An Najah National University Faculty of Engineering Construction Materials Course Eng. Mohammed Abu Neamah

Portland Cement Concrete

High Strength Concrete Many advances have been made in concrete technology. One of the most significant improvements is the production of very high strength concrete, using supplementary cementing materials (SCM) and water reducing admixtures Concrete can be classified based on its strength as follows: Conventional concrete: strength < 50 Mpa and W/C > 0.45 High strength concrete: 50 - 100 Mpa and 0.3< W/C < 0.45 Very high strength concrete: 100 - 150 Mpa and W/C between 0.25 – 0.30 Ultra High strength concrete: strength >150 Mpa and W/C is less than 0.25

High Strength Concrete High strength concrete, mixed with slower reacting supplementary cementing materials, gains a considerable amount of strength after the standard 28 day period. Therefore the design strength be met at 56 or 91 days of age rather than at 28 days. Materials, production, and placing high strength concrete must be controlled very closely.

Mixing, Placing & Curing There are three type of mixing procedures depending on the type of plant and trucks are: Central mixed: mixed in a stationary mixer at the plant and delivered to the site in a truck with a rotating drum. Shrink mixed: concrete is partially mixed at the plant, with mixing completed in the truck. Truck mixed: concrete is mixed completely in the truck, after it has been loaded at the plant.

Mixing, Placing & Curing Specifications for mixing: Mixing require 70 -100 revolutions of the drum at a mixing speed of 6 – 18 rpm for most trucks, followed by agitating at a lower rate (2 – 6 rpm) until the concrete is placed. Minimum mixing time is one minutes for the first cubic meter, and 20 seconds for each additional cubic meter. Concrete must discharged from ready mixed trucks within 2 hours or 300 drum revolutions (whichever comes first) according to ATSM requirement.

Mixing, Placing & Curing Placing Concrete: Concrete should poured closer to its final position and not allowed to fall freely for too great distance to avoid segregation. Buckets, pumps, and belt conveyors are used to move the concrete to forms In forms, the concrete should be deposited in layers 200 – 500 mm thick Vibration is required to consolidate the concrete and fill all voids.

Mixing, Placing & Curing Placing Concrete: Over vibration (more than 15 seconds) can cause segregation or reduce the amount of air entrained in the concrete.

Mixing, Placing & Curing Curing: is one of the most important aspects of concrete construction. Poor curing and addition extra water contributes to poor quality concrete. Suitable curing requires water and favorable temperature. Concrete will gain strength indefinitely (although at slower rate) if kept moist. Excessive evaporation of water = surface shrinkage = cracks. Low temperature slow the rate of hydration

Mixing, Placing & Curing Methods of curing used to ensure the moisture is present are: Ponding Sprinkling or fogging Wet covering (burlap) Water proof paper Plastic film Curing compounds that form a membrane when sprayed on the surface Steam curing

Mixing, Placing & Curing Ponding and continuous sprinkling are fairly expensive methods and are used mainly in small projects. Wet covering are effective but must be kept continuously damp Water proof paper consist of two sheets of paper with an asphalt adhesive, or plastic sheets are effective in preventing evaporation of water. Curing compounds sprayed on the surface are the most common methods used for curing pavement slabs. A membrane forming compounds prevents evaporation if the surface is covered completely with sufficient quantity.

Mixing, Placing & Curing The duration of the curing period varies according to the type of project and type of concrete. A period of 7 days is often specified The use of accelerator reduce the time required for curing. The use of high early strength cement (Type III) allow the period of curing to reduced to three days or less. This concrete will usually reach over 80% of design strength in three days.

Joints Joints must be used in concrete construction to prevent the concrete from cracking. Contraction (control) joints are placed in slabs to control random cracking. To allow for drying shrinkage. It should be spaced at intervals of not over 30 time’s the slab’s thickness in both directions.

Joints Controlled of shrinkage cracking: Unreinforced concrete: contraction joints @ 4-7 m spacing Lightly reinforced concrete, contraction joints @ 12-30 m spacing Heavily reinforced concrete, no contraction joints

Joints Isolation/Expansion Joints: Isolation joints are designed to separate slabs from such structures as column bases and walls, etc. For example, if a slab were placed between two buildings, an expansion joint should be placed adjacent to the face of at least one of the buildings

Joints Construction joints are stopping places in the process of construction. Are usually located at the end of one day’s pour or between lanes in pavement slabs. They are designed to allow load transfer with dowels.

Special Precautions Special precautions and construction practices are required during very hot or cold weather. High air temperature + high concrete temperature +low humidity + high wind velocity = may cause surface cracking During hot weather , ice may be used to improve conditions of placing. Cold weather may require heating of materials ASTM require for minimum temperature of the concrete at placing is 13 oC for sections up to 0.3 m thick. And 10 oC for sections (0.3 – 0.9 m) thick

Concrete Pavement construction Rigid pavement Portland cement used for highway pavements Slump < 3 cm Whitetopping: refer to the growing practice of using concrete overlays over failing asphalt surfaces.

Inspection & Quality Control The quality of concrete and construction operations should be monitored to ensure that final properties are as specified. The plant inspector is responsible for ensuring that the materials and mixing procedures comply with the specifications and the mix design. Specifications usually require that the material be measured in batches, within the following degree of accuracy: Cement (± 1%), Water (± 2%), Aggregate (± 1%), Admixtures (± 3%),

Inspection & Quality Control The quality of concrete and construction operations should be monitored to ensure that final properties are as specified. The plant inspector is responsible for ensuring that the materials and mixing procedures comply with the specifications and the mix design. Specifications usually require that the material be measured in batches, within the following degree of accuracy: Cement (± 1%), Water (± 2%), Aggregate (± 1%), Admixtures (± 3%),

Inspection & Quality Control Strength test (ASTM standard) for ready mix concrete: Specification requires one strength test, consisting of two cylinders, for each 100 m3 of concrete produced. With a minimum of one test for each class of material each day. The strength is the average of the test results from the two cylinders For strength in structures designed using Ultimate strength method, number of tested samples that have values less than required strength should be less than 10% of tested samples & the average of any three consecutive tests should be equal or greater than specified strength.

Inspection & Quality Control Strength test (ASTM standard) for ready mixed concrete: For Other concrete: number of tested samples that have values less than required strength should be less than 20% of tested samples & the average of any six consecutive tests must be as specified strength.

Inspection & Quality Control The standard deviation value is used in quality control and designating a design strength to produce concrete that will adequately meet the specified strength it is calculated as follows: Where: σ : is the standard deviation X: is the test strength X: is the mean strength n: is the number of tests.

Inspection & Quality Control Example: the results of 12 strength tests are 23.8, 24.3, 20.7, 26.2, 24.1, 23.4, 26.8, 22.7, 19.4, 23.4, 21.5, and 19.5 Mpa. Find standard deviation Average = 275.8 / 12 = 22.98 Mpa = 2.36 Mpa Values of standard deviation up to 3.5 Mpa are considered very good

Inspection & Quality Control Quality of hardened concrete Core test: destructive test Rebound hammer: non destructive test

Special Concretes Portland-pozzolan cements: use pozzolanic materials as admixtures that contains silica to reacts with calcium hydroxide to form compounds with cementitious properties (such as fly ash) Advantages: Increase workability Used to replace up to 35% of the Portland cement without serious strength or other problems. Fly ash and ground slag are used with cement type I to produce cement with low heat or sulfate resistance. Replacing type (II, IV, or V) at more economical cost

Special Concretes Light weight concrete can be divided into two categories : Structural light weight concrete have densities of 1400 – 1850 kg/m3 and strength that are usually lower than normal practice Insulating concrete: have low strength with densities as low as 240 kg/m3 Lightweight aggregate, expanded shale and certain types of volcanic rock are used.

Special Concretes Heavy weight concretes are used for shielding purposes in the construction of nuclear reactors at power plants Heavy aggregate: Iron-ore particles are used Density as high as 3200 kg/m3 Architectural concrete usually produced in precast plants Forms that produce special textures and rough surfaces are used

Special Concretes Fiber reinforced concretes contains various types of fibers – steel, glass, added during the mixing operations. Fibers can assist in preventing surface cracks and improve toughness and flexural strength in concrete. Also increase tensile strength of concrete Rollcrete: Used in dam construction with lower w/c ratio and can be placed using large economical earth moving equipment Concrete is compacted by rollers resulting in much higher strength than could be obtained with a similar mix without this type of compaction

Special Concretes Unshrinkable fill concretes used to solve depression problems occurred after some years of trenching conducted in urban and rural roads. This backfill material don’t require compaction and is strong enough to resist all settlement stresses, and can be easily excavated in the future if necessary.

Special Concretes Polymer concretes contains organic materials that combine and grow into polymers , filling small pore space in concrete. These concretes have very high strength & remarkable durability Polymers has been used to protect corrosion of reinforcing steel.

Special Concretes Alumina Cement: is manufactured in the same manner as Portland cement. The main product is Calcium aluminates rather than calcium silicate This cement hydrate rapidly reaching high strength in one day or less Used in many construction or repair situation where time is important and in very cold climates Alumina cement is more resistant to most types of chemical attack than ordinary Portland cement