of building settlement<\/p>\n<\/li>\n
- \n
Water penetration into the structure<\/p>\n<\/li>\n
- \n
Corrosion of reinforcing steel<\/p>\n<\/li>\n
- \n
Reduction in the service life of the building<\/p>\n<\/li>\n<\/ul>\n
Failure to inspect and repair foundation cracks in a timely manner can lead to severe damage and even structural instability.<\/p>\n
\n
5 Formwork Errors That Cause Foundation Cracks<\/h2>\n
Many cases of foundation cracking are not related to design issues but rather to execution errors in formwork. These mistakes may not be visible during construction, but over time they lead to settlement, reduced strength, and the formation of dangerous cracks in the foundation. Below are five critical formwork errors that can significantly shorten the service life of a structure.<\/p>\n
Dimensional Deviation of Formwork and Reduced Foundation Strength<\/h3>\n
Problem Description<\/strong><\/p>\nAccording to the ACI<\/span><\/span> standard, the allowable tolerance for foundation formwork is \u00b112 mm. However, in traditional wooden formwork systems, deviations of 20 to 40 mm are very common. This deviation alters the actual dimensions of the foundation compared to the design.<\/p>\nImpact on Foundation Cracking<\/strong><\/p>\nWhen the width or thickness of the foundation is executed smaller than the design value, its flexural capacity decreases. Even a reduction of 3 centimeters can reduce flexural strength by up to 12%. This loss of capacity leads to stress concentration and ultimately results in cracking of the foundation.<\/p>\n
Proper Engineering Solution<\/strong><\/p>\n\n- \n
Use of precise modular steel formwork systems<\/p>\n<\/li>\n
- \n
Dimensional control before concrete pouring<\/p>\n<\/li>\n
- \n
Use of standard anchoring and fastening systems<\/p>\n<\/li>\n
- \n
Verification of level and dimensions by the supervising engineer<\/p>\n<\/li>\n<\/ul>\n
Inadequate Formwork Bracing and Foundation Deformation<\/h3>\n
Problem Description<\/strong><\/p>\nThe lateral pressure of fresh concrete is very high. If the formwork is inadequately braced, it may deform or bulge outward. This deformation causes non-uniform foundation thickness.<\/p>\n
Impact on Foundation Cracking<\/strong><\/p>\nWhen the foundation thickness varies in different areas, the load is transferred eccentrically. This condition generates additional bending moments, which ultimately result in cracking of the foundation.<\/p>\n
Proper Engineering Solution<\/strong><\/p>\n\n- \n
Calculation of lateral concrete pressure prior to execution<\/p>\n<\/li>\n
- \n
Use of appropriate tie bolts, struts, and walers<\/p>\n<\/li>\n
- \n
Implementation of bracing with an adequate safety factor<\/p>\n<\/li>\n
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Continuous monitoring of formwork during concrete placement<\/p>\n<\/li>\n<\/ul>\n
Omission of Lean Concrete (Blinding Concrete) and Increased Risk of Foundation Cracks<\/h3>\n
Problem Description<\/strong><\/p>\nLean concrete (blinding concrete) is a concrete layer with a thickness of 5 to 10 centimeters that is placed on the soil before foundation construction. Omitting this layer leads to formwork instability and contamination of structural concrete.<\/p>\n
Impact on Foundation Cracking<\/strong><\/p>\nFailure to execute lean concrete results in:<\/p>\n
\n- \n
Irregular foundation geometry<\/p>\n<\/li>\n
- \n
Uneven settlement<\/p>\n<\/li>\n
- \n
Reduced concrete cover for reinforcement<\/p>\n<\/li>\n
- \n
Increased likelihood of foundation cracking<\/p>\n<\/li>\n<\/ul>\n
Proper Engineering Solution<\/strong><\/p>\n\n- \n
Execution of lean concrete with a minimum thickness of 5 cm<\/p>\n<\/li>\n
- \n
Leveling the blinding concrete surface before formwork installation<\/p>\n<\/li>\n
- \n
Preventing direct contact between reinforcement and soil<\/p>\n<\/li>\n<\/ul>\n<\/div>\n
Lack of Formwork Sealing and Resulting Concrete Weakness<\/h3>\n
Problem Description<\/strong><\/p>\nGaps and joints in formwork lead to leakage of cement paste. This results in void formation and a reduction in the quality of foundation concrete.<\/p>\n
Impact on Foundation Cracking<\/strong><\/p>\nLeakage of cement paste causes honeycombing, which leads to the following consequences:<\/p>\n
\n- \n
Reduction in concrete strength<\/p>\n<\/li>\n
- \n
Increased concrete permeability<\/p>\n<\/li>\n
- \n
Initiation of rebar corrosion<\/p>\n<\/li>\n
- \n
Increased likelihood of foundation cracking<\/p>\n<\/li>\n<\/ul>\n
Proper Engineering Solution<\/strong><\/p>\n\n- \n
Complete sealing of all formwork joints<\/p>\n<\/li>\n
- \n
Use of appropriate sealing tapes or materials<\/p>\n<\/li>\n
- \n
Use of precise and standardized formwork systems<\/p>\n<\/li>\n<\/ul>\n
Premature Formwork Removal and Microcracking in Foundations<\/h3>\n
Problem Description<\/strong><\/p>\nRemoving formwork before the concrete reaches sufficient strength causes damage to young concrete. These damages may not be visible initially but can lead to foundation cracking over time.<\/p>\n
Impact on Foundation Cracking<\/strong><\/p>\nPremature formwork removal can result in:<\/p>\n
\n- \n
Formation of microcracks in concrete<\/p>\n<\/li>\n
- \n
Reduction in final concrete strength<\/p>\n<\/li>\n
- \n
Increased moisture penetration<\/p>\n<\/li>\n
- \n
Reduced service life of the foundation<\/p>\n<\/li>\n<\/ul>\n
Appropriate Time for Foundation Formwork Removal<\/strong><\/p>\n\n- \n
Temperatures above 25\u00b0C: minimum 24 hours<\/p>\n<\/li>\n
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Temperatures between 15\u201325\u00b0C: minimum 36 hours<\/p>\n<\/li>\n
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Temperatures between 5\u201315\u00b0C: minimum 48 hours<\/p>\n<\/li>\n
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Temperatures below 5\u00b0C: minimum 72 hours<\/p>\n<\/li>\n
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Mass concrete: minimum 48\u201396 hours<\/p>\n<\/li>\n<\/ul>\n
How Does Formwork Affect Concrete Durability Parameters?<\/h2>\nThe First Line of Defense: Concrete Cover<\/h3>\n
Concrete cover is the layer of concrete between the external surface and the nearest reinforcing bar. Its primary functions are:<\/p>\n
\n- Protection of reinforcement<\/strong> against corrosion<\/li>\n
- Fire resistance<\/strong> (thermal protection of reinforcement)<\/li>\n
- Bond strength<\/strong> between concrete and reinforcement<\/li>\n<\/ul>\n
Relation to formwork:<\/strong> Any displacement of formwork alters the concrete cover. The table below shows the impact of reduced concrete cover on the service life of a foundation:<\/p>\n\n
\n\n\n| Concrete Cover (cm)<\/th>\n | Service Life Until Corrosion (Moderate Environment)<\/th>\n | Service Life Until Corrosion (Aggressive Environment)<\/th>\n<\/tr>\n<\/thead>\n |
\n\n| 7.5 (Standard)<\/td>\n | 50+ years<\/td>\n | 30\u201340 years<\/td>\n<\/tr>\n |
\n| 6.0<\/td>\n | 35\u201340 years<\/td>\n | 20\u201325 years<\/td>\n<\/tr>\n |
\n| 5.0<\/td>\n | 25\u201330 years<\/td>\n | 15\u201320 years<\/td>\n<\/tr>\n |
\n| 4.0<\/td>\n | 15\u201320 years<\/td>\n | 10\u201312 years<\/td>\n<\/tr>\n |
\n| 3.0<\/td>\n | 8\u201312 years<\/td>\n | 5\u20138 years<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n Shocking conclusion:<\/strong> Reducing the concrete cover by only 2.5 cm (from 7.5 to 5.0) cuts the foundation\u2019s service life from 50 years to 25 years\u2014by half.<\/strong><\/p>\nConcrete Compaction<\/h3>\nWhen a concrete vibrator is applied, air bubbles escape and density increases. However, if the formwork lacks sufficient rigidity:<\/p>\n \n- Vibration causes movement and displacement of the formwork<\/strong> instead of compacting the concrete<\/li>\n
- The vibrator operator reduces vibration time<\/strong> to avoid damaging the formwork<\/li>\n
- Result: poorly compacted concrete with internal micro and macro voids<\/strong><\/li>\n<\/ul>\n
Each 1% increase in air voids results in approximately a 5% reduction in compressive strength.<\/strong><\/p>\n![\"\"](\"https:\/\/zemanco.com\/wp-content\/themes\/woodmart\/images\/lazy.svg\") <\/p>\n
Example:<\/p>\n If the designed compressive strength of concrete is:<\/p>\n <\/p>\n That equals a 20% strength reduction<\/strong> \u2014 solely due to improper formwork.<\/p>\nCross-Section Geometry<\/h3>\n<\/p>\n Key point:<\/strong> The parameter d<\/span> is directly dependent on formwork accuracy. If the formwork is not level at the top and foundation thickness is reduced by 2 cm:<\/p>\n<\/p>\n This loss of flexural capacity in a 20-meter-long foundation equals 32 ton-meters<\/strong> of lost bending moment \u2014 a value that can mean the difference between a sound structure and a cracked one<\/strong>.<\/p>\nFive Building Durability Parameters Determined by Foundation Formwork<\/h2>\nThe quality of formwork has a direct impact on structural durability. Proper formwork prevents the ingress of harmful agents and inhibits foundation cracking. In contrast, poor formwork increases concrete permeability, reduces strength, and ultimately leads to foundation cracking.<\/p>\n Below are the most important durability parameters that are directly affected by formwork.<\/p>\n Resistance to Chloride Penetration and Prevention of Foundation Cracking<\/h2>\nChloride ions are among the most destructive agents causing reinforcement corrosion. They are commonly present in coastal areas, industrial environments, and regions where de-icing salts are used. Chloride penetration leads to rebar corrosion and foundation cracking.<\/p>\n Relationship Between Formwork and Chloride Penetration<\/h3>\nIf formwork reduces concrete compaction or creates voids, chloride permeability increases. Porous concrete can allow chloride ions to penetrate 2 to 4 times faster.<\/p>\n The consequences of chloride penetration include:<\/p>\n \n- Initiation of rebar corrosion<\/li>\n
- Increase in rebar volume<\/li>\n
- Development of internal stresses<\/li>\n
- Formation of foundation cracks<\/li>\n<\/ul>\n
Precise formwork increases concrete density and significantly reduces chloride penetration.<\/p>\n Resistance to Sulfate Attack and Prevention of Foundation Deterioration<\/h2>\nSoils in many regions contain sulfates. Sulfates react with cement compounds, causing expansion and deterioration of concrete. This process can lead to cracking in the foundation and a reduction in the structural strength.<\/p>\n Role of Formwork in Sulfate Resistance<\/h3>\nImproper formwork can lead to honeycombing and voids in concrete. These voids increase sulfate penetration and accelerate concrete deterioration.<\/p>\n The results of this process include the following:<\/p>\n \n- \n
Reduction in concrete strength<\/p>\n<\/li>\n - \n
Increase in foundation cracking<\/p>\n<\/li>\n - \n
Reduction in structural service life<\/p>\n<\/li>\n - \n
Increase in repair costs<\/p>\n<\/li>\n<\/ul>\n Accurate formwork reduces porosity and increases concrete resistance to sulfate attack.<\/p>\n Resistance to Carbonation and Reduction of Reinforcement Corrosion Risk<\/h2>\nCarbonation is a process in which carbon dioxide penetrates concrete and reduces its protective properties. This process initiates reinforcement corrosion and leads to cracking in the foundation.<\/p>\n Effect of Formwork on the Rate of Carbonation<\/h3>\nIn dense concrete, the carbonation rate is low; however, in porous concrete, the rate increases by 2 to 3 times.<\/p>\n Poor formwork can result in the following:<\/p>\n \n- \n
Increased concrete porosity<\/p>\n<\/li>\n - \n
Reduction in concrete strength<\/p>\n<\/li>\n - \n
Increase in reinforcement corrosion<\/p>\n<\/li>\n - \n
Formation of foundation cracks<\/p>\n<\/li>\n<\/ul>\n Proper formwork increases concrete density and reduces the carbonation rate.<\/p>\n Seismic Resistance of Foundations and Prevention of Structural Cracking<\/h2>\nThe foundation must be capable of transferring seismic forces without damage. If formwork reduces the dimensions or strength of the foundation, the risk of foundation cracking increases.<\/p>\n According to the requirements of
\n
\nIranian Standard 2800<\/span>
\n<\/span>,
\nthe foundation must have sufficient capacity to transfer seismic forces.<\/p>\nImpact of Improper Formwork on Seismic Performance<\/h3>\nPoor formwork can lead to the following:<\/p>\n \n- \n
Reduction in load-bearing capacity<\/p>\n<\/li>\n - \n
Creation of stress concentrations<\/p>\n<\/li>\n - \n
Increased likelihood of foundation cracking<\/p>\n<\/li>\n - \n
Reduction in structural safety during earthquakes<\/p>\n<\/li>\n<\/ul>\n Precise formwork improves seismic performance and prevents structural damage.<\/p>\n Foundation Waterproofing and Prevention of Moisture Ingress<\/h2>\nOne of the most important causes of foundation cracking is moisture penetration. The quality of the concrete surface, which is directly dependent on formwork, plays a critical role in waterproofing performance.<\/p>\n Effect of Formwork Quality on Waterproofing<\/h3>\nA smooth and dense concrete surface results in better waterproofing performance. In contrast, a rough surface with voids significantly reduces the effectiveness of waterproofing systems.<\/p>\n Comparison of waterproofing performance based on formwork quality:<\/p>\n \n- \n
Smooth surface with metal formwork: waterproofing service life of 15\u201320 years<\/p>\n<\/li>\n - \n
Relatively smooth surface: waterproofing service life of 10\u201315 years<\/p>\n<\/li>\n - \n
Rough surface: waterproofing service life of 3\u20137 years<\/p>\n<\/li>\n - \n
Very rough surface: waterproofing service life of less than 3 years<\/p>\n<\/li>\n<\/ul>\n Moisture penetration leads to reinforcement corrosion and foundation cracking.<\/p>\n Comprehensive Comparison of Foundation Formwork Systems<\/h3>\nSystem 1: Traditional Timber Formwork<\/h4>\nDescription:<\/strong> 20\u201330 mm construction planks + timber studs and braces + nails and wire<\/p>\nAdvantages:<\/strong><\/p>\n\n- Easy availability throughout Iran<\/li>\n
- High flexibility for non-standard geometries<\/li>\n
- Requires simple equipment<\/li>\n<\/ul>\n
Disadvantages:<\/strong><\/p>\n\n- Low dimensional accuracy (\u00b120 to 40 mm)<\/li>\n
- Short service life (3 to 5 uses)<\/li>\n
- Generation of wood waste<\/li>\n
- Long installation and stripping time<\/li>\n
- Leakage of cement paste through joints<\/li>\n
- Swelling in trench moisture<\/li>\n<\/ul>\n
System 2: Custom Steel Formwork<\/h4>\nDescription:<\/strong> Welded steel panels fabricated for project-specific dimensions<\/p>\nAdvantages:<\/strong><\/p>\n\n- High accuracy<\/li>\n
- Good resistance to fresh concrete pressure<\/li>\n
- Long service life<\/li>\n<\/ul>\n
Disadvantages:<\/strong><\/p>\n\n- High initial fabrication cost<\/li>\n
- Heavy weight (difficult transportation and installation)<\/li>\n
- Not adjustable for other dimensions<\/li>\n
- Long fabrication time (2 to 4 weeks)<\/li>\n<\/ul>\n
System 3: Zeman Industrial Modular Formwork<\/h4>\nDescription:<\/strong> Standard steel panels with pin connections, combinable into various dimensions<\/p>\nAdvantages:<\/strong><\/p>\n\n- High dimensional accuracy (\u00b13 mm)<\/li>\n
- Optimized weight (lighter than custom steel)<\/li>\n
- Service life exceeding 100 uses<\/li>\n
- Fast installation and stripping<\/li>\n
- Adjustable for various dimensions<\/li>\n
- No material waste<\/li>\n
- Rental services and free technical consultation<\/li>\n<\/ul>\n
Disadvantages:<\/strong><\/p>\n\n- Requires initial training of installation crews<\/li>\n
- Transportation costs (round trip)<\/li>\n<\/ul>\n
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