Cracking Concrete Surfaces
Concrete, like other construction materials, contracts and expands with changes in moisture and temperature, and deflects depending on load and support conditions. Cracks can occur when provisions to accommodate these movements are not made in design and construction.
Finishing Concrete Flatwork
Finishing makes concrete attractive and serviceable for residential and commercial applications. The final texture, hardness, and joint pattern on slabs, floors, sidewalks, patios, and driveways depend on the concrete’s end use. Warehouse or industrial floors usually have greater durability requirements and need to be flat and level, while other interior floors that are covered with floor coverings do not have to be as smooth and durable. Exterior slabs must be sloped to carry away water and must provide a texture that will not be slippery when wet.
Hot Weather Concreting
Hot weather may be defined as any period of high temperature in which special precautions need to be taken to ensure proper handling, placing, finishing and curing of concrete. Hot weather problems are most frequently encountered in the summer, but the associated climatic factors of high winds, low relative humidity and solar radiation can occur at any time, especially in arid or tropical climates. Hot weather conditions can produce a rapid rate of evaporation of moisture from the surface of the concrete and accelerated setting time, among other problems. Generally, high relative humidity tends to reduce the effects of high temperature.
Jobsite Addition of Water
Jobsite addition of water is the addition of water to ready mixed concrete in a truck mixer after arrival at the location of the concrete placement. Such tempering of concrete may be done with a portion of the design mixing water which was held back during the initial mixing, or with water in excess of the design mixing water, at the request of the purchaser.
Joints in Concrete Slabs on Grade
Concrete expands and shrinks with changes in moisture and temperature. The overall tendency is to shrink and this can cause cracking at an early age, in both commercial and residential projects. Irregular cracks are unsightly and difficult to maintain but generally do not affect the integrity of concrete. Joints are simply preplanned cracks. Joints in concrete slabs can be created by forming, tooling, sawing, and placement of joint formers.
Plastic Shrinkage Cracking
Plastic shrinkage cracks appear in the surface of fresh concrete soon after it is placed and while it is still plastic. These cracks appear mostly on horizontal surfaces. They are usually parallel to each other on the order of 1 to 3 feet apart, relatively shallow, and generally, do not intersect the perimeter of the slab. Plastic shrinkage cracking is highly likely to occur when high evaporation rates cause the concrete surface to dry out before it has set. Plastic shrinkage cracks are unsightly but rarely impair the strength or durability of concrete floors and pavements. The development of these cracks can be minimized if appropriate measures are taken prior to and during placing and finishing concrete.
Dusting Concrete Surfaces
Formation of loose powder resulting from disintegration of surface of hardened concrete is called dusting or chalking. A concrete floor dusts under traffic because the wearing surface is weak.
Scaling Concrete Surfaces
Scaling is local flaking or peeling of a finished surface of hardened concrete as a result of exposure to cycles of freezing and thawing. Generally, it starts as localized small patches which later may merge and extend to expose large areas.
Crazing Concrete Surfaces
Crazing is the development of a network of fine random cracks or fissures on the surface of concrete or mortar caused by shrinkage of the surface layer.
Cracks in Residential Basement Walls
Cast-in-place concrete basements provide durable, high quality living space. Cracking of concrete is a natural occurrence and at times can be undesirable.
Discrepancies in Yield
Concrete yield is defined as the volume of freshly mixed concrete produced from a known quantity of concrete materials.
Low Concrete Cylinder Strength
Strength test results of concrete cylinders are used as the basis of acceptance of ready mixed concrete when a strength requirement is specified.
Strength of In-Place Concrete
Concrete structures are designed to carry dead and live loads during construction and in service. Samples of concrete are obtained during construction and standard ASTM procedures are used to m measure the potential strength of the concrete as delivered.
Curing In-Place Concrete
External curing is a step during construction that involves maintaining newly placed concrete at adequate moisture and temperature conditions so that it can develop properties, such as strength and durability, the mixture was designed to achieve.
Blisters may form on the surface of fresh concrete when either bubbles of entrapped air or bleed water migrate through the concrete and become trapped under the surface, which has been sealed prematurely during the finishing operations.
Chemical Admixtures for Concrete
Admixtures are natural or manufactured chemicals added to the concrete before or during mixing. The most often used chemical admixtures are air-entraining agents, water reducers, water-reducing retarders, and accelerators.
Flexural Strength of Concrete
Flexural strength is an indirect measure of the tensile strength of concrete. It is a measure of the maximum stress on the tension face of an unreinforced concrete beam or slab at the point of failure in bending.
Flowable fill is a self-consolidating and self-leveling low strength cementitious material with a flowable consistency that is used as an economical fill.
Radon Resistant Buildings
Solid concrete is an excellent material for use in constructing radon’s resistant buildings. It is an effective barrier to soil gas penetration if cracks and opening are sealed.
Curling of Concrete Slabs
Curling is the distortion of a slab panel into a curved shape by upward or downward bending of the edges and corners at joints or cracks.
Delamination of Troweled Concrete Surfaces
Delaminations are separations of thin layers of the slab surface from the base concrete. Delamination typically results in break off of surface layers, 1/8 to 1/4 inch in thickness, from a finished slab.
Loss of Air Content in Pumped Concrete
Air loss will occur if the weight of concrete in a vertical downward section of pipe is sufficient to overcome frictional resistance to allow a slightly of concrete to slide down the pipe.
ACI defines grout as “a mixture of cementitious material and water, with or without aggregate, proportioned to produce a pourable consistency without segregation of the constituents.”
Surface discoloration is the non-uniformity of color or hue on the surface of a single concrete placement. It may take the form of dark blotches or mottled discoloration on flatwork surface, gross color changes in large areas of concrete caused by a change in the concrete mix, or light patches of discoloration caused by efflorescence.
Synthetic Fibers for Concrete
Synthetic fibers engineered for use in concrete can withstand the long-term alkaline environment of concrete.
Corrosion of Steel in Concrete
When the steel reinforcement in concrete corrodes, the larger volume of rust formed leads to internal stresses and subsequent delimitation and spalling of the concrete cover.
Cold Weather Concreting
Successful cold-weather concreting requires an understanding of the various factors that affect concrete properties.
Concrete Slab Moisture
Concrete slab moisture can cause problems with the performance of floor-covering materials, such as vinyl tile, wood, resilient sheet flooring, carpet, and cause bond-related failures of non-breathable floor coatings.
Vapor Retarders Under Slab On Grade
Vapor retarders are sheet materials that minimize the transmission of moisture or water vapor from the sub-slab support system into a concrete slab.
Supplementary Cementitious Materials (SCMs)
Today, most concrete mixtures contain supplementary cementitious materials that make up a portion of the cementitious component in concrete.
Ordering Ready Mixed Concrete
Ready mixed concrete is that which is manufactured and delivered to the customer in a freshly mixed and unhardened state.
Concrete Pre-Construction Conference
Prior to the start of a job, especially for major projects, a concrete pre-construction conference (sometimes called a pre-pour meeting) should be held to define and allocate responsibilities of the entire construction team.
High Strength Concrete
High strength concrete is a type of high performance concrete generally with a specified compressive strength of 8,000 psi or greater.
Making Cylinders in the Field
For testing the compressive strength of concrete, cylindrical test specimens are cast and stored in the field until the concrete hardens in accordance with the requirements of ASTM C 31, Standard Practice for Making and Curing Concrete Test Specimens in the Field.
Testing Compressive Strength of Concrete
Compressive strength is measured by breaking cylindrical concrete specimens in a compression-testing machine.
Structural Lightweight Concrete
The primary use of structural lightweight concrete is to reduce the dead load on a structure. This allows the design to reduce the size of columns, footings and load bearing members.
Self-Consolidating Concrete (SCC)
Self-consolidating concrete (SCC), is a highly flowable, non-segregating concrete that can flow into place, fill the formwork and encapsulate the reinforcement without mechanical consolidation.
Pervious concrete is a special type of concrete with a high porosity used for concrete flatwork applications that allows water from precipitation and other sources to pass through it, thereby reducing the runoff from a site and recharging around water.
Maturity Methods to Estimate Concrete Strength
The maturity concept uses the principle that concrete strength (and other properties) is directly related to both age and its temperature history.
A “popout” is a small, generally cone-shaped cavity in a horizontal concrete surface left after a near-surface aggregate particle has expanded and fractured.
Acceptance Testing of Concrete
Acceptance testing is the process of testing representative samples of concrete furnished to a project.
Thermal Cracking of Concrete
Thermal cracking occurs due to excessive temperature differences within a concrete structure or its surroundings.
Alkali Aggregate Reactions (AAR)
AAR results in deleterious expansive cracking of concrete occurring at later ages after construction.
Durability Requirements for Concrete
Conditions that can impact the service life of concrete structures should be identified during design and addressed in project specifications.