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Welcome to our informational page. The goal of this page is to provide basic information in topics related to concrete construction. The target audience is individuals without prior experience in a given subject and strictly serves general knowledge without in-depth analysis. Each topic may be expanded further and if you desire to gain more detailed information please refer to the reference at the bottom of each topic. The information on this page was written by Hercules Concrete staff based on a combination of literature that is referenced and practical experience in the field of work.

Wall footings comprise of continuous concrete strip stretching along the wall length and having a width greater than that of the wall thickness. Main reinforcement must be placed perpendicular to the direction of the wall.

Isolated column footings comprise of square or rectangular concrete slabs, which are constant in thickness, stepped, or sloped toward the cantilever tip. Reinforcement is designed in both directions.

Combined footings support two or more column loads. Generally used when it is more economical to incorporate this type of footing for closely spaced columns, or when placing a column right on the property line and the footing slab cannot project outside of it.

Cantilever or strap footings are similar to the combined footings, except the footings for interior and exterior columns are independently built.

Pile foundations are essential if the supporting ground consists of structurally unsound material layers to large depths. The piles can be precast or cast-in-place by way of drilling a caisson and pouring concrete inside. Other types of piles are made of treated wood or steel.

Floating, raft or mat foundations are necessary if allowable bearing capacity of soil is low to great depths, making choice of pile foundations uneconomical. It is necessary to stretch this type of foundation over the entire area of the structure such that the superstructure can be considered theoretically floating on a raft.

REFERENCES

Edward G. Nawy (2009) Reinforced Concrete a Fundamental Approach 6th edition, Ch. 12 Footings pp. 542-545

Exposed: Exposed surface – foot traffic.

Use: Multiunit residential, decorative, offices, churches.

Covered: Covered surface – foot traffic.

Use: Multiunit residential, offices, churches, commercial, institutional with floor coverings.

Topping: Exposed or covered surface – foot traffic.

Use: Bonded or un-bonded topping over concrete slab for nonindustrial or commercial buildings.

Institutional/Commercial: Exposed or covered surface – foot and light vehicular traffic.

Use: Institutional or commercial.

Industrial: Exposed surface – light- to medium-duty industrial vehicular traffic.

Use: Industrial floors for warehousing, processing, and manufacturing.

Heavy Industrial: Exposed surface – heavy-duty industrial vehicular traffic.

Use: Industrial floors subject to impact loads and heavy traffic.

Heavy Industrial Topping: Exposed surface – heavy-duty industrial vehicular traffic.

Use: Bonded two-course floors subject to impact and heavy traffic.

Commercial/Industrial Topping: Exposed or covered surface – foot traffic and light- to heavy-duty vehicular traffic.

Use: Un-bonded topping on new or old concrete floors.

Critical Surface Profile: Exposed surface – superflat or critical surface tolerance required; robotics requiring specific tolerances or special material-handling vehicles.

Use: Ice rinks, television studios, gymnasiums, or High-bay and narrow-aisle warehouses

REFERENCES

ACI Collection of Concrete Codes, Specifications, and Practices. (2019) Guide to concrete floor and slab construction ACI 302. 1R-15

“A” class – Agricultural, municipal or industrial projects exposed to human or animal wastes.

A-1      Structurally reinforced concrete with exposure to silage gases and/or severe manure, with or without freeze-thaw cycles exposure. Concrete with exposure to vapour above sewage or industrial effluent, where hydrogen sulphide gas may generate.

A-2      Structurally reinforced concrete with exposure to moderate or severe manure and/or silage gases and liquids, with or without freeze-thaw cycles exposure.

A-3      Structurally reinforced concrete with exposure to moderate or severe manure and/or silage gases liquids, with or without freeze-thaw cycles exposure in a continuously submerged condition.

A-4      Non-structurally reinforced concrete with exposure to moderate manure and/or silage gases and liquids, without freeze-thaw cycles exposure.

“C” class – Chloride Exposure.

C-1      Structurally reinforced concrete with exposure to chlorides with or without freeze-thaw cycles exposure.

C-2      Non-structurally reinforced concrete with exposure to chlorides and freeze-thaw cycles.

C-3      Continuously submerged concrete with exposure to chlorides but not freeze-thaw cycles.

C-4      Non-structurally reinforced concrete with exposure to chlorides but not to freeze-thaw cycles.

C-XL    Structurally reinforced concrete with exposure to chlorides or other severe environments with or without freeze-thaw cycles exposure, with higher durability performance expectations than the C-1, A-1, or S-1 classes.

“F” class – Freeze-thaw cycle exposure without chlorides.

F-1      Concrete with exposure to freeze-thaw cycles in a saturated condition, no exposure to chlorides.

F-2      Concrete with exposure to freeze-thaw cycles in an unsaturated condition, no exposure to chlorides.

“N” class – No exposure to chlorides and freeze-thaw cycles.

N         Concrete without exposure to freeze-thaw cycles and chlorides.

“S” class – Sulphate exposure.

S-1      Concrete with very severe exposure to sulphate.

S-2      Concrete with severe exposure to sulphate.

S-3      Concrete with moderate exposure to sulphate.

Air-entraining admixtures

Air-entrained concrete can be achieved by using air-entraining portland cement, or by incorporation of air-entraining agents on a job. One cubic foot of air-entrained concrete contains billions of microscopic air cells, which relieve internal pressure on concrete by providing micro chambers for water to expand into when it freezes.

Water-reducing admixtures

These admixtures generally reduce water content in a concrete mix by approximately 5 to 10 percent. Untreated concrete requires more water to reach required slump than concrete mix containing water-reducing admixtures. Treated concrete can have a lower water-cement ratio, which indicates a higher strength concrete mix can be produced without increase in cement quantity.

Retarding admixtures

Retarding admixtures are used to counteract the accelerated concrete setting in hot weather conditions, by slowing the rate of concrete setting. Rate of concrete hardening in high temperatures is increased which makes concrete placing and finishing very difficult. Retarding admixtures delay initial concrete setting and keep concrete mix in a workable condition during placement.

Accelerating admixtures

Accelerating admixtures reduce time required for concrete curing and protection, increase early strength development rate, and allow for finishing operations to start earlier. Concrete properties modification with accelerating admixtures is especially beneficial in cold weather concreting.

Superplasticizers

Superplasticizers increase workability of concrete during placing operations. They reduce water content in the concrete mix by 12 to 30 percent and can produce a high-slump flowing concrete mix without reducing the strength of the mix. Superplasticizers effect only lasts approximately 30 to 60 minutes and is followed by a rapid loss in concrete workability.

Corrosion-inhibiting admixtures

Are used to reduce the corrosion of steel reinforcing the concrete, they can be used as a defensive strategy for concrete structures.

Shrinkage-reducing admixtures

Promotes concrete expansion at approximately same volume that normal drying shrinkage is contracting it. Small change in the length of hardened concrete helps prevent shrinkage cracks.

Alkali-silica reactivity inhibitors

ASR inhibitors help control problems with durability associated with alkali-silica reactivity.

Damp-proofing admixtures

Are used to reduce transmission of moisture through concrete element that is in contact with water or damp elements. Many damp-proofing admixtures are ineffective, especially if used in concrete that is exposed to water contact under pressure.

Bonding admixtures

Are generally water emulsions of organic materials, they are used in concrete mix to increase the strength of bond between new and old concrete.

Fungicidal, germicidal, and insecticidal admixtures

Hardened concrete fungal and bacteria growth can be partially controlled by way of using these admixtures. High dosages of these admixtures may decrease the concrete strength and effectiveness of these admixtures are generally temporary.

Permeability-reducing admixtures

Are used to reduce rate at which water is transmitted through concrete under pressure.

Coloring admixtures

Coloring admixtures comprise of synthetic and natural materials that are used to produce color in concrete. (Calcium chloride shall not be used to avoid distortions in color).

REFERENCES

ACI Collection of Concrete Codes, Specifications, and Practices. (2019) Report on chemical admixtures for concrete. ACI 212. 3R-16

Slump test is the most commonly used test method to determine workability of fresh concrete. It is used on jobsites to determine if the concrete load should be accepted or rejected. The apparatus consists of a mold shaped as a frustum of cone with a base diameter of 8 inches, top diameter of 4 inches, and overall height of 12 inches. This mold gets filled with concrete in 3 layers of equal volume and each layer is compacted with a tamping rod 25 times. The mold is then lifted vertically upward and is placed beside concrete that was previously inside of it, and the height of concrete is measured against the height of the mold. The only permissible type of slump is referred to as the true slump, where the concrete retains a symmetric shape and remains intact.

REFERENCES

ACI Collection of Concrete Codes, Specifications, and Practices. (2019) Report on measurements of workability and rheology of fresh concrete. ACI 238. 1R-08

Some factors that can affect uniformity of the concrete floors include:

  1. Aggregates moisture content.
  2. Free water added to the concrete mix.
  3. Concrete mix contents variation or blended concrete percentages.
  4. Variations in quantity or dispersion of admixtures in the concrete mix.
  5. Concrete mix delivery times variations, especially in hot weather.
  6. Concrete truck mixer efficiency variations, particularly water addition into the concrete mix.
  7. Change in concrete source or aggregates during the pour.
  8. Concrete placement delays.
  9. Shade and sun area variations on slab surface during the pour.

The single most important factor affecting the uniformity of concrete floors is consistency of the concrete mix. Even if the ideal concrete design mix is not achieved by the concrete supplier first class job can be achieved if the concrete mix is consistent from load to load. Concrete finishers will be able to adapt to the consistent mix design regardless (in majority of cases) of the characteristics, as long as the concrete mix and concrete delivery times are consistent. Bigger issues arise if concrete mix differs from one load to another, especially if some loads arrive with higher water-cement ratio, and if there are delays in concrete delivery from the plant to the jobsite.

REFERENCES

Ban Seng Choo (2003) Advanced Concrete Technology Processes. Ch. 24 Concrete Floors pp. 602-634

Concrete curing practices of newly placed concrete must maintain moisture and temperature conditions throughout the concrete placement operations and immediately after followed for a specific amount of time appropriate to the application. Concrete curing can be attained by some of the following methods: wet curing, wet covering, moisture-retaining covering, plastic film, waterproof paper, and liquid membrane-forming curing compound.

REFERENCES

ACI Collection of Concrete Codes, Specifications, and Practices. (2019) Guide to concrete floor and slab construction. ACI 302.1 R-15

Expectation or requirement that low spots be prevented on level or sloped slabs is inconsistent with standard construction tolerances. Bird baths or puddles on an interior concrete floor after wetting or an outdoor concrete slab after rain may be result of unachievable specified slope and flatness tolerances. Low spots in suspended slabs can be the consequence of deflection subsequent to the removal of shoring. Jointed concrete slab curling may also result in low spot formation. Due to the above-mentioned issues, some low spots that hold water should be expected on all concrete flatwork. On sloped concrete slabs with requirement of even 2 percent positive slope, some birdbaths should be expected due to normal surface deformations over time and construction tolerances.

REFERENCES

ACI Collection of Concrete Codes, Specifications, and Practices. (2019) Guide to concrete floor and slab construction. ACI 302. 1R-15

Concrete dusting or chalking is the formation of loose powder arising from disintegration of hardened concrete surface, composing of cement, water, and fine particles. This concrete surface powder can be easily scratched by nail or by sweeping. Any traffic results in concrete dusting due to the concrete wearing surface weakness. Finishing operations performed before the concrete has finished bleeding can cause surface weakness in concrete slabs. Working bleed water back into the top layer of the concrete slab results in high cement-water ratio, which in turn produces a weak surface layer.

Concrete dusting or chalking can also be caused by other factors:

  1. Dirty aggregates
  2. Surface freezing
  3. Surface carbonation
  4. Absence of or improper concrete curing
  5. Weak concrete mix
  6. Low cement content in the concrete mix
  7. Spreading of dry cement over the concrete slab
  8. Rainfall exposure during finishing stage
  9. Surface water application during finishing stage

References:

  1. Guide for Concrete Floor and Slab Construction, ACI 302. IR, American Concrete Institute, Farmington Hills, MI.
  2. Concrete Slab Surface Defects: Causes, Prevention, Repair, IS177. Portland Cement Association, Skokie, IL.

Cracks in concrete may be categorized into hardened concrete cracking or concrete cracking in plastic stage. Concrete cracking usually affects appearance only, but may be a sign of significant structural distress. Causes of cracking in concrete include plastic shrinkage cracking, thermal stresses, drying shrinkage, settlement cracking, weathering, reinforcement corrosion, construction overloads, poor construction methods, externally applied loads, and design/detailing errors.

REFERENCES

ACI Collection of Concrete Codes, Specifications, and Practices. (2019) Control of cracking in concrete structures. ACI 224 R-01; Causes, evaluation, and repair of cracks in concrete structures. ACI 224 1R-07

Plastic shrinkage cracks form in newly placed concrete slabs when concrete is in a plastic state. They occur when the upward movement of bleed water is lower than the rate of concrete surface drying. Some factors affecting plastic shrinkage cracking are high concrete temperature, high air temperature, low relative humidity, and moderate to high winds.

REFERENCES

ACI Collection of Concrete Codes, Specifications, and Practices. (2019) Guide to concrete floor and slab construction. ACI 302.1 R-15

Concrete Floor Joints

Joints in concrete floors serve multiple purposes. They limit tensile stresses in concrete resulting from thermal and drying shrinkage, and they eliminate or limit random cracking.

Construction joints are used to separate concrete pours where a concrete slab cannot be placed within one single placement, and if there are multiple different elevations within an area of the concrete slab that cannot be poured in one single operation. Construction joints should be eliminated where possible to obtain the best quality concrete floor.

Control joints (or contraction joints) serve the purpose of eliminating or minimizing random cracking within newly placed concrete floors. Ideally, control joints should be formed by way of saw cutting concrete slab with a diamond tipped blade within 24 hours of concrete placement. These joints may also be produced by way of inserting plastics, or forming grooves in the wet concrete, but these methods should be avoided in large pours because they involve disturbing the concrete surface.

Isolation Joints or expansion joints are used if the concrete floor needs to be isolated from the main structure or other intrusions through or partially into it. Failure to adequately isolate the concrete floor can lead to a considerable increase in local stresses, which may result in occurrence of shrinkage cracks.

REFERENCES

  1. Ban Seng Choo (2003) Advanced Concrete Technology Processes. Ch. 24 Concrete Floors pp. 625-636
  2. ACI Collection of Concrete Codes, Specifications, and Practices. (2019) Joints in concrete construction. ACI 224. 3R-95

Suspended slabs not designed as simply supported, should be placed in sections as big as can be practically achieved. Shrinkage crack control is achieved through appropriate design of reinforcement and detailing, control joints are not applicable. Joints in suspended slabs designed as simply supported can be detailed as if ground bearing, subject to control or construction joints perpendicular to the span direction being located directly over the support beams, and accordingly detailed reinforcement.

REFERENCES

Ban Seng Choo (2003) Advanced Concrete Technology Processes. Ch. 24 Concrete Floors pp. 625-636

Minimal requirements of the concrete floors as expected by clients:

  1. Capability of supporting applied loads without deformation or cracking.
  2. Constructed with appropriate tolerances for the materials handling systems to be used.
  3. Flexibility to accommodate future changes in the use of the concrete floor.
  4. Non-slippery, smooth, and easy to clean concrete floor surface.
  5. Dust-free surface.
  6. Contain minimal number of exposed joints.
  7. Maintenance-free joints.

To achieve specific tolerances required in the defined-traffic concrete slabs, the concrete mix should:

  1. Be consistent and suitable for achievement of the required tolerances.
  2. Have good finishability and bleed characteristics.
  3. Have consistent and appropriate workability.
  4. Have appropriate mix characteristics to the method of placement.
  5. Have low shrinkage characteristics.
  6. Be consistent with the available resources and set at the desired rate.
  7. Achieve specified strength within specified time.

Reinforcing concrete with the use of fibres in attempt to modify concrete properties carries the main objectives by modern engineers of:

  1. Improving the rheology or plastic cracking characteristics of concrete in the fresh state
  2. Improving flexural or tensile strength
  3. Improving impact strength
  4. Crack control and failure by means of post-cracking ductility
  5. Improving durability

Main benefit of the use of fibres in hardened concrete is the performance of fibre reinforcement in the post-cracking state. Fibres in hardened concrete contribute to toughness of the composite, failure strain, increased strength by bridging the cracks.

Concrete fibres exist in different sizes and materials, ranging from steel fibres to polypropylene fibres.

REFERENCES

Hannant, D.J. (2001) Fibre-reinforced cements and concretes. In Illston J.M. and Domone, P.L.J. (eds), Construction Materials – Their nature and behaviour, 3rd edn, pp. 386–422

Placing suspended slabs during cold weather requires careful consideration of multiple factors, and careful preparation where placed concrete is not subject to adverse effects of weather. Suspended slabs placed outside in cold weather shall be protected from wind by way of installing a wind breaking barrier around the slab perimeter to at least 4 foot height from the finished slab elevation. Depending on weather conditions, it may be necessary to install heat source below suspended slab to be placed in order to aid newly placed concrete to cure appropriately, however an experienced person must be left responsible for controlling of the temperature of the heat source and locations of heaters. Circulation of warm air under the suspended slab plays a critical role and in general a practice of keeping heat to certain spot locations under the suspended slab must be avoided if the air does not circulate, it will result in inconsistent set time over the entire slab of freshly placed concrete. If air temperature below suspended slab is set too high it may result in excessive moisture loss, which may lead to higher number of plastic shrinkage cracks. Accelerating admixtures must be picked accordingly; insufficient proportions may cause further complications and/or delays in concrete finishing operations.

Curing of concrete suspended slabs in cold weather shall be sufficient solely by the use of heat below the suspended slab and covering the slab with a layer of insulating blankets, however, the planning for the pour should include considerations to finish the suspended slab as soon as possible, and to cover it with insulating tarps progressively as soon as concrete finishers have completed each section of the pour. In consideration for the placement of suspended slabs in cold weather, one must consider the cost of extra labor, materials, and resources for correct placement and curing of the concrete slab.

REFERENCES

ACI Collection of Concrete Codes, Specifications, and Practices. (2019) Guide to cold weather concreting. ACI 306 R-16; Standard specification for cold weather concreting. ACI 306. 1-90

Placing concrete slabs on grade during cold weather requires careful consideration of multiple factors, and careful preparation where placed concrete is not subject to adverse effects of weather. The concrete slab substrate must be protected from freezing by way of laying insulating tarps and if weather is really unfavorable temporary use of glycol heating may be required. Frozen substrate may cause a variety of issues, especially if frost has penetrated deep. Curing slab on grade in cold weather requires careful consideration where heat produced through chemical reaction in freshly placed concrete must be kept within the slab as much as possible by way of covering the slab as soon as finishing operations are complete with insulating blankets and temporary glycol heating if weather is unfavorable.

Ideally, placement of concrete slabs on grade should be postponed until the overnight air temperature is at least 4 Degrees Celsius to avoid complications and higher costs of construction.

REFERENCES

ACI Collection of Concrete Codes, Specifications, and Practices. (2019) Guide to cold weather concreting. ACI 306 R-16; Standard specification for cold weather concreting. ACI 306. 1-90

Hot weather has potential to adversely affect concrete placement and compromise concrete strength and durability. Careful planning must be implemented at early stages of preparation for concrete placement in hot weather conditions. Serious issues may occur if personnel placing concrete lacks experience in hot weather concrete placement. Damage to concrete slabs resulting from hot weather conditions can never be fully alleviated. Some of the potential problems of placing a concrete slab in hot weather conditions include increased water demand, increased slump loss rate, accelerated setting rate, increased tendency for thermal cracking and plastic shrinkage, and difficulty in entrained air control. Careful consideration must be given to conditions in which the concrete placement will take place, including, adding admixtures, controlling slump, analyzing sun and shade over the slab, and immediate curing methods and materials.

REFERENCES

ACI Collection of Concrete Codes, Specifications, and Practices. (2019) Guide to hot weather concreting. ACI 305 R-10; Specification for hot weather concreting. ACI 305. 1-14

Moisture protection of concrete structures falls under two categories, waterproofing and damp-proofing. These terms are often used incorrectly interchangeably. Both of these systems serve the purpose of retarding the passage or absorption of water vapor or water by concrete forming a structure. However, each system has a different specific purpose and levels of moisture protection.

REFERENCES

ACI Collection of Concrete Codes, Specifications, and Practices. (2019) Guide to residential concrete construction. ACI 332. 1R-18

Waterproofing is defined as resistance of the water passage under hydrostatic pressure to a structure. It is intended for use in areas of high or fluctuating water tables and intermittent hydrostatic conditions. Materials used for waterproofing are designed to prevent water absorption by concrete, and moisture transmission through cracks and imperfections.

REFERENCES

ACI Collection of Concrete Codes, Specifications, and Practices. (2019) Guide to residential concrete construction. ACI 332. 1R-18

Damp-proofing is defined as resistance to water penetration in the absence of hydrostatic pressure. It is intended for use if minimal or intermittent exposure to exterior moisture on the structure is expected. Below grade walls that enclose basements shall be damp-proofed as minimum means of protection from moisture infiltration. Materials used for damp-proofing will not bridge cracks in concrete, this limitation makes it unsuitable for waterproofing purposes.

REFERENCES

ACI Collection of Concrete Codes, Specifications, and Practices. (2019) Guide to residential concrete construction. ACI 332. 1R-18

Drainage should be used in conjunction with waterproofing and damp-proofing systems; it is a key factor in performance of concrete walls that are subject to moisture penetration. Drainage systems collect and remove water from the footing by means of a sump pump system or gravity to daylight. The drainage system should be installed with a slight positive slope or level around the structure to prevent collection of water in low areas. The pipe should be placed on top of the footing but in no case should the top of the pipe be above the finish floor elevation inside the structure. The pipe should have a filter fabric layer around it and it must be covered within a clean granular material.

REFERENCES

ACI Collection of Concrete Codes, Specifications, and Practices. (2019) Guide to residential concrete construction. ACI 332. 1R-18

Positive side waterproofing is applied to the exterior side of concrete, it is the most common type of waterproofing and includes many types of available systems. This form of waterproofing protects the structure from water infiltration, as well as, structural components such as, concrete and reinforcement.

Major disadvantage of this form of waterproofing is costly repairs due to the system being inaccessible easily.

Negative side waterproofing is applied on the interior side of the concrete and protects the structure by withstanding hydrostatic pressure against the bond of the material. The main advantage of this form of waterproofing is ease of accessibility for repairs.

Main disadvantage of this form of waterproofing is potential issues with the outside concrete that will be penetrated by moisture and may face corrosion and deterioration.