Effect of depth of epoxy coated lapped splices on bond efficiency

This paper gives a report of research investigation carried out to study the effect of depth of epoxy coated lapped spliced bars on bond strength and efficiency. Thirty-six full size beams of varying lengths and sectional dimensions with lap spliced bars in constant moment region were cast and tested in a four-point bending system. Bond strength and efficiency of epoxy coated top reinforcing bars were investigated using beams with 300 mm and 500 mm depths. Three beam sizes (300 × 180 mm, 300 × 200 mm and 300 × 230 mm) and high yield reinforcing bars (16 mm, 20 mm and 28 mm) were coated with epoxy over the lap length and used with each depth. The ultimate moments from the tests were used to determine the stress developed in the reinforcing bars. Coated top reinforcing bars were found to have lower bond strength and efficiency than coated bottom reinforcing bars in the depths investigated. The location factor, α, which is a reduction factor determined using an average of three beams, for epoxy coated top bars were found to be 0.86, 0.84 and 0.89 respectively for 16 mm, 20 mm and 28 mm diameter bars at 300 mm depth and 0.62, 0.80 and 0.86 respectively for 16 mm, 20 mm and 28 mm diameter bars at 500 mm depth.


Introduction
In reinforced concrete structures, it is sometimes necessary to reinforce members at both the top and bottom (doubly reinforced section). The need arises when the depth of beam is limited and the tensile reinforcement is not adequate to take the moment. In continuous beams, top bars are needed at the supports, so also, cantilevered slabs and beams require the use of top reinforcements to take care of the negative moments. In these cases, it has been found that bond resistance between concrete and uncoated reinforcing bars at the top is reduced as a result of the cavitation created by the air trapped under the bars during vibration of the concrete, the air trapped under the bars may further reduce the bond resistance, of coated top bars. Bond strength is known to be reduced for top bars and coating has been severally confirmed to reduce bond resistance. The extent of further reduction in bond strength when epoxy coated bars are used as top reinforcement is expected to be determined by this investigation.
One of the most influential but least precisely defined variable influencing bond is the position of the embedded bar during casting and hence the amount of concrete poured in a single process under the bar. Uncoated reinforcing bars have been used to date in investigating top reinforcing bars, the results showed a reduced bond strength (Ferguson and Thompson [1], Morita et-al [2]). No attempt has yet been made to investigate epoxy coated top reinforcing bars. Investigation of bond efficiency of epoxy coated top reinforcing bars is worthwhile to increase our knowledge on the extent of reduction or otherwise on bond strength.

Design of test beams
Thirty-six (36) beams were cast for the investigation. Twelve (12) and twenty-four (24) beams were cast for the preliminary and confirmatory investigations respectively. Two depths (300 mm and 500 mm) were investigated and the widths were of three sizes, 180 mm, 200 mm and 230 mm. Epoxy coated high yield, 28 mm, 20 mm and 16 mm diameter bars were used.
Contact arrangement of lapped bars was adopted in this investigation because it is the most probable on construction sites. The loading points were 150 mm from the lap ends thereby making the distance between the point loads to be ls + 300 mm. The ultimate anchorage bond length recommended in BS 8110: Part I 1988 [4] was used. (1) Failure mode in flexure is strongly dependent on the shear-span/effective depth ratio, av/d. Based on reported experimental studies in Kong and Evans [5] the failure pattern to ensure flexural failure was used. av/d was taken as 3, hence av = 3d. The depth of all beams was either 300 mm or 500 mm while the length is a function of lap length and shear span, thus length  = s  + 300 mm + 2a

Preparation and application of coating materials
The reinforcements were sand blasted to remove rust.

Data collection
The Epoxy zinc rich is in two parts. Part A, base and Part B, which is the hardener. The parts were measured out by volume in ratio 3:1, as recommended in the manufacturer's manual, and the two parts were thoroughly mixed together. Thinner was added until the mixture was light enough to pass through the nozzle of a conventional spraying gun. The lengths of the reinforcements to be coated were marked out. The mixture was then sprayed, in one coat, on the marked portions of the reinforcements and allowed to dry in the shade without direct exposure to sunshine.

Manufacture and test procedure of cubes and beams
The Concrete used for the investigation was Grade 35 with targeted cube strength of 35 N/mm 2 . Concrete cubes were cast in batches and cured for 28 days in the laboratory. The strengths achieved at test are in table 2. The reinforcement cages were put in the mould and concrete spacers were placed at interval at various locations; bottom and sides. The ready-mix concrete was poured and vibrated in the mould with poker vibrator. After demoulding, the beams were cured by wetting at intervals for seven days.

Compressive strength of concrete cubes
The aim of the test was to determine the strength of concrete in the beams and to serve as a control so that the compressive strength reach at least the targeted value of 35 N/mm 2 . The cubes were weighed on Avery weighing machine, 50 kg capacity and tested in the Avery Universal testing Machine of 100,000 kg capacity.

Beam test
The beams were tested with a four-point bending arrangement with an I steel section as the spreader beam in a 100,000 kg capacity Avery Universal Testing Machine to determine the ultimate failure loads, modes and crack patterns. The load was gradually applied until the failure of the beam occurred. The failure load was recorded and analysed as follows

Analysis of test results
The steel stress developed in each beam was determined by analysing the section using the general theory for ultimate flexural strength. The ultimate moment for each beam was obtained by applying the principles of statics. The lapped splice was in the constant moment region. The ultimate moment at the point load in figure 2 is: Cross section Strain Stress and Forces Taking moment about the compressive force, the ultimate moment of resistance is The properties of the concrete stress block are expressed in terms of the characteristic ratio k1 and k2. Stress block proposed by Hognestad et-al reported in Kong and Evans [5] and universally accepted was used. Based on the mean cube strength, various values of k1 and k2 and Σcu are tabulated in table 2. The quadratic equation in (5) was solved yielding two values of fs, one of which was always unreasonable because it is not practicable. The reasonable value was the stress developed in the steel.

Test Bond Stress, τt
The steel stress developed in each beam that was determined from equation (5) was substituted in (7) to obtain the τt (τtest).

Theoretical Bond Stress, τcal
The theoretical bond stress, τcal, was determined from a semi-empirical statistical regression equation developed by Orangun et-al [6].
For bars with no transverse reinforcement ( ) Equations (9) is in imperial unit and concrete cylinder strength. The expressions were converted to SI units and modified for use with concrete cube strength by Cairns and Abdullah [3] to give:

Bond Stress using BS 8110's Recommendation, τBS8110
The British Standard, BS 8110's, 1997 [8] recommended bond stress was also used to calculate the bond efficiency.
with the factor β, taken as 0.5 because deformed bars were used in tension.

Effect of Depth of Epoxy Coated Top Reinforcement on Bond Efficiency
The effect of depth of epoxy coated top reinforcing bars on bond efficiency was investigated using beams with two depths, 300 mm and 500 mm. Three beam sectional sizes (300×180 mm, 300×200 mm, 300×230 mm) and varying high yield reinforcing bars (16 mm, 20 mm and 28 mm diameter) were used for each depth. One beam each was cast in the preliminary investigation. The beam data and the test results are in tables 1 and 4; figure 3. Generally, the preliminary tests showed that bond efficiency of coated top reinforcing bars were lower than that of coated bottom reinforcing bars. The decrease in bond efficiency did not follow any regular pattern for the various diameters of bars nor for the depths investigated. Confirmatory tests DCB2020 IOP Conf. Series: Materials Science and Engineering 951 (2020) 012011 IOP Publishing doi:10.1088/1757-899X/951/1/012011 7 were carried out with two additional beams each for the bar diameters and depths used in the preliminary beams. The trend observed in the preliminary tests was confirmed in the confirmatory tests. Coated top reinforcing bars were found to have a lower bond strength and efficiency than coated bottom reinforcing bars at the two depths. This is in line with previous research findings on uncoated top reinforcing bars by Tepfers [9], Jirsa [10], Donahey and Darwin [11]. The lower bond efficiency recorded in top coated reinforcing bars may be due to the cavities created underneath the reinforcing bars by the escaping air bubble during the vibration of the concrete.
If the distance from the base of the beam to the top bar is considered, it may be said that the higher the depth the lower the bond strength and efficiency. This analogy is confirmed in beams with 16 mm diameter bars as the difference in the average bond efficiency of top and bottom cast bars is 0.14 for 300 mm deep beams and 0.44 for 500 mm deep beams. This was also the pattern for beams with 20 mm diameter bars, as the difference in the bond efficiency for 300 mm deep beams is lower than that of 500 mm deep beams, by 0.15 and 0.25 respectively. Beams with 28 mm diameter bars had 0.04 as difference for 300 mm deep beams and 0.14 difference for 500 mm deep beams. If the performance of coated 16 mm, 20 mm and 28 mm diameter top cast bars are compared at the two depths, a reduction in average bond efficiency of 20.2%, -6.98% and 7.5% is observed respectively. The negative reduction in bond efficiency for 20 mm was not expected, more tests should be carried out to have a better knowledge. The bottom cast bars showed the reverse, that is, an increase in bond efficiency. Bond efficiency for bottom cast is expected to be higher with the depth of concrete as in figure 3, depending on the cube strength of concrete, level of compaction, lever arm etc.   figure 3, it is generally observed that the bond efficiency of top cast bars was lower for 500 mm deep beam than for 300 mm deep beam except with 20 mm diameter bars. The reverse is the case for bottom cast bars, bond efficiency of bottom cast bars was generally high for 500 mm deep beams than for 300 mm deep beam. The effect of depth appeared to be more obvious with top coated reinforcement.
Cavities under the reinforcing bars, which reduces bond efficiency, could be caused by the escaping air bubbles which settle underneath the top bars, as well as the effect of settlement of the concrete. The reinforcement is not completely rigid in reality and this reduces the effect of the damage due to settlement. Also, the concrete at the top of a member is generally less compacted and also tends to have greater water content, resulting in lower concrete strength and of course lower bond strength.

Location Reduction factor, α
The variation in concrete strength was eliminated by multiplying the test splice strength with From the test results, in figure 3, the bond strength of coated top reinforcements was found to be lower than the bond strength of coated bottom reinforcements. Attempt was made at deducing a factor for the reduction with increasing height using the average value of the splice strength of the three beams for each bar diameter to determine the ratio of the corrected bond strength of coated top bar to the corrected bond strength of coated bottom reinforcement.
The average of the location reduction factors, α, for coated top bars were found to be 0.86, 0.84 and 0.89 for 300 mm depth and 0.62, 0.80 and 0.86 respectively for 16 mm, 20 mm and 28 mm diameter bars at 500 mm depth. The higher the depth of concrete, the lower the bond strength and efficiency. The effect of bar diameter was not consistent with the depth of concrete below the bar, table 5.