Civil Engineering Basics

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👉Bonding of Old and New Concrete?

🚧In construction, there always comes a time when there is a need to bond hardened concrete (substrate) with fresh concrete topping/overlay. The aim of this post is to explain how to bond old and fresh concrete successfully.
Furthermore, we will review the strength of the interfacial bond between old and new concrete based on laboratory studies.

⏺Proper bonding is very important for adequate performance when the fresh concrete topping is used to overlay an existing hardened concrete. This construction feature is usually found in bridge deck construction, concrete pavements, precast filigree slab, pile caps (in some cases) etc. Proper bonding between the substrate and the topping is not always guaranteed unless simple precautions are taken.

⏺For adequate bonding, it is very important to prepare the surface of the substrate adequately. The preparation of the surface usually involves:

📌Roughening the surface, and removal of all dirt, oil, grease, and loosened or unbonded portions of the existing concrete.

⏺By implication, the surface of the substrate should be hard, firm, clean, and free from loosened particles. This can be achieved by the use of chipping hammers, wire brushing the surface, etc.

📌After this is done, the exposed concrete surface can be cleaned by using pressurized clean water, air, etc. The man hours involved depend on the area of the surface, location, and the ease of cleaning (e.g reinforcement obstruction).

📌After surface preparation, there is usually a need to apply a bonding agent on the surface of the existing concrete in order to facilitate the bonding. Epoxy-based bonding agents are very popular for such operations. It is recommended that a bonding agent is applied prior to casting the fresh concrete.  In essence, the procedure should be ‘wet-to-wet’ as the bonding agent should not be allowed to dry before the fresh concrete topping is placed.

📌In a research carried out by Vandhiyan and Kathiravan (2017), the compressive strength of monolithic and bonded concrete was compared using 150mm x 150mm cube specimens at 28 days. With an epoxy-based bonding agent, the compressive strength of the bonded concrete was about 5% less than the monolithic strength, while without the bonding agent, the compressive strength was about 28% less than the monolithic compressive strength.

📌Research has also shown that the moisture condition of the substrate affects the shear bond strength of bonded concrete. Shin and Wan (2010) investigated the interfacial bond strength of old and new concrete considering saturated surface dry (SSD) and air dry conditions. Saturated surface dry is a condition in which the concrete contains moisture that is equal to its potential absorption, without the surface being wet or damp.

📌At a water/cement ratio of 0.45 (for the topping concrete), the shear bond strength at the interface was about 44% greater when the substrate was at SSD condition than when it was air dry. At a water/cement ratio of 0.6 for the topping layer, an increase in shear bond strength was recorded, but there was a reduction in the compressive strength of the concrete.

⏺So the recommendation in this article is that when casting a topping layer of fresh concrete on old concrete, adhere to the following guidelines;

(1) Prepare the surface properly
(2) Make sure that the substrate is at saturated surface dry condition
(3) Use a bonding agent and follow the manufacturer’s technical recommendation properly.

Bearing Capacity Equation for Square, Rectangular, and Circular Foundations - General Shear Failure

Terzaghi (1943) developed a rational bearing capacity equation for strip footing, by assuming the bearing capacity failure of the foundation in general shear mode.  Terzaghi's bearing equation is  given by:

http://etconp.blogspot.com/2023/04/bearing-capacity-equation-for-square.html

👉Calculation of Lap length in Reinforced Concrete Structures

🔰Lap length in Reinforced Concrete
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Lap length is one of the important term in the reinforcement.

This is usually confused with another important term called development length and anchorage length. In this article, the lap lengths of bars is discussed. During the placement of steel in Reinforced concrete structure, if the required length of single bar may fall short. To get the desired design length, lapping of two bars side by side is done. An alternative to this is to provide mechanical couplers.
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Lapping can be defined as the overlapping of two bars side by side to upto the design length. Usually, the stock length of steel bars is limited to 12m. This is for easy transportation of steel bars to the construction site. For example, imagine there is a need to build a 100ft tall column. But it’s not practically available. Hence the bars are cut every second story.

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Then the tension forces are required to be transferred from one bar to the other bar at the location of discontinuity of the bar. So the second bar is kept closely to the first bar and overlapping is done. This amount of overlapping between two bars is called “lap length”. Lapping is usually done where minimum bending stress is encountered. In general, lap length is 50d which means 50 times the bar diameter, if both bars are of same diameter.

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🔰Lap length in tension:

The lap length including anchorage value of hooks shall be
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For flexural tension – Ld or 30d whichever is greater is considered. For direct tension – 2Ld or 30d whichever is greater is considered.
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The straight length of lapping of bars shall not be less than 15d or 20cm.
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🔰Lap Length in Compression:

The lap length is equal to the development length calculated in compression but not less than 24d.
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🔰For Different Diameter Bars:

When the bars of different diameters are to be spliced, the lap length is calculated considering the smaller diameter bar.
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🔰Lap Splices

Lap splices should not be catered for the bars which are having diameter greater than 36 mm. In such cases, welding should be considered. But if welding is also not feasible in some conditions, then lapping may be allowed for the bars larger than 36 mm diameter. But along with lapping, additional spirals of 6 mm should be provided around the lapped bars.

Types of Admixtures Used In Concrete

Admixtures are natural or manufactured chemicals which are added to the concrete before or during mixing. The most often used admixtures are air-entraining agents, water reducers, water-reducing retarders and accelerators.
We'll discuss The Types of Concrete Admixtures in the following Posts.

1- Water Reducing Admixtures
Water reducing admixtures, the name itself defines that they are used to minimize the water demand in a concrete mix. Workability is the important property of concrete which is improved with the addition of water but if the water is added more than required the strength and durability properties of concrete gets affected.
In addition, to increase in workability it also improves the strength of concrete, good bond between concrete and steel, prevents cracking, segregation, honeycombing, bleeding etc. Water reducing admixtures are also called as plasticizers and these are classified into three types namely plasticizers, mid-range plasticizers and superplasticizers. Normal plasticizer reduces the water demand by up to 10%, mid-range plasticizers reduce the water demand by up to 15% while superplasticizers reduce the water demand by up to 30%.

2- Retarding Admixtures
Retarding admixtures slow down the rate of hydration of cement in its initial stage and increase the initial setting time of concrete. These are also called as retarders and used especially in high-temperature zones where concrete will set quickly.  The quick setting in some situations may lead to discontinuities in structure, the poor bond between the surfaces creates unnecessary voids in concrete etc. Retarders are useful to eliminate this type of problems. Commonly used retarding admixture is calcium sulphate or gypsum. Starch, cellulose products, common sugar, salts of acids are some other retarders. Most of the water-reducing admixtures are also acts as retarding admixtures and they are called as retarding plasticizers.