Understanding the common concreting problems that can occur during concreting practice will help to minimize further mistakes during concreting. Most of the concreting problems arise due to:
- Improper Mixing
- Improper Placing
- Improper Curing
Mentioned below are the common concreting issues and how it can be minimized to a large extent.
Segregation is the tendency of the various constituents of a concrete mix to separate, especially the separation of the large aggregate particles from the cement mortar. Segregation can result in rock pockets or honeycombs in the hardened concrete, sand streaks, porous layers, scaling, laitance, and bond failure at construction joints. Harsh mixes have a tendency to segregate, usually those that are too wet but sometimes those that are too dry.
A well-proportioned mix with a slump of 3 to 4 in. resists segregation, but any mix can segregate if it is not properly handled, transported, and placed. Once segregation has occurred, the aggregate cannot be reintegrated and the mix must be discarded. Segregation can be caused by over-mixing or by improper handling during placement operations.
Discoloration in concrete is observed due to the alterations in the concrete mix and the materials used for batching. This is mainly observed in structures made from ready-mix concrete. Here, there are chances to add more water at the site to meet the workability which is an activity from poor workmanship.
Discoloration in concrete can be avoided by:
- Uniform curing of the concrete surface throughout the entire surface.
- Hard troweling over the exterior concrete surfaces must be avoided.
- The finishing of the concrete must be performed only after all the water has evaporated completely.
The bending of concrete edges will result in distortion of the slab in a curved manner. This is called as curling. The bending can be either in upward or in the downward direction.
The temperature difference or the moisture difference between the bottom and the top surface are the major reason behind the concrete curling.
Curling can be avoided by:
- Proper Curing
- Moisture penetration prevention by using sealers and waxes
- Designing thicker slabs
Concrete spalling is also called as concrete cancer. Weathering, excessive moisture, and corrosion are the major reason behind this surface failure.
Spalling can be avoided by:
- Adequate concrete cover
- Proper curing
- Using air entrained concrete
- Application of water repellent sealer once the concrete is cured
Crystalline deposit of calcium carbonate over the concrete surfaces gives a wheatish appearance. This is called as efflorescence. The reaction of some soluble salts with the concrete result in such deposits. The calcium hydroxide in concrete reacts with salts to form calcium carbonates.
Efflorescence can be avoided by:
- Preventing the amount of calcium hydroxide by the use of fly ash in concrete.
- Application of sealants over the concrete surface
Concrete cracking can be caused by:
- Concrete Shrinkage
- Excessive thermal stresses in concrete
- Reinforcement corrosion
- Presence of excessive water in concrete
- Poor construction practice
Cracking in concrete is tough to prevent completely. But it can be minimized by
- Providing properly designed joints
- Proper curing
- Avoiding moisture penetration
- Making a dense concrete with less pores
Presence of fine cracks or fissures over the concrete surface is called as crazing. These are formed commonly on the concrete surface that is steel trowelled. The causes of crazing are:
- Improper curing
- Floating of excessive cement paste over the concrete surface after compacting
- Excessive laitance on the concrete surface.
Crazing can be avoided by:
- Early and proper curing of concrete
- Avoid sprinkling cement or fine sand over the concrete surface.
The breaking and peeling off of the concrete surface is called as scaling of concrete. Inadequate strength and curing are the main reasons of scaling of the concrete.
Freezing and thawing cycles observed in porous concrete too results in scaling of concrete.
Concrete scaling can be avoided by:
- Concrete must be protected from moisture to prevent it from freezing and thawing cycles.
- Sealing the concrete surface after 28 days of concrete curing.
Bleeding occurs when the cement and aggregate in newly placed concrete begin to settle and surplus water rises to the top surface of the concrete. Bleeding continues until the cement starts to set, until bridging develops between aggregate particles, or until maximum settlement or consolidation occurs.
Bleeding is dependent on:
- Mix proportions
- sand grading
- sand particle shape
- Amount of aggregate fineness
- The fineness of the cement
- Water Content
Some bleeding is a normal part of concrete curing, but excessive bleeding can decrease the durability of the surface, interfere with thebond of cement paste to reinforcing bars, and increase porosity of the hardened concrete.
Bleeding can be avoided by:
- Concrete Air entrainment
- Use of well graded aggregates
- Water content reduction or Increased amount of cement content
10. Plastic Shrinkage
Plastic shrinkage cracking is usually associated with hot-weather concreting. It is caused by rapid evaporation of surface moisture from a slab or other flatwork. The procedures recommended for hot-weather concreting will alleviate the possibility of plastic shrinkage cracking.
Dusting is the wearing away of hardened concrete surfaces under traffic. Dusting is caused by:
- Too much of water content
- Concrete Consolidation
- Use of aggregates that are not clean
- Formation of weak surface layer called laitance
- Concrete segregation
12. False Set & Flash Set
In false set, concrete appears to set or harden after only a few minutes. This is a temporary condition caused by hydration of unstable gypsum (calcium sulfate) in the cement. It usually disappears with prolonged mixing or remixing and is generally not a problem with ready-mixed concrete. Do not add more water. After a few more minutes, with or without additional mixing, the false set will usually disappear on its own.
In flash set, lumps of dry cement are surrounded by a layer of damp or partially hydrated cement, or solid lumps of partially hydrated cement are formed. Flash set is caused by the use of hot-mixing water in cold weather. To avoid this problem, change the batching sequence so that the hot water and aggregates are put in the mixer first and the cement is added after the water has cooled slightly.