The chemical reaction between water and cement (called hydration) causes the concrete to harden. As long as there is moisture in the concrete, this hardening and strength development continue for years but at a decreasing rate.
It is the heat generated during the hydration reaction and causes the rise of the concrete temperature.
It is the hardening and loss of plasticity of cement and concrete after the cement comes in contact with water. There are two setting times:
– Initial set, which occurs in 2 to 4 hours and indicates that the paste is beginning to stiffen.
– Final set, which occurs in 5 to 8 hours and indicates that the cement has hardened to the point at which it can sustain some load.
Pounds per square inch = psi
Kilogram per square centimeter = kg/cm² = 14.2 psi
Megapascal = MPa = 1 N/mm² superscript (Newton per square millimeter) = 145 psi
The water/cement (w/c) ratio. The lower the w/c ratio, the higher the strength of concrete.
It is written sometimes as w/c ratio. It is the weight of water divided by the weight of cement used in the mix. If cementitious additives, such as silica fume are used, then water/cementitious materials (w/cm) ratio is used.
To increase the resistance of hardened concrete to the action of freezing and thawing. Hence, since there is no freezing and thawing action in Saudi Arabia and the Gulf, air entrainment is not required as it does not serve any purpose here, although it is specified by several institutions.
The strength of concrete is reduced, by about 5% for every 1% of air entrained. So, if we have a concrete with an initial strength of 4,000 psi concrete with receives 3% air entrainment, the strength will be reduced to about 3,400 psi.
Honeycombing can be recognized by exposed coarse aggregate on the surface, often without any mortar covering or surrounding the aggregate particles. Honeycombing may extend deep into the concrete. Honeycombing can be caused by insufficient vibration at the time of placement, by a poorly graded concrete mix, or by an oversized of coarse aggregates.
It is the protection of concrete against moisture loss and high temperature. This is necessary for the continuation of the hydration process on which the concrete strength and durability depend.
Ponding, continuous sprinkling, covering concrete with wet burlap, wet sand or polyethylene sheets, or using curing compounds.
Curing should begin immediately after placing and finishing of concrete.
The longer the better. However, the most important and effective curing is the early curing. In general, concrete should be cured uninterrupted for a minimum of 7 days. If adequate curing is maintained, the ultimate strength will generally be increased.
The term “aggregates” refers to the gravel, crushed rock and sand that are mixed with cement and water to make concrete. Aggregates are divided into fine aggregates, also called sand, (size 0 – 5mm) and coarse aggregates (size 5 – 75mm). They are either natural (gravel, dune sand) or artificial (crushed rocks, crushed gravels).
Yes. The water added to concrete needs to be adjusted to take into consideration the amount of water that will be absorbed by the aggregates.
When the aggregates are contaminated with salt, dust or clay it is recommended to wash the aggregates to get rid of such undesirable materials
There are two main sizes: 3/4 ” (20mm) and 3/8” (10mm).
It is simply a concrete mix with a maximum size aggregate size of 3/8” (10mm). There are no 3/4” (20mm) aggregates in screed mixes.
Other Raw Materials
To modify the concrete properties and/or to improve the fresh or hardened concrete properties.
The main types are:
– Water reducers: they allow the use of a lower the water content (from 5-10 %) in the mix without a reduction in fresh concrete workability; resulting in an increase in concrete strength and durability.
– Retarders: they delay the initial setting of concrete to allow more time for transporting, placing and compacting of the concrete.
– Superplasticizers: They allow more reduction of the water content of a given concrete mix by 12 to 30% without reducing its workability, which results in an increase in the concrete’s the strength. Most superplasticizers also act as retarders.
When a retarder is added in normal dosages, it will delay the setting of concrete for at least 3 hours in summer and for about 4 hours in winter. An overdose of a retarder can delay setting of concrete by several days.
If adequate curing is maintained, the ultimate strength will generally be increased. An overdose of a retarder can delay setting of concrete by several days.
It is a very fine powder that is produced as a by-product from the production of silicon metal and ferro-silicon alloys.
When silica fume is added to the mix, it increases the ultimate strength and sulfate resistance, and reduces the permeability and heat of hydration of concrete.
About 5-10% by weight of cement is normally replaced by silica fume.
Other Raw Materials
When concrete is mixed, transported, and placed under conditions of high concrete temperature, high ambient temperature, low relative humidity, solar radiation, or wind, then we consider this condition as hot weather concreting. Special precautions shall be taken in this case to avoid concrete problems.
– Increased water demand
– Increased rate of slump loss, tendency to add water at the job site
– Increased rate of setting, resulting in difficult handling, finishing, and risk of cold joints
– Increased risk of plastic shrinkage cracking
– Decreased 28 day and later strengths
– Increased drying shrinkage and thermal cracking
– Decreased durability due to cracking
– Increased permeability
– Increased potential for reinforcing steel corrosion due to increased cracking
These are cracks occurring that occur in horizontal slabs in diagonal form. They are most common in concrete placed during hot weather, and in windy days, when the rate of moisture evaporation from the concrete surface exceeds the rate of bleeding water rising to the surface.
By preventing the loss of moisture from the surface of concrete in the critical first few hours, using fog sprayers or covering the concrete surface with polyethylene sheeting, for example. Wind-breakers may be erected to reduce wind velocity.
Most of the time, short measures are not due to inaccuracy in delivered quantities of concrete, the main causes of short measures are:
– Error in calculating the volume of concrete required. For example, an error of 3 mm in the thickness of a 10 cm slab would cause a shortage of 3 % or about 1 m3 in a 33 m3 order.
– Deflection or distortion of the formwork by the pressure of the concrete.
– Irregular subgrade, placement over granular fill, and settlement of subgrade prior to placement can increase slab thickness the required concrete quantity.
– Reduction in volume of hardened concrete.
– Some amount of concrete is consumed in the pump, and during sampling and testing of concrete, and casting of concrete cylinders test specimens.
According to ASTM C-94, Note 1: “It should be understood that the volume of hardened concrete may be, or appears to be, less than expected due to waste and spillage, over-excavation, spreading of forms, some loss of entrained air, or settlement of wet mixtures, NONE OF WHICH IS THE RESPONSIBILITY OF THE PROD