Cement is the binding element in both concrete and mortar. Mortar is a mixture of cement and sand while concrete is a mixture of cement, sand and gravel. The term “cement” is often mistakenly used to refer to “concrete”.
The most common type of cement, Portland cement, is made by crushing and sintering the raw materials eg limestone / chalks, clay/shale, silica sand etc at 1450 o C to produce intermediate product called clinker. The clinker is then ground into fine powder, with the help of grinding aids in the form of gypsum, and become Portland cement. Nowadays, supplementary cementitious materials (SCMs) such as ground granulated blast furnace slag (GGBS), fly ash (PFA) and silica fume is sometime added into Portland cement either through intergrinding or blending in order to produce a more durable concrete.
GGBS is a by-product of iron/steel making industry. It is obtained by quenching molten iron slag from blast furnace by water jet spray and the product is a glassy, granular material which is dried and grinded to fine powder. The inclusion of GGBS resulted in a more durable concrete with higher resistance to chloride (seawater), sulphate (underground water), low heat of hydration (mass concreting) and higher long term strength development in comparison with 100% Portland cement mix. The usage and specification of GGBS is governed under British Standard BS EN 197-1 and BS EN 15167-1.
PFA is a by products from the coal-fired power plant. The finely divided ash improved the hardened concrete properties through pozzolanic activity. The inclusion of PFA resulted in concrete with reduced water demand (higher strength), improved durability performance and long term strength development through pozzolanic activity. It is also often used when low heat cement is required. The usage and specification of PFA is governed under British Standard BS EN 197-1 and BS EN 450-1.
The main element to cause concrete to harden is cement. Cement undergoes a chemical reaction called hydration when it is in contact with water. The main reaction production from the aforementioned reaction is calcium silicate hydrate (C-S- H) gels.
Ready-mix concrete is concrete that is mixed in a batching plant, under controlled operations and in accordance to a specified mix design requested by clients. The pre-mixed concrete is then delivered to the specified work site by the means of mixer truck. Other than the basic constituent materials of concrete, chemical admixture is often added to modify the concrete properties such as setting time, water demand etc to suit client’s requirements.
Ready-mix concrete is now often preferred over the conventional site-mixed concrete due to its consistency in performance, precision in mixing, ease of handling and reduced on-site confusion regarding the correct concrete mixture to be used.
The water content in every ready-mix concrete is designed to yield optimum/desired performance. Any additional water added into the ready mix concrete will impair the intended concrete mechanical (strength) and durability properties. In addition, excessive water addition will increased the tendency of segregation/bleeding of concrete upon compaction and caused defect such as honeycomb. Bleeding of concrete will also increase the tendency of plastic shrinkage cracks.
During delivery, depending on client’s specifications and requirements, the most common compliance criteria are slump value of concrete on-site and placement temperature of the concrete. Other than that, the quality of the supplied concrete will be judged based on the 28 days concrete cube compressive strength against the ordered concrete grade for compliance purposes.
Concrete curing is defined as a process to provide the casted concrete with adequate moisture and temperature at an extended period of time. The curing process should start immediately upon placing and finishing process. Due to the large exposed surface area, tendency of cracking is high in slab casting if the slab is not properly cured. Upon finishing process, concrete slab can be cured by wetting the slab surface with soaking hessian, sprinkling of fine mist of moisture or covered with HDPE plastic sheet for an extended period (usually 5-7 days). Curing allows continuous hydration of concrete and prevent/minimize the occurrence of plastic shrinkage cracks.
One of the most common factors that cause concrete to cracks is shrinkage. Concrete literally shrink in volume during drying and hardening process and in restraint condition (reinforced concrete structure), tensile stress developed and when the induced tensile stress exceed the tensile strength of concrete, cracks occurred. The most common types of shrinkage cracks are plastic, autogenous and drying shrinkage cracks. In some cases, especially when casting slab with huge surface area, cracks are inevitable and that is why the provision of joints are imminent to enable the cracks occur in a neat and controlled manner.
Precast concrete is a concrete product produced by casting concrete into pre-determined size of reusable mould, cured and stored in a controlled environment for a pre-determined period of time before being delivered to site. In contrast, cast-in situ concrete is a concrete product produced by casting concrete into site-assembled mould/formwork and subsequent curing will be done on-site. Some examples of precast concrete products are piles, U-drains, pipes, beams, retaining walls etc.
Distinct advantage of precast concrete products include superior intrinsic quality as precast concrete products are manufactured in a controlled environment, high precision method and are independent of weathering condition. Besides, it also brings about reduction in construction time due to the ease of handling, assembling and placement of precast concrete products on-site.
Concrete in nature is excellent in resisting compressive force and weak in resisting tension force. Therefore, in reinforced concrete, steel reinforcement bars are provided to withstand the tension force induced during the service life of the concrete member. In prestressed concrete, an initial compressive axial force was applied to the concrete which greatly reduce the internal tensile stresses. This is done by pre-stressing a steel cable running through the concrete. The main advantages of prestressed concrete are reduced tendency of cracking due to bending load, reduced deflection of the concrete member and member sections are smaller compared to reinforced concrete for the same imposed loads.
The common compliance criteria for precast concrete products include dimensional tolerance, minimum concrete cover tolerance, occurrence of cracks and the strength of concrete. The concrete strength acceptance criteria is based on the 28 days compressive strength of concrete cube samples.