Faq

Frequently Asked Questions

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  • FAQ

    Concrete

     

    Q. What causes hardening in portland cement or concrete?

    A. 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.

     

    Q. What is meant by the heat of hydration?

    A. It is the heat generated during the hydration reaction and causes the rise of the concrete temperature.

     

    Q. What is meant by “setting of cement” or concrete?

    A.  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.

     

    Q. What is the most common measure by which the quality of concrete is judged?

    A. Compressive strength.

     

    Q. What are the common units used for the compressive strength of concrete?

    A.  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

    Q. What is the most important factor affecting the concrete strength?

    A. The water/cement (w/c) ratio. The lower the w/c ratio, the higher the strength of concrete.

     

    Q. What is the water / cement ratio?

    A. 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.

     

    Q. What is the purpose of air entrainment?

    A. 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.

     

    Q. What is the effect of air entrainment on concrete strength?

    A. 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.

     

    Q. What is honeycombing and what causes it?

    A. 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.

     

    Curing


    Q. What is meant by curing?

    A. 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.

    Q. What are the common curing methods?

    A. Ponding, continuous sprinkling, covering concrete with wet burlap, wet sand or polyethylene sheets, or using curing compounds.

     

    Q. When should curing begin?

    A. Curing should begin immediately after placing and finishing of concrete.

     

    Q.  For how long should the concrete be cured?

    A. 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.

     

    Aggregates


    Q. What are aggregates?

    A.  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).

     

    Q. Does the aggregate absorption need to be taken into consideration?

    A. 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.

     

    Q. When should aggregates be washed?

    A. When the aggregates are contaminated with salt, dust or clay it is recommended to wash the aggregates to get rid of such undesirable materials.

     

    Q. What are the common sizes of aggregates used in concrete in our area?

    A. There are two main sizes: 3/4 ” (20mm) and 3/8” (10mm).

     

    Q. What is a screed mix?

    A. 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


    Q. Why do we add chemical admixtures to concrete?

    A. To modify the concrete properties and/or to improve the fresh or hardened concrete properties.

     

    Q. What are the main types of admixtures?

    A. 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.

     

    Q. For how long can the concrete remain workable if a retarder is added?

    A. 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.

     

    Q. What is the effect of an overdose of a retarder on strength?

    A. 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.

     

    Q. What is silica Fume?

    A. It is a very fine powder that is produced as a by-product from the production of silicon metal and ferro-silicon alloys.

     

    Q. What are the benefits of using silica fume?

    A. 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.

     

    Q. What is the normal amount of cement that may be replaced by silica fume?

    A. About 5-10% by weight of cement is normally replaced by silica fume.

     

    Hot Weather Concreting


    Q. What is hot weather concreting?

    A. 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.

     

    Q. What are the potential problems for fresh concrete in hot weather?

    A.  Potential 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

     

    Q. What are the potential problems for hardened concrete in hot weather?

    A.  Potential problems:

    - 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

     

    Q. What are plastic shrinkage cracks?

    A. 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.

     

    Q. How can plastic shrinkage cracks be avoided?

    A. 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.

     

    Short Measures

     

    Q. What are the most probable causes of short measures?

    A. 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


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